A very perplexing ecological problem is unravelling the nature of relationship between diversity and stability of ecosystem. The problem here is that neither the meaning of stability is entirely clear, nor what aspects of diversity are being consided. Are only the number of species in the community and its relation to stability, or the relationship of evenness to stability, or some combination of both is to be considered? Most of the theoretical and empirical studies for long focused only on species diversity, i.e., the number of species present. Only recently works have focussed on the degree of functional diversity represented by species diversity. Advocates of conserving biological diversity have always invoked the diversity-stability hypothesis to justify concern about the loss of individual species. However, if other aspects of diversity also play important roles in the structure and function of ecosystems, a focus on the number of species alone may hinder proper appreciation of the role that evenness plays in the ability of ecosystems to respond to changes in energy and nutrient inputs.
There are at least three ways in which ecosystem stability might be defined:
1. Constancy- The ability of a community to resist changes in species composition and abundances in response to any disturbance.
This is not a particularly useful concept of stability for conservationists because few, if any, ecosystems could be described as truly constant. Even the ecosystems having powerful mechanisms for reacting to environmental fluctuations do so through internal changes that as quickly as possible bring it back to a stable state. But these surely involve responses and changes. These may more appropriately be regarded as examples of resilience than of constancy.
2. Resilience- The ability of a community to return to its pre-disturbance characteristics after changes induced by a disturbance.
Resilience corresponds to stability in the way it is studied in mathematical models. If deviations from an equilibrium are reduced with time, system is stable or if these are amplified with time, system is unstable. This approach still has little applicability to actual ecosystems. It measures a system’s tendency to return to a single stable point, but many ecological sytems appear to have multiple stable points. If disturbance remains below a particular threshold, ecosystem will return to its predisturbance configuration. If it exceeds that threshold, it may move to a new configuration. Furthermore, most ecological systems change not only in response to disturbance but also in response to natural, successional change. There is little evidence that ecological communities ever represent an equilibrium configuration from which it would make sense to study perturbations. Common between the constancy and resilience is focus of both on species persistence and abundance as measures of stability. For example, Selmants et al.[1] show that with decline in species diversity of serpentine grasslands in California, their susceptibility to invasion by exotic species increases. Put differently, diverse grasslands were more resilient than those with lesser diversity.
3. Dynamic stability- A system’s ability to determine its future states largely by its own current state with little or no reference to outside influences.
In many ways, this approach corresponds with our intuitive notions of stability and seems to make sense of the relationship between diversity and stability. It recalls saying of Commoner [2]: “The more complex the ecosystem, the more successfully it can resist a stress.” A dynamically stable system is relatively immune to disturbance like a rapidly spinning gyroscope is dynamically stable because the gyroscopic forces generated by it resist external forces that would alter is plane of rotation. This approach reflects the hope that stable systems would be able to maintain themselves without intervention.
A biological system with high diversity is more likely to be dynamically stable than one that has low diversity. The reason is very important role played by biotic interactions in ecosystem dynamics. This has increasingly been appreciated through many studies. In diverse communities, biotic interactions may often play a larger and very important role in the success of a species than its interactions with the physical environment. To the extent that changes in the system are driven by biotic interactions, it is dynamically stable, since characteristics of the system itself are determining its future state.
However, this formulation of the diversity-stability hypothesis is also not free from problems. How to identify systems whose future state depends primarily on their own internal characteristics?[3] Without a method to identify a dynamically stable system, even testing the truth of this approach to diversity-stability hypothesis is not possible. It seems to verge on circularity. The larger (more diverse) the system considered, fewer things are left out of it. Fewer the things left out, smaller the possible outside influences on the system. Smaller the possible outside influences, greater the degree of dynamic stability. Thus, dynamic stability is (almost) a necessary consequence of diversity simply because diverse systems include many components.[4] Moreover, the argument as presented says nothing about the types of diversity present, e.g., a diverse community assembled from non-native species would be as good as one composed solely of natives.
Ives and Carpenter [5] have suggested a different approach to understanding community stability. Their approach may be quite useful, because it points out firstly, that systems move to a region different from the one from which they were perturbed[6] and secondly, that things other than diversity (like the frequency and character of perturbation) may also affect the stability of ecosystems. A new concept in relation to stability is ‘Biological integrity’ that refers to a system’s wholeness, including presence of all appropriate elements and occurrence of all processes at appropriate rates.[7] But this approach too poses problems.
1. What are ‘appropriate elements’?
2. What are ‘appropriate rates of processes?’
By definition, naturally evolved assemblages possess biological integrity but random assemblages do not. It, therefore, provides justification for management of ecosystems focusing on native species rather than introduced ones. This seems like the logical fallacy of affirming the consequent. However, species composition of lakes exposed to nutrient enrichment or acidification responds more quickly and recovers more slowly than processes like primary production, respiration, and nutrient cycling. Shifts in biotic composition don’t necessarily lead to changes in process rates. These observations mean a focus on integrity rather than diversity makes sense but it makes more sense to conclude that species changes are a more sensitive indicator of what is going on than the process changes. Loss of native species from a system is truly a warning of process changes that may have consequences much larger than are suspected.
From the point of view of conservation, there are still many problems.
1. Can it be psossible that constancy or resilience based approaches to diversity-stability hypothesis probably are not true and may not provide a solid conceptual basis for arguing that conservation of biological diversity is an important goal.
2. Is it possible that a less specific version defining stability as a dynamic property related to the degree that the components of a system determine their own future state, provides a plausible basis for the hypothesis. Unfortunately, this version of the hypothesis verges on circularity and is almost immune to empirical investigation. It may also be pointed out that a system that is “stable” with respect to some perturbations like hurricanes, drought, or other extreme weather events may not be stable to others like invasion by exotic plants or animals, extinctions of component species, or other biotic changes. From the point of view of practical conservation applications, the diversity-stability hypothesis seems to provides merely a useful heuristic.
There seems something more useful for practical applicability in the idea of biological integrity. Easily observable changes in species composition and community structure may act as pointer to underlying changes in ecosystem processes more quickly than attempts to directly measure these processes. Diverse systems provide more indicators of change in these underlying processes and if the systems are managed so that they are protected then the underlying processes will remain intact too. Chapin et al. [8] summarized that:
1. High species richness maximizes resource acquisition at each trophic level and the retention of resources in the ecosystem. 2. High species diversity reduces the risk of large changes in ecosystem processes in response to directional or stochastic variation in the environment. 3. High species diversity reduces the probability of large changes in ecosystem processes in response to invasions of pathogens and other species. 4. Landscape heterogeneity most strongly influences those processes or organisms that depend on multiple patch types and are controlled by a flow of organisms, water, air, or disturbance among patches.
Wang and Loreau [9] developed the last point more formally and suggested that when thinking about a meta-community or meta-ecosystem it is useful to decompose the variability in response across the entire system into components analogous to those used in partitioning species diversity i.e.
1. Variation within the individual components of a meta-community or meta-ecosystem. 2. Variation among different components of a meta-community or meta-ecosystem. 3. Variation across the entire system, the sum (or product) of alpha and beta variation.
Thinking about ecosystem functioning at various scales, as Wang and Loreau suggest, leads to recognition that the experimental focus on diversity and variation in functioning leaves out a vital component for those trying to manage ecosystems that includes a variety of different habitats. Stability at the whole-system level may depend as much or more on retaining those distinct components as it does on stability within any one of them.
Bibliography
1. Paul C Selmants, Erika S Zavaleta, Jae R Pasari, and Daniel L Hernandez. Realistic plant species losses reduce invasion resistance in a California serpentine grassland. Journal of Ecology,
100(3):723-731, 2012.
2. B Commoner. The Closing Circle. Alfred Knopf, New York, NY, 1972. 3. R MacArthur. Fluctuations of animal populations and a measure of community stability. Ecology, 35:533-536, 1955.
4. B G Norton. Why Preserve Natural Variety? Princeton University Press, Princeton, NJ, 1987.
5. Anthony R Ives and Stephen R Carpenter. Stability and Diversity of Ecosystems. Science, 317(5834):58-62, 2007.
6. Laurie J Raymundo, Andrew R Halford, Aileen P Maypa, and Alexander M Kerr. Functionally diverse reef-fish communities ameliorate coral disease. Proceedings of the National Academy of Sciences,
106(40):17067-17070, 2009.
7. Noah A Rosenberg, Jonathan K Pritchard, James L Weber, Howard M Cann, Kenneth K Kidd, Lev A Zhivotovsky, and Marcus W Feldman. Genetic structure of human populations. Science, 298(5602):2381-2385, 2002. 8. F S Chapin III, O E Sala, I C Burke, J P Grime, D U Hooper, W K Lauenroth, A Lombard, H A Mooney, A R Mosier, S Naeem, S W Pacala, J Roy, W L Steffen, and D Tilman. Ecosystem consequences of changing biodiversity: experimental evidence and a research agenda for the future. BioScience, 48:45-52, 1998.
9. Shaopeng Wang and Michel Loreau. Ecosystem stability in space: , and variability. Ecology Letters, 17(8):891-901, 2014.
The temperature of a planet irradiated by solar radiation can be estimated by balancing the amount of radiation absorbed (Ra) against the amount of outgoing radiation (Ro). The Ra will be the product of:
(i) Solar irradiance (I)
(ii) Area of planet. Area of planet relevant for such calculations is the area of the planet as seen by incoming radiation which is given by r2 where r = radius of the planet.
(iii) Absorbed fraction of radiation. The fraction of radiation that is absorbed is given by (1 – A where A = albedo of planet. This albedo represents the fraction of radiation that is reflected back from the planet.
Figure. 1. Energy balance of the Earth. (Components values in kcal/cm2/year).
Thus the energy absorbed by the planet will be:
Ra = I (1 – A)
Intensity of outgoing radiation of a body is given by Stefan-Boltzmann Law i.e.
Io = T4
where = Stefan-Boltzmann Constant = 5.6 x 10-8 Wm-2 K-4. The total energy radiated by the planet will be the product of the intensity of outgoing radiation (Io) and the area of the whole planetary surface giving out radiation (4 r 2). Thus, the outgoing radiation (Ro) from the planet is given by:
Ro = 4r2 T4
Effective planetary temperature
Since Ra = Ro i.e. system is assumed to be in steady-state where radiation absorbed and outgoing radiation are equal, an expression for the effective planetary temperature (Te) can be obtained from the above equation and it may be given by:
Te = [I – (1 – A)/4 ]0.25
In this expression of effective planetary temperature, effect of atmosphere has not been taken into account. For Earth, solar irradiance (I) at the top of atmosphere is about 1.4 x 103/m2/s and albedo of Earth as a whole is about 0.33. From these values, the calculated equilibrium temperature of Earth comes to be 254 K. However, the actual observed average ground level temperature of Earth is about 288 K. This higher effective temperature of Earth from the calculated value is due to the greenhouse-effect of atmosphere.
The black-body spectrum of Earth at 288 K shows that radiation from Earth is of much longer wavelength and is at much lower intensity than radiation from Sun. The absorption spectrum of Earth’s atmosphere overlaps fairly well with the solar emission spectrum. Except for a very narrow window in the absorption bands, much of the long-wave radiations from Earth correspond with the region of absorption in the atmosphere. This means that much of the incoming radiation reaches the Earth’s surface while the outgoing thermal radiation is largely absorbed by the atmosphere rather than being lost to space. Thus, the effect of atmosphere is to trap the outgoing thermal radiation. This effect is termed green-house effect.. The thermal radiation i.e. the heat trapped by the atmosphere due to green-house effect is responsible for the effective temperature of Earth being higher than the temperature calculated without taking into account the effect of atmosphere. In general, absorption of re-emitted long-wave radiation and vertical mixing processes determine the temperature profile of the lower part of atmosphere (troposphere) which in turn determine the Earth’s temperature.
Optical depth of atmosphere and Earth’s surface temperature
The atmosphere is not transparent to the outgoing long-wave radiation and much of this radiation is absorbed in the lower part of the atmosphere, which is warmer than the upper parts. Simple radiative equilibrium models have been developed for Earth and to account for this effect, these models divide the atmosphere into layers that are just thick enough to absorb the outgoing radiation. These atmospheric layers are said to be optically thick and the atmosphere is discussed in terms of its optical depth based on the number of these atmospheric layers of different optical thickness. Earth’s atmosphere is sometimes said to have two layers while that of planet Venus has almost 70 layers which are largely due to enormous amount of CO2 in the atmosphere of Venus. The radiation equilibrium model indicates that the effective planetary temperature (Te) is thus related to ground-level planetary temperature (Tg) by the equation:
The optical depth of atmosphere increases with increase in atmospheric concentrations of carbon dioxide and water vapor because both these are principal atmospheric absorbers of outgoing long-wave radiation. With increasing concentrations of CO2 in lower layers of atmosphere, other such gases that are responsible for radiating heat to outer space are pushed to slightly higher and colder levels of atmosphere. The radiating gases will radiate heat less efficiently because they are colder at higher altitudes. Thus, the atmosphere becomes less efficient radiator of heat and this results in rise of atmospheric temperature. This rise in atmospheric temperature, in turn, leads to more evaporation and increase in atmospheric water vapor, which is a greenhouse gas and further increases the absorption of outgoing long-wave thermal radiation. This positive feedback results in further increase in atmospheric temperature. The model also suggests that increase in atmospheric CO2 is associated with decrease in temperatures of upper (stratospheric).
Vertical heat transport and Earth’s surface temperature
Simple models of radiation balance of atmosphere do not take into account various other processes that transport heat vertically in the atmosphere and, therefore, overestimate the surface temperature of Earth. Convection is major process of vertical heat transport and is very important in lowering the surface temperature. Convection occurs because warm air is lighter than cool air and so rises upwards carrying heat from Earth’s surface to the upper atmosphere. As warm air rises up, it expands due to fall in pressure and work done in expansion causes it to cool adiabatically. Thus,
CvT = – P V (where Cv = molar heat capacity at constant volume)
Ideal gas equation PV = RT takes the differential form P dV + V dP = R dt which may be rearranged in incremental form as:
– P V + R = V P
This equation may be combined with equation CvT = – P V using the fact that Cp – Cv = R, where Cp = molar heat capacity at constant pressure = 29.05 J/mol/K. This results in following equation:
CpT = (Cv + R) T = – P V + R T = V P = (RT/P) P ………….(a)
It can be shown that P/P = – Mmg z /RT where Mm = mean molecular weight of air = 0.028966 kg/mol; g = acceleration due to gravity = 9.8065 m/s/s; z = altitude. This gives:
RT/P = – Mmg z/ P……………………………………………….(b)
Substitution of the above equation (b) in equation (a) gives:
CpT = – Mmgz
or,T/ z = – (Mmg/Cp)
For Earth’s atmosphere, the lapse rate ( T/ z) works out to be -9.8 k/km for dry air. However, the air is usually wet and as it rises up, it releases latent heat so the measured lapse rate is -6.5 K/km.
If atmospheric temperature falls much less slowly with height than the lapse rate (or even rises with height) then inversion conditions exist and air is very stable with respect to vertical convective mixing. Conversely, if temperature falls very rapidly with height, at a rate greater than lapse rate, then the atmosphere is unstable and convective mixing will be active.
Short-wave radiation and temperature
The discussion till now has assumed total transparency of atmosphere to incoming solar radiation. Though it is true for visible range of radiation, it is not true for ultra-violet region of the solar spectrum. Though the amount of such short-wave radiation is very small, it has important consequences for the temperature of Earth-atmosphere system.
Various ultra-violet wavelengths are absorbed in the atmosphere at different heights. At just over 40 km, absorption of ultra-violet radiation by ozone results in considerable warming of stratosphere and in this zone, temperature rises with altitude. Average temperature of stratosphere is 250 K. Considering it to be a black-body radiator, maximum power radiation would be expected at 11.5 µm. This value is very close to absorption band of carbon dioxide which means that this gas also plays important role in stratospheric temperature. Increase in concentration of carbon dioxide in stratosphere might allow more effective radiation from stratosphere and, therefore, its cooling. This effect is quite opposite to that noted for troposphere.
Further, at the altitude of thermosphere, atmosphere is very thin. In this zone, molecules are exposed to unattenuated solar radiation of extremely short wavelength i.e. of high energy. This radiation arises from the outer region of Sun. At wavelengths below 50 nm, effective emission temperature exceeds 10,000 K. High-energy solar protons of such wavelengths are absorbed by gas molecules giving them high transitional energies i.e. high temperatures. The energies may be large enough to dissociate oxygen and nitrogen. Temperatures in thermosphere undergo wide variations depending upon the state of Sun. During solar disturbances, output of high-energy protons is very much enhanced that results in very high atmospheric temperatures. Temperature in this zone may further be increased by another mechanism. The temperature is normally defined in terms of transitional energy but absorption and emission of radiation occur through vibrational and rotational changes. In upper atmosphere, the frequency of molecular collisions is relatively low and so exchange of translational, vibrational and rotational energies is infrequent. Hence the cooling of thermosphere by re-radiation is very inefficient. The temperature of thermosphere increases with height so it is also stable against convection. Heat can be lost only by very inefficient diffusion processes and as a result, thermospheric temperatures are extremely high.
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Electromagnetic radiation consists of the waves of enery combining electical and magnetic fields. These include whole range of electromagnetic wave spectrum: from very long-wave radio waves at one end to X-rays and gamma rays at the other end of spectrum. Visible light falls in a very narrow wave band in the middle of the spectrum. Every iving and non-living object in nature is constantly being exposed to a natural background of electromagnetic radiation that comes from space as well is produced by radioactive elements in the Earth’s crust. A large proportion of the cosmic radiation coming from space is absorbed by the atmosphere and only a very small portion reaches the ground. However, there is no such filtering of radiation originating from the Earth itself. All living organisms are evolutionarily adapted to such natural radiations in their natural environments. In fact, animals and plants use electromagnetic radiation for a variety of their living activities e. g. communication, control and regulation of their various physiological, psychlogical and behavioural functions. Though essential for living organisms, exposure to excess such radiation beyond the naturally evolved tolerence limits causes various harmful effects in them. The effects of increased exposure to electromagnetic radiation on human, animal and plant bodies are now coming to light and are being increasingly studied.
In the present urban, domestic and working place environments, sources of electromagnetic radiations are increasing rapidly. Increasing radiations from sources like power lines, microwave, telecommunication, electrical appliances, radar, transmissions of radio and television etc. are causing the problem of increasing electromagnetic pollution of environment.
The electromagnetic radiation may be classified into two broad categories according to the frequency and their effects. First category includes relatively low-frequency radiation, from visible light wave band down through infra-red, microwave, radar, television and radio waves and constitutes non-ionizing radiation. Second category includes relatively high frequency gamma and X-rays and constitutes ionizing radiation. Exposure to excessive dose of both the types of radiations casuses various harmful effects on living organisms.
The effects of extremely low-frequency electromagnetic radiations are dependent on dose and duration of exposure and are cummulative. It may take years of exposure before symptoms will appear. Usually symptoms of such electromagnetic pollution manifest as constant headaches, lack of energy, loss of apetite, mental blocks, decreased ability to concentrated, insonia & sleep disturbances, palpitaions, dizziness, trembling and rashes. After prolonged exposure, the symptoms may proceed to blackouts, nervous & psychological disorders like depression, feeling of being trapped, anxiety attacks, increased suicidal impulses, epilepsy, lowered libido & fertility, increased risk of arthritis and even cancer. White blood cells (WBC) are particularly sensitive to electromagnetic radiation and the risk of leukaemia is increased in those areas where exposure to such radiations is high e.g. around around power lines. Exposure to alternating magnetic fields accompanies exposure to electromagnetic radiations around power lines and from a variety of electrical appliances. Such exposure causes build up of serum triglycerides i.e. The fats found in blood stream that are implicated in heart diseases. Constant or frequent overexposure to such radiations may contribute to onset of heart problems.
A fully loaded 400 kv power line creates an electromagnetic field for 350 metres on either side of the line. This can generate electrical currents in the body, which produce an effect comparable to those that gives relief from pain. This effect of electromagnetic radiation on body is possibly due induced production of endorphins (natural pain-killers in body). It has been observed that continuous exposure to such radiations results in development of addictive dependency in cows and they start to prefer standing under power pylons for grazing or resting. They show withdrawl symptoms when away from such stimulation. It has been found that excessive concentration of positive ions builds up under power pylons and laboratory animals have died after constant exposure of 3 months to such conditions. A 50 Hz electromagnetic field has been found to adversely affect E. coli bacteria and water of wells underneath power lines has been shown to be devoid of naturally occurring bacteria present in waters away from such exposure. Laboratory experiments on rats, mouse and animal cells have shown an increased activity of enzyme ornithine decarboxylase in electromagnetic field of strength comparable to that produced by power lines. This enzyme speeds up the growth of cancer cells. Similar frequencies of radiation have been found to affect loss of calcium from the brain. it has been suggested that the problems arising from exposure around power lines may partly be due the effects of such radiation on calcium metabolism of the body.
When an object, living or non-living, enters an electromagnetic field, the field folds over the object so that its strength may become several hundred times more than that of unperturbed field. In the animal body, head houses the most vital organ, the brain which is most sensitive to electromagnetic field. Therefore, the head is the most affected part by the field-strength enhancement effect of electromagnetic field folded over animal body. Researches on the effects of electric blankets had shown alarming results because the body of user is entirely exposed to electromagnetic field for quite a few hours regularly. Among user women, seasonal (September to June) increase in miscarriages has been reported. The menstrual cycle is also disturbed in user women, which may be a contributing factor in the incidence of hormone related cancers such as breast cancers. The effect of electromagnetic field on such cancers is thought to be due to the effect of the field on melatonin production in the body. Electrically heated beds and blankets have also been linked to slower foetal development and learning problems in children, especually if mothers also used these during pregnancy.
The strength of electrical field drops off quite quickly with distance from the source and its frequency is clearly that of the source. However, the magnetic field fluctuates more, has ‘contaminating frequencies’ and its strength does not decrease with distance as quickly as that of electrical field. Furthermore, with increased distance, there is more likelihood that distribution of magnetic field over the body is uniform. A 400 kv power line may be generating underneath it a magnetic field of 1 microtesla (when current is around 100 A) to 10 microtesla (at greater loads, usual maxima being 5000 A) depending upon many variables like load, capacity, ground and weather conditions. It has been observed that people living near incoming supply in high-rise flats or ground floors are exposed to average magnetic field of 0.2 to 0.4 microtesla. These people have been found to be more susceptible to risks of heart diseases, cancers, depression and thyroid problems than those living on top floors away from the supply lines where magnetic field may be only around 0.015 microtesla. in cities, there may be continuous exposure to magnetic field of less than 0.1 microtesla in normal households. Such exposure may cause depressive feelings in the inmates. Electrical workers may experience exposure to levels up to 5 microtesla and the risks of various bodily and psychological disturbances to them may well be more harmful than moving in and out of similar field. Probably unbalanced, non-uniform magnetic field causes greater risk of various diseases and disturbances in the exposed subject than a balanced and unform field.
Apart from human beings, a number of detrimental effects of electromagnetic radiations have also been observed on animals and plants. It has been observed that earthworms are distributed and move away from underground power cables. Hens living near power lines lay scrambled eggs in thin shells, bees seal up their hives and become aggressive, cows loose apetite znd birds such as homing pigeons become disoriented. Plants exposed to electromagnetic radiation show disturbed root growth, seed germination, growth of pollen tubes, ion & water uptake and photosynthesis.
Exposure to microwaves is also increasing particularly in households due to increasing use of microwave-producing gadgets. Such exposure is also posing health risks as microwave exposure is known to cause cataracts and have detrimental effects on nervous and cardio-vascular systems.
Atmospheric pollution becomes problem over a large area because atmosphere transports relatively uniform concentrations of air pollutant(s) over considerable distances. Rhdhe et al. (1982) categorized the scale of atmospheric transport as follows:
Local transport: This occurs from individual point or line source for a distance of few kilometres only. It is mainly associated with plumes and is affected by local meteorological conditions. As the plume disperses, most of the pollutant falls back to the ground as dry deposition.
Regional transport: This occurs to distances less than a thousand kilometres. At this scale, individual plumes merge together and development of a relatively uniform profile of pollutant after about 1-2 hours becomes possible. Such transport causes spread of air pollution problem from urban/industrial sources to the surrounding downwind countryside. In such transport, pollutants come down both by dry and wet deposition depending upon meteorological conditions.
Sub-continental and continental transport: Such transport occurs over several to a few thousand kilometres. In such transport, relatively uniform pollutant profile becomes well developed the pollution undergoes several diurnal cycles. At this scale, wet deposition of pollutants becomes more important than dry deposition and interchange of pollution between troposphere and stratosphere becomes possible.
Global transport: Such transport extends from a few thousand kilometres to the entire global atmosphere. At this scale, definable pathways of pollutant transport disappear due to inevitable mixing in atmosphere. Continental and global transports of air pollution are termed long range transport (LRT). Such transport requires an organized meteorological system of atleast synoptic scale that will allow movement of pollution without much dispersion or loss.
Most unnfavourable conditions for LRT are windy-blustry conditions associated with heavy rain and strong turbulent mixing. Under such conditions, any organised mass of pollution will almost immediately be blown apart. Scavenging from within and below the clouds will remove virtually all the pollutant material quite close to source region. Further, there would be little time available for secondary chemistry to develop.
Most favourable conditions for LRT involve circulation associated with the back side of a high pressure system where two major features become crucial:
1. Persistence of a synoptic-scale inversion over a wide area reducing vertical distribution of pollution and restricting it to a fairly shalow depth of atmosphere.
2. Existence of conditions allowing slow horizontal advection in the layer containing air pollution.
Both these situations are assisted by flat terrain that allows uninturrupted horizontal movement of air mass. During development of LRT, pollutants collect initially under the influence of a shallow, stable and high-pressure system that has been stagnating over a source region for previous several days. During this period, the surface wind speeds have been less than 3 metres per second, mixing heights below the inversion have been restricted to 1500 metres or less, hours of sunshine have been close to maximum possible and there have been no weather fronts or precipitation. Over the period, which is most likely to persist for four days, the concentration of air pollutants increases significantly, extensive secondary reactions occur and poor visibility persists. Under such circumstances, LRT of air pollution can occur in three major ways with first one having strongest impact.
1. The high pressure slowly begins to move out of the area and polluted air is drawn towards the trailing edge (in the back side) where it starts moving northwards (in northern hemisphere) in the prevailing circulation. If a weak cold or warm front is in the vicinity, transport is improved and is often beteer directed by the enhanced pressure gradient. Inversion and stable atmosphere often restrict LRT to layers below 700 mb pressure altitude and the polluted air mass may move several thousand kilometres without much dispersion or dilution. Separation from the friction layer near the surface is also important if polluted air mass is to maintain its cohesion. Over continental areas, extreme stability of lowest atmospheric layers in winter suggests that at airflow at 850 mb, pressure level might be the best indicator of LRT. During summers, transport is usually not so well defined as the solar heating and vertical convection tend to dominate the stability situation.
2. Variants to LRT situation provide smaller but still important concentrations of pollutants to great distances downwind of sources. If edge of a high is associated with cool, cloudy weather and a stiff breeze, the transport will be quite rapid in a dynamic and unstable atmosphere. high humidity and cloud cover will allow oxidation of precurssors such as SO 2 or NOx without a great deal of dispersion away from the air mass core. This results in transfer of moderate concentrations of pollutants that are not removed by rainwater because of the absence of enough vertical diffusion for precipitation.
3. If circulation system on the edge of a high pressure draws moist air from ocean regions, which then moves over source areas of pollution, weak turbulent mixing will draw the polluted material into the air stream. This material will mix rapidly through the depth of the air flow creating quite uniform and mild pollution concentrations. This pollution will be transported for considerable distances. If it meets a mountain barrier in the way, the mildly polluted air can be drawn up orographically and forced over the top. Pollutants are then scavenged resulting in mildly acidic rainfall on the windward side of mountain, which may persist for several hours. In the tropics, LRT largely depends on the persistence and frequency of trade winds associated with emission source regions. Transport occurs at the altitude of trade wind inversion and in stable atmospheric conditions over long distances. This allows minimal dispersion of pollutant material in the air mass.
In southern hemisphere, there are very few air pollution sources and, therfore, no important LRT associated with air pollution. In the northern hemisphere, four major continental to hemispherical scale air pollution problems due to long range transport of air pollution. These are Arctic haze, Western Atlantic ocean air quality, Saharan dust and Asian dust.
Trajectory analysis
Measurements of LRT are often difficult and expensive. Therefore, trajectory analysis is often used to establish back trajectories (general source area of pollution origin) and forward trajectories (general location to which pollutants are being transported). Trajectories are calculated from synoptic level upper wind data and are normallly based on isobaric or isentropic principles. Two-dimensional models can not describe the vertical movements of either the airflow or the polluted air mass. Three-dimensional models, using a series of grid points at various altitudes have been designed to establish the sources of polluted air masses that have moved several thousand kilometres. Trajectories are best used as a support method to establish general transport movements and not to estimate the changes in pollution concentrations over the time. Benefits of such use are:
Support for chemical tracer experiments from different source regions;
Simulation of dry deposition;
Evaluation of acute air pollution problems;
Establishment of the source of a chronic pollution problem. At best, trajectories show general accuracy for five days assuming that air flow is consistent with little vertical or horizontal changes.
Most often trajectories lose accuracy after 48 hours since spatial distribution of upper synoptic grid and the number of measurements from them are too thin to obtain accurate interpolations of air flow variations. Distortions occur with the presence of turbulent eddies, evolving synoptic patterns, increasing diffusion and inaccuracies in determination of mean wind. Trajectory analysis fails in presence of fronts or other rapidly changing atmospheric conditions.
The climate is most important factor controlling the environment. The type of soil and the native vegetation in a given region are basically the product of the climate of the area. The scientific study of climate requires a scheme of climate classification. Such schemes aim to categorize all the variations in the climates found in different areas into several clearly defined and easily distinguishable groups. Many useful systems of climate classification can be devised by taking different weather elements such as temperature, pressure, winds, precipitation as the basis of classification. the distribution of natural vegetation and soils may suggest still other types of classification schemes.
Temperature as basis of climate classification
The general parallelism of isotherms with parallels of latitude was perhaps the first basis of climate classification. With intelligent application, temperature has been made a fundamental factor in most of the schemes. Three major climate groups are clearly recognizable on the basis of this criterion:
Equatorial tropical group: characterized by uniformly warm temperature throughout the year and absence of a winter season.
Middle-latitude group: characterized by alternating summer and winter seasons.
Polar-arctic group: characterized by the absence of true summer season.
A common boundary between polar-arctic climates and middle-latitude climates is 10 degrees celsius (50 degrees F) isotherm of the warmest month (i.e. july in northern hemisphere). The boundary between middle-latituse and equatorial climates is marked by 18 degrees celsius (64.4 degrees F) isotherm of the coldest month.
The use of temperature alone as the basis of climate classification is unsatisfactory because humid and desert regions are not distinguished in such scheme.
Precipitation as basis of climate classification
The climate map in such a scheme would be same as the mean annual rainfall map. Such system may be refined by subdividing classes according to distribution of precipitation throughout the year, whether uniform or seasonal. However, such classification scheme fails because it groups cold arctic climates together with hot deserts of low latitudes controlled by air temperature. The cold climates, in general, are effectively humid with same meager precipitaion that produces very dry deserts in hot subtropics and tropics.
Vegetation as basis of climate classification
Different plant types require special condition of temperature and precipitation for their survival. Thus, plants form an index of climate and limits of growth of key plant types provide meaningful boundaries of climstic zones. There is much merit in such vegetational zones based climate classificstion scheme. However, vegetation is an effect rather than a primary cause of climate. Therefore, it can not give so satisfactory a climate classification as the one based on the primary cause(s) of climate.
Koppen climate classification system
After various attempts of classification schemes based on some single characteristic feature, it became evident that a meaningful system should devise climate classes that combine temperature and precipitation characteristics but also set limits and boundaries that fit into obviously known vegetational distributions. Dr. Wladimir Koppen (1918) devised such a system that was later revised and extended by him and his students. this Koppen climate classification system has become most widely used system for biogeographical purposes.
Koppen system is strictly empirical in nature. Each climate group is defined according to fixed values of temperature and precipitation, computed according to averages of the year or of individual months. No attention is paid to the causes of climate in terms of pressure belts, wind belts, air masses, air fronts or storms. A given place can be assigned to a particular climate subgroup solely on the basis of the records of temperature and precipitation of that area provided that the records are long enough to yield meaningful averages. Since air temperature and precipitation are most easily obtainable surface weather data, the system has great advantage that the areas covered by each subtype of climate can be computed or estimated for large regions of the world. This system thus, incorporates an empirical-quantitative approach.
Koppen system has a shorthand code of letters disignating major climate groups (A, B, C, D), subgroups within major groups(S, W, f, m, s, w) and further subdivisions to distinguish particular seasonal characteristics of temperature and precipitation (a, b, c, d, h, k).
Air mass source regions and frontal zones as basis of climate classification
Increasingly detailed studies of vsrious charateristics of weather elements, global circulation, air mass properties, source regions and cyclonic storms have yielded many principles related to the causes of weather patterns and their seasonal variations at global scale. Therefore, such knowledge has also been incorporated in the explanatory-descriptive system of climate classification based on the cause and effect. Thus, such system is based on the location of air mass source regions and nature and movement of air masses, fronts and cyclonic storms. Koppen code symbols have been incorporated into this system by interrelating both systems. This system simply provides a reasonable scientic explanation for the existence of Koppen’s climate groups.
NASA satellites give us a global view of what’s happening on our planet. To explore how key parts of Earth’s climate system change from month to month, click on the following link and see the various global maps.
Sulfur dioxide, or SO2, is a colorless, pungent gas that can be both air pollutant and important atmospheric component, depending on where it resides. Along with water vapor and carbon dioxide, sulfur dioxide is one of the most abundant gas emissions during volcanic eruptions.
The following link shows the emission and transport of sulfur dioxide from the Grímsvötn Volcano in Iceland at the end of May 2011.
More and more such studies are crucial in understanding the patterns and factors associated with long-range transport of sulphur dioxide emitted from volcanic eruptions or otherwise.
India’s Forest Cover accounts for 20.6% of the total geographical area of the country as of 2005. In addition, Tree Cover accounts for 2.8% of India’s geographical area.
Over the last two decades, progressive national forestry legislations and policies in India aimed at conservation and sustainable management of forests have reversed deforestation and have transformed India’s forests into a significant net sink of CO2.
Following link is a report of Ministry Of Environment and Forests, Government of India summarizing impact of various programmes and policies in this direction.
Louis Aggasiz was one of the first scientists to study the clues of the ice age. An erratic is a large boulder, and when Aggasiz told some scientists that the boulders had been left there by a glacier they thought that he was out of his mind. The reason Louis Aggasiz proved that they had been put there by glaciers is because they were made of a kind of rock that you can’t find naturally in that area – granite. Because of that he proved that they can’t be from there, they were from somewhere else. Other proof that the ice age really existed is: polished bedrock, sand and gravel piles, big valleys, and rough mountain tops.
About 1/3 of the earth was ice. The most recent ice age was almost 10,000 years ago. As the earth started warming up the ice started to melt. The last ice age left traces that it was there. It left GLACIERS!!! Sheets of ice covered valleys and rivers. Ice spread to different parts of the world. Scientists called it the ice age. It kept melting, then froze again. This went on for about a million years. About 10,000 years ago the earth started to warm up. Sheets of ice started to melt. As the ice melted it left lakes and broad valleys with a mixture of rocks and soil. The only ice left was up high in the mountains. The glaciers that you see now are what is left over from the ice age.
The Geography of the Last Glacial Period
At the time of the LGM (map of glaciation), approximately 10 million square miles (~ 26 million square kilometers) of the earth was covered by ice. During this time, Iceland was completely covered as was much of the area south of it as far as the British Isles. In addition, northern Europe was covered as far south as Germany and Poland. In North America, all of Canada and portions of the United States were covered by ice sheets as far south as the Missouri and Ohio Rivers.
The Southern Hemisphere experienced the glaciation with the Patagonian Ice Sheet that covered Chile and much of Argentina and Africa and portions of the Middle East and Southeast Asia experienced significant mountain glaciation.
During the ice age countries like the Britain, France, Spain and Germany were very cold. At the northern and southern part of the earth the sheets of ice were much colder than they are today. Nobody knows why the ice age started, or why it stopped after 25,000 years. All we know is that it came and went very slowly.
Glacial Climate and Sea Level
The North American and European ice sheets of the last glaciation began forming after a prolonged cold stage with increased precipitation (mostly snow in this case) took place. Once the ice sheets began forming, the cold landscape altered typical weather patterns by creating their own air masses. The new weather patterns that developed reinforced the initial weather that created them, plunging the various areas into a cold glacial period.
The warmer portions of the globe also experienced a change in climate due to glaciation in that most of them became cooler but drier. For example rainforest cover in West Africa was reduced and replaced by tropical grasslands because of a lack of rain.
At the same time, most of the world’s deserts expanded as they became drier. The American Southwest, Afghanistan, and Iran are exceptions to this rule however as they became wetter once a shift in their air flow patterns took place.
Finally, as the last glacial period progressed leading up to the LGM, sea levels worldwide dropped as water became stored in the ice sheets covering the world’s continents. Sea levels went down about 164 feet (50 meters) in 1,000 years. These levels then stayed relatively constant until the ice sheets began to melt toward the end of the glacial period.
Flora and Fauna
During the last glaciation, shifts in climate altered the world’s vegetation patterns from what they had been prior to the formation of the ice sheets. However, the types of vegetation present during the glaciation are similar to those found today. Many such trees, mosses, flowering plants, insects, birds, shelled mollusks, and mammals are examples.
Because of all the ice the land was shaped much, much differently. The land looked bare because it was too cold for beech and oak trees to grow. There would be an few fir trees here and there. No grass grew, just shrubs, bushes, and moss grass. In the northern parts of North America, Europe, and Asia there is still tundra.
The animals were different from today too. Back then there were woolly mammoth, woolly rhinos, cave bears, bison, wolves, horses, and herds of reindeer like modern day reindeer. Woolly mammoth, cave bear, and woolly rhino are now extinct.
Some mammals also went extinct around the world during this time but it is clear that they did live during the last glacial period. Mammoths, mastodons, long-horned bisons, saber toothed cats, and giant ground sloths are among these.
Human history also began in the Pleistocene and we were heavily impacted by the last glaciation. Most importantly, the drop in sea level aided in our movement from Asia into North America as the landmass connecting the two areas in the Alaska’s Bering Straight (Beringia) surfaced to act as a bridge between the areas.
During the ice age the men would set a trap for their food. When an animal fell for the trap the men would go kill it. Then the men would work on cutting the mammoth into big chunks, and then carried the chunks of meat to their cave. There the women and children would cut the mammoth meat into pieces that they were able to cook. The ice age people lived 35,000 years ago.
The ice age with glaciation came and very slowly in comparison to the span of human life. Therefore, the people that lived at the time didn’t realize that it was getting colder and colder, nor did they know that they were becoming the ice age hunters. Most of the ice age hunters lived in the western, central part of Europe.
The ice age people painted pictures of various animals e.g. woolly mammoth, woolly rhinos, bison etc. on the sides of their caves, and the skeletons of the animals have been found in caves.There are cuts from the hunters’ knives in the bones and the knives were sitting beside them.
Today’s Remnants of the Last Glaciation
Though the last glaciation ended about 12,500 years ago, remnants of this climatic episode are common around the world today. For example, increased precipitation in North America’s Great Basin area created enormous lakes (map of lakes) in a normally dry area. Lake Bonneville was one and once covered most of what is today Utah. Great Salt Lake is today’s largest remaining portion of Lake Bonneville but the old shorelines of the lake can be seen on the mountains around Salt Lake City.
Various landforms also exist around the world because of the enormous power of moving glaciers and ice sheets. In Canada’s Manitoba for instance, numerous small lakes dot the landscape. These were formed as the moving ice sheet gouged out the land beneath it. Over time, the depressions formed filled with water creating “kettle lakes.”
Finally, the many glaciers still present around the world today are some of the most famous remnants of the last glaciation. Most ice today is located in Antarctica and Greenland but some is also found in Canada, Alaska, California, Asia, and New Zealand. Most impressively though are the glaciers still found in the equatorial regions like South America’s Andes Mountains and Mount Kilimanjaro in Africa.
Most of the world’s glaciers are famous today however for their significant retreats in recent years. Such a retreat represents a new shift in the earth’s climate- something that has happened time and time again over the earth’s 4.6 billion year history and will no doubt continue to do in the future.
Just The Facts
There were about 11 different ice ages.
The ice ages were during the earth’s 4.6 billion years of history.
The last ice age was called “The Great Ice Age” and was 11,000 years ago.
During the “Great Ice Age” over a third of the earth was covered in ice. During the ice age the air had less carbon dioxide in it.
Right now we are living in a mini ice age.
There are two explanations of why the ice ages might have occurred: 1.The temperatures were much colder so it never rained, only snowed. 2. The earth changed its tilt away from the sun
Timeline of glaciation
From Wikipedia, the free encyclopedia
There have been five known ice ages in the Earth’s history, with the Earth experiencing the Quaternary Ice Age during the present time. Within ice ages, there exist periods of more severe glacial conditions and more temperate referred to as glacial periods and interglacial periods, respectively. The Earth is currently in an interglacial period of the Quaternary Ice Age, with the last glacial period of the Quaternary having ended approximately 10,000 years ago with the start of the holocene.
Known ice ages
500 million year record shows current and previous two major glacial periods
The second ice age, and possibly most severe, is estimated to have occurred from 850 to 635 Ma (million years) ago, in the late Proterozoic Age and it has been suggested that it produced a second[1] “Snowball Earth” in which the earth iced over completely. It has been suggested also that the end of this second cold period[1] was responsible for the subsequent Cambrian Explosion, a time of rapid diversification of multicelled life during the Cambrian era. However, this hypothesis is still controversial[2][3], though is growing in popularity among researchers as evidence in its favor has mounted.
A minor series of glaciations occurred from 460 Ma to 430 Ma. There were extensive glaciations from 350 to 250 Ma. The current ice age, called the Quaternary glaciation, has seen more or less extensive glaciation on 40,000 and later, 100,000 year cycles.
Quaternary glacial cycles
Glacial and interglacial cycles as represented by atmospheric CO2, measured from ice core samples going back 650,000 years
Originally, the glacial and interglacial periods of the Quaternary Ice Age were named after characteristic geological features, and these names varied from region to region. It is now more common to refer to the periods by their marine isotopic stage number.[4] The marine record preserves all the past glaciations; the land-based evidence is less complete because successive glaciations may wipe out evidence of their predecessors. Ice cores from continental ice accumulations also provide a complete record, but do not go as far back in time as marine data. Pollen data from lakes and bogs as well as loess profiles provided important land-based correlation data.[5] The names system has not been completely filled out since the technical discussion moved to using marine isotopic stage numbers. For example, there are five Pleistocene glacial/interglacial cycles recorded in marine sediments during the last half million years, but only three classic interglacials were originally recognized on land during that period (Mindel, Riss and Würm).[6]
Land-based evidence works acceptably well back as far as MIS 6, but it has been difficult to coordinate stages using just land-based evidence before that. Hence, the “names” system is incomplete and the land-based identifications of ice ages previous to that are somewhat conjectural. Nonetheless, land based data is essentially useful in discussing landforms, and correlating the known marine isotopic stage with them.[5]
The last glacial and interglacial periods of the Quaternary are named, from most recent to most distant, as follows. Dates shown are in thousand years before present.
Land-based chronology of Quaternary glacial cycles
This section’s factual accuracy is disputed. Please see the relevant discussion on the talk page. (May 2008)
Ice cores are used to obtain a high resolution record of recent glaciation. It confirms the chronology of the marine isotopic stages. Ice core data shows that the last 400,000 years have consisted of short interglacials (10,000 to 30,000 years) about as warm as the present alternated with much longer (70,000 to 90,000 years) glacials substantially colder than present. The new EPICA Antarctic ice core has revealed that between 400,000 and 780,000 years ago, interglacials occupied a considerably larger proportion of each glacial/interglacial cycle, but were not as warm as subsequent interglacials.
References
1. ^ ab Miracle Planet: Snowball Earth, (2005) documentary, Canadian Film Board, rebroadcast 25 April 2009 on the Science Channel (HD)
2. ^ van Andel, Tjeerd H. (1994) New Views on an Old Planet: A History of Global Change 2nd ed. Cambridge University Press, Cambridge, UK, ISBN 0521447550
6. ^ Kukla, George (2005) “Saalian supercycle, Mindel/Riss interglacial and Milankovitch’s dating” Quaternary Science Reviews 24(14/15): pp. 1573-1583
Origin and definition of last glacial period
The last glacial period is sometimes colloquially referred to as the “last ice age”, though this use is incorrect because an ice age is a longer period of cold temperature in which ice sheets cover large parts of the Earth, such as Antarctica. Glacials, on the other hand, refer to colder phases within an ice age that separate interglacials. Thus, the end of the last glacial period is not the end of the last ice age. The end of the last glacial period was about 12,500 years ago, while the end of the last ice age may not yet have come: little evidence points to a stop of the glacial-interglacial cycle of the last million years.
The last glacial period is the best-known part of the current ice age, and has been intensively studied in North America, northern Eurasia, the Himalaya and other formerly glaciated regions around the world. The glaciations that occurred during this glacial period covered many areas, mainly on the Northern Hemisphere and to a lesser extent on the Southern Hemisphere. They have different names, historically developed and depending on their geographic distributions: Fraser (in the Pacific Cordillera of North America), Pinedale, Wisconsinan or Wisconsin (in central North America), Devensian (in the British Isles), Midlandian (in Ireland), Würm (in the Alps), Mérida (in Venezuela), Weichselian (in Scandinavia and Northern Europe), Vistulian (in northern Central Europe), Valdai in Eastern Europe and Zyryanka in Siberia, Llanquihue in Chile, and Otira in New Zealand.
The last glaciation centered on the huge ice sheets of North America and Eurasia. Considerable areas in the Alps, the Himalaya and the Andes were ice-covered, and Antarctica remained glaciated.
Canada was nearly completely covered by ice, as well as the northern part of the USA, both blanketed by the huge Laurentide ice sheet. Alaska remained mostly ice free due to arid climate conditions. Local glaciations existed in the Rocky Mountains and the Cordilleran ice sheet and as ice fields and ice caps in the Sierra Nevada in northern California.[2] In Britain, mainland Europe, and northwestern Asia, the Scandinavian ice sheet once again reached the northern parts of the British Isles, Germany, Poland, and Russia, extending as far east as the Taimyr Peninsula in western Siberia.[3] Maximum extent of western Siberian glaciation was approximately 18,000 to 17,000 BP and thus later than in Europe (22,000–18,000 BP).[4] Northeastern Siberia was not covered by a continental-scale ice sheet.[5] Instead, large, but restricted, icefield complexes covered mountain ranges within northeast Siberia, including the Kamchatka-Koryak Mountains.[6]
The Arctic Ocean between the huge ice sheets of America and Eurasia was not frozen throughout, but like today probably was only covered by relatively shallow ice, subject to seasonal changes and riddled with icebergscalving from the surrounding ice sheets. According to the sediment composition retrieved from deep-sea cores there must even have been times of seasonally open waters.[7]
Outside the main ice sheets, widespread glaciation occurred on the Alps–Himalaya mountain chain. In contrast to the earlier glacial stages, the Würm glaciation was composed of smaller ice caps and mostly confined to valley glaciers, sending glacial lobes into the Alpine foreland. To the east the Caucasus and the mountains of Turkey and Iran were capped by local ice fields or small ice sheets.[8],[9] In the Himalaya and the Tibetan Plateau, glaciers advanced considerably, particularly between 47,000–27,000 BP[10] and in contrast to the widespread contemporaneous warming elsewhere.[11] The formation of a contiguous ice sheet on the Tibetan Plateau is controversial.[12][13][14]
Other areas of the Northern Hemisphere did not bear extensive ice sheets but local glaciers in high areas. Parts of Taiwan for example were repeatedly glaciated between 44,250 and 10,680 BP[15] as well as the Japanese Alps. In both areas maximum glacier advance occurred between 60,000 and 30,000 BP[16] (starting roughly during the Toba catastrophe). To a still lesser extent glaciers existed in Africa, for example in the High Atlas, the mountains of Morocco, the Mount Atakor massif in southern Algeria, and several mountains in Ethiopia. In the Southern Hemisphere, an ice cap of several hundred square kilometers was present on the east African mountains in the Kilimanjaro Massif, Mount Kenya and the Ruwenzori Mountains, still bearing remnants of glaciers today.[17]
Glaciation of the Southern Hemisphere was less extensive because of current configuration of continents. Ice sheets existed in the Andes (Patagonian Ice Sheet), where six glacier advances between 33,500 and 13,900 BP in the Chilean Andes have been reported.[18]Antarctica was entirely glaciated, much like today, but the ice sheet left no uncovered area. In mainland Australia only a very small area in the vicinity of Mount Kosciuszko was glaciated, whereas in Tasmania glaciation was more widespread.[19] An ice sheet formed in New Zealand, covering all of the Southern Alps, where at least three glacial advances can be distinguished.[20] Local ice caps existed in Irian Jaya, Indonesia, where in three ice areas remnants of the Pleistocene glaciers are still preserved today.[21]
Named local glaciations
A. Pinedale or Fraser glaciation, in the Rocky Mountains, USA
The Pinedale (central Rocky Mountains) or Fraser (Cordilleran ice sheet) glaciation was the last of the major glaciations to appear in the Rocky Mountains in the United States. The Pinedale lasted from approximately 30,000 to 10,000 years ago and was at its greatest extent between 23,500 and 21,000 years ago.[22] This glaciation was somewhat distinct from the main Wisconsin glaciation as it was only loosely related to the giant ice sheets and was instead composed of mountain glaciers, merging into the Cordilleran Ice Sheet.[23] The Cordilleran ice sheet produced features such as glacial Lake Missoula, which would break free from its ice dam causing the massive Missoula floods. Geologists estimate that the cycle of flooding and reformation of the lake lasted on average of 55 years and that the floods occurred approximately 40 times over the 2,000 year period between 15,000 and 13,000 years ago.[24]Glacial lake outburst floods such as these are not uncommon today in Iceland and other places.
B. Wisconsin glaciation, in North America
The Wisconsin Glacial Episode was the last major advance of continental glaciers in the North American Laurentide ice sheet. This glaciation is made of three glacial maxima separated by interglacial warm periods (such as the one we are living in). These glacial maxima are called, from oldest to newest, Tahoe, Tenaya, and Tioga. The Tahoe reached its maximum extent perhaps about 70,000 years ago, perhaps as a byproduct of the Toba super eruption. Little is known about the Tenaya. The Tioga was the least severe and last of the Wisconsin Episode. It began about 30,000 years ago, reached its greatest advance 21,000 years ago, and ended about 10,000 years ago. At the height of glaciation the Bering land bridge permitted migration of mammals such as humans to North America from Siberia.
It radically altered the geography of North America north of the Ohio River. At the height of the Wisconsin Episode glaciation, ice covered most of Canada, the Upper Midwest, and New England, as well as parts of Montana and Washington. On Kelleys Island in Lake Erie or in New York’s Central Park, the grooves left by these glaciers can be easily observed. In southwestern Saskatchewan and southeastern Alberta a suture zone between the Laurentide and Cordilleranice sheets formed the Cypress Hills, which is the northernmost point in North America that remained south of the continental ice sheets.
The Great Lakes are the result of glacial scour and pooling of meltwater at the rim of the receding ice. When the enormous mass of the continental ice sheet retreated, the Great Lakes began gradually moving south due to isostatic rebound of the north shore. Niagara Falls is also a product of the glaciation, as is the course of the Ohio River, which largely supplanted the prior Teays River.
With the assistance of several very broad glacial lakes, it carved the gorge now known as the Upper Mississippi River, filling into the Driftless Area and probably creating an annual ice-dam-burst.
In Northwest Greenland, ice coverage attained a very early maximum in the last glacial period around 114,000. After this early maximum, the ice coverage was similar to today until the end of the last glacial period. Towards the end glaciers readvanced once more before retreating to their present extent.[25] According to ice core data, the Greenland climate was dry during the last glacial period, precipitation reaching perhaps only 20% of today’s value.[26]
D. Devensian & Midlandian glaciation, in Britain and Ireland
The effects of this glaciation can be seen in many geological features of England, Wales, Scotland, and Northern Ireland. Its deposits have been found overlying material from the preceding Ipswichian Stage and lying beneath those from the following Flandrian stage of the Holocene.
E. Weichselian glaciation, in Scandinavia and northern Europe
Alternative names include: Weichsel or Vistulian glaciation (named after the Polish river Vistula or its German name Weichsel). During the glacial maximum in Scandinavia, only the western parts of Jutland were ice-free, and a large part of what is today the North Sea was dry land connecting Jutland with Britain. It is also in Denmark that the only Scandinavian ice-age animals older than 13,000 BC are found.[citation needed] In the period following the last interglacial before the current one (Eemian Stage), the coast of Norway was also ice-free.[citation needed]
The Baltic Sea, with its unique brackish water, is a result of meltwater from the Weichsel glaciation combining with saltwater from the North Sea when the straits between Sweden and Denmark opened. Initially, when the ice began melting about 10,300 ybp, seawater filled the isostatically depressed area, a temporary marine incursion that geologists dub the Yoldia Sea. Then, as post-glacial isostatic rebound lifted the region about 9500 ybp, the deepest basin of the Baltic became a freshwater lake, in palaeological contexts referred to as Ancylus Lake, which is identifiable in the freshwater fauna found in sediment cores. The lake was filled by glacial runoff, but as worldwide sea level continued rising, saltwater again breached the sill about 8000 ybp, forming a marine Littorina Sea which was followed by another freshwater phase before the present brackish marine system was established. “At its present state of development, the marine life of the Baltic Sea is less than about 4000 years old,” Drs. Thulin and Andrushaitis remarked when reviewing these sequences in 2003.
Overlying ice had exerted pressure on the Earth’s surface. As a result of melting ice, the land has continued to rise yearly in Scandinavia, mostly in northern Sweden and Finland where the land is rising at a rate of as much as 8–9 mm per year, or 1 meter in 100 years. This is important for archaeologists since a site that was coastal in the Nordic Stone Age now is inland and can be dated by its relative distance from the present shore.
F. Würm glaciation, in the Alps
The term Würm is derived from a river in the Alpine foreland, approximately marking the maximum glacier advance of this particular glacial period. The Alps have been the area where first systematic scientific research on ice ages has been conducted by Louis Agassiz in the beginning of the 19th century. Here the Würm glaciation of the last glacial period was intensively studied. Pollen analysis, the statistical analyses of microfossilized plant pollens found in geological deposits, has chronicled the dramatic changes in the European environment during the Würm glaciation. During the height of Würm glaciation, ca 24,000–10,000 ybp, most of western and central Europe and Eurasia was open steppe-tundra, while the Alps presented solid ice fields and montane glaciers. Scandinavia and much of Britain were under ice.
During the Würm, the Rhône Glacier covered the whole western Swiss plateau, reaching today’s regions of Solothurn and Aarau. In the region of Bern it merged with the Aar glacier. The Rhine Glacier is currently the subject of the most detailed studies. Glaciers of the Reuss and the Limmat advanced sometimes as far as the Jura. Montane and piedmont glaciers formed the land by grinding away virtually all traces of the older Günz and Mindel glaciation, by depositing base moraines and terminal moraines of different retraction phases and loess deposits, and by the pro-glacial rivers’ shifting and redepositing gravels. Beneath the surface, they had profound and lasting influence on geothermal heat and the patterns of deep groundwater flow.
G. Merida glaciation, in the Venezuelan Andes
The name Mérida Glaciation is proposed to designate the alpine glaciation which affected the central Venezuelan Andes; during the Late Pleistocene. Two main moraine levels have been recognized: one between 2600 and 2700 m, and another between 3000 and 3500 m elevation. The snow line during the last glacial advance was lowered approximately 1200 m below the present snow line (3700 m). The glaciated area in the Cordillera de Mérida was approximately 600 km2; this included the following high areas from southwest to northeast: Páramo de Tamá, Páramo Batallón, Páramo Los Conejos, Páramo Piedras Blancas, and Teta de Niquitao. Approximately 200 km2 of the total glaciated area was in the Sierra Nevada de Mérida, and of that amount, the largest concentration, 50 km2, was in the areas of Pico Bolívar, Pico Humboldt (4,942 m), and Pico Bonpland (4,893 m). Radiocarbon dating indicates that the moraines are older than 10,000 years B.P., and probably older than 13,000 years B.P. The lower moraine level probably corresponds to the main Wisconsin glacial advance. The upper level probably represents the last glacial advance (Late Wisconsin).[27][28][29][30]
H. Llanquihue glaciation, southern Andes
The Llanquihue glaciation takes its name from Llanquihue Lake in southern Chile which is a fan-shaped piedmontglacial lake. On the lake’s western shores there are large moraine systems of which the innermost belong to the last glacial period. Llanquihue Lake’s varves are a node point in southern Chile’s varve geochronology. During the last glacial maximum the Patagonian Ice Sheet extended over the Andes from about 35°S to Tierra del Fuego at 55°S. The western part appears to have been very active, with wet basal conditions, while the eastern part was cold based. Palsas seems to have developed at least in the unglaciated parts of Isla Grande de Tierra del Fuego. The area west of Llanquihue Lake was ice-free during the LGM, and had sparsely distributed vegetation dominated by Nothofagus. Valdivian temperate rainforest was reduced to scattered remnants in the western side of the Andes.[31]
Lake Pippa was also affected by the Pleistocene, a glacier ripped through the center of it causing a very deep lake in the south atlantic.
I. Antarctica glaciation
During the last glacial period Antarctica was blanketed by a massive ice sheet, much like it is today. The ice covered all land areas and extended into the ocean onto the middle and outer continental shelf.[32][33] According to ice modelling, ice over central East Antarctica was generally thinner than today.[34]
5. ^ Lyn Gualtieri et al.: Pleistocene raised marine deposits on Wrangel Island, northeast Siberia and implications for the presence of an East Siberian ice sheet. Quaternary Research, Vol. 59, No. 3, pp. 399–410, May 2003. Abstract: doi:10.1016/S0033-5894(03)00057-7
6. ^ Zamoruyev, V., 2004. Quaternary glaciation of north-east Asia. In: Ehlers, J., Gibbard, P.L. (Eds.), Quaternary Glaciations: Extent and Chronology: Part III: South America, Asia, Africa, Australia, Antarctica. Elsevier, Netherlands, pp. 321–323.
7. ^ Robert F. Spielhagen et al.: Arctic Ocean deep-sea record of northern Eurasian ice sheet history. Quaternary Science Reviews, Vol. 23, No. 11-13, pp. 1455–1483, 2004. Abstract: doi:10.1016/j.quascirev.2003.12.015
10. ^ Lewis A. Owen et al.: A note on the extent of glaciation throughout the Himalaya during the global Last Glacial Maximum, Quaternary Science Reviews, V. 21, No. 1, 2002, pp. 147–157. Abstract: doi:10.1016/S0277-3791(01)00104-4
12. ^Matthias Kuhle, 2002: A relief-specific model of the ice age on the basis of uplift-controlled glacier areas in Tibet and the corresponding albedo increase as well as their positiv climatological feedback by means of the global radiation geometry.- Climate Research 20: 1–7.
13. ^Matthias Kuhle, 2004: The High Glacial (Last Ice Age and LGM) ice cover in High and Central Asia. Development in Quaternary Science 2 (c, Quaternary Glaciation – Extent and Chronology, Part III: South America, Asia, Africa, Australia, Antarctica, Eds: Ehlers, J.; Gibbard, P.L.), 175–199. (Elsevier B.V., Amsterdam)..
14. ^ Lehmkuhl, F.: Die eiszeitliche Vergletscherung Hochasiens – lokale Vergletscherungen oder übergeordneter Eisschild? Geographische Rundschau 55 (2):28–33, 2003. English abstract
15. ^ Zhijiu Cui et al.: The Quaternary glaciation of Shesan Mountain in Taiwan and glacial classification in monsoon areas. Quaternary International, Vol. 97–98, pp. 147–153, 2002. Abstract: doi:10.1016/S1040-6182(02)00060-5
16. ^ Yugo Ono et al.: Mountain glaciation in Japan and Taiwan at the global Last Glacial Maximum. Quaternary International, Vol. 138–139, pp. 79–92, September–October 2005. Abstract: doi:10.1016/j.quaint.2005.02.007
24. ^ Richard B. Waitt, Jr.: Case for periodic, colossal jökulhlaups from Pleistocene glacial Lake Missoula, Geological Society of America Bulletin, v.96, p.1271-1286, October 1985. Abstract
25. ^ Svend Funder (ed.) Late Quaternary stratigraphy and glaciology in the Thule area, Northwest Greenland. MoG Geoscience, vol. 22, 63 pp., 1990. Abstract
26. ^ Sigfus J. Johnsen et al.: A “deep” ice core from East Greenland. MoG Geoscience, vol. 29, 22 pp., 1992. Abstract
32. ^ Anderson, J.B., S.S. Shipp, A.L. Lowe, J.S. Wellner, J.S., and A.B. Mosola, 2002, The Antarctic Ice Sheet during the Last Glacial Maximum and its subsequent retreat history: a review. Quaternary Science Reviews. vol. 21, pp. 49–70.
33. ^ Ingolfsson, O., 2004, Quaternary glacial and climate history of Antarctica. in: J. Ehlers and P.L. Gibbard, eds., pp. 3–43, Quaternary Glaciations: Extent and Chronology 3: Part III: South America, Asia, Africa, Australia, Antarctica. Elsevier, New York.
34. ^ P. Huybrechts: Sea-level changes at the LGM from ice-dynamic reconstructions of the Greenland and Antarctic ice sheets during the glacial cycles, Quaternary Science Reviews, V. 21, no. 1-3, pp. 203–231, 2002. Abstract: doi:10.1016/S0277-3791(01)00082-8
Further reading
Bowen, D.Q., 1978, Quaternary geology: a stratigraphic framework for multidisciplinary work. Pergamon Press, Oxford, United Kingdom. 221 pp. ISBN 978-0080204093
Ehlers, J., and P.L. Gibbard, 2004a, Quaternary Glaciations: Extent and Chronology 2: Part II North America. Elsevier, Amsterdam. ISBN 0-444-51462-7
Ehlers, J., and P L. Gibbard, 2004b, Quaternary Glaciations: Extent and Chronology 3: Part III: South America, Asia, Africa, Australia, Antarctica.ISBN 0-444-51593-3
Gillespie, A.R., S.C. Porter, and B.F. Atwater, 2004, The Quaternary Period in the United States. Developments in Quaternary Science no. 1. Elsevier, Amsterdam. ISBN 978-0-444-51471-4
Harris, A.G., E. Tuttle, S.D. Tuttle, 1997, Geology of National Parks: Fifth Edition. Kendall/Hunt Publishing, Iowa. ISBN 0-7872-5353-7
Matthias Kuhle, 1988: The Pleistocene Glaciation of Tibet and the Onset of Ice Ages- An Autocycle Hypothesis. In: GeoJournal 17 (4), Tibet and High-Asia I. 581–596.
Mangerud, J., J. Ehlers, and P. Gibbard, 2004, Quaternary Glaciations : Extent and Chronology 1: Part I Europe. Elsevier, Amsterdam. ISBN 0-444-51462-7
Sibrava, V., Bowen, D.Q, and Richmond, G.M., 1986, Quaternary Glaciations in the Northern Hemisphere, Quaternary Science Reviews. vol. 5, pp. 1–514.
Pielou, E.C., 1991. After the Ice Age : The Return of Life to Glaciated North America. University Of Chicago Press, Chicago, Illinois. ISBN 0-226-66812-6 (paperback 1992)
Ice Ages
About 1/3 of the earth was ice. The most recent ice age was almost 10,000 years ago. As the earth started warming up the ice started to melt. The last ice age left traces that it was there. It left GLACIERS!!! Sheets of ice covered valleys and rivers. Ice spread to different parts of the world. Scientists called it the ice age. It kept melting, then froze again. This went on for about a million years. About 10,000 years ago the earth started to warm up. Sheets of ice started to melt. As the ice melted it left lakes and broad valleys with a mixture of rocks and soil. The only ice left was up high in the mountains. The glaciers that you see now are what is left over from the ice age.
Louis Aggasiz was one of the first scientists to study the clues of the ice age. An erratic is a large boulder, and when Aggasiz told some scientists that the boulders had been left there by a glacier they thought that he was out of his mind. The reason Louis Aggasiz proved that they had been put there by glaciers is because they were made of a kind of rock that you can’t find naturally in that area – granite. Because of that he proved that they can’t be from there, they were from somewhere else. Other proof that the ice age really existed is: polished bedrock, sand and gravel piles, big valleys, and rough mountain tops.
The Geography of the Last Glacial Period
At the time of the LGM (map of glaciation), approximately 10 million square miles (~ 26 million square kilometers) of the earth was covered by ice. During this time, Iceland was completely covered as was much of the area south of it as far as the British Isles. In addition, northern Europe was covered as far south as Germany and Poland. In North America, all of Canada and portions of the United States were covered by ice sheets as far south as the Missouri and Ohio Rivers.
The Southern Hemisphere experienced the glaciation with the Patagonian Ice Sheet that covered Chile and much of Argentina and Africa and portions of the Middle East and Southeast Asia experienced significant mountain glaciation.
Glacial Climate and Sea Level
The North American and European ice sheets of the last glaciation began forming after a prolonged cold stage with increased precipitation (mostly snow in this case) took place. Once the ice sheets began forming, the cold landscape altered typical weather patterns by creating their own air masses. The new weather patterns that developed reinforced the initial weather that created them, plunging the various areas into a cold glacial period.
The warmer portions of the globe also experienced a change in climate due to glaciation in that most of them became cooler but drier. For example rainforest cover in West Africa was reduced and replaced by tropical grasslands because of a lack of rain.
At the same time, most of the world’s deserts expanded as they became drier. The American Southwest, Afghanistan, and Iran are exceptions to this rule however as they became wetter once a shift in their air flow patterns took place.
Finally, as the last glacial period progressed leading up to the LGM, sea levels worldwide dropped as water became stored in the ice sheets covering the world’s continents. Sea levels went down about 164 feet (50 meters) in 1,000 years. These levels then stayed relatively constant until the ice sheets began to melt toward the end of the glacial period.
Flora and Fauna
During the last glaciation, shifts in climate altered the world’s vegetation patterns from what they had been prior to the formation of the ice sheets. However, the types of vegetation present during the glaciation are similar to those found today. Many such trees, mosses, flowering plants, insects, birds, shelled mollusks, and mammals are examples.
Some mammals also went extinct around the world during this time but it is clear that they did live during the last glacial period. Mammoths, mastodons, long-horned bisons, saber toothed cats, and giant ground sloths are among these.
Human history also began in the Pleistocene and we were heavily impacted by the last glaciation. Most importantly, the drop in sea level aided in our movement from Asia into North America as the landmass connecting the two areas in the Alaska’s Bering Straight (Beringia) surfaced to act as a bridge between the areas.
People During the Ice Age
During the ice age the men would set a trap for their food. When an animal fell for the trap the men would go kill it. Then the men would work on cutting the mammoth into big chunks, and then carried the chunks of meat to their cave. There the women and children would cut the mammoth meat into pieces that they were able to cook. The ice age people lived 35,000 years ago.
During the ice age countries like the Britain, France, Spain and Germany were very cold. At the northern and southern part of the earth the sheets of ice were much colder than they are today. Nobody knows why the ice age started, or why it stopped after 25,000 years. All we know is that it came and went very slowly. So that is the reason why the people that lived at the time didn’t realize that it was getting colder and colder, nor did they know that they were becoming the ice age hunters. Most of the ice age hunters lived in the western, central part of Europe.
Because of all the ice the land was shaped much, much differently. The land looked bare because it was too cold for beech and oak trees to grow. There would be an few fir trees here and there. No grass grew, just shrubs, bushes, and moss grass. In the northern parts of North America, Europe, and Asia there is still tundra.
The animals were different from today too. Back then there were woolly mammoth, woolly rhinos, cave bears, bison, wolves, horses, and herds of reindeer like modern day reindeer. Woolly mammoth, cave bear, and woolly rhino are now extinct. How do we know that they existed? Well the ice age people painted pictures of these animals on the sides of their caves, and the skeletons of the animals have been found in caves.There are cuts from the hunters’ knives in the bones and the knives were sitting beside them.
Today’s Remnants of the Last Glaciation
Though the last glaciation ended about 12,500 years ago, remnants of this climatic episode are common around the world today. For example, increased precipitation in North America’s Great Basin area created enormous lakes (map of lakes) in a normally dry area. Lake Bonneville was one and once covered most of what is today Utah. Great Salt Lake is today’s largest remaining portion of Lake Bonneville but the old shorelines of the lake can be seen on the mountains around Salt Lake City.
Various landforms also exist around the world because of the enormous power of moving glaciers and ice sheets. In Canada’s Manitoba for instance, numerous small lakes dot the landscape. These were formed as the moving ice sheet gouged out the land beneath it. Over time, the depressions formed filled with water creating “kettle lakes.”
Finally, the many glaciers still present around the world today are some of the most famous remnants of the last glaciation. Most ice today is located in Antarctica and Greenland but some is also found in Canada, Alaska, California, Asia, and New Zealand. Most impressively though are the glaciers still found in the equatorial regions like South America’s Andes Mountains and Mount Kilimanjaro in Africa.
Most of the world’s glaciers are famous today however for their significant retreats in recent years. Such a retreat represents a new shift in the earth’s climate- something that has happened time and time again over the earth’s 4.6 billion year history and will no doubt continue to do in the future.
Just The Facts
There were about 11 different ice ages.
The ice ages were during the earth’s 4.6 billion years of history.
The last ice age was called “The Great Ice Age” and was 11,000 years ago.
During the “Great Ice Age” over a third of the earth was covered in ice. During the ice age the air had less carbon dioxide in it.
Right now we are living in a mini ice age.
There are two explanations of why the ice ages might have occurred: 1.The temperatures were much colder so it never rained, only snowed. 2. The earth changed its tilt away from the sun
Timeline of glaciation
From Wikipedia, the free encyclopedia
There have been five known ice ages in the Earth’s history, with the Earth experiencing the Quaternary Ice Age during the present time. Within ice ages, there exist periods of more severe glacial conditions and more temperate referred to as glacial periods and interglacial periods, respectively. The Earth is currently in an interglacial period of the Quaternary Ice Age, with the last glacial period of the Quaternary having ended approximately 10,000 years ago with the start of the holocene.
Known ice ages
500 million year record shows current and previous two major glacial periods
The second ice age, and possibly most severe, is estimated to have occurred from 850 to 635 Ma (million years) ago, in the late Proterozoic Age and it has been suggested that it produced a second[1] “Snowball Earth” in which the earth iced over completely. It has been suggested also that the end of this second cold period[1] was responsible for the subsequent Cambrian Explosion, a time of rapid diversification of multicelled life during the Cambrian era. However, this hypothesis is still controversial[2][3], though is growing in popularity among researchers as evidence in its favor has mounted.
A minor series of glaciations occurred from 460 Ma to 430 Ma. There were extensive glaciations from 350 to 250 Ma. The current ice age, called the Quaternary glaciation, has seen more or less extensive glaciation on 40,000 and later, 100,000 year cycles.
Quaternary glacial cycles
Glacial and interglacial cycles as represented by atmospheric CO2, measured from ice core samples going back 650,000 years
Originally, the glacial and interglacial periods of the Quaternary Ice Age were named after characteristic geological features, and these names varied from region to region. It is now more common to refer to the periods by their marine isotopic stage number.[4] The marine record preserves all the past glaciations; the land-based evidence is less complete because successive glaciations may wipe out evidence of their predecessors. Ice cores from continental ice accumulations also provide a complete record, but do not go as far back in time as marine data. Pollen data from lakes and bogs as well as loess profiles provided important land-based correlation data.[5] The names system has not been completely filled out since the technical discussion moved to using marine isotopic stage numbers. For example, there are five Pleistocene glacial/interglacial cycles recorded in marine sediments during the last half million years, but only three classic interglacials were originally recognized on land during that period (Mindel, Riss and Würm).[6]
Land-based evidence works acceptably well back as far as MIS 6, but it has been difficult to coordinate stages using just land-based evidence before that. Hence, the “names” system is incomplete and the land-based identifications of ice ages previous to that are somewhat conjectural. Nonetheless, land based data is essentially useful in discussing landforms, and correlating the known marine isotopic stage with them.[5]
The last glacial and interglacial periods of the Quaternary are named, from most recent to most distant, as follows. Dates shown are in thousand years before present.
Land-based chronology of Quaternary glacial cycles
This section’s factual accuracy is disputed. Please see the relevant discussion on the talk page. (May 2008)
Ice cores are used to obtain a high resolution record of recent glaciation. It confirms the chronology of the marine isotopic stages. Ice core data shows that the last 400,000 years have consisted of short interglacials (10,000 to 30,000 years) about as warm as the present alternated with much longer (70,000 to 90,000 years) glacials substantially colder than present. The new EPICA Antarctic ice core has revealed that between 400,000 and 780,000 years ago, interglacials occupied a considerably larger proportion of each glacial/interglacial cycle, but were not as warm as subsequent interglacials.
References
1. ^ ab Miracle Planet: Snowball Earth, (2005) documentary, Canadian Film Board, rebroadcast 25 April 2009 on the Science Channel (HD)
2. ^ van Andel, Tjeerd H. (1994) New Views on an Old Planet: A History of Global Change 2nd ed. Cambridge University Press, Cambridge, UK, ISBN 0521447550
6. ^ Kukla, George (2005) “Saalian supercycle, Mindel/Riss interglacial and Milankovitch’s dating” Quaternary Science Reviews 24(14/15): pp. 1573-1583
Origin and definition
The last glacial period is sometimes colloquially referred to as the “last ice age”, though this use is incorrect because an ice age is a longer period of cold temperature in which ice sheets cover large parts of the Earth, such as Antarctica. Glacials, on the other hand, refer to colder phases within an ice age that separate interglacials. Thus, the end of the last glacial period is not the end of the last ice age. The end of the last glacial period was about 12,500 years ago, while the end of the last ice age may not yet have come: little evidence points to a stop of the glacial-interglacial cycle of the last million years.
The last glacial period is the best-known part of the current ice age, and has been intensively studied in North America, northern Eurasia, the Himalaya and other formerly glaciated regions around the world. The glaciations that occurred during this glacial period covered many areas, mainly on the Northern Hemisphere and to a lesser extent on the Southern Hemisphere. They have different names, historically developed and depending on their geographic distributions: Fraser (in the Pacific Cordillera of North America), Pinedale, Wisconsinan or Wisconsin (in central North America), Devensian (in the British Isles), Midlandian (in Ireland), Würm (in the Alps), Mérida (in Venezuela), Weichselian (in Scandinavia and Northern Europe), Vistulian (in northern Central Europe), Valdai in Eastern Europe and Zyryanka in Siberia, Llanquihue in Chile, and Otira in New Zealand.
The last glaciation centered on the huge ice sheets of North America and Eurasia. Considerable areas in the Alps, the Himalaya and the Andes were ice-covered, and Antarctica remained glaciated.
Canada was nearly completely covered by ice, as well as the northern part of the USA, both blanketed by the huge Laurentide ice sheet. Alaska remained mostly ice free due to arid climate conditions. Local glaciations existed in the Rocky Mountains and the Cordilleran ice sheet and as ice fields and ice caps in the Sierra Nevada in northern California.[2] In Britain, mainland Europe, and northwestern Asia, the Scandinavian ice sheet once again reached the northern parts of the British Isles, Germany, Poland, and Russia, extending as far east as the Taimyr Peninsula in western Siberia.[3] Maximum extent of western Siberian glaciation was approximately 18,000 to 17,000 BP and thus later than in Europe (22,000–18,000 BP).[4] Northeastern Siberia was not covered by a continental-scale ice sheet.[5] Instead, large, but restricted, icefield complexes covered mountain ranges within northeast Siberia, including the Kamchatka-Koryak Mountains.[6]
The Arctic Ocean between the huge ice sheets of America and Eurasia was not frozen throughout, but like today probably was only covered by relatively shallow ice, subject to seasonal changes and riddled with icebergscalving from the surrounding ice sheets. According to the sediment composition retrieved from deep-sea cores there must even have been times of seasonally open waters.[7]
Outside the main ice sheets, widespread glaciation occurred on the Alps–Himalaya mountain chain. In contrast to the earlier glacial stages, the Würm glaciation was composed of smaller ice caps and mostly confined to valley glaciers, sending glacial lobes into the Alpine foreland. To the east the Caucasus and the mountains of Turkey and Iran were capped by local ice fields or small ice sheets.[8],[9] In the Himalaya and the Tibetan Plateau, glaciers advanced considerably, particularly between 47,000–27,000 BP[10] and in contrast to the widespread contemporaneous warming elsewhere.[11] The formation of a contiguous ice sheet on the Tibetan Plateau is controversial.[12][13][14]
Other areas of the Northern Hemisphere did not bear extensive ice sheets but local glaciers in high areas. Parts of Taiwan for example were repeatedly glaciated between 44,250 and 10,680 BP[15] as well as the Japanese Alps. In both areas maximum glacier advance occurred between 60,000 and 30,000 BP[16] (starting roughly during the Toba catastrophe). To a still lesser extent glaciers existed in Africa, for example in the High Atlas, the mountains of Morocco, the Mount Atakor massif in southern Algeria, and several mountains in Ethiopia. In the Southern Hemisphere, an ice cap of several hundred square kilometers was present on the east African mountains in the Kilimanjaro Massif, Mount Kenya and the Ruwenzori Mountains, still bearing remnants of glaciers today.[17]
Glaciation of the Southern Hemisphere was less extensive because of current configuration of continents. Ice sheets existed in the Andes (Patagonian Ice Sheet), where six glacier advances between 33,500 and 13,900 BP in the Chilean Andes have been reported.[18]Antarctica was entirely glaciated, much like today, but the ice sheet left no uncovered area. In mainland Australia only a very small area in the vicinity of Mount Kosciuszko was glaciated, whereas in Tasmania glaciation was more widespread.[19] An ice sheet formed in New Zealand, covering all of the Southern Alps, where at least three glacial advances can be distinguished.[20] Local ice caps existed in Irian Jaya, Indonesia, where in three ice areas remnants of the Pleistocene glaciers are still preserved today.[21]
[edit] Pinedale or Fraser glaciation, in the Rocky Mountains, USA
The Pinedale (central Rocky Mountains) or Fraser (Cordilleran ice sheet) glaciation was the last of the major glaciations to appear in the Rocky Mountains in the United States. The Pinedale lasted from approximately 30,000 to 10,000 years ago and was at its greatest extent between 23,500 and 21,000 years ago.[22] This glaciation was somewhat distinct from the main Wisconsin glaciation as it was only loosely related to the giant ice sheets and was instead composed of mountain glaciers, merging into the Cordilleran Ice Sheet.[23] The Cordilleran ice sheet produced features such as glacial Lake Missoula, which would break free from its ice dam causing the massive Missoula floods. Geologists estimate that the cycle of flooding and reformation of the lake lasted on average of 55 years and that the floods occurred approximately 40 times over the 2,000 year period between 15,000 and 13,000 years ago.[24]Glacial lake outburst floods such as these are not uncommon today in Iceland and other places.
The Wisconsin Glacial Episode was the last major advance of continental glaciers in the North American Laurentide ice sheet. This glaciation is made of three glacial maxima separated by interglacial warm periods (such as the one we are living in). These glacial maxima are called, from oldest to newest, Tahoe, Tenaya, and Tioga. The Tahoe reached its maximum extent perhaps about 70,000 years ago, perhaps as a byproduct of the Toba super eruption. Little is known about the Tenaya. The Tioga was the least severe and last of the Wisconsin Episode. It began about 30,000 years ago, reached its greatest advance 21,000 years ago, and ended about 10,000 years ago. At the height of glaciation the Bering land bridge permitted migration of mammals such as humans to North America from Siberia.
It radically altered the geography of North America north of the Ohio River. At the height of the Wisconsin Episode glaciation, ice covered most of Canada, the Upper Midwest, and New England, as well as parts of Montana and Washington. On Kelleys Island in Lake Erie or in New York’s Central Park, the grooves left by these glaciers can be easily observed. In southwestern Saskatchewan and southeastern Alberta a suture zone between the Laurentide and Cordilleranice sheets formed the Cypress Hills, which is the northernmost point in North America that remained south of the continental ice sheets.
The Great Lakes are the result of glacial scour and pooling of meltwater at the rim of the receding ice. When the enormous mass of the continental ice sheet retreated, the Great Lakes began gradually moving south due to isostatic rebound of the north shore. Niagara Falls is also a product of the glaciation, as is the course of the Ohio River, which largely supplanted the prior Teays River.
With the assistance of several very broad glacial lakes, it carved the gorge now known as the Upper Mississippi River, filling into the Driftless Area and probably creating an annual ice-dam-burst.
In Northwest Greenland, ice coverage attained a very early maximum in the last glacial period around 114,000. After this early maximum, the ice coverage was similar to today until the end of the last glacial period. Towards the end glaciers readvanced once more before retreating to their present extent.[25] According to ice core data, the Greenland climate was dry during the last glacial period, precipitation reaching perhaps only 20% of today’s value.[26]
[edit] Devensian & Midlandian glaciation, in Britain and Ireland
The effects of this glaciation can be seen in many geological features of England, Wales, Scotland, and Northern Ireland. Its deposits have been found overlying material from the preceding Ipswichian Stage and lying beneath those from the following Flandrian stage of the Holocene.
[edit] Weichselian glaciation, in Scandinavia and northern Europe
Alternative names include: Weichsel or Vistulian glaciation (named after the Polish river Vistula or its German name Weichsel). During the glacial maximum in Scandinavia, only the western parts of Jutland were ice-free, and a large part of what is today the North Sea was dry land connecting Jutland with Britain. It is also in Denmark that the only Scandinavian ice-age animals older than 13,000 BC are found.[citation needed] In the period following the last interglacial before the current one (Eemian Stage), the coast of Norway was also ice-free.[citation needed]
The Baltic Sea, with its unique brackish water, is a result of meltwater from the Weichsel glaciation combining with saltwater from the North Sea when the straits between Sweden and Denmark opened. Initially, when the ice began melting about 10,300 ybp, seawater filled the isostatically depressed area, a temporary marine incursion that geologists dub the Yoldia Sea. Then, as post-glacial isostatic rebound lifted the region about 9500 ybp, the deepest basin of the Baltic became a freshwater lake, in palaeological contexts referred to as Ancylus Lake, which is identifiable in the freshwater fauna found in sediment cores. The lake was filled by glacial runoff, but as worldwide sea level continued rising, saltwater again breached the sill about 8000 ybp, forming a marine Littorina Sea which was followed by another freshwater phase before the present brackish marine system was established. “At its present state of development, the marine life of the Baltic Sea is less than about 4000 years old,” Drs. Thulin and Andrushaitis remarked when reviewing these sequences in 2003.
Overlying ice had exerted pressure on the Earth’s surface. As a result of melting ice, the land has continued to rise yearly in Scandinavia, mostly in northern Sweden and Finland where the land is rising at a rate of as much as 8–9 mm per year, or 1 meter in 100 years. This is important for archaeologists since a site that was coastal in the Nordic Stone Age now is inland and can be dated by its relative distance from the present shore.
The term Würm is derived from a river in the Alpine foreland, approximately marking the maximum glacier advance of this particular glacial period. The Alps have been the area where first systematic scientific research on ice ages has been conducted by Louis Agassiz in the beginning of the 19th century. Here the Würm glaciation of the last glacial period was intensively studied. Pollen analysis, the statistical analyses of microfossilized plant pollens found in geological deposits, has chronicled the dramatic changes in the European environment during the Würm glaciation. During the height of Würm glaciation, ca 24,000–10,000 ybp, most of western and central Europe and Eurasia was open steppe-tundra, while the Alps presented solid ice fields and montane glaciers. Scandinavia and much of Britain were under ice.
During the Würm, the Rhône Glacier covered the whole western Swiss plateau, reaching today’s regions of Solothurn and Aarau. In the region of Bern it merged with the Aar glacier. The Rhine Glacier is currently the subject of the most detailed studies. Glaciers of the Reuss and the Limmat advanced sometimes as far as the Jura. Montane and piedmont glaciers formed the land by grinding away virtually all traces of the older Günz and Mindel glaciation, by depositing base moraines and terminal moraines of different retraction phases and loess deposits, and by the pro-glacial rivers’ shifting and redepositing gravels. Beneath the surface, they had profound and lasting influence on geothermal heat and the patterns of deep groundwater flow.
The name Mérida Glaciation is proposed to designate the alpine glaciation which affected the central Venezuelan Andes; during the Late Pleistocene. Two main moraine levels have been recognized: one between 2600 and 2700 m, and another between 3000 and 3500 m elevation. The snow line during the last glacial advance was lowered approximately 1200 m below the present snow line (3700 m). The glaciated area in the Cordillera de Mérida was approximately 600 km2; this included the following high areas from southwest to northeast: Páramo de Tamá, Páramo Batallón, Páramo Los Conejos, Páramo Piedras Blancas, and Teta de Niquitao. Approximately 200 km2 of the total glaciated area was in the Sierra Nevada de Mérida, and of that amount, the largest concentration, 50 km2, was in the areas of Pico Bolívar, Pico Humboldt (4,942 m), and Pico Bonpland (4,893 m). Radiocarbon dating indicates that the moraines are older than 10,000 years B.P., and probably older than 13,000 years B.P. The lower moraine level probably corresponds to the main Wisconsin glacial advance. The upper level probably represents the last glacial advance (Late Wisconsin).[27][28][29][30]
The Llanquihue glaciation takes its name from Llanquihue Lake in southern Chile which is a fan-shaped piedmontglacial lake. On the lake’s western shores there are large moraine systems of which the innermost belong to the last glacial period. Llanquihue Lake’s varves are a node point in southern Chile’s varve geochronology. During the last glacial maximum the Patagonian Ice Sheet extended over the Andes from about 35°S to Tierra del Fuego at 55°S. The western part appears to have been very active, with wet basal conditions, while the eastern part was cold based. Palsas seems to have developed at least in the unglaciated parts of Isla Grande de Tierra del Fuego. The area west of Llanquihue Lake was ice-free during the LGM, and had sparsely distributed vegetation dominated by Nothofagus. Valdivian temperate rainforest was reduced to scattered remnants in the western side of the Andes.[31]
Modelled maximum extent of the Antarctic ice sheet 21,000 years before present
Lake Pippa was also affected by the Pleistocene, a glacier ripped through the center of it causing a very deep lake in the south atlantic.
During the last glacial period Antarctica was blanketed by a massive ice sheet, much like it is today. The ice covered all land areas and extended into the ocean onto the middle and outer continental shelf.[32][33] According to ice modelling, ice over central East Antarctica was generally thinner than today.[34]
5. ^ Lyn Gualtieri et al.: Pleistocene raised marine deposits on Wrangel Island, northeast Siberia and implications for the presence of an East Siberian ice sheet. Quaternary Research, Vol. 59, No. 3, pp. 399–410, May 2003. Abstract: doi:10.1016/S0033-5894(03)00057-7
6. ^ Zamoruyev, V., 2004. Quaternary glaciation of north-east Asia. In: Ehlers, J., Gibbard, P.L. (Eds.), Quaternary Glaciations: Extent and Chronology: Part III: South America, Asia, Africa, Australia, Antarctica. Elsevier, Netherlands, pp. 321–323.
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Further reading
Bowen, D.Q., 1978, Quaternary geology: a stratigraphic framework for multidisciplinary work. Pergamon Press, Oxford, United Kingdom. 221 pp. ISBN 978-0080204093
Ehlers, J., and P.L. Gibbard, 2004a, Quaternary Glaciations: Extent and Chronology 2: Part II North America. Elsevier, Amsterdam. ISBN 0-444-51462-7
Ehlers, J., and P L. Gibbard, 2004b, Quaternary Glaciations: Extent and Chronology 3: Part III: South America, Asia, Africa, Australia, Antarctica.ISBN 0-444-51593-3
Gillespie, A.R., S.C. Porter, and B.F. Atwater, 2004, The Quaternary Period in the United States. Developments in Quaternary Science no. 1. Elsevier, Amsterdam. ISBN 978-0-444-51471-4
Harris, A.G., E. Tuttle, S.D. Tuttle, 1997, Geology of National Parks: Fifth Edition. Kendall/Hunt Publishing, Iowa. ISBN 0-7872-5353-7
Matthias Kuhle, 1988: The Pleistocene Glaciation of Tibet and the Onset of Ice Ages- An Autocycle Hypothesis. In: GeoJournal 17 (4), Tibet and High-Asia I. 581–596.
Mangerud, J., J. Ehlers, and P. Gibbard, 2004, Quaternary Glaciations : Extent and Chronology 1: Part I Europe. Elsevier, Amsterdam. ISBN 0-444-51462-7
Sibrava, V., Bowen, D.Q, and Richmond, G.M., 1986, Quaternary Glaciations in the Northern Hemisphere, Quaternary Science Reviews. vol. 5, pp. 1–514.
Pielou, E.C., 1991. After the Ice Age : The Return of Life to Glaciated North America. University Of Chicago Press, Chicago, Illinois. ISBN 0-226-66812-6 (paperback 1992)
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