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What's Happening & Why

Evidence from the top of the atmosphere to the depths of the oceans, collected by scientists and engineers from around the world, tells an unambiguous story: the planet is warming, and over the last half century, this warming has been driven primarily by human activity—predominantly the burning of fossil fuels.
 
U.S. average temperature has increased by 1.3°F to 1.9°F since 1895, and most of this increase has occurred since 1970. The most recent decade was the Nation’s and the world’s hottest ever recorded, and 2012 was the hottest year on record in the continental United States. Temperatures are projected to rise another 2°F to 4°F in most areas of the U.S. over the next few decades. 
 

This NASA visualization shows how global temperatures have risen from 1950 through 2013, as compared to average global temperature for the period 1951-1980.

Climate change means more than hotter weather. Certain types of extreme weather events with links to climate change have become more frequent and/or intense, including prolonged periods of heat, heavy downpours, and, in some regions, floods and droughts. In addition, warming is causing sea level to rise and glaciers and Arctic sea ice to melt. Oceans are becoming more acidic as they absorb carbon dioxide from the atmosphere, and climate change is impacting biodiversity and disrupting ecosystems.

Multiple lines of independent evidence confirm that human activities are the primary cause of the global warming of the past 50 years. The burning of coal, oil, and gas, and clearing of forests have increased the concentration of carbon dioxide in the atmosphere by more than 40% since the industrial revolution, and methane and nitrous oxide emissions from agriculture and other human activities add to the atmospheric burden of heat-trapping gases. 

Because human-induced warming is superimposed on a background of natural variations in climate, change is not uniform over time: short-term fluctuations in the long-term upward trend are natural and expected. Nonetheless, global temperatures are still on the rise and are expected to rise further. Climate change will accelerate significantly if global emissions of heat-trapping gases continue to increase.

 
NASA visualizations developed to support the National Climate Assessment show projections of temperature and precipitation patterns from today through the year 2100—revealing how different emissions scenarios could impact the U.S. climate. 

 

To learn more about climate change and the information on this page, explore Chapter 2 of the National Climate Assessment.