Environment of Earth

February 27, 2008

Climate theory and analysis of climate

Filed under: Environment — gargpk @ 1:58 pm

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CLIMATE THEORY AND ANALYSIS OF CLIMATE

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. For this numerical models of climate are devised 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 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 shows the influence of the circulation of the circulation of ocean waters on climatic conditions while model given by Holloway and Manabe shows the distribution of basic components of heat and water balance at the Earth’s surface similar to that present actually 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 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 this 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 inhibiting further increase in temperature.

2. Heat transfer in atmosphere and air temperature gradient: The usual flow of heat in 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:

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 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 the influence of change in solar constant on air temperature at Earth’s surface in condition of constant relative humidity is almost twice as when absolute humidity is stable. This particular feedback relationship is important in numerical models of thermal regimes employed in studies of climatic changes.
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.
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 is about 0.6 in Antarctic region. 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.
Air temperature and Earth’s albedo: Creation of ice and snow covers at the Earth’s surface due to reduction in air temperature creates 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 atmosphere are absent. In such condition Earth’s albedo will change from its present value of 0.33 to value for dry snow cover i.e. 0.8. This 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 present times, the average temperature in lower layers of atmosphere rises substantially almost everywhere at the Earth’s surface as a result of green-house effect that is associated with absorption of long-wave radiation by water vapour and carbon dioxide present in atmosphere. But at very low temperature resulting in 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 radiations. The average temperature of the Earth’s surface for such transparent atmosphere is determined by the formula 4/So(1- αs /4 σ where So = Solar constant; αs = albedo of Earth’s surface; σ = Stefen’s constant. This formula shows that Earth’s average surface temperature at albedo value of 0.8 will be -87o C (186o K). Thus if ice or snow were to cover the entire Earth even for a short period, the Earth’s average temperature will decline by approximately 100o C from its present value of 15o C. This shows the enormous influence of snow cover on the thermal regime.
A number of studies have attempted to calculate the influence of sea polar ice on Arctic thermal regime. On the basis of available data on thermal balance in central regions of Arctic Ocean and of approximate values of 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 that the Arctic Ocean could be free of ice in the present age but 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.

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 the 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. This study showed that if Solar constant increases by more than 2%, average yearly air temperature at low latitudes increases by approximately 2o C, more at higher latitudes and by about 10o C at 80o N. As a result Earth’s ice cover is reduced. The conclusion of this study are similar to those obtained by using semi-empirical models of thermal regime of atmosphere though 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.

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