Heat Waves

Heat Waves Are Increasing

Trend from 1961-1990 in the Karl-Knight heat wave index, which tracks the warmest average minimum temperature over three consecutive nights in a year.

Gutowski et al. 2008

Across the globe in recent decades, there has been an increase in the number of hot extremes, particularly very warm nights.1 Hot days have also been hotter and more frequent.2 Since 1950 the number of heat waves has increased and heat waves have become longer.3

The effect of warming has been substantial. In the United States the record high temperature each year now outnumber the record lows by 2:1.4 The rise in heat-trapping gases in the atmosphere has increased the probability of U.S. temperatures breaking a record by 15 fold compared to pre-industrial times.5 In Europe, global warming is now responsible for an estimated 29% of the new record highs set each year.6 China has witnessed a remarkable increase of hot days across all regions.7 Globally, extremely warm nights that used to come once in 20 years now occur every 10 years.8

Change in the Frequency of Hot Days in North America

Changes in the annual percent of days in North America exceeding the temperature thresholds set by the historical average of warmest 10% of days, warmest 5% of days and the warmest 2.5% of days.

Kunkel et al. 2008

Natural variation alone cannot explain the increase in hot weather. Only with the inclusion of human influences can computer models of the climate reproduce the observed changes in frost days, growing season length, the number of warm nights in a year, warming on the warmest night of the year, warming on the coldest nights and days of the year, warming on the hottest day of the year, unusually hot days throughout the year, and heat waves.9 The increase in hot weather is a direct result of climate change, and human influence is estimated to have more than doubled the likelihood of the warming trends experienced recently in virtually every region of the globe.10

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(Full List of References)


  1. Gutowski et al. 2008
  2. Trenberth et al. 2007
  3. Trenberth et al. 2007]
  4. Meehl et al. 2009
  5. Hoerling et al. 2007
  6. Wergen and Krug 2010
  7. Ding et al. 2009
  8. Zwiers et al. 2010
  9. Meehl et al 2007, Gutowski et al. 2008, Stott et al. 2010, and Christidis et al. 2011
  10. Christidis et al. 2009, and Stott et al. 2010