The Impact of Global Warming on Health and Mortality

W. R. Keatinge, MA, MB, BCHIR, PHD, FRCP; G. C. Donaldson, BA, PHD


South Med J. 2004;97(11) 

In This Article

How Is Global Warming Changing Mortality in Practice?

Global warming has been under way for at least 25 years, and there is strong evidence that it is largely man-made and is continuing.[22] A large part of the warming is due to the burning of fossil fuels and consequent increase of carbon dioxide in the atmosphere. Since heat-related deaths are generally much fewer than cold-related deaths, the overall effect of global warming on health can be expected to be a beneficial one. Inevitably, though, when it was recognized in the 1990s that global warming was under way, attention shifted from the hazards of cold to those of hot weather.

The main concern at first was that diseases transmitted by insects, such as malaria, would spread to cooler regions of the world and would become a problem there. Closer examination showed that this was unlikely to happen to a serious extent. Malaria, for example, was once prevalent in most of Europe and even in Russia but had already been eliminated. The main reason was that modern farming methods and changes in human living conditions had reduced the number of the mosquitoes that spread the disease and had reduced their access to people.[23] From time to time, global warming, together with rapid air travel, is likely to cause new health problems from insect borne illnesses, such as the recent outbreak of West Nile Fever in New York. These should remain relatively easy to contain by measures such as spraying to kill mosquito larvae and preventing access of mosquitoes to infected patients.

A simple assessment of the immediate effect of rising temperature can be made on the assumption that particular degrees and patterns of heat or cold will continue to produce the same mortality rates as they did previously. Lack of daily statistics has prevented accurate assessment of this kind for some regions, but outside the tropics, it indicates that rises in temperature over the next few years would increase heat-related deaths less than they decrease cold-related deaths. For example, on this assumption, the rise in temperature of 3.6°F expected over the next 50 years would increase heat-related deaths in Britain by about 2,000, but reduce cold-related deaths by about 20,000.[5]

Of course, people dying as a result of heat will not be reassured by being told that fewer people will die next winter as a result of cold. It is important to minimize any increase in heat-related mortality rates, regardless of falls in cold-related mortality rates. Studies of populations living in widely different climates show that they have in fact adjusted to their own climates remarkably effectively over time. The Eurowinter study, which made active surveys of 8,000 people in 8 regions of Europe, and related mortality rates to daily temperature in each region, showed that people in cold regions such as the north of Finland had no more winter deaths than people in regions with much milder winters such as London and Athens.[1] Studies in Siberia provided even more striking examples. There was little excess winter death in the big industrial town of Yekaterinburg[24] in western Siberia. There was none at all in Yakutsk, in eastern Siberia, the coldest city in the world with temperatures averaging -33°F in winter.[25] The heat-related mortality rate showed evidence of corresponding adjustment. Such mortality rates were not significantly greater in the hot summers of Athens than in the cooler summers of the north of Finland.[3]

The surveys, covering a thousand homes in each region, showed how people had adjusted. Those in the colder regions not only kept their houses warmer than people in warm regions, even at the same level of outdoor temperature, they also dressed more warmly and were more likely to keep moving when outside. These comparisons were made for the same outdoor temperature and for people of the same ages. People in cold countries were simply much more effective in keeping warm. People in Yakutsk wore massive fur clothing outdoors, and on days of extreme cold reduced the time they spent outdoors. The ways in which people in hot regions adjusted to heat are less well documented, but people in European countries with hot summers have long-standing and well known strategies for this. The siesta, ceiling fans, and outside shutters that prevent sunlight from entering windows to cause greenhouse warming are obvious examples of these in southern Europe.

North American data showed a discrepancy from the European data with regard to winter as well as summer mortality, which appears to be due to better heating in winter and more air conditioning in summer. In North Carolina, the mortality rate in cold weather rose no more steeply with a fall in outside temperature than it did in the much colder European region of north Finland[1] (see Figure 3). It rose much more steeply in parts of Europe with winter climates similar to North Carolina, but with poorer defenses against cold, such as London or Athens.[3,4] Until recently, central heating was widely installed in Europe only in the coldest countries, whereas it was usual for most homes to be kept fully warm in even moderately cold weather in the United States.

Mortality/mean daily temperature relation, in Europe and North Carolina. Age 55+, years 1971 to 1997. Areas of symbols proportionate to number of days at that temperature. Reprinted with permission from Donaldson GC, Keatinge WR, Nayha S. Changes in summer temperature and heat-related mortality since 1971 in North Carolina, South Finland, and Southeast England, in Environmental Research Vol 91, Philadelphia, Elsevier, 2003, pp 1-7.

In recent years, temperature and mortality data from several countries shows that cold-related deaths in each age group are falling in most countries. Much of that was due to rising climatic temperature and better home heating. The reduced frequency and severity of new epidemics of winter influenza has contributed,[26] but since 1976, campaigns for warmer housing and advice on clothing and exercise to keep warm outdoors can take most of the credit.

A surprising finding is that the heat-related mortality rate has stabilized or fallen, despite rising temperatures. Air conditioning has been a major factor in the United States. Heat-related deaths there are lower among people with air conditioning.[20,27,28] An extension of air conditioning was accompanied by the virtual disappearance of heat-related death in North Carolina, despite summers becoming hotter.[4] In 1971, such a mortality rate in North Carolina was similar to that in Athens. In North Carolina, summer temperatures then rose around 1.8°F, humidity increased and wind decreased, but heat-related death virtually disappeared. The spread of air conditioning in the South Atlantic region of the United States, which includes North Carolina, from 56 to 72% of homes from 1976 to 1997, provides an explanation. A recent study confirms falling heat-related mortality rates in many cities of the United States,[29] although the method used to estimate it makes quantitative comparison difficult.

Britain and the rest of northern Europe still have little air conditioning, and the heat-related mortality rate in London has not fallen. Nor has it risen, however, despite a 3.6°F rise in summer temperature since 1971. Such factors as more relaxed lifestyle, more informal clothing, and purchase of electric fans as prosperity increased have apparently countered heat stress there.[4]


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