Abstract
The outgoing long wave radiation to space is significantly reduced as a result of heat absorption by the so-called greenhouse gases, notably water vapour, carbon dioxide, methane, nitrous oxides and chlorofluorocarbons. The dominating ones are water vapour and carbon dioxide. The direct effect of the greenhouse gases is to increase the average temperature of the earth's surface by some 33°C, that is from -18°C to the present +15°C. Since the industrial revolution the combustion of fossil fuels and deforestation has led to an increase of 26 in carbon dioxide in the atmosphere. It is presently increasing by about 0.5 per year. Based on strict radiative forcing calculations, the total additional warming because of the increase in greenhouse gases, not including the effect of associated changes in water vapour, is equivalent to about 2.5 W m-2 and is estimated to increase to some 10 W m-2 at the end of the next century. The solar constant, by comparison, is equal to 341 W m-2 globally averaged. The effect on climate cannot satisfactorily be estimated on radiation balance calculation only but must incorporate the large scale atmospheric circulation and the important feedbacks with water vapour (positive feedback), clouds (both positive and negative feedbacks), surface albedo and the oceans. For that reason it is necessary to build comprehensive mathematical models of the coupled ocean/land atmosphere system and carry out numerical simulation experiments. Several such studies have been undertaken during the last few years both in Europe and the USA. Results from these experiments, and in particular from those carried out at the Max-Planck-Institut für Meteorologie in Hamburg, will be presented and discussed. Because of the enormous complexity of the problem, a number of simplified assumptions have been used and the results so far must be cautiously assessed. The overall global warming from these experiments is about 3°C with a difference of about 0.5°C between the experiments. There are large regional differences and the warming is generally larger over land than over sea. A particular problem is the temperature of the North Atlantic and also in the Antarctic waters where changes in the deep ocean circulation are significant. The strong coupling to the deep ocean circulation in the North Atlantic reduces the warming in this area and consequently moderates the effect on the European climate. Sea level is estimated to increase by around 15 cm because of thermal expansion of sea water; however, it appears likely that there may also be an increased accumulation of ice on Antarctica and probably Greenland which may at least partly compensate for this increase. Possible changes in the vegetation following a climate change have been estimated by evaluating the response of a biome model. This shows minor changes in the vegetation; over Europe and North America a small north-eastward movement of the vegetation zones. © 1994.
