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A number of new transient AOGCM simulations for the SRES A2 and B2 scenarios have recently become available and a preliminary analysis was conducted by the lead authors. This follows the procedure similar to that described in this section in relation to Figures 10.3 to 10.6. The results are presented in Box 10.1.
Box 10.1: Regional climate change in AOGCMs which use SRES emission scenarios
The SRES results may be compared with the earlier results summarised in Figures 10.4 and 10.6 (which will be referred to here as the IS92a results). However, it should be noted that these two sets of results differ in the set of models used (both in the model versions and in the total number of simulations), and in the scenarios contrasted in each case (for IS92a it is GHG-only versus GHG+sulphate and for SRES it is A2 versus B2). Also, due to differences in the number of models, thresholds for agreement are not the same in each case (although they have been chosen to be as nearly equivalent as possible).
SRES versus IS92a
The analysis described above is for broad area-averages only and the results described should not be assumed to apply to all areas within these regions. More focused regional studies have examined within-region spatial patterns of change (Joubert and Tyson, 1996; Machenhauer et al., 1996, 1998; Pittock et al., 1995; Whetton et al., 1996b; Carril et al., 1997; Labraga and Lopez, 1997). Such studies can reveal important features which are consistent amongst models but are not apparent in area-average regional results. For example, Labraga and Lopez (1997) noted a tendency for simulated rainfall to decrease in northern Amazonia and to increase in southern parts of this region. Jones R.N. et al. (2000) noted a predominance of rainfall increase in the central equatorial Pacific (northern Polynesia), but in the areas to the west and south-west the direction of rainfall change was not clearly indicated.
Figure 10.7: For the European region, simulated change in annual precipitation, averaged by latitude and normalised to % change per °C of global warming. Results are given for twenty-three enhanced GHG simulations (forced by CO2 change only) produced between the years 1983 and 1998. The earlier experiments are those used in the SCENGEN climate scenario generator (Hulme et al., 1995) and include some mixed-layer 1x and 2xCO2 equilibrium experiments; the later ones are the AOGCM experiments available through the DDC. From Hulme et al. (2000).
To illustrate further inter-model variations in simulated regional precipitation change, results obtained in model inter-comparison studies for the Australian, Indian, North American and European regions are examined. All of these regions have been extensively studied over the years using equilibrium 2xCO2 experiments (such as those featured in IPCC, 1990), first generation transient coupled AOGCMs (as in the SAR), and more recent AOGCMs available in the DDC (Table 9.1). This comparison also enables an assessment of how the regional precipitation projections have changed as the models evolved.
In the Australian region, the pattern of simulated precipitation change in winter (JJA) has remained broadly similar across these three groups of experiments and consists of rainfall decrease in sub-tropical latitudes and rainfall increase south of 35 to 40°S (Whetton et al., 1996a, 2001). However, as the latitude of the boundary between these two zones varied between models, southernmost parts of Australia lay in the zone where the direction of precipitation change was inconsistent amongst models. In summer (DJF) the equilibrium 2xCO2 experiments showed a strong tendency for precipitation to increase, particularly in the north-west of the continent. This tendency was replaced in the first coupled AOGCMs by one of little change or precipitation decrease, which has remained when the most recent coupled models are considered. Whetton et al. (1996a) partly attributed the contrast in the regional precipitation response of the two types of experiments to contrasts in their hemispheric patterns of warming.
Lal et al. (1998b) surveyed the results for the Indian subcontinent of seventeen climate change experiments including both equilibrium 2xCO2 and transient AOGCM simulations with and without sulphate aerosol forcing. In the simulations forced only by GHG increases, most models show wet season (JJA) rainfall increases over the region of less than 5% per degree of global warming. A minority of experiments show rainfall decreases. The experiments which included scenarios of increasing sulphate forcing all showed reduced rainfall increases, or stronger rainfall decreases, than their corresponding GHG-only experiments.
For the central plains of North America, IPCC (1990) noted a good deal of similarity in the response of equilibrium 2xCO2 experiments, with precipitation decreases prevailing in the summer and increases in the winter of less than 10%. In the second group of experiments (nine transient runs with AOGCMs) a wider range of responses was found (in the SAR). In winter, changes in precipitation ranged from about -12 to +20% for the time of CO2 doubling, and most of the models (six out of nine) exhibited increases. In summer, the range of change was narrower, within ±10%, but there was no clear majority response towards increases or decreases. Doherty and Mearns (1999) found that the CGCM1 and HadCM2 models simulated opposite changes in precipitation in both seasons over North America. While overall there is a tendency for more decreases to be simulated in the summer and more increases in the winter, there does not seem to be a reduction in the uncertainty for this region through the progression of climate models.
Many studies have considered GCM-simulated patterns of climate change in the European region (e.g., Barrow et al., 1996; Hulme and Brown, 1998; Osborn and Hulme, 1998; Räisänen, 1998; Benestad et al. 1999; Osborn et al., 1999). Hulme et al. (2000) provide an overview of simulated changes in the region by considering the results of twenty-three climate change simulations (forced by GHG change only) produced between the years 1983 and 1998 and including mixed-layer 1x and 2xCO2 equilibrium experiments as well as transient experiments. Figure 10.7 shows their results for simulated change in annual precipitation, averaged by latitude and normalised to percentage change per degree of global warming. It may be seen that the consensus amongst current models for drying in southern Europe and wetter conditions in northern Europe represents a continuation of a pattern established amongst the earlier simulations. The effect of model development has primarily been to intensify this pattern of response.
Variations across simulations in the regional enhanced GHG results of AOGCMs, which are particularly evident for precipitation, represent a major uncertainty in any assessment of regional climate change. Such variation may arise due to differences in forcing, systematic model-to-model differences in the regional response to a given forcing or differences due to natural decadal to inter-decadal scale variability in the models. Giorgi and Francisco (2000a,b) analysed AOGCM simulations including different models, forcing scenarios and ensembles of simulations, and found that the greatest source of uncertainty in regional climate change simulation was due to inter-model differences, with intra-ensemble and inter-scenario differences being less important (see Figures 10.3 and 10.5). However, it should be noted that Giorgi and Francisco (2000a,b) used long (thirty year) means and large (sub-continental scale) regions and that the uncertainty due to simulated natural variability would be larger when shorter averaging periods, or smaller regions, are used. The results of Hulme et al. (1999) also suggest that low-frequency natural climatic variability is important at the sub-regional scale in Europe and can mask the enhanced GHG signal.
Regional changes in the mean pattern of atmospheric circulation have been noted in various studies, although typically the changes are not marked (e.g., Huth, 1997; Schubert, 1998). Indeed, the work of Conway (1998) and Wilby et al. (1998b) suggests that the contribution of changes in synoptic circulation to regional climate change may be relatively small compared to that of sub-synoptic processes.
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