Early Warning Signs of Global Warming: Heat Waves

Worldwide temperature measurements, corrected for instrumental and measurement errors that result from such factors as changing instruments or urbanization, indicate that global mean annual surface temperatures have increased about 0.3 to 0.6°C during the last 150 years (Nicholls et al., 1996). Since 1975, the increase of the 5-year mean temperature is about 0.5°C - a rate that is faster than for any previous period of equal length in the instrumental record (NASA, 1999). Global surface temperatures in 1998 were the warmest since reliable instrumental measurements began, and seven of the ten warmest years occurred in the 1990s. Temperature reconstructions using proxy climate records such as tree rings, fossil pollen, corals and ice cores and borehole measurements indicate that the 20th century warming is unusual relative to the last 400 to 600 years (e.g., Mann et al., 1998, Overpeck et al., 1997; Pollack et al., 1998).

The observed magnitude and patterns of temperature change during the last century correspond well with model predictions when greenhouse gas, solar, and aerosol forcings are considered together (Wigley, 1999). Most of the recent warming has been in winter over the high mid-latitudes of the Northern Hemisphere continents, between 40 and 70° N (Nicholls et al., 1996). There has also been a general trend toward reduced diurnal temperature range, mostly because nights have warmed more than days. Since 1950, minimum temperatures on land have increased about twice as fast as maximum temperatures (Easterling et al., 1997). This may be attributable in part to increasing cloudiness, which reduces daytime warming by reflection of sunlight and retards the nighttime loss of heat (Karl et al., 1997). Increased soil moisture also reduces daytime warming because part of the solar energy is used to evaporate the excess moisture.

Climate model simulations predict an increase in average surface air temperature of about 2.5°C by the year 2100 (Kattenberg et al., 1996). A warming of global mean temperature will result in an increase in the frequency of warm temperature extremes at all time scales (e.g., days, seasons, and years). Regionally and locally, a small upward shift in mean temperature can cause relatively large increases in the number of extremely hot days, increasing the likelihood of "killer" heat waves during the warm season (Karl et al., 1997). In temperate climates, for example, the number of very hot days would approximately double for an increase of 2 to 3°C in average summer temperatures (Kattenberg et al., 1996). A recent analysis suggests that the number of days per year that temperature thresholds for mortality are surpassed has increased significantly over the last half-century for some US cities (Gaffen and Ross, 1998).

Increased heat waves due to climate change would cause more heat-related illness and death. It is still unclear whether the excess mortality will be offset by a decrease in deaths due to extreme cold (McMichael, 1996). Deaths due to hot weather are predominantly associated with preexisting cardiovascular and respiratory disorders, with the elderly, very young, and ill being disproportionately affected (McMichael, 1996). The degree of acclimatization (biological adaptation) of the population is important, as is the quality of housing and exposure to urban heat island effects. In 1995, nearly 500 people died during a severe heat wave in Chicago. Most were poor, elderly residents in homes or apartments without air conditioning. The unusally warm nights during which the nighttime heat index (a measure that includes humidity) failed to drop below 89° F (32°C) for two days (Karl and Knight, 1997) contributed to the high mortality. Continued urbanization and other socioeconomic trends can further increase the number of vulnerable persons.


Easterling, D.R., B. Horton, P.D. Jones, T.C. Peterson, T.R. Karl, D.E. Parker, M.J. Salinger, V. Razuvayev, N. Plummer, P. Jamason, and C.K. Folland, 1997. Maximum and minimum temperature trends for the globe. Science 277, 364-367.

Gaffen, D.J. and R.J. Ross. 1998. Increased summertime heat stress in the U.S. Nature 396, 529-530.

Karl, T.R. and R.W. Knight, 1997. The 1995 Chicago heat wave: How likely is a recurrence? Bulletin of the American Meteorological Society 78, 1107-1119.

Karl, T.R., N. Nicholls, and J. Gregory, 1997. The coming climate. Scientific American, 78-83.


Mann, M. E., R. S. Bradley, and M. K. Hughes, 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392, 779-787.

McMichael, A.J., 1996. Human population health. In Climate Change 1995 - Impacts, Adaptations, and Mitigation of Climate Change: Scientific-Technical Analyses, 561-584 (Eds RT Watson, MC Zinyowera, RH Moss), Cambridge University Press, Cambridge, UK.

NASA, 1999. Global temperature trends: 1998 global surface temperature smashes record. NASA Goddard Institute for Space Studies. 16 December 1998. http://www.giss.nasa.gov/research/observe/surftemp/

Nicholls, N., G. V. Gruza, J. Jouzel, T. R. Karl, L. A. Ogallo, and D. E. Parker. 1996. Observed climate variability and change. In Climate Change 1995: The Science of Climate Change, (Eds J.T. Houghton, L.G.M. Filho, B.A. Callander, N. Harris, A. Kattenberg, and K. Maskell), 133-192, Cambridge University Press, Cambridge, UK.

Overpeck, J., K. Hughen, D. Hardy, R. Bradley, R. Case, M. Douglas, B. Finney, K. Gajewski, G. Facoby, A. Jennings, S. Lamoureaux, A. Lasca, G. MacDonald, J. Moore, M. Retelle, S. Smith, A. Wolfe, and G. Zielinski, 1997. Arctic environmental change of the last four centuries. Science 278, 1251-1256.

Pollack, H. N., S. Huang, and P.-Y. Shen. 1998. Climate change record in subsurface temperatures: A global perspective. Science 282, 279-281.

Wigley, T.M.L., 1999. The Science of Climate Change: Global and U.S. Perspectives. Pew Center on Global Climate Change, Arlington, Virginia, 48 p.

Additional Resources

NASA GISS Common Sense Climate Index The "Common Sense Climate Index" is a measure of whether an area has experienced a temperature change that should be noticeable to most people who have lived at that location for a few decades. GISS maintains a web site with clickable maps that can be used to display the Common Sense Index for United States and world cities. http://www.giss.nasa.gov/data/update/csci/

NCDC Extreme Weather and Climate Events This website is a gateway to climatic data and reports on extreme weather events throughout the United States and the world. http://www.ncdc.noaa.gov/ol/climate/severeweather/

National Weather Service Heat Stress Information Describes the "heat index" and heat stress, and provides links to forecasts and further information about heat waves. http://weather.noaa.gov/weather/hwave.html

"What Does Global Warming Mean for Your City?" A clickable map showing the potential increases in the number of hot days in selected US cities, based on an analysis using IPCC warming scenarios by NASA and Columbia University scientists. http://www.edf.org/programs/GRAP/90Plus/

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