Global climate change is a widespread and growing concern that has led to extensive congressional and international discussions and negotiations. Climate change mitigation strategies have focused on reducing emissions of greenhouse gases (GHGs), especially carbon dioxide (CO 2 ). One significant source of CO 2 emissions is deforestation. Reducing deforestation to lower CO 2 emissions is seen as one of the least costly methods of mitigating climate change. Forests are carbon sinks in their natural state (i.e., they store more carbon than they release). Trees absorb CO 2 and convert carbon into leaves, stems, and roots, while releasing oxygen. Forests account for more than a quarter of the land area of the earth, and store more than three-quarters of the carbon in terrestrial plants and nearly 40% of soil carbon. When forests are cleared, some of their carbon is released to the atmosphere—slowly through decay or quickly through burning. One estimate shows that land use change, primarily deforestation, releases about 5.9 GtCO 2 (gigatons or billion metric tons of CO 2 ) annually, about 17% of all annual anthropogenic GHG emissions. This contribution to GHG emissions makes efforts to reduce deforestation significant in international strategies to mitigate climate change. There has also been some discussion of the relationship between forests and methane (CH 4 ), a less prominent but far more potent GHG than CO 2 . However, the evidence of the relationship is still limited. It generally shows forests to be net CH 4 sinks, except in water-saturated soils (i.e., forested wetlands), and it is unclear whether activities that modify forest cover (e.g., deforestation) affect CH 4 absorption and release. Thus, this report addresses only the relationship between forests and carbon as it affects climate change. The loss of tropical forests is of particular concern. The existing data show little, if any, net deforestation in boreal and temperate forests, and thus the carbon consequences of deforestation in these ecosystems might not be significant. In contrast, the loss of tropical forests is substantial and continuing. Tropical deforestation has significant climate impacts because of the large amount of CO 2 sequestered in the vegetation—nearly half of all the carbon in terrestrial plants. Thus, the lowest cost and largest carbon benefit of reducing deforestation is with tropical forests. In the United States, tropical forests are largely limited to Hawaii and Puerto Rico. Congress has addressed international deforestation through laws that authorize funding to conserve forests and in proposed climate change legislation that would provide resources to reduce deforestation in developing countries. The Tropical Forest Conservation Act of 1998 (22 U.S.C. §2431 et seq. ), for example, authorizes the United States to conduct debt-for-nature swaps with developing countries to conserve their tropical forests. Under pending climate change legislation (e.g., H.R. 2454 , the American Clean Energy Act of 2009, and S. 1733 , the Clean Energy Jobs and American Power Act), Congress is considering providing resources for developing countries to establish programs and implement projects to reduce deforestation and forest degradation, and creating policy mechanisms to establish standards and markets for international offsets to reduce GHGs. Three deforestation issues are likely to be of particular importance to Congress. The first two are geographic variation in the causes and the consequences of deforestation. These then suggest approaches for efforts to reduce deforestation. The third issue is the poor quality of information about forests generally, which might point to needed research and infrastructure as well as suggesting caution in relying on existing data for decision-making. Congressional interest in reducing deforestation to lower CO 2 emissions parallels several international initiatives that aim to accomplish the same objective. International proposals focus on reducing emissions from deforestation and forest degradation (REDD) in developing countries. These proposals were discussed and debated in climate meetings associated with the United Nations Framework Convention on Climate Change (UNFCCC) in Copenhagen in December 2009. Forests cover more than a quarter of the land area in the world but are not uniformly distributed. They account for less than 5% of the land in many countries—such as Greenland, Egypt, Pakistan, and Haiti—but cover more than 90% of the land in a few places such as Suriname and French Guiana. Some countries have naturally low forest cover (Greenland and Egypt), whereas others have diminished forest cover because of deforestation, possibly centuries ago (e.g., United Kingdom and Algeria). Forests store enormous quantities of carbon, and contain more biomass per hectare in vegetation than other biomes. Carbon sequestration and release vary by forest type, although generalizations can be made about the three major forest biomes—boreal, temperate, and tropical forests. Table 1 shows global average carbon levels in the vegetation and soils for major terrestrial biomes, including the forest biomes. The quantities shown in Table 1 should be recognized as global averages, with substantial variation of carbon stocks within each biome; for example, wetlands can be dominated by trees (a swamp) or by grasses (a marsh), while tropical forests can be very wet (rainforests) or quite dry (trees in a savannah). There are also continuous gradations across biomes (e.g., warm, humid temperate forests—subtropical forests—have traits in common with both temperate forests and tropical forests). Deforestation is the loss of tree cover, usually as a result of forests being cleared for other land uses such as farming or ranching. Some limit the definition of deforestation to the permanent conversion of forests to another habitat. Others add to this definition by including the conversion of natural forests to artificial forests such as plantations. Deforestation activities affect carbon fluxes in the soil, vegetation, and atmosphere. The effects of these activities can vary, depending on the type of activity. For example, logging can lead to carbon storage if trees are converted to wood products (e.g., lumber) and deforested areas are restored. (The issues surrounding tree planting to offset deforestation are discussed below.) However, logging can also lead to carbon emissions if the surrounding trees and vegetation are damaged, and if not all woody biomass is processed into products. Other activities that alter the carbon cycle in forests and affect climate are discussed below. The impact of deforestation on soils, and the release of soil carbon, depends on the magnitude of soil disturbance and the type of soil. Soil carbon content is related to the lifecycle of the vegetation it supports. When vegetation dies, it decomposes and releases carbon. Some carbon is deposited in the soil; some is dissolved and leaches into surface waters or groundwater; and some is released directly to the atmosphere as CO 2 . Deforestation exposes soils to sunlight, which increases soil temperature and the rate of soil carbon oxidation. This process increases the rate of CO 2 release to the atmosphere. Soil carbon can also be released at high rates if soils are disturbed, for example, by logging operations or tillage. Peat soils are particularly important for climate because of their very high carbon content (as well as CH 4 content and release). Peat soils generally occur in forests where natural decomposition rates are low, such as in periodically flooded forests or forests with a short growing (and thus decomposition) season. Peat soils may partly account for the high soil carbon levels of boreal forests, but some occur in temperate and tropical forests. Peat soils are considered major carbon sinks, and could potentially be large sources of carbon emissions, if disturbed. Deforestation can lead to carbon emissions from decomposing vegetation left on the forest floor. However, wood converted into products—such as lumber and plywood—could store carbon for many years, ranging from an average of 10 years for shipping pallets to 100 years or more for lumber. Paper products store carbon for a brief duration, often less than a year. The proportion of a tree converted to products varies widely, and depends on the size (diameter) and form (taper and branching) of the tree as well as the particular species. The purpose of the tree cutting also affects utilization and waste. Harvesting pulpwood for paper production, for example, can include much more of the woody biomass than harvesting veneer bolts for plywood or sawlogs for lumber. Cutting to clear a site for agriculture yields much more waste, as the woody biomass is generally burned to prepare the site for crop or pasture production. In addition, the harvest method can affect wood utilization and waste. Selective logging, where certain trees or species are harvested, can lead to large quantities of wood waste because more roads are needed and because the harvest and extraction procedures often damage the remaining trees. Clear-cutting can reduce wood waste, when the majority of the trees can be removed for wood products, but can increase biomass waste if done to clear land for agriculture. One technique—reduced impact logging (RIL)—has been developed to reduce timber harvest damage to soils and residual trees. Descriptions of RIL are typically either lacking in details or highly site-specific with limited general applicability, because the practices that will reduce logging damages depend on a variety of site conditions, such as soil type and water content, and tree species diversity. Nonetheless, one source reported that RIL reduces wood waste by more than 60% and soil disturbance in roads, landings, and skid trails by almost 50%. However, a major barrier to increased use of RIL is illegal logging in the tropics. Biomass not removed for products remains on the site and decomposes. Some of the carbon is deposited in the soil and some is released into the atmosphere. If the remaining biomass is burned, as is common in clearing lands for agriculture and in preparing sites for reforestation, the carbon is quickly released to the atmosphere.