Heat Waves: The Details

A heat wave is generally defined as a period of several days to weeks of abnormally hot weather.

In the past 3-4 decades, there has been an increasing trend in high-humidity heat waves, which are characterized by the persistence of extremely high night-time temperature.1 The combination of high humidity and high night-time temperature can make for a deadly pairing, offering no relief and posing a particular threat for the elderly. Extreme heat events are responsible for more deaths annually than hurricanes, lightning, tornadoes, floods, and earthquakes combined.2

At the same time, low-humidity heat waves associated with droughts and fueled in part by climate change contribute to the dry conditions that are driving wild fires.3 4

Numerous studies have documented that human-induced climate change has increased the frequency and severity of heat waves across the globe.5 6 7 8 9

For instance, a thorough statistical analysis of the Russian heat wave suggests that there was an approximate 80% probability that the 2010 July Russian heat record would not have occurred without climate warming, or alternatively the probability increased by a factor of five.10 11

Globally, extremely warm nights that used to come once in 20 years now occur every 10 years.12 And extremely hot summers, those more than three standard deviations above the historic average, are now observed in about 10% of the global land area, compared to 0.1-0.2% for the period 1951-1980.13

These trends cannot be explained by natural variation alone. Only with the inclusion of human influences can computer models of the climate reproduce the observed changes. These changes include an increase in the number of warm nights, unusually hot days, and heat waves, as well as warming of the warmest night of the year, warming of the coldest nights and days of the year, and warming of the hottest day of the year.14 15 16 17 18 19


Figure 3: The ratio of record daily temperature highs to record daily lows observed at about 1,800 weather stations in the 48 contiguous United States from January 1950 through September 2009. Source: Meehl et al. 2009

While natural variability continues to play a key role in extreme weather, climate change has shifted the odds and changed the natural limits, making heat waves more frequent and more intense. In an unchanging climate both new record highs and new record lows are set regularly, even while the total number of new records set each year may decrease as time goes on. However, the new records would be, on average, evenly balanced between record highs and record lows. In contrast, the balance shifts in a warming climate, and on average more new records highs than new record lows are set over any time period. It is worth noting that even in a warming climate new record lows are still set, though fewer in number, due to natural variation even as the climate warms.

Sixty years ago in the continental United States, the number of new record high temperatures recorded around the country each year was roughly equal to the number of new record lows. Over the past decade, however, the number of new record highs recorded each year has been twice the number of new record lows, a signature of a warming climate, and a clear example of its impact on extreme weather.20 (See Figure 3.)

NOAA’s National Climatic Data Center reports that from January 1 – June 18 of 2012 new high temperature records outnumbered cold records across the United States by nearly 10:1. As noted above, this ratio cannot be expected to remain at that level for the rest of the year, but it illustrates how unusual 2012 has been, and how these types of extremes are becoming more likely.

For the U.S., the rise in heat-trapping gases in the atmosphere has increased the probability of record-breaking temperatures 15-fold.21 In Europe, global warming is now responsible for an estimated 29% of the new record highs set each year.22


Figure 4: Ratios of Record High Temperatures to Record Low Temperatures.

Figure 4 illustrates that during the 1950s in the U.S. about as many new record highs were recorded as new record lows, roughly consistent with expectations of an unchanging climate. However, in 2009, 2010, and 2011, roughly twice as many new record highs were recorded compared to record lows, qualitatively consistent with a warming climate. The data for the year-to-date in 2012 reflect a very high ratio, much more than 2 to 1, and that highlights the fact that in any given year, a region as small as the U.S. can have very large departures from the global average. Atmospheric circulation variability, El Niños and La Niñas, and many other phenomena are involved in determining which regions will experience extremes in any given year.

A small change in average global temperature leads to a dramatic change in the frequency of extreme events.23 24 25 The following graphs in Figure 5 help to illustrate this point. In a normal climate the probability for extreme events can be visualized like a traditional bell curve.

Moderate weather events are much more common than extreme events. So a small shift in temperature has an outsized effect on the frequency of extreme events due to relative impact of this shift.26

Climate change shifts the curve to the hotter side, moving the average over. Climate change also flattens the curve, providing for a greater spread of events, an increase in variability. The combination provides for a dramatic increase in record hot weather.27 Here “variance” is a measure of the spread of temperatures around the “mean” or average temperature.


Figure 5: IPCC (2001) graph illustrating how a shift and/or widening of a probability distribution of temperatures affects the probability of extremes

The graph below, Figure 6, plots historical temperature data from the Northern Hemisphere, with each colored line representing a different decade. A “positive temperature anomaly” means temperatures are higher than the long-term average, while a “negative temperature anomaly” means they are lower. Thus, the graph illustrates both the shift and the flattening of the curve representing the distribution of unusual temperatures.28 The overall effect corresponds with graph (c) in Figure 5 above.

Figure 6: Frequency of summer temperature anomalies (how often they deviated from the historical normal of 1951-1980) over the summer months in the northern hemisphere. Source: NASA/ Hansen et al. 2012

Next Page (Heat Waves and Wildfires) →

(Full List of References)


  1. Global Climate Change Impacts in the United States. op. cit.
  2. Lugber G., and McGeehin. (2008)
  3. Trenberth, K. E. (2011)
  4. Lau, W. K. M. and K-M Kim. (2012)
  5. Gutowski, et al., op. cit.
  6. Stott, P.A., et al. (2010)
  7. Christidis, N., P.A. Stott, and S.Brown. (2011)
  8. Seneviratne et al., op. cit.
  9. Hansen, Sato, and Ruedy, op. cit.
  10. Rahmstorf, S. and D. Coumou (2011)
  11. Otto, F. E. L., et al. (2012)
  12. Zwiers F., X. Zhang, and Y. Feng. (2010)
  13. Hansen, Sato, and Ruedy, op. cit.
  14. Meehl, G.A., J.M. Arblaster and C. Tebaldi. (2007)
  15. Gutowski, et al., op. cit.
  16. Stott et al., op. cit.
  17. Christidis, N., P.A. Stott, and S. Brown, op. cit.
  18. Seneviratne et al., op. cit.
  19. Hansen, Sato, and Ruedy, op. cit.
  20. Meehl et al., op. cit.
  21. Hoerling, M., J. Eischeid, X. Quan, and T. Xu. (2007)
  22. Wergen, G. and J. Krug. (2010)
  23. Trenberth et al., op. cit.
  24. Karl et al., op. cit.
  25. Gutowski et al., op. cit.
  26. Gutowski et al., op. cit.
  27. Rahmstorf, S. and D. Coumou, op. cit.
  28. Hansen, Sato, and Ruedy, op. cit.