SW-NCA-color-FINALweb

102 assessment of climate change in the southwest united statesSouthwest and is closely related to Chapter 7, which is concerned with the implicationsof climate change on shorter period phenomena, especially extreme events. Theprojections derive from the outcomes of several global climate models, and associated“downscaled” regional climate simulations, using two emissions scenarios (“A2” or“high-emissions,” and “B1” or “low-emissions”) developed by the IntergovernmentalPanel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES; Nakićenovićand Swart 2000). The key findings are:• Temperatures at the earth’s surface in the Southwest will rise substantially (bymore than 3°F [1.7°C] over recent historical averages) over the twenty-first centuryfrom 2001–2100. (high confidence)• The amount of temperature rise at the earth’s surface in the Southwest will behigher in summer and fall than winter and spring. (medium-high confidence)• Climate variations of temperature and precipitation over short periods (yearto-yearand decade-to-decade) will continue to be a prominent feature of theSouthwest climate. (high confidence)• There will be lower precipitation in the southern portion of the Southwest regionand little change or increasing precipitation in the northern portion. (mediumlowconfidence)• There will be a reduction of Southwest mountain snowpack during Februarythrough May from 2001 through 2100, mostly because of the effects of warmertemperature. (high confidence)• Substantial parts of the Southwest region will experience reductions in runoffand streamflow from the middle to the end of the twenty-first century. (mediumhighconfidence)6.1 Global Climate Models: Statistical and DynamicalDownscalingGlobal climate models (GCMs) are the fundamental drivers of regional climate-changeprojections (IPCC 2007). GCMs allow us to characterize changes in atmospheric circulationassociated with human causes at global and continental scales. However, because ofthe planetary scope of the GCMs, their resolution, or level of detail, is somewhat coarse.A typical GCM grid spacing is about 62 miles (100 km) or greater, which is inadequatefor creating projections and evaluating impacts of climate change at a regional scale.Thus, a “downscaling” procedure is needed to provide finer spatial detail of the modelresults.Downscaling is done in two ways—statistical (or empirical) downscaling and dynamicaldownscaling—each with its inherent strengths and weaknesses. Statisticaldownscaling relates historical observations of local variables to large-scale measures.For climate modeling, this means taking the observed relationship of atmospheric circulationand regional-scale surface data of interest (temperature and precipitation) andapplying those empirical relationships to GCM data for some future period (Wilby etal. 2004; Maurer et al. 2010). Many of the results shown here, involving projected temperatureand precipitation, are based upon a “bias correction and spatial downscaling”