Environment of Earth

March 9, 2008

CLIMATE THEORY AND ANALYSIS OF CLIMATE

Filed under: Climate,Environment — gargpk @ 3:00 pm
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The global climate is the result of innumerable interactions occurring amongst components of the global environment, chiefly amongst hydrosphere (particularly the oceans) and the atmosphere. These interactions result in particular meteorological conditions and spatial average fields of these conditions produce climatic regimes. The purpose of climate theory is to identify the average distribution of meteorological elements in space and time as well as their responsiveness to external factors. Numerical models of climate are devised for this purpose, which make it possible to calculate average fields of meteorological elements. With the development of high-power computers, it has also become possible to design numerical models that can reproduce non-average fields of meteorological elements and calculate such fields for prolonged time intervals thus allowing calculation of average fields describing climatic conditions. Various theoretical/numerical models have been developed in the past half a century. These allow description and/or study of present climate as well as changes in climatic conditions produced by natural or anthropogenic activities. For example, numerical model of climate given by Manabe and Bryan (1969) shows the influence of the circulation of the ocean waters on climatic conditions. The model given by Holloway and Manabe (1971) shows the distribution of basic components of heat and water balance at the Earth’s surface similar to that actually present on Earth. As has been pointed above, the numerical models of climate can be employed for the study of current climate as well as climatic changes. However, the models used to study climatic change have to meet more rigorous requirements than the models used to study current climatic regime. The important requirements for such models are:

  1. Model should not include empirical data concerning the distribution of individual elements of climate, particularly those that change substantially during the changes in climate.

  2. Model must recognize realistically all types of inflow of heat that influence the temperature field appreciably; in particular, the law of conservation of energy.

  3. Model must include major feedback relations among various elements of climate.

The third requirement is most important and has been discussed below.

Feedback Relations Among Elements Of Climate

Feedback relations among climatic elements are highly complex and interrelated. Such relations include both negative and positive feedback relations.

Negative feedback relationships

These relationships reduce the anomalies of meteorological elements and contribute to the approximation of the values of these elements towards their climatic normals. Thus, the climatic stability is maintained by negative feedback relationships among climatic elements. Major such negative feedback relationships are:

1. Long-wave radiation and temperature at Earth’s surface: Intensity of long-wave radiation increases with increase in temperature at Earth’s surface. This produces a greater expenditure of heat energy, which in turn inhibits further increase in temperature.

2. Heat transfer in atmosphere and air temperature gradient: The usual flow of heat into the atmosphere from a zone of higher temperature produces a smoothing process that eliminates the differences in temperature distribution.

Positive feedback relationships

Such relationships among climatic elements play major role in climatic change since these increase the anomalies in meteorological elements. Thus, positive feedback relationships reduce the climatic stability. Major such relationships are:

  1. Absolute air humidity and air temperature: Absolute air humidity increases with rise in air temperature. With rise in temperature, evaporation increases leading to comparative constancy of relative humidity in most climatic zones (except in dry continental regions). Increase in absolute air humidity decreases long-wave radiation. Thus, the increase in absolute air humidity with rise in temperature partly compensates the increased long-wave radiation attributable to increased temperature. Manabe and Wetherald (1967) showed that influence of the change in solar constant on air temperature at Earth’s surface is almost twice in the condition of constant relative humidity as compared to the condition when absolute humidity is stable. This particular feedback relationship is important in numerical models of thermal regimes employed in studies of climatic changes.

  2. Snow and ice cover and albedo of Earth’s surface: Positive feedback relationship between snow and ice covers on the albedo of Earth’s surface plays a very great influence on the patterns of changes in atmospheric thermal regimes. Ice or snow covers have high albedo and so reduce air temperature above them and climatic changes are intensified by formation and melting of ice. Available data shows that during summer months, albedo over ice cover in Central Arctic is about 0.7 while in Antarctica, it is about 0.8 to 0.85. In regions free of ice and snow, albedo of Earth’s surface does not exceed 0.15. This indicates that other conditions being same, snow and ice covers reduce the radiation absorbed by Earth’s surface by several times.

  3. Snow and ice covers and Earth-atmosphere system: The data shows that albedo of Earth-atmosphere system during summers in Central Arctic region is 0.55 and in Antarctic region is about 0.6. This is approximately twice the value of the estimated albedo for the planet as a whole which is about 0.33. Such large differences in values of albedo must exert considerable influence on the atmospheric thermal regime.

  4. Air temperature and Earth’s albedo: Ice and snow covers are created at the Earth’s surface due to reduction in air temperature. These covers cause a sharp decline in the absorbed radiation. This contributes to a further reduction in Earth’s temperature and consequently further increases the area under snow and ice. The reverse process may be effected by increases in temperature, which results in melting of ice and snow. Budyko (1968) showed that inclusion of this feedback relationship into a numerical model of atmospheric thermal regime invariably always exerts a very substantial influence on the distribution of air temperature at Earth’s surface. This influence can be shown by a simple example, which shows how average global temperature will change if Earth’s surface becomes fully covered by snow and ice and clouds in the atmosphere are absent. In such a condition, Earth’s albedo will change from its present value of 0.33 to the value for dry snow cover i.e. 0.8. Thi
    s increase in albedo will reduce the air temperature. The absence of clouds will further reduce the temperature of lower layers of atmosphere near Earth’s surface. In the present times, the average temperature in lower layers of atmosphere rises substantially almost everywhere at the Earth’s surface. This is due to greenhouse effect associated with absorption of long-wave radiation by water vapour and carbon dioxide present in the atmosphere. However, at very low temperature resulting in the formation of snow and ice covers at Earth’s surface, green-house effect would become insignificant and dense clouds which perceptibly change the radiation flows are not formed. Under such conditions, atmosphere will become more or less transparent to both short-wave and long-wave radiation. The average temperature of the Earth’s surface for such transparent atmosphere is determined by the formula

4/So(1- s) /4

where So = Solar constant; s = albedo of Earth’s surface; = Stefen’s constant.

The above formula shows that Earth’s average surface temperature at albedo value of 0.8 will be _ 87o C (186o K). Thus, the Earth’s average temperature will decline by approximately 100o C from its present value of 15o C if ice or snow were to cover the entire Earth even for a short period. Thus, the enormous influence of snow cover on the thermal regime of Earth becomes quite clear.

A number of studies have attempted to calculate the influence of sea polar ice on Arctic thermal regime. These studies are based on the available data of thermal balance in central regions of Arctic Ocean and approximate values of the proportions derived from a semi-empirical theory of climate. It has been established that polar ice reduces average air temperature in the Central Arctic during summer months by several degrees and by about 20o C in winters. It has been concluded from these studies that the Arctic Ocean could be free of ice in the present age. However, this state would be extremely unstable and it could develop an ice cover as a result of a relatively small change in climate.

Since a permanent ice cover exerts a substantial influence on the atmospheric thermal regime even when it covers only a small part of the Earth’s surface, this must be taken into account in studies of climatic changes.

Present-Day Climate

The climatic conditions of present century have been established on the basis of meteorological data collected from worldwide network of climatic stations. The data shows that elements of meteorological regime change perceptibly over time. These changes are both periodic (daily or yearly) oscillations as well as non-periodic oscillations of different time intervals.

Short interval non-periodic changes (of days or months) occurring in meteorological regime determine the oscillations in weather. These changes are not spatially homogeneous and are largely explained by the instability of atmospheric circulation. For longer time intervals (of several years), irregular oscillations of individual elements of meteorological regime occur along with long-term changes that are similar over large territories. Such changes characterize fluctuations in climate.

Since climatic fluctuations at present time are relatively modest, average values of meteorological elements over a period of several decades could be used in order to describe the climatic features of the present age. The use of such average values makes it possible to exclude the influence of unstable atmospheric circulation pattern. Following Table shows average temperature in January and July and also the average yearly atmospheric precipitation at various latitudes.

Data show that difference between average temperatures at Earth’s surface at various latitudes is almost 700 C. The Earth’s surface temperature is maximum at Equator and lowest at South Pole. Earth’s spherical shape exerts a substantial influence on the distribution of these temperatures by producing variations in the total radiation reaching the upper boundary of atmosphere. Further, permanent ice covers are found at high latitudes where air temperature does not rise above freezing point almost throughout the year. Apart from substantial changes in meridonial direction, average air temperature at Earth’s surface also changes substantially in most latitudinal zones at various longitudes. This is largely explained by the distribution of continents and oceans.

The influence of ocean’s thermal regime extends to a large part of the surface of continents on which maritime climate exists. This influence is characterized by relatively modest yearly oscillations in air temperature at middle and high latitudes. The amplitude of yearly temperature fluctuations increases sharply in those extratropical continental regions where influence of oceanic thermal regime is less pronounced, characterizing the continental climate.

The distribution of average latitudinal precipitation values produces a pattern in which the principal maximum value occurs in the equatorial zone, total precipitation declines at subtropical latitudes, two secondary maxima lie at middle latitudes and precipitation declines in polar latitudes. Changes in average precipitation at different latitudes are explained by the distribution of average air temperatures and by specific characteristics of atmospheric circulation. Other conditions being equal, total precipitation increases with temperature because it increases the volume of atmospheric water vapour. Vertical air currents that carry water vapour through condensation level producing clouds also have important role in precipitation.

The atmosphere’s overall circulation is closely associated with geographical distribution of stable pressure systems. Particularly important such systems are:

  1. low-pressure belt near the equator,

  2. high-pressure region at high tropical and subtropical latitudes

  3. the region of frequent cyclone-formations at middle latitudes.

Downward movements of air within high-pressure zones substantially reduce the precipitation. On the other hand, pronounced upward air movements increase the precipitation at equatorial latitudes and in several regions at middle latitudes.

Largest desert areas on Earth having negligible precipitation are found in the subtropical high-pressure zone. Total precipitation also declines in continental regions at middle latitudes, which are distant from ocea
ns because very small quantity of water vapour carried by air currents from oceans reaches these regions.

Thus in the continents, zones of humid climates are largely located at equatorial latitudes and in regions of maritime climate, at middle and high latitudes. Similarly, at high tropical and subtropical latitudes and in regions of continental climates, conditions of insufficient moisture prevail.

Table : Present-day latitude-wise air temperatures and precipitations

Latitude      January temperature (0C)        July temperature (0C)           Precipitation(cm/year)

90-80 N/S             -31/-13                                      1/-42                                        19/11

80-70 N/S             -25/-8                                        2/-30                                       26/25

70-60 N/S             -22/0                                       12/-12                                        52/67

60-50 N/S            -10/5                                        14/1                                           80/101

50-40 N/S             -1/12                                       20/8                                          75/108

40-30 N/S            -11/20                                     26/14                                         77/103

30-20 N/S             19/25                                     28/18                                         73/91

20-10 N/S             25/26                                     28/24                                      114/122

10-0 N/S               27/27                                     27/26                                       201/150

Stability Of Climate

Budyko (1968) first used a semi-empirical model of atmospheric thermal regime to study the single-valued character and stability of current global climatic regime. The study using distribution of average air temperatures among different latitudes showed that the present climate is not the only possible one for existing climate-forming factors. Aside from the existing climate, current external conditions may produce a climate corresponding to a ‘white Earth’ as well as other variants of climate. Relatively small changes in external climate-forming factors may greatly alter the existing climate. Numerous subsequent studies employing similar models of climate have confirmed this conclusion.

The study of Wetherald and Manabe (1975) about the stability of existing climate is especially interesting. Unlike studies using semi-empirical models of t
he distribution of average air temperatures among different latitudes, their study applied a three-dimensional model of a general theory of climate that includes a detailed consideration of dynamic processes occurring in the atmosphere. The model takes into account the influence of state transformations of water on thermal regime including the feedback relationships between the snow cover, polar ice and air temperature. The study showed that if solar constant increases by more than 2%, the average yearly air temperature shall increase by approximately 2o C at low latitudes, more at higher latitudes and by about 10o C at 80o N. The ice cover on Earth shall be reduced by such rise in average yearly air temperature. The conclusions of this study are similar to those obtained by using semi-empirical models of thermal regime of atmosphere. However, the calculations from such models show slightly larger changes in average yearly air temperature following increases in the solar constant by two percent i.e. 3o to 4o C at low latitudes and 12 to 14o C at 80o N.

Thus, it may be concluded that the contemporary climate of Earth is neither unambiguous nor highly stable. It is highly sensitive to small changes in inflows of heat arriving at the upper boundary of atmosphere.

References

Budyko, M.I. (1968) On the origin of Ice Ages. Meteorologiya I gidrologiya No. 11.

Holloway, J.L. & Manabe, S. (1971) A Global General Circulation Model with Hydrology and Mountains. Monthly Weather Review Vol. 99, No. 5

Manabe, S. & Bryan, K. (1969) Climate Calculation with a Combined Ocean- Atmosphere Model. Journal of the Atmospheric Sciences. Vol. 26, No. 4.

Manabe, S. & Wetherald, R.T. (1967) Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity. Journal of the Atmospheric Sciences Vol. 24, No. 3.

Wetherald, R.T. & Manabe, S. (1975) The Effect of Changing the Solar Constant on the Climate of a General Circulation Model. Journal of the Atmospheric Sciences Vol. 32, No. 11.

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