Hoerling et al. 2008
Hoerling, M., G. Hegerl, D. Karoly, A. Kumar, and, 2008: Attribution of the causes of climate variations and trends over North America during the modern reanalysis period. In Reanalysis of Historical Climate Data for Key Atmospheric Features: Implications for Attribution of Causes of Observed Change, U.S. Climate Change Science Program Synthesis and Assessment Product 1.3. National Oceanic and Atmospheric Administration, National Climatic Data Center, 47-92.
Increasingly, climate scientists are being asked to go beyond descriptions of what the current climate conditions are and how they compare with the past, to also explain why climate is evolving as observed; that is, to provide attribution of the causes for observed climate variations and change.
Today, a fundamental concern for policy makers is to understand the extent to which anthropogenic factors and natural climate variations are responsible for the observed evolution of climate. A central focus for such efforts, as articulated in the Intergovernmental Panel on Climate Change (IPCC) Assessment Reports has been to establish the cause, or causes, for globally averaged temperature increases over roughly the past century. However, requests for climate attribution far transcend global temperature change alone, with notable interest in explaining regional temperature variations and the causes for high-impact climate events, such as the recent multi-year drought in the western United States and the record setting U.S. warmth in 2006. For many decision makers who must assess potential impacts and management options, a particularly important question is: What are the causes for regional and seasonal differences in climate variations and trends, and how well do we understand them? For example, is the recent drought in the western United States due mainly to factors internal to the climate system (e.g., the sea surface temperature variations associated with ENSO), in which case a return toward previous climate conditions might be anticipated, or is it a manifestation of a longer-term trend toward increasing aridity in the region that is driven primarily by anthropogenic forcing? Why do some droughts last longer than others? Such examples illustrate that, in order to support informed decision making, the capability to attribute causes for past and current climate conditions can be a major consideration.
The recently completed IPCC Fourth Assessment Report (AR4) from Working Group I contains a full chapter devoted to the topic "Understanding and Attributing Climate Change". This Chapter attempts to minimize overlap with the IPCC Report by focusing on a subset of questions of particular interest to the U.S. public, decision makers, and policy makers that may not have been covered in detail (or in some cases, at all) in the IPCC Report. The specific emphasis here is on present scientific capabilities to attribute the causes for observed climate variations and change over North America.