warming; and the strong signal of warming in the semi-arid to arid regions especially in the Mediterranean basin and nearby portions of the Asian continent in June-July-August. Areas of disagreement between models and observations remain, particularly in the form of more homogeneous patterns of warming at the subcontinental scales in the modeled patterns than in observations, with one example being the area of cooling in the southeast of the United States appearing in the HadCRU data and not represented in the models.
As described in Section 4.1, it is reasonable to assume that the local temperature response to an increase in a well-mixed greenhouse gas is proportional to the global mean temperature response, with a well-defined spatial and seasonal pattern. There are also good reasons to assume that the local precipitation response scales with the global mean surface temperature response, although the uncertainties are greater, both with regard to the spatial and seasonal structure of the pattern and with regard to the limitations of this pattern scaling assumption.
Using the CMIP3 archive and computing the precipitation response, measured as a percentage change, divided by the global mean warming and then averaging over models and scenarios, just as for the temperatures in Figure 4.1, one obtains the pattern shown in Figure 4.6, both globally for the annual average and the summer and winter seasons, and focusing on North America (which is not found in the IPCC AR4 report [IPCC, 2007a]). The patterns are very similar to those shown for a particular scenario and time frame in the Summary for Policy Makers of the AR4/WG1 report (IPCC, 2007e).
There is a general increase in precipitation in subpolar and polar latitudes, and a decrease in the subtropics, and an increase once again in many equatorial regions. The boundary between the subtropical decrease and subpolar increase cuts through the continental United States, but with the boundary moving north in the summer and south in the winter. As a result, this ensemble mean projection is for an increase in precipitation in much of the continental United States in the winter and a reduction in the summer. In contrast, Canada is more robustly wetter and Mexico drier, being located closer to the centers of the subpolar region of increasing precipitation and the subtropical region of decreasing precipitation, respectively. The Mediterranean/Middle East and southern Australia are other robust regions of drying in these projections.