GCM projections, though undergoing continuous improvements, remain too uncertain over a region such as Europe to draw more than very tentative conclusions. Nested regional climate model (RCM) simulations also are too tenuous to provide firm conclusions about climatic change in Europe. Temperature changes in large parts of Europe will be a function not only of positive radiative forcing resulting from increases in the atmospheric concentrations of a number of greenhouse gases but also of the countereffects of aerosols. Although the magnitude of the aerosol effects is still quite uncertain, the regional negative forcing of sulfate aerosols in central Europe could offset almost half the positive radiative forcing of CO2 (Mitchell et al., 1995). The summary of results that follows is based on GCM simulations without aerosol forcing that were published in the Working Group I volume of the Second Assessment Report (SAR) (IPCC 1996, WG I, Chapter 6).
Most GCM-based projections for the European region indicate that there may be an overall increase in winter annual temperatures (IPCC 1996, WG I, Figure 6.32) and that this increase could be larger in boreal latitudes than in mid-latitude Europe. The diurnal temperature range, according to the model used, is 2.5-4.5°C for northerly latitudes, compared with 1.5-4.5°C for southern Europe. The range of summer temperatures forecast by the different models is larger than for the winter period, but the upper limit of the range is about 4.5°C increase in a 2xCO2 climate for southern and northern Europe.
Projected precipitation patterns are more uncertain. Most models show an increase in precipitation for Europe as a whole as a consequence of a higher content of water vapor in the atmosphere. Winter precipitation in high latitudes of Europe may increase by as much as 20% (IPCC 1996, WG I, Figure 6.32), according to most models. Rainfall during the summer months may remain unchanged in many parts of Europe. Some models show decreases in the Mediterranean region and in central and eastern Europe, though others show increases; in northern Europe, most models suggest an increase in summer precipitation. There is much uncertainty associated with future precipitation trends, however; for instance, GCM simulations incorporating the aerosol effect provide conflicting evidence for future precipitation trends in parts of Europe, compared with greenhouse-gas-only simulations.
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