Conclusion 1: Climate change is occurring, is caused largely by human activities, and poses significant risks for—and in many cases is already affecting—a broad range of human and natural systems.

This conclusion is based on a substantial array of scientific evidence, including recent work, and is consistent with the conclusions of recent assessments by the U.S. Global Change Research Program (e.g., USGCRP, 2009a), the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC, 2007a-d), and other assessments of the state of scientific knowledge on climate change. Both our assessment—the details of which can be found in Chapter 2 and Part II (Chapters 6-17) of this report—and these previous assessments place high or very high confidence¹ in the following findings:

• Earth is warming. Detailed observations of surface temperature assembled and analyzed by several different research groups show that the planet’s average surface temperature was 1.4°F (0.8°C) warmer during the first decade of the 21st century than during the first decade of the 20th century, with the most pronounced warming over the past three decades. These data are corroborated by a variety of independent observations that indicate warming in other parts of the Earth system, including the cryosphere (snow- and ice-covered regions), the lower atmosphere, and the oceans.

• Most of the warming over the last several decades can be attributed to human activities that release carbon dioxide (CO₂) and other heat-trapping greenhouse gases (GHGs) into the atmosphere. The burning of fossil fuels—coal, oil, and natural gas—for energy is the single largest human driver of climate change, but agriculture, forest clearing, and certain industrial activities also make significant contributions.

• Natural climate variability leads to year-to-year and decade-to-decade fluctuations in temperature and other climate variables, as well as substantial regional differences, but cannot explain or offset the long-term warming trend.

• Global warming is closely associated with a broad spectrum of other changes, such as increases in the frequency of intense rainfall, decreases in Northern Hemisphere snow cover and Arctice sea ice, warmer and more frequent hot days and nights, rising sea levels, and widespread ocean acidification.