B.3.2. Precipitation

On increasing CO₂, all models produce an increase in global mean precipitation. Precipitation increases in high latitudes in winter (except in k around the Norwegian Sea where there is cooling and a reduction in precipitation), and in most cases the increases extend well into mid-latitudes (e.g., Figure 6.11a in IPCC 1996, WG I). The warming of the atmosphere leads to higher atmospheric water vapor content and enhanced poleward water vapor transport into the northern high latitudes-hence enhanced water vapor convergence and precipitation (e.g., Manabe and Wetherald, 1975). In the tropics, the patterns of change vary from model to model, with shifts or changes in intensity of the main rainfall maxima. However, many produce more rainfall over India and/or southeast Asia as seen in Figure B-1. This is consistent with an increase in atmospheric water vapor concentration leading to enhanced low level moisture convergence associated with the strong mass convergence into the monsoon surface pressure low. All models considered apart from p and q produce a general reduction in precipitation over southern Europe. In general, changes in the dry subtropics are small.

With the inclusion of aerosol forcing (y,z), there is only a small increase in global mean precipitation. The patterns of change in precipitation in northern winter are broadly similar to that in a parallel simulation with greenhouse gases only (x,w respectively), but less intense. In northern summer, there is a net reduction in precipitation over the Asian monsoon region (Figure B-2), because the aerosol cooling reduces the land-sea contrast and the strength of the monsoon flow. This is in contrast to the models run with CO₂ increase only that showed increases of monsoon precipitation (Figure B-1). Precipitation increases on average over southern Europe (it decreases when aerosol effects are omitted) and over North America, where changes were small with increases in greenhouse gases only.

There is now mounting evidence to suggest that a warmer climate will be one in which the hydrological cycle will in general be more intense (IPCC, 1992), leading to more heavy rain events (ibid, pp. 119). It should be noted, however, that as the GCM grid sizes are much larger than convective elements in the atmosphere, daily precipitation is poorly reproduced by GCMs.

B.3.3. Soil Moisture

Soil moisture may be a more relevant quantity for assessing the impacts of changes in the hydrological cycle on vegetation than precipitation since it incorporates the integrated effects of changes in precipitation, evaporation, and runoff throughout the year. However, simulated changes in soil moisture should be viewed with caution because of the simplicity of the land-surface parameterization schemes in current models (e.g., experiments a,e-i,m,n,p,q, and r use an unmodified "bucket" formulation; see Section 5.3.2 of IPCC 1996, WG I).

Most models produce a general increase in soil moisture in the mean in high northern latitudes in winter, though in some (a,k) there are also substantial areas of reduction. The increases are due mainly to the increased precipitation discussed above, and the increased reaction of precipitation falling as rain in the warmer climate. At the low winter temperatures, the absolute change in potential evaporation is small, as expected from the Clausius-Clapyeron relation, so evaporation increases little even though temperature increases are a maximum in winter. Hence, the increase in soil moisture in high altitudes in winter is consistent with physical reasoning and the broad scale changes are unlikely to be model-dependent. However, it should be noted that in general the models considered here do not represent the effects of freezing on groundwater.

Most models produce a drier surface in summer in northern mid-latitudes. This occurs consistently over southern Europe (except q, which produces an excessively dry surface in winter in its control climate) and North America (except d,k, and q). The main factor in the drying is enhanced evaporation in summer (see Wetherald and Manabe, 1995): The absolute rate of increase in potential evaporation increases exponentially with temperature if other factors (wind, stability, and relative humidity) are unchanged.

As noted in the IPCC (1990), the following factors appear to contribute to summer drying:

• The soil in the control simulation is close to saturation in late winter or spring; this ensures that much of the extra precipitation in winter is not stored in the soil but lost as runoff.
• There is a substantial seasonal variation in soil moisture in the simulation of present climate; some of the simpler models may exaggerate the seasonal cycle of soil moisture (see Chapter 5 of IPCC 1996, WG I) leading to an exaggerated response in the warmer climate.
• In higher latitudes, earlier snowmelt leading to enhanced solar absorption and evaporation may contribute.
• Changes in soil moisture may be amplified by cloud feedbacks in regions where evaporation is being limited by low soil moisture values (e.g., Wetherald and Manabe, 1995).
• The drying is more pronounced in regions where precipitation is reduced in summer.

Given the varying response of different land-surface schemes to the same prescribed forcing (IPCC 1996, WG I, Chapter 5), the consistency from model to model of reductions over southern Europe in summer might be regarded as surprising. All models submitted (except p,q) produced a reduction in summer precipitation over southern Europe: Here changes in circulation and precipitation may be more important in determining soil moisture changes than the details of the land-surface scheme. Reductions over North America are less consistent, and there is a still wider model-to-model variation in the response over northern Europe and northern Asia.

With aerosol forcing included (y,z), the patterns of soil moisture change in northern winter are similar but weaker than with greenhouse gas forcing only (x,w). However, soil moisture increases over North America and southern Europe in summer when aerosol effects are included (y,z), presumably because of the reduced warming and its effect on evaporation, and because of increases in precipitation. The changes in the hydrological cycle are likely to be sensitive to the distribution of aerosol forcing and the coupled model used. However, it is clear that aerosol effects have a strong influence on simulated regional climate change.

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