CR 10:155-162 (1998)  -  doi:10.3354/cr010155

The IPCC future projections: are they plausible?

Vincent Gray*

Climate Consultant, 75 Silverstream Road, Crofton Downs, Wellington 6004, New Zealand

ABSTRACT: IPCC (Intergovernmental Panel on Climate Change) future projections are based on a set of emission scenarios, IS92a to f, which are used to calculate future atmospheric concentrations of greenhouse gases. These, in turn, are used to calculate projections of radiative forcing, and then projections of future temperature and sea level change to the year 2100, using computer climate models. The assumptions of these 6 IPCC emission scenarios for the years 1995 and 2000 are compared with currently available information on greenhouse gas emissions, world population trends, and trends in world coal production. All of the scenarios exaggerate one or more of these quantities. Calculations of confidence limits on the net human-induced contribution of carbon dioxide to the atmosphere show a very high level of inaccuracy. When added to the even greater uncertainties connected with assumptions on the main greenhouse gas, water vapour, and also on clouds, plus the uncertainties of the computer models themselves, the current IPCC future projections of global temperature and sea level must be regarded as extremely unreliable. Fossil fuel emissions assumed by the IPCC scenarios for the year 2000 are plausible for scenarios IS92a, b, c and d, but not for e and f. The calculated rate of increase of atmospheric carbon dioxide concentration since 1990 assumed by the IPCC is exaggerated by 13% for all scenarios. The calculated rates of increase in atmospheric methane from 1990 to 2000 are exaggerated by 3 to 7 times, world population increases by up to 5.5%, and world coal production increases by 60 to 510%. The rate of increase of carbon dioxide in the atmosphere has been almost constant, at 0.4% a year, between 1971 and 1996, despite a 54% increase in emissions from the combustion of fossil fuels over that period. Currently suggested reductions from present emission levels are therefore unlikely to influence carbon dioxide concentrations, or global temperatures. Since all of the IS92 scenarios exaggerate one or more current climate and economic trends, the calculated future projections of atmospheric greenhouse gas concentrations are thus correspondingly exaggerated. A more realistic set of scenarios, which would include a mechanism for continuous updating, needs to be developed, thus scaling down the current values. Even if this is done, however, the accumulated inaccuracies inherent in the final calculations of climatic effects are so great as to render them unreliable as a guide to public policy.

KEY WORDS: Climate change · Intergovernmental Panel on Climate Change · Emission scenarios

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