Regardless of the public conversation, the Earth's climate is changing. Changes are exhibited in observations of average temperatures over land and in the oceans, melting glaciers and ice caps, shifting precipitation patterns, modified growing seasons, shifting distributions of plants and animals, and a variety of additional observations. (Many but not all elements of climate show distinct trends.) Regional climates in the United States have shifted as well ( Figure 1 ). A variety of factors contribute to the changes, their weights differing depending on the time periods and geographic locations under examination. In public media, the controversy over causes may appear much greater than the broad scientific agreement that exists: the scientific evidence best supports rising atmospheric concentrations of "greenhouse gases" (GHG) (particularly carbon dioxide, methane, nitrous oxides) and other air pollutants as having contributed to the majority of global average temperature increase since the late 1970s. (See box.) The rise of GHG concentrations is due to emissions from human-related activities. Other air pollution, irrigation, the built environment, and depletion of ozone in the stratosphere may be more important for changing temperature and/or precipitation patterns in some locations over the past 30 years but have small overall effect on global average temperature. For short periods, such as a few years, volcanic eruptions and solar cycles may have noticeable influence. Over time scales of hundreds to tens of thousands of years, cycles of the Sun's radiation and the features of the Earth's orbit and wobble have dominated, triggering effects amplified by feedbacks in the climate system and visible in glacial cycles. Regardless of causes, 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. Congress has engaged, over the past three decades, in authorizing and funding federal programs to improve understanding of climate changes (past and predicted) and their implications. Science programs predominated prior to 1990. In 1992, the Senate gave its advice and consent to U.S. ratification of the United Nations Framework Convention on Climate Change (UNFCCC), effectively agreeing to its objective: ... to achieve ... stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner. This commitment is legally binding, though not practicably enforceable, and has guided some subsequent federal actions, including the first U.S. climate action plan in 1992, published under President George H. W. Bush. Since the 1990s, some federal programs and many legislative proposals have sought to slow greenhouse gas (GHG)-induced climate change through regulatory, voluntary, and financial efforts to abate emissions. Many such proposals remain controversial and few have been enacted. In 2007, the Supreme Court ruled in Massachusetts v. EPA that the Clean Air Act's (CAA's) "sweeping" definition of "air pollutant" embraces "any air pollutant ... including any physical, chemical ... substance or matter which is emitted into or otherwise enters the ambient air." Also, the Court ruled that EPA could not use policy considerations in deciding whether to regulate GHG emissions; EPA can avoid taking further action "only if it determines that greenhouse gases do not contribute to climate change or if it provides some reasonable explanation as to why it cannot or will not exercise its discretion." Following this decision, EPA found that GHG-induced climate change endangers human health and welfare and has acted to promulgate regulations to control six GHG. Most experts, and also the Obama Administration, would prefer new legislation strategically addressing GHG abatement, rather than authorities under the CAA, as a policy vehicle to address climate change. The 111 th Congress debated bills that would have established comprehensive climate change policy, and that would have included new regulatory authority to cap emissions of GHG and to allow emissions sources the flexibility to trade the emissions allowances like a commodity ("cap-and-trade"). Debate in the 112 th Congress has focused more on restricting authorities of the EPA to control GHG as pollutants under the CAA, or to reduce or eliminate funding for climate change-related federal programs. New federal programs (especially in the Department of the Interior) aimed at planning for adaptation to climate change, regardless of its cause, have emerged in the 2000s. Many agencies and some in Congress consider projections of impacts and preparation to be among the stewardship responsibilities of the federal government for publicly held resources that may be affected by climate change, as well as for protecting human health and general welfare. Some of those in Congress who consider such programs to be warranted may not, however, fully support Administrative proposals for funding, in light of budget pressures or concerns about strategies or program design. Underlying efforts explicitly to address climate change are other programs enacted for other purposes that influence U.S. contributions and vulnerabilities to climate change. For example, regulations and financial incentives for agriculture, energy, and infrastructure shape these sectors' emissions of GHG, technological opportunities, and vulnerabilities in the face of changing seasonality, water availability and temperatures, inundation of flood plains, winds, and other climate-linked phenomena. Ongoing public concerns and international pressures for U.S. collaboration to mitigate and adapt to climate change are likely to keep climate change on Congress's legislative agenda for the foreseeable future. To support congressional considerations, this report outlines (1) conceptual approaches to setting goals for policies, and (2) brief descriptions of the principal "policy tools" that could be wielded to achieve policy goals. Neither domestically nor internationally have policy-makers converged on a common approach to setting goals or managing climate change-related risks. Some do not consider that there are sufficient risks of climate change to merit governmental intervention. For policy-makers who may wish to consider addressing climate change, this section articulates four competing 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. For several decades, policy-makers have been aware of the large range of projections of GHG-induced climate change and adverse impacts, as well as of the potentially large costs associated with avoiding GHG-induced climate change. In the face of these uncertainties, arguably the primary strategy followed by the U.S. federal government has been to support research and to "wait-and-see." Proponents assume that scientific research will yield more certainty about climate change that would help make better policy decisions, and yield answers in a timeframe consistent with making effective policy decisions. They may also assume that investment in technology research will reduce GHG abatement costs, making it cheaper to reduce emissions later. Following this approach, the U.S. government has invested many billions of dollars in climate research and "clean" technologies over the past two decades, with a small fraction allocated to policies that directly mitigate the risks or promote adaptation to them. Some scientists and advocates for emissions abatement action have welcomed the resources for research, but also expressed the likelihood that research may well widen, not narrow, uncertainties. No matter how much is invested in research, critical uncertainties are likely to remain. Some people have argued that research cannot provide "right" policy answers and that this strategy constitutes avoidance of difficult decisions. In contrast, others have pointed to analysis that wrong actions taken in the context of uncertainties may result in unnecessary costs. Some have suggested that economic conditions in the future may make addressing climate change more affordable than the present: increasing incomes and improving technologies could make it easier for future generations to pay to address climate change than for people today. Research has made significant scientific progress over the past three decades. Still, it may be easier to argue that uncertainties may now be better characterized but not narrowed; some may have widened. It is unclear that further research within the next decade or two will significantly narrow crucial uncertainties, such as prediction of precipitation patterns over the next 50 years (needed to estimate climate change risks, such as impacts on costs of agricultural production or flood control, as examples). If over that period GHG emissions continue to rise, future GHG policies would need to make greater and more rapid reductions in order to avoid any particular level of risk reduction. Federally supported research also has made new technologies available and reduced the costs of others. The lowering of technology costs during that period may or may not offset the added costs of starting later, with greater, more rapid GHG reductions to achieve a given level of risk reduction. Some advocates propose a science-centric approach, looking to physical or biological criteria to identify appropriate policy goals. This assumes that science alone can provide an objective standard of a "safe" or "tolerable" level or rate for climate change, or at least an inflection point beyond which the projected damages of climate change may rise more steeply. Proponents of this approach may look to past rates of temperature change, past (i.e., pre-industrial) atmospheric concentrations of GHG, or indicators of ecosystem adaptability, for identifying the policy goal. Typically, the science-based approach draws on the estimated relationships between GHG emissions, GHG atmospheric concentrations, global average temperature changes, and projected impacts of climate change for identifying equivalent targets across these different parameters ( Figure 2 and Table 1 ).