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6.9.1. GCM Climate Simulations

During the last decade or so, a number of complex GCMs have attempted to simulate future anthropogenic climate change. Earlier models (e.g. Manabe & Stouffer, 1980; Hansen et al., 1984; Mitchell et al., 1989; see also IPCC, 1990a) studied equilibrium climate change associated between two, presumably stable, states of climate. The results of these are discussed in the first IPCC report (1990a). Associated with a doubling of pre-industrial atmospheric CO2, the following conclusions have been made:

a) a global average warming at or near the Earth's surface of between 1.5 and 4.5C, with a "best guess" of 2.5C, will occur;

b) the stratosphere will experience a significant cooling;

c) surface warming will be greater at high latitudes in winter, but less during the summer;

d) global precipitation will increase by 3 to 15%;

e) year-round increases in precipitation in high-latitude regions are expected, whilst some tropical areas may experience small decreases.

More recent time-dependent GCMs (e.g.. Washington & Meehle, 1989; Stouffer et al., 1989; Cubasch et al., 1990; see also IPCC, 1992) which couple the atmospheric and oceanic components of the climate system together, provide more reliable estimates of greenhouse-gas-induced climate change. For steadily increasing greenhouse forcing, the global rise in temperature is typically less than the equilibrium rise corresponding to an instantaneous forcing. Significant results indicate:

a) a global average warming of 0.3C per decade, assuming non-interventionist greenhouse gas emission scenarios (see section 6.9.2);

b) a natural variability of about 0.3C in global surface air temperature on decadal time scales;

c) regional patterns of temperature and precipitation change similar to equilibrium experiments, although warming is reduced in high latitude oceans where deep water is formed.

The ability to model the time-dependent nature of the climate system more adequately has allowed scientists to investigate the damping effects of the oceans on climate change (section 2.4). Because the response time of the oceans, in particular the deep ocean, is much longer than for the free atmosphere, they have a regulating or delaying effect on the warming associated with enhanced greenhouse forcing. In addition, the transient GCMs have allowed increased attention to focus on the critical role of feedback processes (section 2.7) in determining the climate's response to forcing perturbations.