Global warming is the warming of the Earth’s atmosphere due to increased concentrations of greenhouse gases emitted from human and other activities. It occurs as an amplification of a natural phenomenon, the Earth’s greenhouse effect, whereby a delicate balance of greenhouse gases helps keep the temperature of the Earth at a level that supports life. When the sun heats the Earth, some of this heat escapes back to space. The rest of the heat is trapped in the atmosphere by clouds and greenhouse gases in the form of infrared radiation. While this “greenhouse effect” occurs naturally, human activities in the past century have substantially increased the amount of greenhouse gases in the atmosphere, causing the atmosphere to trap more heat and leading to changes in the Earth’s climate. The major greenhouse gases emitted into the atmosphere through human activities are carbon dioxide, methane, nitrous oxide and fluorinated gases.

The addition of greenhouse gases to the atmosphere from human activities has warmed the atmosphere – the average temperature of the Earth’s surface increased by about 1.4 degrees Fahrenheit over the past 100 years, with about 1.0 degrees Fahrenheit of this warming occurring just over the past three decades. This global warming has resulted in observed changes to the weather and climate, sea level rise, melting of ice and glaciers, and other impacts. According to a 2013 overview of the evidence for and causes of climate change prepared by the National Academy of Sciences, “It is now more certain than ever, based on many lines of evidence, that humans are changing Earth’s climate.”

During the 2009 international climate change negotiations in Copenhagen, Denmark, President Barack Obama announced a goal of reducing U.S. greenhouse gas emissions by 17 percent below 2005 levels by 2020. Since that time, the President has proposed and begun implementing a series of measures in order to achieve this reduction target. More recently, on June 25, 2013, the President issued his Climate Action Plan, a multi-pronged approach to address global warming emissions from new and existing power plants.

Multi-state oranizations and state and local governmental agencies are implementing their own global warming initiatives to reduce greenhouse gas emissions. For example, the Regional Greenhouse Gas Initiative (RGGI or "ReGGIe") is a cooperative effort among the states of Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New York, Rhode Island and Vermont under which the states have capped and will reduce carbon dioxide emissions from the power sector 10 percent by 2018. Thirty four states and D.C. have completed Climate Action Plans.

In 1992, 188 countries negotiated the U.N. Framework Convention on Climate Change (UNFCCC). The UNFCCC went into effect in 1993 after it was ratified by 50 nations, including the United States. The UNFCCC contains voluntary commitments by industrialized nations to reduce their greenhouse gas emissions to 1990 levels by 2000, a target that was not met.

Research on the causes and impacts of global warming is conducted by organizations at all levels of society: academic institutions, nongovernmental groups, the U.S. and other governments, and international institutions. The Intergovernmental Panel on Climate Change (IPCC) is the leading international body for the assessment of climate change.