GREENHOUSE EFFECT AND GLOBAL WARMING
The atmosphere over the earth contains certain gases that play significant role in maintaining the mean effective planetary temperature. In this respect, the atmosphere acts like the glass walls of a greenhouse. The glass allows short-wave solar radiation to enter the greenhouse but the long-wave and infrared heat-producing re-radiation from inside the greenhouse can not pass through the glass walls. Thus the temperature within the greenhouse becomes higher than outside. The solar radiation entering the atmosphere is predominantly short-wave whereas the terrestrial re-radiation leaving the earth is mainly long-wave or infrared. The composition of atmosphere is such that it allows the short-wave radiation from space to enter but does not allow the heat-producing infrared re-radiation from earth to pass through it. Due to the presence of certain gases, the atmosphere strongly absorbs the infrared radiation resulting in atmospheric heating. The gases that absorb strongly in long-wave and particularly in infrared region are commonly called greenhouse gases. The heating of atmosphere due to presence of these greenhouse gases is commonly called greenhouse effect.
These include those gases present in the atmosphere that have the property of trapping and absorbing strongly the long-wave and infrared terrestrial re-radiation thus causing the atmospheric heating. Important such gases are discussed briefly.
1) Carbon dioxide (CO2): The percentage of this gas by volume in dry air is about 0.035%. The gas is involved in a complex global cycle. It is released from the interior if the earth and is produced by respiration of living organisms, soil processes, combustion of organic matter and oceanic evapouration. On the other hand, it is dissolved in the water bodies on the earth and is also consumed in the photosynthesis of plants.
2) Methane (CH4): In dry air, the proportion of this gas by volume is about 0.0017%. It is produced primarily by anaerobic processes in the natural wetlands, rice paddies, digestive processes of animals, biomass burning and other human activities. It is destroyed in the troposphere by a reaction with hydroxyl (OH) ion.
3) Nitrous oxide (N2O): This gas is produced by biological processes in the oceans as well as soils, by industrial combustion, burning of fossil fuels and by use of chemical fertilizers. It is destroyed by photochemical reactions in the stratosphere involving the production of nitrogen oxides (NOx). The gas has quite long lifetime of about 132 years in the atmosphere.
4) Ozone (O3): The volumewise proportion of this gas in atmosphere is about 0.00006%. It is produced by high level breakup of oxygen molecules by solar ultra-violet radiation and is destroyed by photochemical reactions involving nitrogen oxides and chlorine in the middle and upper stratosphere.
5) Chlorofluorocarbons (CFCs): These gases are produced solely by human activities i.e. by aerosol propellants, refrigeration coolents, cleansers and air conditioning plants. Chief among these are CFCl2 and CFCl3. These gases released into the atmosphere near ground level quickly move up and reach the stratosphere. Within the stratosphsphere, the gases move towards poles. These gases are decomposed to chlorine by photochemical reactions. The average lifetime of these gases in atmosphere is 55-116 years.
6) Water vapour: Though water vapour is not a gas in the strict chemical sense, it is an important atmospheric constituent. Its average proportion in the air by volume is about 1.0%. The amount of water vapour in the atmosphere is highly variable depending upon the place ant time.
The atmospheric concentrations of greenhouse gases are quite low. Therefore, the relative effects of these gases increase approximately linearly with increasing concentrations except in case of CO2 where the effect is related to the logarithm of the concentration. The ozone in stratosphere absorbs significant amounts of both incoming ultra-violet solar radiation and outgoing long-wave terrestrial re-radiation. Thus, overall thermal role of ozone is quite complex. Its net effect on surface temperature of earth depends on the altitude at which the absorption occurs. The net effect, to some extent, is a balance between shortwave and long-wave absorption. An increase of ozone above 30 km altitude absorbs relatively more incoming short-wave radiation, causing a net decrease in surface temperature. However, ozone increase below 25 km results in relatively more absorption of outgoing long-wave radiation causing a net decrease in surface temperature.
The mean ‘effective’ planetary temperature of earth is 255oK corresponding to the emitted infrared radiation. On the global scale, annual average emission of long-wave radiation from the surface of earth is about 390 W per square meter. At the top of atmosphere, the long-wave radiation emitted to the outer space from the earth-atmosphere system is about 237 W per square meter. The balance of about 153 W per square meter long-wave radiation is trapped by the clouds and greenhouse gases present naturally in the atmosphere. This trapped long-wave radiation is re-radiated back towards lower troposphere and earth and is crucial for maintenance of the global mean temperature. The warming contribution of natural concentrations of atmospheric greenhouse gases to the mean planetary temperature is approximately 33oK. Water vapour accounts for 21oK, carbon dioxide for 7oK and ozone for 2oK. Nitrous oxide and methane together contribute about 3oK. If long-wave radiation flux of earth changes by as little as 1 W per square meter, it may cause important changes in the planetary temperature and as a result, in the climate system of the earth.
It has been observed that since the dawn of industrial age, the atmospheric concentrations of various greenhouse gases have been increasing at increasing rate. This is causing increased trapping of long-wave radiation in the atmosphere resulting in increased planetary temperature. The increase in global mean temperature as a consequence of increase in concentrations of atmospheric greenhouse gases is commonly called greenhouse problem or global warming.
The capacity of atmospheric gases to absorb the long-wave re-radiation from the surface of earth depends on the following factors:
1) Concentration of the gas: The absorption of long-wave radiation by a gas occurs in specific absorption lines. Each absorption line is surrounded by extended wings and a series of lines creates an absorption band. The gas molecules collect radiation by their vibrational energy transitions creating lines and bands through rotational splitting. Absorption by major greenhouse gases like carbon dioxide and ozone soon reaches saturation in the main absorption bands. However, with the increase in the concentration, the capacity of various gases to absorb more long-wave radiation increases in the band wings. If the absorption in a band is weak (i.e. unsaturated), significant capture of radiation occurs only along the specific absorption lines. Such absorption increases linearly with concentration of the gas. If absorption in a band is strong (i.e. saturated), extra absorption of long-wave radiation occurs mainly in the wings. With increase in concentration of the gas the wings may even overlap with other bands. In such cases, absorption of long-wave radiation increases logarithmically at a much slower rate with increase in the concentration of gas.
2) Wavelength of the radiation: Carbon dioxide and water vapour are most important in relation to atmospheric absorption of terrestrial re-radiation. These tow gases absorb about 90% of the total long-wave radiation absorbed by the atmosphere. Particularly in the bands around 8 mm and above 15 mm, water vapour is more active and other gases have little influence on the absorption of long-wave radiation. Absorption in the band range from 8.5 mm to 11.5 mm is dominated by methane, ozone, nitrous oxide and CFCs and is unsaturated. This band range is commonly called atmospheric window. In natural conditions, concentrations of these gases are very low and long-wave radiation in the earth-atmosphere system can escape to outer space through the atmospheric window. With increasing concentrations of these relatively active gases, such radiation is increasingly held back and absorbed in the lower troposphere. Such absorption results in increased re-radiation of long-wave radiation from troposphere towards earth’s surface resulting in the rise in temperature near the ground. This surface warming causes increase in evapouration from the water bodies leading to further increase in water vapour in the atmosphere and more absorption of radiation around 8 mm and above 15 mm.
3) Altitude of the gas above the surface: About 20% of the total long-wave radiation from the earth’s surface is absorbed in the lowest 80 km of the atmosphere and 99% is absorbed in the first 4 km from the earth’s surface. The earth is not in pure radiation equilibrium. Heat in the troposphere is transferred through sensible and latent heat fluxes and by convection mixing. The earth-atmosphere thermal equilibrium is at present maintained at effective radiating temperature of –18oC in the atmosphere. This equilibrium is maintained at the altitude of 5-6 km. In the colder upper troposphere, resultant release of long-wave radiation must decrease with altitude. If there is net increase in the trapped radiation in the troposphere, more energy would become available to drive the climatic circulation and the present equilibrium would be changed. Thus with increase in concentrations of greenhouse gases would result in rise in the altitude of the radiation equilibrium balance, temperature of earth’s surface and temperature of the troposphere until a new balance is reached between planetary emission and the absorbed energy. The altitude also influences the greenhouse warming due to absorption of solar energy. The greenhouse gases are largely transparent to short-wave radiation. Only water vapour and ozone have some short-wavelength absorption bands. Absorption of short-wavelength radiation by the gases in the lower troposphere will add to the warming while absorption in the upper troposphere and lower stratosphere will have a cooling effect.
Potential impacts in the stratosphere may be opposite to the troposphere and net cooling may be expected. due to:
1) Less absorption of solar radiation by the lower levels of ozone,
2) Increase in the emission of long-wave radiation in both upward and downward directions from increased concentrations of greenhouse gases in the stratosphere.
Loss of stratospheric ozone will allow more short-wave radiation to troposphere. However,, a cooler stratosphere emits less long-wave radiation to troposphere depending on the altitude of ozone layer. The two countering effects are comparable in magnitude. Model calculations suggest that cooling in upper stratosphere at 40-50 km altitude may be upto 6oK.
On the global average, the impact of atmospheric carbon dioxide on the long-wave absorption and global temperature is likely to be as much as that of all the other major greenhouse gases combined together. Estimates suggest that by the mid 21st century, earth’s temperature will increase by 2-4oK from the present level. This rise in temperature will be created by perhaps 4 W per square meter net radiation heating in the atmosphere. However, it may be emphasized that the greenhouse theory works only when atmosphere is cooler than the earth’s surface and the interactions between the cryosphere, biosphere, ocean and land are on a scale smaller than global. Therefore, the greenhouse effect i.e. global warming effects will not be globally uniform and will show spatial variations.
Trends in greenhouse gases
More than 35 gases covering a wide range of chemical groups could contribute to global warming. Five of these gases seem to be particularly important and these have shown a steady rise in their concentrations over the period of their recorded measurements at global background stations of the World Meteorological Organization. The comparison of trends in two hemispheres shows that there are major seasonal differences in the two hemispheres. This fact emphasizes the importance of vegetation in Northern Hemisphere in relation to carbon dioxide and methane. The concentrations of all the major greenhouse gases are higher in the Northern Hemisphere except the N2O that has an important oceanic source. Ozone in troposphere shows high spatial variability across the globe. Trends of major greenhouse gases have been summarized below.
1) Carbon dioxide: Since industrial revolution in mid 1800s, the amount of carbon dioxide emitted into the atmosphere is gradually increasing due to increasing use of fossil fuels for the energy needs of human activities. Analysis of air bubbles trapped in the ice cores from Green land and Antarctica has revealed a pre-industrial CO2 concentration of about 279-280 ppm. However, presently about 82% of human energy needs are met by fossil fuels on a global scale. This is divided mainly among oil (41%), gas (17%) and coal (24%). As a result, recorded CO2 concentrations since 1957 has been increasing at the rate of 1.8% per year with concentrations having exceeded by 325 ppmv by 1975. This upward global trend has resulted in increase of CO2 concentration to 350 ppmv which is still rising. Presently the global emission of CO2 due to human activities is estimated to be about 5.1 Gt per year that is about 150 times more than the global emission of SO2.
About 90% of the global anthropogenic CO2 emission occurs in Northern Hemisphere though there are marked spatial and seasonal variations. Maximum emission occurs in middle latitudes in Northern Hemisphere where most of the sources exist. The concentrations show peak in late April or early May associated with season of maximum growth of plants in the middle to high latitudes in Northern Hemisphere. Minimum concentrations occur at the end of summer. The maximum and minimum values in the latitudinal band of 60-70o N show a difference of about 15.0 ppm, which diminishes to about 1.0 ppm near equator. Differences in maximum and minimum seasonal concentrations in Southern Hemisphere are very small owing to lack of land vegetation in that area. However, due to good atmospheric mixing and effective atmospheric exchange between hemispheres over a time period of about a year, CO2 concentrations differ by only 1.5-2.0 ppmv between the two hemispheres. Further, the addition CO2 to the atmosphere due to deforestation is about 2 Gt per year. However, the impact of changes in the biosphere on atmospheric CO2 is quite complex.
2) Methane: Present concentration of methane in the atmosphere is about 1650 ppbv and the rate of increase has been of the order of 1-2% per year. January measurements at South Pole show an increase of 17.5+1.3 ppbv per year. Analysis of the trends over past 300 years shows an exponential increase in human population across the globe and the rise in agricultural production (SCOPE, 1986).
Increase in methane is caused due to decline in OH concentrations as well as human activities. Research indicates that OH concentrations presently are about 20% lower than several centuries ago. However, this decline in OH has contributed only about 30% to methane increase. The seasonal variations, out of phase between hemispheres are mainly due to seasonal changes in the sources of methane listed above.
3) Nitrous oxide: Measurements of ice cores indicates a pre-industrial N2O concentration of about 281-191 ppbv which are about 8% lower than the present concentrations. N2O has increased by about 0.2% per year from 8-9 to 14 Tg per year (SCOPE, 1986) between 1977 and 1987. Measurements in January at the south pole show an increase of 1.04+0.14 ppbv from 1975-1985.
4) Tropospheric ozone: Evidence about ozone as a greenhouse gas is somewhat inconclusive but there is indication that absorption in the 9.6 mm wavelength band is increasing because of the increase on its concentration. In the northern hemisphere, tropospheric ozone is estimated to have increased by 0.8-1.5% per year since 1967 but in the southern hemisphere, there does not seem to be such an increasing trend.. The impact of ozone in global warming depends on whether tropospheric or stratospheric ozone is being altered and by what amount. By measurements of vertical distributions, it has been established that increasing trends in tropospheric ozone occur mainly in 2-8 km altitude levels.
5) Chloroflurocarbons: Concentrations of CFCs have doubled in the past 10 years. January measurements at south pole have shown an yearly increase of 10.6+ 0.8 pptv for CFC-11 and 18.5+0.9 pptv for CFC-12 between 1975 and 1985. Concentration of CFC-11 is about 250 pptv and of CFC-12 is about 400 pptv. Though these concentrations are quite small compared to CO2, their relative Impacts on long-wave radiation absorption are quite high because CFCs absorb in the 8.5-11.5 mm atmospheric window and their wavelengths of absorption are unsaturated.
Future projections of greenhouse gases
With increasing awareness of the potential dangers of increasing atmospheric pollutant gases, particularly greenhouse gases, there have been global efforts to reduce the atmospheric loading of these gases by a variety of preventive measures. However, given that the present industrial and agricultural situation around the world is most likely to continue and expand, it seems unlikely that the increase of CH4, N2O and O3 shall be limited in the near future though there may be some reduction in the increase of CO2 and CFCs.
Concentrations of CO2 by the end of 21st century shall depend on the developments in fossil-fuel combustion technologies that may minimize CO2 emission, possible developments in the fields of alternative sources of energy and renewable fuel sources. There have been various speculative estimates of CO2 emission by the end of year 2100 and these range from a high figure of 20 Gt per year to a low estimate of 2 Gt per year.
Though decreasing trend of greenhouse gases is desirable, it seems that this shall not be feasible and much will depend on the location and the rate of global population growth in the next 50 years, a wide range of social, economical and political factors. Various workers have speculated that increasing trends in all the major greenhouse gases will continue well into the 21st century resulting in increasing global warming and related global impacts.
Evidences of greenhouse warming
The evidence that global temperature is rising due to greenhouse gases has its base in the temperature curves calculated by Jones et al. (1986) and Jones (1988). Various other analyses have also verified the overall trends with minor differences in details. Temperature plots show a rise in global temperature since 1800s. There has been global warming between 0.3 to 0.7oC on a global scale since 1800s with some differences between northern and southern hemispheres and also between land and ocean locations. The land temperatures are calculated for 5o latitude x 10o longitude grids based on the temperature measurements from reliable land and fixed-position weather ships. The data are considered reliable back to 1890 in the Southern Hemisphere and 1875 in the Northern Hemisphere with acceptable trends back to 1860s.
The temperature plots show some important temporal variation between the two hemispheres. Most of the overall warming trends have been observed in the polar regions of northern hemisphere but all other latitudinal zones show a similar significant warming trend except the 44.64oS. Temperature trends in Antarctica show no relationship to those for the rest of the world. This suggests that climatic regime of this zone is independent and not coupled with rest of the global circulation.
In the Northern Hemisphere, a cooling of 0.3oC has been observed between 1940 and 1970. Such cooling trend is much less apparent in Southern Hemisphere. In view of the greenhouse theory, no satisfactory explanation for this decrease in temperature has been given till date. However, temperature has increased by 0.3oC in Northern Hemisphere and by 0.23oC in Southern Hemisphere between 1967 and 1987. Much of this warming has been recorded in the mid-latitudinal regions rather than in Polar regions. These observations emphasize that much of the warming trend has occurred abruptly in 10-15 years in the Northern Hemisphere while in Southern Hemisphere, the warming has been more consistent throughout the recorded period. The block of warmest years in both hemispheres during 1980s has been put forward as clear evidence for greenhouse-gas induced global warming by many atmospheric scientists.
The trends of warming seen at the surface have also been observed throughout the troposphere. Data between 1960 and 1985 from 63 stations of upper air network has shown general warming throughout the troposphere but cooling across the tropopause and stratosphere. With greater changes occurring in the Southern Hemisphere. In contrast to greenhouse theory, tropospheric warming has been more in the tropics as compared to Polar regions. In the stratosphere, cooling of 3.0 to 5.0oC in the 26-55 km altitude layer followed by minimal cooling thereafter was measured between 1970 and 1976. This has not been fully explained by greenhouse theory.
Despite evidence of increasing trend in global temperature in the past few decades, there is disagreement amongst atmospheric scientists regarding impacts of human influences on the global warming. The trends of temperature and CO2 show very poor correlation; impact of oceanic thermal inertia is poorly understood and there are major gaps in the understanding of relative influence of carbon dioxide versus water vapour absorption and the influence of aerosol scattering and absorption. There are questions whether water vapour concentrations will increase the long-wave radiation absorption as the greenhouse warming theory suggests and if so, to what extent water vapour concentration will increase as a result of global temperature rise.
However, it should be pointed out that most of the critics of the greenhouse theory used surface energy balance approach and did not consider the impacts on the earth-atmosphere system as a whole or they tried to apply an empirical non-equilibrium situation. The impacts of urban warming effect on the overall temperature is still being debated hotly. Jones et al. (1989) found that urban warming influences, at the most, may account for 0.1oC of the global warming trend since 1860. Therefore, they concluded that urban temperature effects are unlikely to account for significant proportion of the hemispherical temperature trend. Jones (1988) has also pointed out that potential regional variations and the possibility of the differences in hemispheric factors should not be ignored as is usually done in incorporating all temperature data into a global set. Regions where distinct cooling has occurred over the 1967-1986 period include most of the northern and central Europe, northeast Canada, parts of central U.S.A., Japan, central south America and Antarctica. In most other areas across the globe, there has been distinct rise in temperature.
Models of greenhouse impacts
The impacts of rising trends in green house gases in the near future are mainly estimated by two methods:
1) Computer models: These are simulation models of the earth’s atmosphere that in some way link with the earth’s surface.
2) Palaeoclimatic proxy models: These models show climatic conditions during warmer periods in the past.
Both methods attempt to ‘predict’ the future climatic changes due to greenhouse gases. Though both methods have major limitations, they give somewhat similar general indications about the climate in the near future.
Estimates from the computer models
The three-dimensional (3-D) general circulation models (GCM models) are most appropriate for establishing the broad spatial variations of potential warming effects due to rising trends of greenhouse gases. The GCM models incorporate many of the features of 1-D and 2-D models and present an overall picture on a global or hemispherical basis. The models focus on modeling temperature variations assuming mostly a doubling of CO2 which is considered to be similar to a combination of all greenhouse gases working together. Attempts have also been made through these models to estimate the impacts on soil moisture, sea ice distribution, albedo and cloud cover. However, the success of such attempts has been quite limited. All such models differ in the estimates of the magnitudes of the impact and the detailed location of major changes. In general, the GCM models suggest following global impacts:
1) Average global warming of 3.5 to 4.2oC.
2) Increase in global precipitation by 7-11% by mid 21st century.
3) Warming throughout the troposhpere and across tropopause, particularly in the tropics and mid-latitudes.
4) Significant cooling in the stratosphere. At altitude above 25 km over the poles, temperature may decrease by more than 6oC during December to February.
5) Stratospheric cooling becoming most apparent over equatorial regions between June and August.
6) Stratospheric cooling increasing with the altitude at all latitudes in all the seasons.
In both the hemispheres, the warming near the earth’s surface is strongest in higher latitudes during winters. Maximum near surface warming during summers occurs over central Australia (over 8oC) in Southern Hemisphere and over eastern North America (by about 6oC) in Northern Hemisphere. Predictions of models differ about Antarctica as to whether warming is greatest in the interior or in coastal regions. During winters, greatest warming occurs in North Atlantic, North Pacific and Antarctic ocean reagions. The warming effects are stronger in Northern Hemisphere as compared to Southern Hemisphere.
The models also suggest that global atmospheric warming due to increase in greenhouse gases would create a more active hydrological cycle resulting in enhanced global evapouration, cloudiness and precipitation. Very rough estimates show that largest precipitation changes should occur in the tropics and a wide pariety of changes should occur outside the tropics. The models show much variation in the estimates of precipitation changes. The models show even greater variation in the estimates of the changes in cloudiness, soil moisture and albedo. This indicates that models are too coarse in areal details and are not sensitive enough to evaluate the impact of greenhouse-gas warming on these features.
This method estimates the possible warming impacts by use of the palaeoclimatic data of past periods that are similar in temperatures to the expected effects. In this method, all the influences of greenhouse gases are lumped together and no attempt is made to give explanation for the warming in terms of theory. Three main methods are used to evaluate the impacts of greenhouse gases from the proxy palaeoclimatic data:
1) Meteorological records of about past century are used to define the groups of warm and cold years. The groups of years represent more correctly the slower warming changes expected to occur due to increasing trends of greenhouse gases. These groups of past warm and cold years are compared and contrasted with each other. The method is often used for analyses of changes over large regions such as Europe or North America. However, the method is based on temperature changes smaller than expected for warming due to greenhouse gases and does not include the oceanic or cryospheric influences.
2) The palaeoclimate of a region for a period such as altithermal or climatic optimum (6,000-9,000 BP) that is supposed to have had temperature similar to that expected from warming due to increase in greenhouse gases is reconstructed from a wide variety of proxy data. It is assumed that the response of atmosphere shall be same irrespective of the cause. Though the knowledge of the past climates is incomplete, some general results can be obtained from such analyses.
3) Atmospheric dynamics in combination with empirical climatic relationships is used to make reasonable estimates of possible climatic changes that may result from warming due to increasing trends of greenhouse gases. This method is least developed amongst the three methods.
Use of palaeoclimate proxies has several advantages, particularly the following:
1) Quantification of natural background variability in a situation where levels of the greenhouse gases were not high is possible with these methods.
2) It is possible to obtain insight into the ways in which climate responds to external forces in a real sense because the proxy is based on real data and is not a model.
3) The use of proxy allows comparison testing between present and possible future situations that is again based on the real data.
4) There is some evidence of direct link between CO2 concentrations and degree of atmospheric warming over quite long geological times in the distant past.
However, use of the past proxy climates to estimate the impact of greenhouse gases on global warming has been criticized for two main reasons:
1) The separation of warming signal due to greenhouse gases from the background climate variability or ‘noise’ is very difficult since such separation requires accurate data and sound methods and both of these are not firmly established.
2) The warming impacts due to greenhouse gases may be different in all sorts of ways from the warming that is caused by natural climatic variability.
Some of the results of proxy analyses are broadly similar to those from GCM models but there are regional differences also.
In very general terms, proxy analyses suggest that:
1) Conditions shall become drier in most of Europe, North America except Alaska and extreme Northern Europe.
2) Conditions shall become wetter in most of Europe, north and east Africa and perhaps India, China and western half of Australia.
3) Polar regions shall experience more warming than the tropics.
4) Circulation zones shall shift towards poles.
Potential implications of greenhouse-gas warming
The GCM models and proxy methods have attempted to visualize and estimate the possible changes in various climatic parameters and their impacts on the biosphere. Though there are still controversies surrounding greenhouse-theory and regarding the results of GCM models and proxy methods, atmospheric scientists are confident enough to establish scenarios of the general changes that might from warming due to increasing trends of greenhouse gases. In general, following potential impacts on global environment, sea level, biosphere and ecosystems are visualized as a result of such global warming.
Global environmental changes: Some of the major visualized impacts of global greenhouse-gas warming on the global environment are:
1) Changes in the global thermal regimes and the energy balance of the earth-atmosphere system.
2) Change in the movements of air masses and consequent change in the distribution of monsoons.
3) Increase in temperature towards higher latitudes leading to gradual spread of warmer sub-tropical climatic conditions towards poles.
4) Increase in temperature at equator turning presently sub-tropical and tropical zones into deserts.
5) Melting of polar ice caps and other ice masses, rise in snowlines on mountains resulting in increase in the mean global sea levels and changes in the salinity of seas.
Sea level changes: All the estimates of sea level changes over the past century suggest that global sea level has risen between 12 and 15 cm during this century with a more rapid rate 22 cm per century since 1930. A correlation between sea level changes and hemispherical temperature trends shows a coefficient of 0.6 when annual mean data is used. In general, best correlations are obtained when both data sets show increases. From the correlation between air temperature and sea level change, it has been postulated that the sea level shall rise by 20-140 cm by 2030 A.D.
Sea levels may rise due to various reasons. Following reasons have been suggested to particularly important.
1) Thermal expansion of the sea water.
2) Melting of small glaciers.
3) Melting of Greenland and polar ice sheets.
4) Changes in runoff due to changes in precipitation.
5) Changes in evapouration due to warmer atmosphere.
Thermal expansion due to changes temperature and salinity is likely to be limited upto a depth of 1000 m from the surface on a short time scale. The top 100 m of sea water might expand by 10 cm sea level equivalent and the next 900 m by 20 cm sea level equivalent by mid 21st century (SCOPE, 1986).
If major melting of Antarctica ice sheet does not occur, impact of ice melting on the sea level rise would be small. Analyses have suggested that the chance of the melting of Antarctica ice sheet is as much as the chance of its expanding due to changes in precipitation associated with changes in atmospheric circulation.
There is no threat of extensive melting og the ice sheets of Antarctica or ice caps of Greenland and Arctic region. However, more extensive knowledge of the polar oceanography and the relationship between ice cap changes and the estimated future atmospheric warming is required for definite estimates of the melting of polar ice due to global greenhouse warming and its impact on the rise of global sea levels.
Sea levels might be influenced more by regional or local changes on the e.g. subsidence and local elevations, atmospheric pressure, wind stress and ocean circulation. Sea levels have large spatial and temporal variability on a smaller regional scale suggesting that many areas might not follow the global trend. Small rise in sea level may be linked to increased frequency of storms caused by warming due to increase in greenhouse gases.
Some of the major impacts of rise in sea levels may be as follows.
1) Coastal land with an elevation of one meter or less alongwith parts of many coastal cities worldwide would be submerged. The parts of coast presently reclaimed from the ocean such as large areas of Netherlands would be flooded.
2) Rise in sea levels would increase the coastal erosion, remove the sand from present beaches and create changes in the structure and shape of the coastline.
3) There would be more frequent incursions of salt-water into the coastal wetlands changing their salinity.
4) Supply of fresh groundwater presently used for agriculture and human consumption might become saline and unfit for use.
Changes in biosphere and ecosystems: General visualized consequences of global warming due to greenhouse gases are as following:
1) Spread of sub-tropical and tropical vegetation towards higher latitudes and development of desert vegetation in the presently sub-tropical and tropical zones.
2) Change of the species composition of vegetation due to increase in the percentage of megatherms in all regions of the earth.
3) Decrease in total evotranspiration from the vegetation covered areas of the earth due to higher stomatal resistance at higher CO2 concentrations.
4) Ultimate destruction of phytoplankton in all regions of earth due to decreasing concentration of carbon dioxide dissolved in water with increasing temperature.
5) Changes in the distribution of distribution of animal populations worldwide in response to changed climatic and vegetational conditions.
6) A variety of different stresses being imposed on the ecosystems such as pests, droughts and floods due to changed climatic conditions.
7) Extinction of many plant and animal species because changed environmental conditions will be beyond their tolerance range.
8) Evolution of many new plant and animal species due to increased chances of mutations.
9) Increased productivity of ecosystem due to increased photosynthesis during initial phases of greenhouse warming. Potential impacts of global warming due to greenhouse gases have mainly been analyzed for agricultural and forest ecosystems. Extensive work has been done to evaluate the direct impact of increased CO2 concentration on plant growth. However, data is mostly from controlled laboratory type conditions and its projection on the real environmental change is highly speculative. There are basic, mutually interactive ways in which global warming due to greenhouse gases might affect the ecosystems.
a) Photosynthesis would be stimulated due to warmer temperature and higher CO2 concentration. This would increase the plant growth and development at all levels i.e. productivity of ecosystems would increase. However, estimation of the magnitude of this benefit is difficult because different species have widely varying responses to such conditions. Induced partial stomatal closure in increased carbon dioxide concentration would reduce transpiration and thus result in better water conservation in plants. It is estimated that the increase in growth and yield would be about 10-50% in C3 species and about 1-10% in C4 species.
b) Any benefits from higher CO2 concentrations may be nullified or enhanced due to changing climatic patterns associated with possible global warming. Changes in the temperature and precipitation in different areas of the earth would result in changes in the distributions of species and ecosystems. The degree of response to changed climatic conditions would vary between species resulting in changes in their densities. This would bring about major changes in the species diversity and species composition of all the ecosystems. Warmer temperatures would decrease the yield of major mid-latitude crops such as wheat and maize in Northern Hemisphere by 3-17% depending on the temperature change. Changes in the precipitation in different areas may increase or decrease this loss in yield. Increasing temperatures and precipitation alongwith increasing CO2 concentrations may create a shift in feasible agricultural areas by several hundred kilometers per degree increase across a present-day climatic boundary.
February 25, 2008
GREENHOUSE EFFECT AND GLOBAL WARMING