CO2 and the greenhouse effect: present assessment and perspectives

Our present knowledge on the increasing greenhouse effect is based on the 1990 assessment of the Intergovernmental Panel on Climate Change (IPCC) and its 1992 supplement. Model predictions suggest that a doubling of atmospheric CO2 concentration would increase global temperature by 2-4 degrees C. Because time is needed for the upper layers of the oceans to warm up, there is a delay between the realized increase and the estimated equilibrium increase. The 0.5 degrees C increase in global temperature over the past 100 years is in accordance with model predictions but also within the range of natural variability. The IPCC's assessment suggests that if fossil fuel consumption continues according to a 'business as usual' scenario, global temperature will increase at 0.3 degrees C per decade; droughts, flooding and storms would become more frequent and more severe, and sea-level would continue to rise. Analysis of gas in ice cores from Antarctica and Greenland provides estimates of CO2 concentrations in pre-industrial ages; accurate measurement of atmospheric CO2 began in 1958. Atmospheric CO2 concentration has increased from 280 p.p.m. in AD 1800 to 355 p.p.m. at present. Between 1945 and 1973 global emissions of CO2 increased at a rate of 4.4% per year; after 1973 the rate of increase decreased to 1.6% per year. This change permits a test of the CO2 model. Reconstructed CO2 emission agrees within less than 10% with estimated fossil fuel-generated CO2 emission; contributions to CO2 emission from non-fossil fuel sources must be smaller than assumed previously. There are strong indications that in the past changes in atmospheric greenhouse gas concentrations inherent to the glacial-interglacial cycles did play an important climatic role. For example, they were probably responsible for interhemispheric coupling during the major climatic changes. Measures which might stabilize the greenhouse effect include energy conservation and improved energy efficiency, a transition to hydrogen rather than carbon as a source of fuel, and reforestation.