The lack of regionally specific climate information from global climate models poses a major challenge, because many climate-related decisions, especially those related to adaptation, demand information on regional to local scales. A variety of downscal-ing approaches have been developed to obtain this regional information. One widely used approach is statistical downscaling, wherein empirical relationships between past observations of local- and regional-scale climate variations are used to translate large-scale projections from global climate models to smaller space scales and shorter time scales. Alternatively, finer-scale regional models can be "nested" within coarser-resolution global models to simulate regional climate changes (e.g., Hay et al., 2002; Leung et al., 2003; UCAR, 2007). A related approach is linking models currently used to predict weather and seasonal to interannual climate variations with those that predict climate change on decadal to centennial time scales (this is sometimes called "seamless prediction").
In general, downscaling techniques are not as well developed or understood as global models, and key technical and scientific issues remain. For example, regional modeling efforts have been limited by constraints on computing resources, uncertainties and complexities associated with data assimilation and parameterization, the lack of a well-developed framework for downscaling, and the limitations of the large-scale simulations on which the downscaling is performed (Held and Soden, 2006; NRC, 2009k). An additional challenge for regional projections is representing regional modes of variability, such as ENSO and the Pacific Decadal Oscillation (described earlier in this chapter). Not only do these regional modes have a strong influence on local and regional climate change, but many also have global signatures, and they could potentially change themselves as the climate system warms. Finally, climate forcing scenarios that project human influences on local and regional climate, such as regional aerosol loading and land use change, are needed because these forcings may have a large influence on local and regional climate change (CCSP, 2008c).
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