Global Warming Effects Around the World

Chicago, IL, USA

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People (Health)

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Temperature (Air)

People (Costs)

Graphic depicts projected number of 1995-like heat waves over the next century in Chicago, Illinois

Scientists expect Chicago to face life-threatening heat waves more often as the planet warms. This graph shows projections that extreme heat waves could hit Chicago three times a year, on average, by the end of the century if we do nothing to curb our carbon emissions.1

Key Facts

Chicago is projected to face severe heat waves more often as the climate changes. The effects on residents of the third-largest U.S. city are likely to be deadly1,13—unless we take steps now to reduce heat-trapping emissions and develop adaptation strategies.

  • A 1995 heat wave took the lives of more than 700 Chicagoans,1 and emergency room visits spiked by 3,300.3
  • If we do nothing to curb our carbon emissions,13 scientists project that heat-related deaths are likely to quadruple by 2050,1,13 and an event like the deadly 1995 heat wave could occur three times a year by the end of this century.1,11,12
  • Even if we make significant efforts to reduce our emissions,12 deaths caused by extreme heat and high humidity could still more than double by 2050,1,13 and a 1995-type heat wave could hit Chicago nearly every other year by the end of the century.1,11,12


Chicago ranked as the third-largest U.S. city in 2008.2 With nearly three million people2 living in a crowded urban environment with many energy-inefficient historic buildings, Chicago is likely to be particularly affected by extreme heat events, which scientists expect to occur more often as the planet warms.1

A 1995 heat wave took the lives of more than 700 Chicagoans1, and emergency room visits spiked by 3,300.3 Despite an overall decline in heat-related illness because of greater use of air-conditioning since the 1970s, excessive heat remains the leading cause of weather-related deaths in the United States.1,4

Humans overheat if core body temperatures much above 98.6° F (37° C) are sustained. Normally, when skin temperature is somewhat cooler than 98.6° F (37° C), the body loses its metabolically generated heat by conducting that heat outward from the core. Extremely hot and humid conditions, however, can make it difficult to keep this heat balance maintained.5

Ironically, people in cold regions can be most vulnerable to heat waves, because they are not acclimated to extremely hot weather, and because buildings designed for cold climates may not offer protection against extreme heat and high humidity.6

Extreme heat, especially when combined with high humidity, can be particularly dangerous to old, young, or frail people, as well as those suffering from cardiovascular, respiratory or diabetic disease.1,6 Lower-income people are also at higher risk, because they often do not have access to well-insulated housing or air-conditioning.7 In the 1995 Chicago heat wave, non-Hispanic blacks had 50 percent higher mortality rates than non-Hispanic whites.7

What the Future Holds

Cities such as Chicago, Philadelphia, New York, Milwaukee, and St. Louis are all projected to experience more damaging heat waves in the future.8,9 If we do nothing to curb our carbon emissions, scientists project that by the end of this century the number of heat-wave days in Chicago is likely to quadruple,1,10 and an event like the deadly 1995 heat wave could happen three times a year, on average.1,11,12 Deaths caused by such extreme heat events are projected to quadruple by mid-century.1,13

When humans are exposed to heat, humidity, and minimal air movement for a sustained time, our health may be at risk. That's why extreme heat waves are so dangerous. One study estimates that there are likely to be places on Earth where unprotected humans without cooling mechanisms, such as air conditioning, would die in less than six hours if global average surface temperature rises by 12.6° F (7° C).5 If global average temperatures were to increase even further—by 19.8-21.6° F (11-12° C)—this same study projects that regions where approximately half of the world's people now live could become intolerable.5

With no adaptive planning measures in place, heat-related deaths are likely to quadruple by 2050.1,13 If, however, we make significant efforts to reduce our carbon emissions, we could save lives by limiting a 1995-type Chicago heat wave to once every other year—rather than three times a year—by the end of the century.1,11,12,13

Given the potential for more frequent and intense heat events owing to climate change, local governments must be prepared for economic as well as public health effects. Emergency response costs money and requires constant vigilance.1,14 Greater demand for air-conditioning can produce power outages—and electricity blackouts will exacerbate the health effects of heat waves.6

Fortunately, adaptation to extreme heat events is possible and already occurring across the nation. Heat-related deaths are preventable through timely, specific actions such as opening air-conditioned shopping malls at night to those who are threatened.6,15 Other life-saving measures include ensuring access to weather forecasts, directly contacting people known to be at high risk, expanding outreach to homeless individuals, and suspending utility shutoffs.6



  1. U.S. Global Change Research Program. 2009. Global climate change impacts in the United States. Edited by T.R. Karl, J.M. Melillo, and T.C. Peterson. Cambridge University Press. Graphic: Union of Concerned Scientists, Climate Hot Map 2010.
  2. U.S. Census Bureau. 2009. 2008 population estimates, Table 1. Washington, DC: Population Division. Online at popest/ cities/ tables/ SUB-EST2008-01.xls. Accessed March 25, 2010.
  3. World Health Organization. 2003. Climate change and human health: Risks and responses. Geneva.
  4. Kalkstein, L.S. 1997. Climate and human mortality: Relationship and mitigating measures. Advances in Bioclimatology 5:161-177.
  5. Sherwood, S.C., and M. Huber. 2010. An adaptability limit to climate change due to heat stress. Proceedings of the National Academy of Sciences 107 (21). Online at cgi/ doi/ 10.1073/ pnas.0913352107.
  6. U.S. Environmental Protection Agency. 2006. Excessive heat events guidebook. EPA 430-B-06-005. Washington, DC: Office of Atmospheric Programs.
  7. Congressional Black Caucus Foundation. 2004. African Americans and climate change: An unequal burden. Washington, DC.
  8. McGeehin, M., and Mirabelli, M. 2001. The potential impacts of climate variability and change on temperature-related morbidity and mortality in the United States. Environmental Health Perspectives 109(S2):185-189.
  9. Meehl, G.A., and C. Tebaldi. 2004. More intense, more frequent, and longer lasting heat waves in the 21st century. Science 305:994-997.
  10. Vavrus, S., and J. van Dorn. 2008. Projected future temperature and precipitation extremes in Chicago. Journal of Great Lakes Research, forthcoming.
  11. Hayhoe, K., S. Sheridan, J.S. Greene, and L. Kalkstein. 2009. Climate change, heat waves, and mortality projections for Chicago. Journal of Great Lakes Research doi:10.1016/j.jglr.2009.12.009. Published online May 2010.
  12. Hayhoe, K., D. Wuebbles, and the Climate Science Team. 2008. Climate change and Chicago: Projections and potential impacts. City of Chicago. Online at Accessed March 25, 2010.
  13. Hayhoe, K., K. Cherkauer, N. Schlegal, J. VanDorn, S. Vavrus, and D. Wuebbles. 2009. Regional climate change projections for Chicago and the Great Lakes. Journal of Great Lakes Research doi:10.1016/j.jglr.2010.03.012. Published online May 2010.
  14. Wilbanks, T.J., P. Kirshen, D. Quattrochi, P. Romero-Lankao, C. Rosenzweig, M. Ruth, W. Solecki, J. Tarr, P. Larsen, and B. Stone. 2008. Effects of global change on human settlements. In: Analyses of the effects of global change on human health and welfare and human systems. Edited by J.L. Gamble; authored by K.L. Ebi, F.G. Sussman, and T.J. Wilbanks. Synthesis and assessment product 4.6. Washington, DC: U.S. Environmental Protection Agency, pp. 89-109.
  15. Patz, J.A., D. Campball-Lendrum, T. Holloway, and J.A. Foley. 2005. Impact of regional climate change on human health. Nature 438:310-317.
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