The focus of policy-makers on climate change science has shifted from debate over whether the Earth's climate has changed and whether human-related greenhouse gases are responsible for a major portion of it. Focus has turned to debate over the magnitude and patterns of future climate change, how adverse such changes may be, and how projections may inform mitigation and adaptation policy choices. There is also growing recognition of a wider variety of human-related "forcings" of climate change than the six "Kyoto gases" to include other kinds of pollution, newly developed chemicals, and land use patterns. Understanding of potential impacts of climate change has deepened as well, but remains hindered by poor time and spatial resolution of predictions and, especially, by the wide divergence of projections across different climate models. This divergence reflects continuing uncertainties concerning clouds, oceans, and vegetation feedbacks to climate change, among other questions. It also reflects re-emerging recognition of the importance of socio-economic factors in technological change, climate change projections, potential damages (or opportunities) of those changes, and appropriate policy responses. Confidence has grown in some aspects of climate change science, allowing policy-makers to deliberate over appropriate risk management strategies, but needs for further research continue. This report highlights major scientific observations, conclusions, and issues. The principal scientific findings from the 2007 Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) continue to stand, and will be summarized in this report. A fuller explanation of climate change processes, analytical methods, uncertainties, and controversies is provided in CRS Report RL33849, Climate Change: Science and Policy Implications , by [author name scrubbed]. The IPCC in 2007 declared that "[w]arming of the climate system is unequivocal.... Observational evidence from all continents and most oceans shows that many natural systems are being affected by regional climate changes." The Earth's climate has warmed by 0.6 o to 0.9 o Celsius (1.1 to 1.6 o Fahrenheit) since the Industrial Revolution and approximately 0.5 o C compared to the 20 th century average (see Figure 1 ). Precipitation has increased over the past century, although some regions have become wetter and some have become drier, consistent with scientists' understanding of how heightened greenhouse gas concentrations affect climate regionally. Observed increases in ocean temperatures, altered wind patterns, extreme weather events, melting glaciers and sea ice, and timing of seasons are also attributed in part to greenhouse gas forcing. Although there is substantial natural variability in the climate system, a warming trend continued through 2008, with the year tied with 2001 as the eighth warmest globally since reliable measurements began in 1880. Figure 1 also shows that the global climate varies from year-to-year and over longer cycles, as well as showing a century-long trend to warming. Some influences are natural and some are very likely human-related. At regional and local scales, climate is generally even more variable. This illustrates the caution that should be exercised in trying to detect changes in trends in the context of variability. One or a series of hot or cool years, or a few extreme weather events, do not necessarily represent more than normal climate variability. For example, better scientific understanding of natural phenomena like La Nina, and solar variability have helped scientists to understand their cooling influence on global average temperatures since the year 2000, or the impact of El Nino on the extreme high temperature of 1998. Detecting changes in the climate system requires measurements and assessment over decades or longer. The U.S. Climate Change Science Program (CCSP) produced a new synthesis report in 2008, "reanalyzing" the U.S. climate from 1950 to the present, evaluating observed and modeled changes, and attributing changes to different factors. The researchers estimated average warming over North America to have been 0.9 o C (1.6 o F) from 1951 to 2006, with almost all of the warming later than 1970. The reanalysis concluded, among other findings, that: •    The largest yearly average regional temperature increases have occurred over northern and western North America, with up to 2.0°C (3.6°F) warming in 56 years over Alaska, the Yukon Territories, Alberta, and Saskatchewan. On the other hand, there have been no significant yearly average temperature changes in the southern United Sates and eastern Canada. •    There has not been a significant trend in North American precipitation since 1951, although there have been substantial changes from year to year and even decade to decade. The report did not find a significant trend in continental precipitation. Nor did it find systematic changes in how often or where severe droughts have occurred in the contiguous United States over the past decades. However, the demands and competition for water resources have increased in some regions, so that the vulnerability to drought is more severe. In 2007, the IPCC fourth assessment report concluded that "[m]ost of the observed increase in globally-averaged temperatures since the mid-20 th century is very likely due to the observed increase in anthropogenic GHG concentrations." According to the report, natural phenomena, such as volcanoes, solar variability and land cover change, have undoubtedly influenced the observed climate change, but the dominant driver of change since the 1970s is estimated to be the increase of greenhouse gases (GHG) in the Earth's atmosphere due to emissions from human-related activities. The CCSP reanalysis of North American climate data concluded that there is more than a 66% likelihood that more than half the average continental warming since 1951 has been due to human activities, but also that the regional differences in summer surface temperatures are unlikely to have been driven by human influences alone. The reanalysis found that sea surface temperatures likely have affected temperature trends, regional differences in temperature and regional and seasonal differences in precipitation. It also concluded that sea surface temperatures have likely contributed to multi-year droughts. Although the most potent greenhouse gas in the Earth's atmosphere is water vapor, it is thought not to be directly influenced at a large scale by human activities. Most policy attention has been given to the "basket" of six gases covered by the Kyoto Protocol: Carbon dioxide (CO 2 ) , methane (CH 4 ), nitrous oxide (N 2 O), sulfur hexafluoride (SF 6 ), hydrofluorocarbons (HFC), and perfluorocarbons (PFC). CO 2 is the most important human-influenced GHG globally. Less policy attention has been give to other GHG and other human-related "forcings" of climate change, which might also offer climate change abatement opportunities: certain synthetic chlorinated and fluorinated chemicals (e.g., chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC), the production of which is controlled to reverse destruction of the ozone layer in the stratosphere (but which continue to be emitted from certain sources); tropospheric ozone (or "smog"), which is controlled in many countries as an air pollutant with adverse health and environmental effects, and is not emitted but is formed in the atmosphere due to emissions of nitrogen oxides (NOx), volatile organic compounds (VOC), and carbon monoxide (CO); regional scale air pollutants, such as sulfates, and tiny carbon-containing particles called black carbon aerosols . In some regions and over some periods, these air pollutants may dominate local climate changes, including how much precipitation falls and where. Aerosols also darken reflective surfaces, such as snow, and absorb more of the Sun's radiation. On snow, scientists have shown particles to increase melt, and hence create feedbacks that may accelerate climate changes. Some researchers argue that abatement of black carbon aerosols could help to slow atmospheric warming and melting in the Arctic and mountainous regions. New types of emissions are emerging that also have small, but potentially growing, effects on climate change. For example, nitrogen trifluoride (NF 3 ) is a gas used in certain manufacturing processes, introduced as a more benign alternative to CFC, but with a strong Global Warming Potential. While the current influence of this new, substitute gas is very small (well less than half a percent of CO 2 forcing), its appearance points to the need for attention to Global Warming Potentials as new chemicals are developed, and for flexibility in incorporating new climate change influences into policy approaches over time. Land use and land use changes have long been recognized for their influence on local climates, and increasingly on global climate. For example, the built environment is often constructed with dark materials, such as asphalt roads or roof shingling, that absorbs the Sun's radiation and heats the environment. These urban heat islands have a very small global effect. However, other changes, such as the warming influences of loss of snow cover, and the influences of changing vegetation (e.g. forest losses in the tropics) , and possibly of agricultural irrigation, have been raised as meriting attention for their regional effects and potential global influence. While these influences do not reduce the importance of CO 2 emissions and other GHG forcing, they raise opportunities for enhancing mitigation and management of climate changes locally and globally. Carbon dioxide (CO 2 ) concentrations have grown from a pre-industrial concentration of about 280 parts per million volume (ppm) to 386 ppm in 2008 (see Figure 2 ). The IPCC had concluded in 2007 that "[a]tmospheric concentrations of CO 2 (379ppm) and methane (1774 parts per billion - ppb) in 2005 exceed[ed] by far the natural range over the last 650,000 years." The IPCC found that the increases in CO 2 concentrations since the Industrial Revolution were due primarily to human use of fossil fuels, with land-use changes (primarily deforestation) making a significant but smaller contribution.