The reality of global warming, reflected in a broad spectrum of climate system change, is now unequivocal. Moreover, human complicity in global warming has also been established beyond a reasonable doubt. With these and other advances embodied in the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) has come a shift in scientific focus toward efforts designed to improve our understanding of what will happen in the future, and to what can be done to deal with the reality of global to regional climate change. Major efforts are now needed to develop strategies for adapting to climate change that is already in the pipeline, and also to identify climate changes that may be deemed unacceptable, and thus worthy drivers of mitigation strategies designed to reduce the rate of atmospheric greenhouse gas increases.

There may be many aspects of future global climate change that will ultimately be deemed undesirable and worth mitigation efforts, but two major issues are already coming into focus. The first is global sea level rise coupled with increasing tropical storm intensities. There is no doubt that global sea level is rising, and little doubt that the rates of sea level rise are likely to increase. In contrast to what some biased media and individuals are saying the IPCC AR4 did not lower estimates of future sea level rise. Indeed, the most recent estimates suggest that 1m or more of sea level rise could occur by 2100, as well as a commitment to a much larger sea level rise over subsequent centuries. The wildcard will be the future behavior of the large polar ice sheets, and there is growing evidence that the ice sheets are more vulnerable to global warming than widely thought.

Although coastal areas could thus be big losers in the face of continued climate change, recent climate change coupled with climate change projections indicate that the American West could be a more near-term casualty. Surface air temperatures are already rising faster than elsewhere in the coterminous United States, and will likely continue to rise steadily. These temperature increases are already causing snow to fall increasingly as rain, and also to melt earlier in the year. Thus, even in the absence of a precipitation decrease, there will be less snow-related run-off and related surface water flow. This trend is also being exacerbated in some parts of the West by human-caused increases in atmospheric dust loading. Unfortunately, nearly all state-of-the-art climate models being forced with increasing greenhouse gases (and other human-caused pollution) are also simulating a steady decline in average wintertime precipitation in the Southwest. More troubling is the fact that these simulated changes are also in accord with what has been happening in the real world – there is a growing scientific consensus that winters will become much hotter and significantly drier due to the greenhouse-gas climate forcing. On top of these trends in average condition is the likelihood that multi-year, even multi-decade, drought will also become more common.  Thus, the recent western drought – already the worst of the instrumental era – could be a harbinger of greater aridity to come, and also a significant threat to the West as we know it. Fortunately, there are solutions if we choose to act aggressively.

Jonathan Overpeck is the Director of the Institute for the Study of Planet Earth at the University of Arizona, where he is also a Professor of Geosciences and a Professor of Atmospheric Sciences. He has published over 110 papers in specialty areas including: a) climate dynamics, and paleoclimatology, b) abrupt climate system change, c) climate and ecosystem interaction, d) climate assessment, and e) environmental decision-support. Overpeck recently shared the 2007 Nobel Peace Prize for his role as a Coordinating Lead Author for the UN Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment (2007). He has been awarded the US Department of Commerce Bronze and Gold Medals, as well as the Walter Orr Roberts award of the American Meteorological Society for his interdisciplinary research. Overpeck was awarded a Guggenheim Fellowship to spend his sabbatical year (2005-6) investigating paleoenvironmental perspectives on the future, and was the 2005 American Geophysical Union Bjerknes Lecturer. He serves on the Board of Reviewing Editors for Science Magazine.