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  • Agriculture, fisheries, and food production;

  • Public health;

  • Cities and the built environment;

  • Transportation systems;

  • Energy systems;

  • Solar radiation management;

  • National and human security; and

  • Designing, implementing, and evaluating climate policies.

The research progress in each of these topics is explored in additional detail in Part II of the report, but even those chapters are too brief to provide a comprehensive review of the very large body of research on these issues. Likewise, this report does not cover all scientific topics of interest in climate change research, only those of most immediate interest to decision makers. Readers interested in additional information should consult the extensive assessment reports completed by the USGCRP,2 the IPCC,3 the National Research Council (NRC),4 and other groups, as well as the numerous scientific papers that have been published since their completion.


Earth’s physical climate system, which includes the atmosphere, oceans, cryosphere, and land surface, is complex and constantly evolving. Nevertheless, the laws of physics and chemistry ultimately govern the system, and can be used to understand how and why climate varies from place to place and over time.

The Greenhouse Effect is a Natural Phenomenon That Is Critical for Life as We Know It

GHGs—which include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and several others—are present in relatively low concentrations in the atmosphere, but, because of their ability to absorb and re-radiate infrared energy, they trap heat near the Earth’s surface, keeping it much warmer than it would otherwise be (Figure 2.1). The atmospheric concentrations of GHGs have increased over the past two centuries as a result of human activities, especially the burning of the fossil

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