Congress has, over the past three decades, authorized and funded federal programs to improve understanding of climate changes and their implications. Climate changes have potentially large economic and ecological consequences, both positive and negative, which depend on the rapidity, size, and predictability of change. Some of the impacts of past change are evident in shifting agricultural productivity, forest insect infestations and fires, shifts in water supply, record- breaking summer high temperatures, and coastal erosion and inundation.People and natural systems respond to climate changes regardless of whether the government responds. Over time, the consequences of climate change for the United States and the globe will be influenced by choices made or left to others by the U.S. Congress.Different factors contribute to climate change, their contributions depending on the time periods and geographic locations under examination. Current scientific evidence best supports rising atmospheric concentrations of “greenhouse gases” (GHG) (particularly carbon dioxide, methane, nitrous oxides) and other air pollutants as having driven the majority of global average temperature increase since the late 1970s. The increase in concentrations is due almost entirely to GHG emissions from human activities. Hence, the policy debate has focused on whether and how to abate GHG emissions from human-related activities. Locally, human-related air pollution, irrigation, the built environment, land use change, and depletion of ozone in the stratosphere may be more important but have small overall effect on global average temperature.Policy proposals take different approaches to setting goals or managing climate change-related risks. This report describes four strategies for setting climate change policies: (1) research and wait-and-see, (2) science-based goal setting, (3) economics-based policies, and (4) incrementalism or adaptive management. Each may take into account the concerns, values, and skepticisms of some constituencies, but each also has limitations. It is unclear whether any single conceptual approach could cover all elements of the policy debate, though hybrid approaches may help to build political consensus over whether and how much policy intervention is appropriate.If climate change merits federal action, a variety of generic policy tools may be available (some in use already) to achieve policy goals:•regulatory, including market-based, tools to reduce GHGs; •distribution of potential revenues from GHG programs; •non-regulatory tools that help markets work more efficiently; •tools to stimulate technological change; •options to ease the economic transition to a lower GHG economy; •instruments to encourage international actions; and •tools to stimulate adaptation to climate change.Analysts have elucidated the potential usefulness and limitations of each option. Many experts have concluded that, to achieve a given policy goal, strategies using complementary policy tools can increase cost-effectiveness, alleviate burdens on particular constituencies, and address additional concerns of policy-makers. This report seeks to support Congress as it debates and modifies the mix of federal programs that may influence the climate or adaptation to its changes.