Greenhouse gases—including carbon dioxide, methane, and nitrous oxide, among other gases—trap some of the sun’s heat in the Earth’s atmosphere and prevent the heat from returning to space. This insulating effect, known as the greenhouse effect, moderates atmospheric temperatures, keeping the Earth warm enough to support life. However, according to the IPCC, global atmospheric concentrations of these greenhouse gases have increased markedly as a result of human activities over the past 200 years and now far exceed preindustrial levels. The IPCC further reports that these increases have contributed to a warming of the Earth’s climate. Table 1 shows the shares of greenhouse gas emissions and their global warming potentials from U.S. sources. The IPCC attributes increases in average global air and ocean temperatures, widespread melting of snow and ice, and rising mean global sea levels to a warming of the Earth’s climate system. The IPCC reports that 11 of the 12 years between 1995 and 2006 rank among the 12 warmest years since 1850 (the first year that global temperatures were recorded) and are indicative of a strong upward warming trend over the last 50 years. Furthermore, according to the IPCC, since 1961, average global ocean temperatures have increased, because the oceans have absorbed more than 80 percent of the heat added to the Earth’s climate system. Such warming causes seawater to expand, contributing to sea level rise. The IPCC also reports that mountain glaciers and snow cover have declined, on average, in both hemispheres, and that widespread decreases in the sizes of glaciers and polar ice caps, combined with losses in the ice sheets of Greenland and Antarctica, have very likely contributed to a sea level rise of 0.17 meters during the 20th century. The federal government manages nearly 30 percent of the land in the United States. Three federal agencies within Interior—BLM, FWS, and NPS—and Agriculture’s FS administer over 90 percent of these lands. NOAA, within Commerce, administers 14 Marine Protected Areas. These agencies manage their resources for a variety of purposes related to preservation; recreation; and in some cases, resource use, yet each agency has distinct responsibilities for the resources it administers. The statutes governing these agencies’ resource management activities generally do not require the agencies to manage for specific outcomes, such as a certain number of visitors, or to provide a specific response to changes in ecological conditions, such as insect outbreaks. Instead, these laws give the agencies discretion to decide how best to carry out their responsibilities in light of their respective statutory missions as well as the need to comply with or implement specific substantive and procedural laws, such as the Clean Air Act, the ESA, or NEPA. The agencies are generally authorized to plan and manage for changes in resource conditions, regardless of the cause that brings about the change. As such, federal resource management agencies are generally authorized, but are not specifically required to address in their actions and planning efforts changes in resource conditions resulting from climate change. BLM, FS, FWS, NOAA, and NPS have unique management missions, as follows: BLM’s mission is to sustain the health, diversity, and productivity of public lands for the use and enjoyment of present and future generations under the principles of multiple use and sustained yield as provided in the Federal Land Policy Management Act. BLM manages more than 260 million acres located primarily in 12 western states. The agency manages and issues permits for activities such as recreation, livestock grazing, timber harvesting, and mining. FS’s mission is to sustain the health, diversity, and productivity of the nation’s forests and grasslands to meet the needs of present and future generations under the Multiple Use and Sustained Yield Act and the National Forest Policy Management Act. FS manages more than 190 million acres throughout the country. The agency manages and issues permits for activities such as skiing, livestock grazing, recreation, timber harvesting, and mining and for rights-of-way for road construction. FWS’s mission is to work with others to conserve, protect, and enhance fish, wildlife, and plants and their habitats for the continuing benefit of the American people. FWS also has a regulatory function, in that it enforces laws, including the ESA and others. FWS manages 547 national wildlife refuges and 37 large, multiple-unit Wetland Management Districts on more than 96 million acres of land throughout the nation; 69 national fish hatcheries; and 46 administrative sites. The National Wildlife Refuge System, which performs FWS’s land management functions, administers a national network of lands and waters for the conservation; management; and, where appropriate, restoration of the fish, wildlife, and plant resources and their habitats within the United States. In addition to these 4 agencies, NOAA seeks to preserve the biological and ecological integrity of U.S. marine systems, which include its National Marine Sanctuary System, consisting of 14 Marine Protected Areas that encompass more than 150,000 square miles of marine and Great Lakes waters. (See fig. 1.) NOAA also manages the waters of the Exclusive Economic Zone, the outer boundary of which is 200 nautical miles from the U.S. coastline. NPS’s mission is to conserve the scenery, the natural and historic objects, and the wildlife of the national park system so that they will remain unimpaired for the enjoyment of current and future generations. NPS manages 391 national park units covering more than 84 million acres throughout the United States and its territories. The agency manages many of the nation’s most precious natural and cultural resources. Other federal entities that manage various federal land and water resources include the Bureau of Indian Affairs, the Bureau of Reclamation, the Department of Defense, and the Tennessee Valley Authority, among others. Figure 2 shows federal resources according to the entity responsible for their management. The resource units managed by the federal government encompass four principal ecosystem types, as shown in table 2. Experts at our workshop told us that climate change is likely to affect federal resources in a number of ways. For example, the experts said that climate change has already caused—and will likely continue to cause— physical changes, including drought, floods, glacial melting, sea level rise, and ocean acidification. Climate change will also cause biological changes, such as increases in insect and disease infestations, shifts in species distribution and abundance, and changes in the timing of natural events (referred to as phenological changes), among others. The experts further said that climate change is likely to adversely affect economic and social goods and services supported by federal resources, including recreation, tourism, infrastructure, water supplies, fishing, ranching, and other resource uses. Officials at our four case study sites provided us with additional examples of some of these climate-related effects already occurring on federal resources. As we have previously mentioned, these four sites are the Florida Keys National Marine Sanctuary in southern Florida (coasts and oceans ecosystem); the Chugach National Forest in south-central Alaska (forests ecosystem); Glacier National Park in northwestern Montana (fresh waters ecosystem); and the BLM Kingman Field Office in northwestern Arizona (grasslands and shrublands ecosystem). Experts at our workshop and officials at our four case study sites stated that climate change can result in physical changes, and that many such changes have already occurred and are likely to continue in the future. Physical effects of climate change include warmer temperatures, drought, glacial melting, floods, sea level rise, and ocean acidification, among others. Experts from the workshop identified increased temperatures as one of the key physical effects of climate change. According to these experts, effects of increasing temperatures that have already been observed include the declining duration of lake and river ice cover throughout the northern hemisphere and increased temperatures of 7 to 10 degrees Fahrenheit in eastern water bodies after storm events. A scientist at Glacier National Park told us that, while the estimated global average temperature has risen 0.6 degree Celsius (about 1 degree Fahrenheit) since about 1897, the estimated average temperatures of the highest elevations in the park have risen 1.6 degrees Celsius (about 3 degrees Fahrenheit) over the same period. This scientist expects this trend to continue. Winter and summer temperatures within the park also are increasing. The data suggest that the number of frost days below 0 degree Celsius (32 degrees Fahrenheit) in Montana from 1900 to 2005 declined by 3 weeks. Furthermore, in 1900, the estimated number of extreme heat days—defined as days with temperatures above 90 degrees Fahrenheit—was about 5 days per year in Glacier National Park (and western Montana); in 2005, this estimated number was about 20 days per year. A record was set in 2003, with 31 days above 90 degrees Fahrenheit. Experts consider 90 degrees to be a major threshold beyond which there are negative impacts on human health and plants in the region. Increased moisture stress due to increased temperatures makes plants more vulnerable to fire, and fire activity in the region has increased in recent years. In 2003, 74,000 hectares in Glacier National Park—about 13 percent of the park’s total area—burned, in what was the largest fire year since 1910, when the park was founded. Other major fires occurred in 1988 and 2001, according to NPS and U.S. Geological Survey (USGS) officials. According to experts at the workshop, warmer spring seasons—due at least partly to climate change—have already resulted in earlier snowmelt, longer summer dry periods, and increased wildland fire activity in western U.S. forest ecosystems, where the experts stated that fires are linked more to climatic conditions than to land management techniques.