Climate Fact: State of Knowledge about Heat and Cold Waves, Floods, and Droughts

The scientific community has observed that extreme weather and climate events in the United States are changing over various time scales. Extreme weather events impact human health, agriculture, the economy and nature. The causes of changes in heat waves and cold waves are better understood than floods and droughts, because changes in river flooding are a product of intermingling factors and droughts are difficult to define and measure. Some of the observation highlights and trends are:

Heat Waves and Cold Waves

  • Heat is the number one weather-related killer. When heat is combined with high humidity it exacerbates the impacts of heat waves on humans, especially in urban heat islands.
  • The highest number of heat waves at a national level occurred in the 1930s, followed by the 2001-2010 decade; however, the western regions of the United States (including Alaska) had their highest number of heat waves in the 2000s.
  • The number of cold waves was highest in the 1980s and lowest in the 2000s. There has been a prevalent lack of cold waves in the 2000s.
  • North America‚Äôs coldest air masses from artic and subarctic Canada are warming.

Floods and Droughts

  • Most of the United States shows almost no change in flooding magnitude, with some exceptions. Flood magnitudes decreased in the Southwest and increased in parts of the Midwest, the northern half of the eastern prairies, and from the northern Appalachians to New England.
  • Days with heavy precipitation have increased in the eastern United States, especially in New England. Total precipitation for the United States has increased about five percent over the past 50 years.
  • Snowpack is melting earlier and affecting the rain-to-snow ratio in some regions of the U.S.
  • Droughts of the 12th and 13th centuries exceed those in the 20th century in duration and spatial extent.
  • Records over the past century indicate a general drying trend across the western United States in the past 50 years (1959-2008) compared to the previous 50 years (1909-1958).

(Source: Peterson, T.C., R.R. Heim, R. Hirsch, D.P. Kaiser, H. Brooks, N.S. Diffenbaugh, R.M. Dole, J.P. Giovannettone, K. Guirguis, T.R. Karl, R.W. Katz, K. Kunkel, D. Lettenmaier, G.J. McCabe, C.J. Paciorek, K.R. Ryberg, S. Schubert, V.B.S. Silva, B.C. Stewart, A.V. Vecchia, G. Villarini, R.S. Vose, J. Walsh, M. Wehner, D. Wolock, K. Wolter, C.A. Woodhouse and D. Wuebbles. 2013. Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States. 94(6):821)

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