El Niño exerts a deadly effect

Lighter rains mean more fires — and more pollution.

Fires for land clearance, such as this one in Sumatra, are more prevalent in El Niño years, reducing air quality and increasing the number of pollution-related deaths. Credit: Reuters/Corbis

An unexpected link between the climate event El Niño and a rise in the number of deaths in southeast Asia is revealed in research published today in Nature Climate Change1.

El Niño events, which displace warm water into the eastern Pacific Ocean and cool waters near Indonesia, exert their effect by suppressing the monsoon rains that usually put a dampener on the use of fire to clear land for agriculture. The resulting additional pollution can account for as many as 15,000 deaths in El Niño years, the study says.

“We usually think of deforestation and fires in terms of global carbon emissions, but we are seeing this regional impact on public health as well,” says Miriam Marlier, a graduate student at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, who led the study.

The work focused on an area that is home to 540 million people, stretching from Indonesia to the Philippines, Vietnam and Thailand. Marlier and her team used emissions estimates derived from satellite observations for 1997–2006, which they plugged into a pair of atmospheric chemistry models to analyse air pollution in the region.

During the strong El Niño of 1997, the analysis revealed that there was a spike in the amount of fine particles in the atmosphere, exceeding air-quality standards recommended by the World Health Organization by 300% for some 200 days of the year. There was also an increase in the level of ozone, a component of smog, for the same period. By contrast, when a La Niña event occurred in 2000, which saw warmer water enhance the monsoon rains, the level of carbon particulates was 98% lower than in 1997. The pollution is worst over Indonesia and Malaysia but also extends further into the mainland.

The researchers calculate that the increased air pollution in El Niño years would exact a heavy toll on the population. Carbon particulates, which contribute to heart disease, would have the greatest effect, causing 6,800–14,300 more deaths. Ozone would be responsible for an additional 2,300–5,900 deaths.

The study brings together disparate data to offer a fresh perspective on the effects of deforestation on human health, says Guido van der Werf, who studies global fire emissions at VU University Amsterdam and was not involved in the research. “Hopefully these results will somehow trickle down to the ones that are most exposed to poor air quality,” he says. The good news, van der Werf adds, “is that these fires are all caused by humans, so in principle they can be prevented”.

Drew Shindell, a co-author on the study and modeller at NASA’s Goddard Institute for Space Studies in New York, says he was surprised at the role fires played in determining air quality. Although much of the burning takes place in Sumatra and Borneo, the effects are noticeable even in large cities such as Singapore and Kuala Lumpur in Malaysia. “I wasn’t surprised that there would be a big impact in rural areas,” Shindell says, “but it turned out that the influence of these fires was so large that even in urban areas it was really important.


  1. 1

    Marlier, M. E. et al. Nature Clim. Change http://dx.doi.org/10.1038/nclimate1658 (2012).

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Tollefson, J. El Niño exerts a deadly effect. Nature (2012). https://doi.org/10.1038/nature.2012.11158

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