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Climate Change and Variability

Ten Indicators of a Warming World Graphic from the third National Climate Assessment

Climate is defined as long-term averages and variations in weather measured over a period of several decades. The Earth’s climate system includes the land surface, atmosphere, oceans, and ice. Many aspects of the global climate are changing rapidly, and the primary drivers of that change are human in origin. Evidence for changes in the climate system abounds, from the top of the atmosphere to the depths of the oceans. See Observed Change in the National Climate Assessment to learn more about the indicators of a warming world.

According to the third U.S. National Climate Assessment, “Global climate is changing and this is apparent across the United States in a wide range of observations. The global warming of the past 50 years is primarily due to human activities, predominantly the burning of fossil fuels.”

Mandated at least every four years by the Global Change Research Act of 1990, the U.S. National Climate Assessment documents climate change related impacts and responses for various sectors and regions, with the goal of better informing public and private decision making at all levels.

Third U.S. National Climate Assessment Key Findings

  • Global climate is changing and this is apparent across the United States in a wide range of observations. The global warming of the past 50 years is primarily due to human activities, predominantly the burning of fossil fuels.
  • Some extreme weather and climate events have increased in recent decades, and new and stronger evidence confirms that some of these increases are related to human activities.
  • Human-induced climate change is projected to continue, and it will accelerate significantly if global emissions of heat-trapping gases continue to increase.
  • Impacts related to climate change are already evident in many sectors and are expected to become increasingly disruptive across the nation throughout this century and beyond.
  • Climate change threatens human health and well-being in many ways, including through more extreme weather events and wildfire, decreased air quality, and diseases transmitted by insects, food, and water.
  • Infrastructure is being damaged by sea level rise, heavy downpours, and extreme heat; damages are projected to increase with continued climate change.
  • Water quality and water supply reliability are jeopardized by climate change in a variety of ways that affect ecosystems and livelihoods.
  • Climate disruptions to agriculture have been increasing and are projected to become more severe over this century.
  • Climate change poses particular threats to Indigenous Peoples’ health, wellbeing, and ways of life.
  • Ecosystems and the benefits they provide to society are being affected by climate change. The capacity of ecosystems to buffer the impacts of extreme events like fires, floods, and severe storms is being overwhelmed.
  • Ocean waters are becoming warmer and more acidic, broadly affecting ocean circulation, chemistry, ecosystems, and marine life.
  • Planning for adaptation (to address and prepare for impacts) and mitigation (to reduce future climate change, for example by cutting emissions) is becoming more widespread, but current implementation efforts are insufficient to avoid increasingly negative social, environmental, and economic consequences.

To view the full report, visit the third National Climate Assessment’s interactive website at For a synopsis of the key findings from the Assessment, see the Highlights report or download the 20-page Overview booklet.

See the Intergovernmental Panel on Climate Change for more in-depth information on climate change and to view various assessments and technical reports.

For general information on climate and climate change, visit

Climate Change and Extremes

See Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States for more information on these extremes and scientists’ collective state of knowledge.

See Analyses of Maximum Precipitation Estimates: Impacts of a Wetter Future for more information on a recent report about rising atmospheric concentrations of greenhouse gases and their effects on extreme precipitation events.

See NOAA’s Atlantic Hurricanes, Climate Variability, and Global Warming Fact Sheetfor a summary and assessment on the relationship between Atlantic hurricanes and climate change.

See the Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation to learn more about policies to avoid, prepare for, respond to, and recover from the risks of extreme events.