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Climate change presents a "global ethical challenge" as most of its health impacts will be felt in countries least able to face them and least responsible for causing them, say researchers.

Reviewing evidence linking climate change and human health in Nature today (17 November), they warn that sub-Saharan Africa and coastal regions along the Indian and Pacific Oceans could be most seriously affected.

According to the World Health Organization, human-induced climate change already causes 150,000 deaths a year.

The two main ways that climate change affects health are: directly, through heat waves and droughts; and indirectly by increasing the spread of infectious diseases.

Research shows that some infectious diseases are affected by the El Niño Southern Oscillation (ENSO) — a periodic reversal of currents in the Pacific Ocean that disrupts the world's climate.

"Some climatologists predict more ENSO with global warming, therefore greater climate variability which could mean more diseases in some parts of the world," says lead author Jonathan Patz of the University of Wisconsin-Madison, United States.

There is evidence linking ENSO to malaria in Latin America, rift valley fever in east Africa, and dengue fever and its more lethal form, dengue hemorrhagic fever, in Thailand.

Patz points out though that urban development could also contribute to rising dengue cases as the mosquito that transmits the virus survives well in towns and cities.

Estimated deaths due to climate change in 2000, based on comparison with
1961-1991 climate
(Source: WHO)

His team's review shows that while some research in East Africa suggests a link between rising temperatures and malaria (see African malaria rise parallels warming trend), other long-term studies found none.

"Climate change could increase the prevalence of malaria," says Patz. "But this is still difficult to prove."

The team also looked at direct heat-related effects, such those of heat waves, adding that temperate regions could be vulnerable, as they would warm disproportionately more than the tropics.

Patz and colleagues say climate change poses a "global ethical challenge", pointing out that the populations at most risk from global warming, are also those least responsible for it.

"The United States is the number one emitter of greenhouse gases, and as a developed nation must take a leadership role," to deal with these health problems, says Patz.

But he adds that China, the second biggest emitter, must adopt strategies to reduce its emissions too, despite Chinese per-capita emissions being a fraction of the United States.

"I am fearful of the health impacts of global warming, and I'm glad that [Patz's] summary has been written," says Laurence Kalkstein of Center for Climatic Research at University of Delaware, United States.

Kalkstein says that more work should be done to determine whether cold related deaths will decline and compensate for increased heat related deaths. But he adds that: "My belief is that cold related mortality will not decline much in a warmer world, and heat related mortality will increase significantly".

Link to full article in Nature

Reference: Nature 438, 310 (2005)

Read more about climate change in Africa in SciDev.Net's news focus.