B.2. Simulations with Greenhouse Gas and Aerosol Forcing

Two simulations (x,w) were forced with the historical increase in equivalent CO₂, then a 1%/yr increase in equivalent CO₂. The patterns of change are qualitatively similar to those in the experiments above. In Sections 6.2.1.2 and 8.4.2.3 of the Working Group I contribution to IPCC (1996), we saw that the inclusion of the direct sulfate aerosol forcing can improve the simulation of the patterns of temperature change over the last few decades. Here we consider the effect of combined greenhouse gas and direct sulfate aerosol forcing derived from IS92a on simulated patterns of temperature change to 2050 and beyond (y,z). Figure B-2 shows area averages of summer and winter surface temperature, precipitation, and soil moisture from two models-one set with CO₂ increase only, the other with CO₂ increase and the direct effects of sulfate aerosols. The areas considered include five from Figure B-1 (i.e., Central North America, South East Asia, Sahel, Southern Europe, and Australia).

Figure B-2: Simulated regional changes from 1880-1889 to 2040-2049 (experiments x, y) or from pre-industrial to 2030-2050 (experiments w, z). Experiments x and w include greenhouse gas forcing only, whereas y and z also include direct sulfate aerosol effects (see Table B-1): (a) Temperature (December to February); (b) Temperature (June to August); (c) Precipitation (December to February); (d) Precipitation (June to August); (e) Soil Moisture (December to February); and (f) Soil Moisture (June to August).

Increasing CO₂ alone leads to positive radiative forcing everywhere, with the largest radiative heating in regions of clear skies and high temperatures (experiment w shown in Figure 6.7a of IPCC 1996, WG I). The surface temperature warms everywhere except in the northern North Atlantic (Figure 6.7b of IPCC 1996, WG I). In transient simulations to 2050, the inclusion of aerosols based on IS92a (y,z) reduces the global mean radiative forcing, and leads to negative radiative forcing over southern Asia. This leads to a muted warming or even small regions of cooling (y) in mid-latitudes. In (z), China continues to warm, albeit at a very reduced rate, even though the local net radiative forcing becomes increasingly negative (Figure 6.7c in IPCC 1996, WG I). The rate of warming over North America and western Europe, where the aerosol forcing weakens, remains below that in the simulation with greenhouse gases only (w). The cooling due to aerosols is amplified by sea ice feedbacks in the Arctic (Figure 6.7f in IPCC 1996, WG I).

In assessing these results, one should bear in mind the possible exaggeration of the sulfate aerosol concentrations under this scenario, the uncertainties in representing the radiative effects of sulfate aerosols, and neglect of other factors including the indirect effect of sulfates. Nevertheless, these experiments suggest that the direct effect of sulfate aerosols could have strong influence on future temperature changes, particularly in northern mid-latitudes.

B.3. Seasonal Changes in Temperature, Precipitation, and Soil Moisture

IPCC (1990) reported some broad scale changes that were evident in most of the equilibrium 2xCO₂ experiments that were then available. The detailed regional changes differed from model to model. In the transient experiments reported in IPCC (1992), it was found that the large-scale patterns of response at the time of doubling CO₂ were similar to the corresponding equilibrium experiments (IPCC, 1990), except that there was a smaller warming in the vicinity of the northern North Atlantic and the Southern Ocean in transient experiments. Here we summarize the main features in the seasonal (December to February and June to August) patterns of change in temperature, precipitation, and soil moisture in those experiments with a 1%/yr increase in CO₂ for which data were available. The changes are assessed at the time data of CO₂ doubling (after 70 years). In experiments w-z, we also contrast the continental-scale response under the IS92a scenario with and without aerosol forcing at around 2040.

B.3.1. Temperature

With increases in CO₂, all models produce a maximum annual mean warming in high northern latitudes (Figures 6.6 and 6.7b of IPCC 1996, WG I). The warming is largest in late autumn and winter, largely due to sea ice forming later in the warmer climate. In summer, the warming is small; if the sea ice is removed with increased CO₂, then the thermal inertia of the mixed-layer prevents substantial warming during the short summer season, otherwise melting sea ice is present in both control and anomaly simulations, and there is no change in surface temperature (see Ingram et al., 1989). The details of these changes are sensitive to parameterization of sea ice and, in particular, the specification of sea ice albedo (e.g., Meehl and Washington, 1995). In one simulation (k), there is a marked cooling over the northeastern Atlantic throughout the year, which leads to a cooling over part of northWest Europe in winter. There is little seasonal variation of the warming in low latitudes or over the southern circumpolar ocean.

When aerosol effects are included (y,z cf. x,w respectively), the maximum winter warming in high northern latitudes is less extensive. In mid-latitudes, there are some regions of cooling (e.g., over China), and the mean warming in the tropics is greater than in mid-latitudes. In northern summer, there are again regions of cooling in mid-latitudes and the greatest warming now occurs over Antarctica. Again, including the direct forcing by sulfate aerosols has a strong effect on simulated regional temperature changes, though the reader should bear in mind the limitations of these experiments as noted earlier.

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