Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science ProgramChapter 3Figure 3.17. Modeled changes in annual mean precipitation minus evaporation (P–E) over southwesternNorth America (125°W.–95°W., 25°N.–40°N., land areas only) averaged over ensemble members for 19models participating in IPCC AR4. The historical period used known and estimated climate forcings andthe projections used the SResA1B emissions scenario (IPCC, 2007). Shown are the median (red line) and25th and 75th percentiles (pink shading) of the P–E distribution amongst the 19 models, and the ensemblemedians of P (blue line) and E (green line) for the period common to all models (1900–2098). Anomaliesfor each model are relative to that model’s climatology for 1950–2000. Results have been 6-year low-passfiltered to emphasize low frequency variations. Units are millimeters per day (mm/day). The model ensemblemean P–E in this region is around 0.3 mm/day. From Seager et al. (2007c).however, that GCMs are unable to capture themesoscale processes that underlie the NorthAmerican monsoon (e.g., Castro et al., 2007a),so there is uncertainty regarding the impact ofthese changes on monsoon season precipitationin the American Southwest and NorthernMexico.The area encompassing the Mediterraneanregions of southern Europe, North Africa, andthe Middle East dries in the model projectionseven more strongly, with even less disagreementamongst models, and also beginning towardthe end of the last century. Both here and insouthwestern North America, the drying is notabrupt in that it occurs over the same time scaleas the climate forcing strengthens. However, theseverity is such that the aridity equivalent tohistorical droughts—but as a new climate ratherthan a temporary state—is reached within thecoming years to a few decades. Assessed onthe time scale of water-resource development,demographic trends, regional development, oreven political change, this could be describedas a “rapid” if not abrupt climate change and,hence, is a cause for immediate concern.causes of historical droughts. The latter arerelated to particular patterns of tropical SSTanomalies, while the former arises as a consequenceof overall, near-uniform warmingof the surface and atmosphere and how thatimpacts water-vapor transports and atmosphericcirculation. Both mechanisms involve a polewardmovement of the mid-latitude westerliesand similar changes to the eddy-driven meanmeridional circulation. However, a polewardexpansion of the Hadley Cell has not beeninvoked to explain the natural droughts. Furtherfuture drying is expected to be accompanied bya maximum of warming in the tropical uppertroposphere (a consequence of moist convectionin the deep tropics), whereas natural droughtshave gone along with cool temperatures in theThe future subtropical drying occurs in themodels for reasons that are distinct from the108