Intended for healthcare professionals


Global warming

BMJ 2004; 328 doi: (Published 27 May 2004) Cite this as: BMJ 2004;328:1269
  1. Jonathan A Patz, assistant professor (jpatz{at}
  1. Department of Environmental Health Sciences, Johns Hopkins University, Bloomberg School of Public Health, 615 N. Wolfe St., Baltimore, MD 21205

    Health impacts may be abrupt as well as long term

    The doomsday film thriller The Day After Tomorrow is based on global warming theory, whereby the infusion of freshwater into the north Atlantic from the melting of Greenland's glaciers stops the circulation of water via the Gulf Stream. Although the probability of this event is low, according to climatologists, the scenario of abrupt climate change has certainly caught Hollywood's imagination.

    Not surprisingly, the prospect of extreme weather events also has caught the real concern of health experts (not just their imaginations), following on the heels of last year's devastating heat wave, as a result of which an estimated 15 000 people in France died in a matter of a weeks. The extent to which the severity of the European heat wave falls far outside the current distribution of weather is consistent with expectations of future climate change scenarios.1 Climatologists have long remarked that global warming will not simply manifest itself by a gradual climb in average temperatures. Rather, it is the frequency and intensity of extreme climatic events—such as heat waves, droughts, floods, and storms—that are expected to occur.2

    Extreme weather events such as severe storms, floods, and drought have claimed millions of lives during the past 20 years and have adversely affected the lives of many more as well as costing enormous amounts in property damage.3 On average, the number of people killed annually by weather disasters between 1972 and 1996 was about 123 000, most of them in Africa and Asia.4 For every one person killed in a natural disaster, 1000 people are affected, either physically or through loss of property or livelihood.5

    River floods in central Europe left over 200 000 people homeless; more than 100 people were killed,5 and due to climate change such floods are projected to increase. Degradation of the local environment can also contribute to vulnerability from flooding. For example, Hurricane Mitch, the most deadly hurricane to strike the western hemisphere in the past two centuries, caused 11 000 deaths and thousands of others were missing in Central America. Many fatalities occurred as a result of mudslides in deforested areas.6

    Studies of the effect of climate change on food production show that yields of cereal grains are likely to decrease in the tropics where many countries are already under water stress. In particular there is concern that climate change may increase the extent of malnutrition in Africa, and there is currently widespread evidence of under-nutrition in countries of central, southern, and eastern Africa.7 Drought also leads to forest fires, which in some locations (especially Malaysia and Brazil) have been associated with an increased risk of respiratory disease, eye problems, injuries, and fatalities.

    The El Niño phenomenon is the strongest short term driver of climate variability worldwide (excluding seasonal variability). It already causes natural disasters that pose health risks, particularly droughts, on a global scale. The difference in numbers of people affected by disasters between a pre-El Niño and post-El Niño year is on average around 2.7% of the world's population.8 A large number of case reports and a smaller number of time series analyses over more than one event show a range of impacts of El Niño on health.9 The most consistent associations are with malaria epidemics in parts of Latin America and South Asia, but outbreaks of cholera, hantavirus infection, Rift Valley fever, and other diseases have also been associated with El Niño.10 Although this is still being debated, more and more climatologists believe that global warming may increase the frequency and intensity of El Niño events: not good news for the health sector.

    Although extreme weather variability affects injuries, fatalities, and the incidence of diseases such as malaria, we must not lose sight of the myriad of other diseases and health outcomes affected by more subtle long term climate change. Mosquito borne diseases, such as dengue fever and encephalitis, are generally more influenced by ambient conditions than diseases passed directly from human to human. Formation of ozone air pollution is hastened by warmer temperatures.11 Excessive rainfall and runoff can lead to large numbers of micro-organisms entering drinking water, and outbreaks of waterborne disease have been associated with heavy rainfall events in the United States and elsewhere.12

    Although the doomsday scenarios may be far from reality, the slower march of climate change still presents a formidable challenge for the health sector and society as a whole. A tidal wave inundating a city is an easily identifiable disaster that, given enough warning, people may escape from. The many health effects posed by climate change will arrive through numerous convoluted pathways and will require interdisciplinary analyses and integrated prevention planning.


    • See also Reviews p 1323

    • Competing interests None declared.