Human Dimensions of Global Warming

Direct Impacts of Climate on Health


Direct impact of climate on human health occurs when the human body is physically stressed or injured immediately by some element of the climate system. The direct impacts of climate include heat waves, cold waves, and severe weather. Heat waves have been more frequent and more intense in the last few decades as the number of very hot days and hot nights has been increasing. Along with these changes, heat wave mortality has also been going up. Significant heat waves in the US, South and Southeast Asia, southern Europe, and Russia have killed thousands of people. The worst of these was the southern European heat wave of 2003 that killed an estimated 35,000 people.

Map and chart of France showing mortality rates from heatwaves.Mortality rates were steady until a heat wave in August 2003 when they spiked
Figure 7.2: a) The distribution of excess mortality in France from 1 to 15 August 2003 compared with the previous three years; (b) daily mortality and temperature in Paris in June-August 2003.
Credit: Parry, M. L., et al. (2007). Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, U.K.: Cambridge University Press, Chapter 8.2.1. Retrieved June 29, 2011 from

Heat waves primarily affect elderly people in ill health, causing them to die a few weeks to few months before they would have died anyway. Some recent heat waves, such as the one in southern Europe, have been so severe that this mortality displacement has been only a minor component of the total number of deaths, and mortality has gone beyond the elderly to include outdoor workers. It is interesting to note that heat wave mortality varies with location: in Russia, South Asia, and Southeast Asia, deaths were primarily in rural areas; in the US, deaths were almost exclusively in urban areas; and in southern Europe, deaths were in both rural and urban areas.

Although very cold days, very cold nights, and total frost days have declined over the past few decades, cold waves are still a problem in northern latitudes, causing frostbite, hypothermia, and death. Most mortality today occurs because of accidental exposures among homeless, alcoholic, and drug-addicted people, as well as the old and young. Even in well-adapted countries, mortality can occur when heating systems fail during a severe cold wave.

Other weather- and climate-related natural hazards kill and injure many more people than heat waves and cold waves combined. Severe storms and floods take tens of thousands of lives each year. Low-lying coastal regions with high-density populations experience the greatest problems, with environmentally degraded areas suffering the worst. Less-developed countries suffer greater mortality than developed countries because they have less-developed emergency warning systems and emergency preparedness, response, and recovery programs.

Thus, recent direct impacts of climate have been getting worse, but future climate change will, for the most part, make them even more damaging. Climate change is projected to increase heat wave illness and death significantly; at the same time, it is projected to decrease cold wave-related health problems. Even the most conservative global climate models project a major increase in the number of very hot days as the 21st century progressses. Although exposure to heat will be going up across the globe, it is important to remember that people acclimatize to heat physiologically and technologically, thus decreasing heat stress. Moreover, as time goes on, continual “harvesting” of the sickest elderly people will decrease the number of likely candidates to experience mortality. In contrast, all models show a marked decrease in the number of very cold days in those areas experiencing potentially lethal cold today, thereby decreasing the geographic range where frostbite and hypothermia are possible.

 Bar graph of projected number of days over 90 degrees in Harrisburg, Pennsylvania. See link to text version in caption.
Figure 7.3: Projected number of days over 90 degrees Fahrenheit in Harrisburg, Central Pennsylvania. Grey bars represent the number of observed days during the base period 1961-1990; tan bars denote a lower-emissions scenario and red bars indicate a higher-emissions scenario. The inset indicates days over 100 degrees Fahrenheit.
Click Here for a text description of figure 7.3
Observed and projected number of days over 90 degrees Farenheit in Harrisburg, PA, 1961- 2099
Years Observed days Projected number of days
over 90 degrees (lower-emissions scenario)
Projected number of days
over 90 degrees (higher-emissions scenario)
1961 - 1990 15 -- --
2010 - 2039 -- 25 26
2040 - 2069 -- 35 52
2070 - 2099 -- 40 77
Observed and projected days over 100 degrees Farenheit in Harrisburg, PA.
Years Observed Days Projected number of days
over 100 degrees (lower-emissions scenario)
Projected number of days
over 100 degrees (higher-emissions scenario)
1961 - 1990 less than 1 -- --
2070 - 2099 -- 6 25


Credit: Union of Concerned Scientists. (October 2008). Climate Change in Pennsylvania, Impacts and Solutions for the Keystone State. p. 14. Retrieved June 29, 2011 from

Global climate models are too coarse to resolve storms, but most models indirectly suggest that the intensity of severe storms will increase in the future. Some imply that intensities will increase, too. Interactions with other climate-related changes such as sea-level rise further suggest that most climate-related natural hazards –– floods, hurricanes, heavy downpours, and thunderstorms –– will be much worse in the future. Given the steady, steep population rise, especially in risky coastal locations, it is clear that there will be increased morbidity and mortality from other natural hazards as climate change deepens.

Photo of refugees
Refugees in Darfur.