LegiStorm

Summary:

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.