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. 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]. Attribution of Observed Changes Mostly to Greenhouse Gases

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. 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 . Trends in Atmospheric Concentrations of Greenhouse Gases

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 ). Observed Impacts of Climate Changes

The IPCC concluded in 2007 that: "... discernible human influences extend beyond average temperature to other aspects of climate." Results from a few released in 2008 are highlighted here. Without Further GHG Mitigation Policies, GHG Emissions Will Grow

The U.S. This research produced new scenarios of future GHG emissions and concluded that "In the reference scenarios, economic and energy growth, combined with continued fossil fuel use, lead to changes in the Earth's radiation balance that are three to four times that already experienced since the beginning of the industrial age." Wet regions are expected to get more precipitation and dry regions are expected to become drier. Concern About Abrupt "Tipping Points" in the Climate System

Some people are concerned less about chronic damages due to slow and continuing climate change than they are about the potential for abrupt "tipping points" in the current climate system. In other words, according to the estimates represented in this figure, it is most likely that greenhouse gas emissions from 1750 to 2005 will lead to global average warming of 1 o -3 o C, and potentially result in ice-free Arctic summers, major reduction of area and volume of the Hindu-Kush-Himalaya-Tibetan (HKHT) glaciers, major melting of the Greenland Ice Sheet, and so non. Snowfall in some regions is expected to increase, however, with greater atmospheric moisture. As the degree and distribution of climate changes continue, ranges of species are likely to change. Afterwards, temperature projections increasingly depend on specific emission scenarios." (p. 11)
"Sea level rise under warming is inevitable....
Scientific conclusions have become more compelling regarding the influence of human activities on the Earth's climate. In 2007, the Intergovernmental Panel on Climate Change (IPCC) declared that evidence of global warming was "unequivocal." It concluded that "[m]ost of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic [human-related] greenhouse gas [GHG] concentrations."
The IPCC concluded that human activities have markedly increased atmospheric concentrations of "greenhouse gases" (GHG), including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and gases (such as chlorofluorocarbons, CFC) that are controlled under the Montreal Protocol to protect the stratospheric ozone layer. From the beginning of the Industrial Revolution, CO2 has risen from about 280 parts per million (ppm) to about 386 ppm today (up 38%). The concentration of CO2 is higher now than in at least 800,000 years before present.
Additional human influences on the climate that are not easily compared to GHG emissions could, nonetheless, be managed to moderate regional and global climate change. These include tropospheric ozone pollution (i.e., smog), particulate and aerosol emissions, and land cover change. New chemicals, such as nitrogen trifluoride (NF3), also may play a small role.
Without radical changes globally from current policies and economic trajectories, experts uniformly expect that GHG emissions will continue to grow and lead to continued warming of the Earth's climate. Experts disagree, however, on the timing, magnitude and patterns of future climate changes. In the absence of concerted climate change mitigation policies, for a wide range of plausible GHG scenarios to 2100, the IPCC projected "best guess" increases in global average temperatures from 1.8oC to 4.0oC (3.2oF to 7.2oF). Although these temperature changes may seem small, they compare to the current global, annual average temperature of around 14oC (57oF). While precipitation overall is expected to increase, its distribution may become more uneven: regions that now are dry are likely to get drier, while regions that now are wet, are likely to get wetter. Extreme precipitation and droughts are expected to become more frequent. Experts project that warming ocean waters will expand, and melting glaciers and ice sheets will further add to sea level rise. The Arctic Ocean could become ice free in summers within a few decades. Ocean salinity is expected to fall, and the Meridional Overturning Circulation in the Atlantic Ocean could slow, reducing ocean productivity and altering regional climates in both North America and Europe. The climate would continue changing for hundreds of years after GHG concentrations were stabilized, according to most models. There are also possibilities of abrupt changes in the state of the climate system, with unpredictable and potentially catastrophic consequences. Much concern is focused now, among scientists and economists, about the likelihoods and implications of exceeding such thresholds of abrupt change, sometimes called "tipping points."
This report summarizes highlights of scientific research and assessments related to human-induced climate change. For more extensive explanation of climate change science and analytical methods, see CRS Report RL33849, Climate Change: Science and Policy Implications.