As global maximum temperatures rise, heat waves are becoming more pronounced. Europe experienced the hottest summer on record in 2003, with average temperatures 3.5°C above normal.  During a 2-week period in August 2003, between 30,899 and 49,004 Europeans died from heat-related causes. Mortality was highest among the old, chronically ill, and isolated. 
Was this a natural disaster, independent from human activities? Simulations run on the HadGCM3 global climate model indicate that human-induced climate change more than doubled the risk of such an event in 2003.  Under the warming anticipated during the next 40 years, similar heat waves will be 100 times more likely to occur.
Climate models also predict that California will experience more severe heat waves. From 1961 to 1990, an average of 165 people in Los Angeles died each year from heat-related causes. A computer model that relates mortality to temperature suggests that Los Angeles will suffer two times to eight times more heat-related deaths annually (319 to 1429) by the end of the century. 
Most of the studies on heat waves and human health have been conducted in developed countries located in temperate zones. One exception  studied São Paulo, Brazil, a developing city in the subtropics, and found that mortality increased at extreme temperatures, hot or cold, in a manner similar to that observed in North American and European cities. This finding suggests that the anticipated upsurge in heat-related deaths from global warming will know few boundaries.
Mortality (% change from the average daily rate) in response to temperature (°C) averaged over the current and previous day in São Paulo, Brazil, 1991–1994. Mortality was adjusted for season, day of week, pollution, and humidity. Shown are means (red) and confidence intervals (gray). Mortality increased when temperatures become colder or hotter than 20°C. After Gouveia et al. 2003.
Human bodies cool themselves via perspiration. We feel less comfortable when high temperature combines with high humidity because of our inability to lose heat when the concentration gradients that drive evaporation from our bodies are small. Consequently, the apparent temperature or heat index—a measure of how hot a person feels—is a function of relative humidity as well as air temperature. Regional changes in relative humidity and precipitation will interact with global warming to influence the impact of future heat waves.
Apparent temperature (°C) as a function of relative humidity (%) and actual air temperature (°C). For example, when relative humidity is 60% and actual temperature is 33°C, apparent temperature is 40°C (purple line). Below 27°C and 40% relative humidity, apparent and actual temperatures are nearly equal.
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 Stott, P. A., D. A. Stone, and M. R. Allen (2004) Human contribution to the European heatwave of 2003. Nature 432:610-614.
4 Hayhoe, K., D. Cayan, C. B. Field, P. C. Frumhoff, E. P. Maurer, N. L. Miller, S. C. Moser, S. H. Schneider, K. N. Cahill, E. E. Cleland, L. Dale, R. Drapek, R. M. Hanemann, L. S. Kalkstein, J. Lenihan, C. K. Lunch, R. P. Neilson, S. C. Sheridan, and J. H. Verville (2004) Emissions pathways, climate change, and impacts on California. Proceedings of the National Academy of Sciences of the United States of America 101:12422-12427.
5 Gouveia, N., S. Hajat, and B. Armstrong (2003) Socioeconomic differentials in the temperature-mortality relationship in Sao Paulo, Brazil. International Journal of Epidemiology 32:390-397.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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