Until recently studies of causes underlying changes in climate were made difficult by the absence of a corresponding physical theory. However, in recent years, numerical models of climate theory including the semi-empirical theory of atmospheric thermal regimes have significantly contributed to the studies of causes underlying climatic changes.
Quantitative data concerning climatic conditions in the distant past are not very reliable. Therefore, study of the causes of climatic changes in Prequaternary period is quite difficult while that of Quaternary period is relatively more reliable.
The study of the causes of climatic change during Prequaternary period can only be limited to an examination of the secular trend of air temperature during second half of the Mesozoic era and the Tertiary period. Palaeotemperatures determined through the method of isotopic analysis can be reliably used in such studies. In has been established from the studies of palaeotemperatures that during the last 130 million years there has been a tendency for average air temperatures at the Earth’s surface to decline. This tendency was interrupted several times by perceptible increases in air temperature but these were always followed by periods of cooling and did not alter the overall course of climate evolution. In order to explain the changes in climatic conditions, consideration of the influence of fluctuations in the composition of atmosphere and in the structure of the Earth’s surface on the thermal regime is necessary.
1. Effect of atmospheric carbon dioxide on thermal regime of atmosphere
Since carbon dioxide influences the absorption of long-wave radiation, thus sustaining the atmospheric green-house effect, a reduction in its mass produces a decline in air temperature at Earth’s surface. It has been established that only for relatively low concentrations (<0.1%) the dependence of temperature on the concentration of carbon dioxide becomes perceptible. At higher concentrations, changes in the volume of carbon dioxide in the atmosphere influence air temperature insignificantly. As a result, only during Pliocene and Pleistocene epochs the reductions in the atmospheric carbon dioxide content have produced considerable changes in air temperature. Calculations based on semi-empirical theory of thermal regime have shown that during that period the average air temperature at the Earth’s surface has declined by several degrees.
2. Effect of structure of Earth’s surface on thermal regime of atmosphere
During the second half of Mesozoic epoch of Tertiary period, the average elevation of continents was relatively low. As a result, a large part of continental plateform was covered by shallow seas. During that time a large number of uplifts and sinkings of different continental areas occurred but this did not alter the overall pattern in which straights and seas of various sizes separated various continents.
During Neogene epoch of Tertiary period, intensive tectonic movements produced the elevation of continents and caused a gradual vanishing of many intercontinental seas. Particularly, the sea on the present-day tertiary of Western Siberia that linked tropical oceans with the polar basin ceased to exist. This resulted in transformation of Arctic Ocean into its today’s condition of a relatively isolated body of water that is linked only with Atlantic Ocean and poorly connected to Pacific Ocean through the very narrow Bering Strait.
Possibly the slow drift of Antarctic was completed during the Tertiary period. It is presumed that such changes in the structure of Earth’s surface produced substantial decline in meridonial heat exchanges in the oceans. Such reductions in meridonial flows of heat in oceans resulting from increases in the levels of continents have contributed to the reduction in temperatures that have taken place at middle and high latitudes during the last 100-150 million years.
The reasons why the temperatures fluctuate over intervals extending to tens of millions of years is an interesting problem. It may be assumed that cause was local changes in circulation processes in the atmosphere and oceans.
The Quaternary period was preceded by a prolonged evolution of climate in the direction of a more pronounced thermal zonality resulting from changes in Earth’s surface structure. This was largely expressed in a continual decline in air temperature at middle and high latitude. Peculiar climatic conditions during Quaternary period appear to have emerged due to further decline in concentration of carbon dioxide in the atmosphere and also due to drifts and elevation of continents.
1. Effect of decline in atmospheric Carbon dioxide
During Pliocene epoch, climatic conditions began to be influenced by a reduction in the atmospheric carbon dioxide concentration. Such reduction led to a decline in average global air temperature by 2o to 3o C ( 3o to 5o C at high latitudes). This resulted in development and extension of polar ice caps which in turn, caused further decline in average global temperature, particularly at higher latitudes.
2. Effect of shifts and elevation of continents
Development of polar ice caps sharply increased the sensitivity of thermal regime to even very small changes in climate-forming factors. This made possible the very large oscillations in the boundaries of snow and ice covers on land and in oceans as a result of changes in the location of land in relation to the Sun; earlier this factor had not influenced the climate substantially.
Continuos reduction in atmospheric CO2 contributed to the advance of glaciations, even though the major influence on their scope was a combination of astronomical factors determining the location of Earth’s surface in relation to the Sun. These factors include the eccentricity of Earth’s orbit, the inclination of Earth’s axis in relation to the plane of its orbit and the time of equinoxes. These astronomic changes occur in periods of tens of thousands of years. By comparison with changes in astronomic factors, all other factors appear to have exerted a lesser influence on climatic fluctuations during Quaternary period.
The above conclusions have been verified by using a numerical model that makes possible the calculation of individual ice covers as influenced by external factors. Budyko and Vashishcheva (1971) studied the climatic conditions during glaciation periods using a model describing the distribution of average latitudinal temperature in different seasons showed that fluctuation in the radiation regime caused by changes in the position of Earth’s surface in relation to Sun may lead to substantial changes in climate. Though average global temperature does not fluctuate much at such times, such modest fluctuations are accompanied by perceptible shifts in the boundaries of ice covers.
Climatic changes in 20th century
Meteorological observations for first half of present century show a trend of increased temperatures which attained a maximum in 1930s. From the studies of reasons explaining climatic change during this period it appears that increase in temperature has been caused by an increase in transparency of stratosphere resulting in increased inflow of solar radiation (i.e. increased meteorological Solar constant) entering the troposphere. This has led to an increase in average global air temperature at Earth’s surface.
The changes in air temperature at different latitudes and in different seasons have depended on the optical thickness of the stratospheric aerosol and on the shifts in the boundaries of sea polar ice. The shrinking Arctic sea ice due to rise in air temperature has further contributed to increase in air temperature during cold seasons at high latitudes in Northern Hemisphere.
It seems probable that changes in transparency of stratosphere during first half of this century have been associated with the regime of volcanic activities and in particular with changes in the inflow of the products of volcanic eruptions (including sulphur dioxide) into stratosphere. There are grounds to believe that during last 3 to 4 decades, changes in climate have also begun to depend, atleast in some measure, on human activities.