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This report, the Synthesis and Assessment Product 4.6 (SAP 4.6), focuses on impacts of global climate change, especially impacts on three broad dimensions of the human condition: human health, human settlements, and human welfare. The SAP 4.6 has been prepared by a team of experts from academia, government, and the private sector in response to the mandate of the U.S. Climate Change Science Program's Strategic Plan (2003). The assessment examines potential impacts of climate change on human society, opportunities for adaptation, and associated recommendations for addressing data gaps and near- and long-term research goals.

Over the past several years, our understanding of global environmental change and our ability to estimate its future effects has improved significantly. In order to summarize the key conclusions of this research, the U.S. Climate Change Science Program (CCSP) has undertaken a national scale "Scientific Assessment of the Effects of Global Change on the United States." The conclusions in this assessment build on the vast body of observations, modeling, decision-support, and other types of activities conducted under the auspices of CCSP. It draws on findings from previous assessments of the science, including reports and products by the Intergovernmental Panel on Climate Change, (IPCC), CCSP, and others. Together with CCSP's 21 Synthesis and Assessment Products, this is arguably the most comprehensive assessment to date of the effects of global change, and especially the climate, on the United States

This report focuses on public health impacts associated with climate change. Summer temperatures in California under scenarios of future climate change are projected to increase by 2°C to 7°C (3.6°F to 12.6°F) by the year 2100. Increases will be accompanied by longer, more frequent, and more severe extreme heat conditions. These increases are expected to affect human health directly through heat-related mortality, and indirectly through air pollution, potential effects on various infectious diseases, and wildfires. By the end-of-century, the likelihood of extreme heat is projected to be 2-4 times greater under the climate change scenarios examined. For an increasingly urbanized population, extreme heat waves create a significant risk of adverse health effects and heat-related mortality.