Experts project that if greenhouse gas (GHG) emissions are not abated well below current levels, the Earth's climate will warm further—to levels never experienced by human civilizations. In parallel, growing attention is being given to characterizing and supporting adaptations to changes already observed or expected future changes. Domestic actions to address climate change are moving independently across many fronts. In the 110 th Congress, numerous bills have been proposed to address climate change research and policy; one bill ( S. 2191 ) that would cap and reduce greenhouse gas emissions was reported by the Senate Committee on Environment and Public Works and the Senate agreed on June 2, 2008 to consider the bill (now S. 3036 ). Primary concerns are the costs, which could reach trillions of dollars over coming decades, depending on policy choices; the distribution of those costs; and the effectiveness of policies, with the knowledge that U.S. greenhouse gas reductions would achieve success only if sufficient international cooperation can be achieved as well. Precipitation has increased over the past century, although some regions have become wetter while some have become drier. Most scientists conclude that a majority of the Earth's warming since the 1970s is due to GHG emissions from human activities, especially use of fossil fuels, clearing of land, and some industrial processes. The long atmospheric residence also means a long lag between policies to abate GHG emissions and their full effects on the climate system. To limit future risks, many experts propose targets to cap or "stabilize" the concentrations of GHGs in the atmosphere; any level of stabilization is associated with a wide range of possible temperature outcomes. Program Design and the Costs of GHG Mitigation

Greenhouse gas control programs raise concerns about costs: that the costs may be large; that the costs of mitigation may exceed the benefits of mitigation; or, that the distribution of costs may not be "fair." Climate Change and Other Policy Issues: Commonality or Conflict? Many technologies and policy options can serve all or many policy objectives. Technically and politically, though, an "efficiently designed" program may not be realistic. Available for others who are considering additional actions is an assortment of policy tools that they see as stimulating further reductions of GHG emissions and reducing risks to the economy, specific populations, and natural systems. Tools to Stimulate Technological Change

Achieving deep GHG reductions from projected levels—necessary to avoid most projected climate change—would require extraordinary changes in how energy is used and supplied over time. Some policy tools that could be applied, although some could encounter potential challenges under the World Trade Organization (WTO) rules, include
border tax adjustments that would raise the prices of imports from countries without GHG controls comparable to those of the United States; "international reserve" allowances that importers of certain goods must purchase (raising the cost of imports) if the country of origin does not apply GHG controls comparable to those of the United States; giving, over some period, allowances or revenues from sales of allowances to affected industries in order to facilitate adjustment; in the process of crafting domestic policies, negotiating with potentially affected WTO Members to seeks ways to avoid imposing restrictive import measures; working within the WTO to change or clarify rules to permit the imposition of import restrictions by countries adopting trade-vulnerable GHG control requirements; and working multilaterally to have GHG emission controls applied equitably to sources internationally (see discussion below) and to avoid WTO challenges. In considering the alternatives, policy makers may wish to answer for themselves such questions as
whether the risks of human-induced climate change are sufficiently certain to call for policy actions to mitigate the causes and/or adapt to perceived inevitable change; how a domestic policy package can be designed that balances the risks of climate change with costs that are accepted as reasonable and fairly distributed; how domestic policy would interact with international negotiations to avoid unacceptable climate risks while distributing the effort equitably among countries and sectors; and when and how to promote appropriate adaptation by private and public decision-makers to the uncertain climate ahead.
On June 2, 2008, the Senate agreed to consider a bill (S. 3036) to control greenhouse gas emissions in the United States. In the 111th Congress, leadership in both chambers have announced their intentions to pass bills in 2009 to reduce greenhouse gas emissions. These actions are indicative of the pressures Members of Congress increasingly face on whether and how to address human-induced climate change. Contentious debates scrutinize issues of science, economics, values, geopolitics and a host of other concerns. Deliberations also weigh the appropriateness of alternative policy tools and program designs. The economic stakes are potentially large—with both the costs of controls and the "costs of inaction" ranging, by some estimates, into trillions of dollars over several decades.
A major international assessment released in 2007 concluded that the Earth's climate had warmed unequivocally over the past century, and that elevated levels of so-called "greenhouse gases" (GHGs) were likely responsible for a major portion of the observed warming. Elevated concentrations of GHGs in the atmosphere are due mostly to human activities, especially emissions from use of fossil fuels, clearing of land, and some industrial processes. Continued population and economic growth, with dependence on fossil fuels and needs for expanding agricultural lands, are expected to drive GHG emissions and induced climate change over the 21st Century to levels never experienced by human civilizations. While benefits may accrue to some people who may experience a limited amount of climate change, the aggregate effects are expected to become increasingly adverse, with people living in dry regions or along low-lying coasts, and people with low incomes, expected to be especially vulnerable. Adaptations can moderate the impacts and expand opportunities, but at a cost. Besides the overall costs of climate change, key concerns include the distributional effects within and across generations, how to value ecological impacts, and the potential for abrupt and irreversible changes. While important uncertainties remain concerning future climate change and its impacts, many experts are convinced that the evidence calls for U.S. action to abate GHG emissions. Others argue that mandatory controls would be premature, unnecessary or too costly.
For decision-makers considering actions to address climate change, an assortment of policy instruments is available; studies suggest that a combination could be most effective in achieving various climate policy objectives. Current policy attention has focused on "cap and trade" strategies to reduce GHG emissions, with additional policy tools aimed at promoting the technology development considered necessary to slow climate change significantly. In parallel, growing attention is being given to supporting adaptations to expected future changes, as well as to strategies to gain effective international engagement in reducing GHGs. One significant obstacle to consensus is concern about the potential costs of abating GHG emissions, since deep reductions would require extraordinary changes in energy use and technologies. Studies suggest that efficiently designed programs could moderate the costs of reducing GHG emissions; technically and politically, though, an "efficiently designed" program may not be realistic. Policy options can ease the adjustments required and modify the distribution of costs—or potential wealth embodied in distribution of emission allowances—across specific sectors or populations. A core challenge of policy design, then, is balancing the climate effectiveness of a policy, the economic costs, and its distributional effects.