During the next century Earth's atmospheric carbon dioxide concentration is expected to double, i.e. to reach 700 conventional units. However, there are different points of view as to the question of when this point could occur exactly. Most forecasts made during 1984-88 anticipated that carbon dioxide concentrations of 700 units would be reached by 2050. A more detailed analysis has been accomplished recently by a group brought together under the auspices of the United Nations, the Intergovernmental Panel on climate Change (IPCC). This group considered different factors of the natural environment, new data on the future use of organic fuels and the measures to be undertaken to limit carbon dioxide emission to the atmosphere, with the result that lower concentrations are now predicted. The diagram on the right shows one of the variants of these new estimates which although anticipating the most intensive growth in concentration, predicts that they will double only by the end of the next century.
Carbon dioxide concentration increases result in a rising global air temperature, with the extent of the warming varying through a wide range of values depending of the premises accepted and a detailed consideration of the forcing factors.
For example, the recent estimates consider that atmospheric aerosols noticeably attenuate solar energy income which thus contributes to air cooling.
The values given above for potential changes in global air temperature, however reliable they are, are insufficient even for the most approximate estimates of future water resources. Such estimates can be obtained only on the basis of data on potential regional changes in climatic conditions, primarily precipitation and air temperature by season or month. Unfortunately, such assessments are extremely unreliable, even for the largest regions and river basins.
To forecast changes in regional climate with global warming, atmospheric general circulation models (GSMs) are widely used, as are palaeoclimatic reconstructions of the climate of past warm epochs. There is a wide variety of type of GCM developed in different countries of the world. They produce values for monthly variations in air temperature and precipitation for the entire land surface which would result from a doubling in carbon dioxide concentration: some of these models even postulate values for smaller concentrations. A general feature of all types of GCM used with palaeoclimatic analogues is that with a 2-3º C global warming in individual regions, especially those in high latitudes, air temperature is expected to rise by up 5-6ºC, with much smaller values (0-1ºC) likely in the subequatorial regions.
