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Updated 10/28/2021: Greenhouse gases (GHGs) in the atmosphere trap radiant energy, warming earth’s surface and oceans. Scientific assessments conclude that GHGs very likely have been the main driver of warming of the earth’s lower atmosphere since 1979. The most recent global assessment projected that surface temperature would continue to rise until at least mid-century even under the lowest GHG emission scenarios considered. A range of actions to mitigate GHG emissions and the risks of climate change (i.e., for adaptation or resilience) are under way or being developed on the international, national, and subnational levels. U.S. federal policymakers and stakeholders have different viewpoints regarding what, if anything, to do about future climate change and related impacts.Brief History of U.S. Climate Policy ― Historically, the United States has demonstrated varying approaches and intent with regard to addressing climate change generally and to participating in GHG abatement under the 1992 United Nations Framework Convention on Climate Change (UNFCCC). International negotiations led to, among other agreements, the adoption of the Paris Agreement (PA) in 2015. President Obama accepted the PA without submitting it to the Senate for advice and consent, and the United States became a Party to the agreement when the PA entered into force in 2016. President Trump announced U.S. withdrawal from the PA in June 2017, which became effective in November 2020. President Biden again accepted the PA, and the United States became a Party on February 19, 2021.U.S. climate change policy has involved actions implemented under various legal authorities. Prior to 2007, the federal government implemented voluntary programs to address climate change and regulatory programs that indirectly limited GHG emission increases from vehicles, appliances and equipment, and buildings. A shift toward direct regulation of GHG emissions occurred following the 2007 decision in Massachusetts v. EPA, i