Climate change, coming home: global warming's effects on populations.

• Author: DeWeerdt, Sarah

Since the 1970s, rainfall has been scarce in the Sahel, the wide belt of semi-arid land that stretches across Africa on the southern edge of the Sahara Desert. One of the worst-affected areas has been the Tigray region of northern Ethiopia, where a series of prolonged droughts exacerbated by war caused widespread famine in the 1970s and 1980s.

To help increase the productivity of farmers' fields, the local government decided in the late 1980s to build a series of small dams to trap the unreliable rainfall and connect these to simple irrigation systems. Sure enough, harvests increased and fewer people went hungry--but health researchers also found that children in villages near the dams were seven times as likely to suffer from malaria. The water stored behind the dams provided perfect breeding habitat for the mosquitoes that carry the disease.

The people of this isolated rural region of Ethiopia offer a glimpse into the human future--a view of how global climate change can play havoc with populations' lives and livelihoods, and how addressing one climate-related problem can sometimes cause another. The World Health Organization (WHO) has calculated that by 2020 human-triggered climate change could kill 300,000 people worldwide every year. By 2000, in fact, climate change was already responsible for 150,000 excess deaths annually--deaths that wouldn't have occurred if we humans weren't burning vast quantities of fossil fuels and loading up the air with carbon dioxide and other greenhouse gases.

Jonathan Patz, a professor with the University of Wisconsin/Madison's Center for Sustainability and the Global Environment, praises the WHO's sober accounting as the most comprehensive, scientific estimate available of the health effects of climate change. The agency combined models of recent and projected climate change with data on several health dangers that are known to be affected by climate (including malaria, diarrheal diseases, and malnutrition) to calculate the disease burden due to changes in climate. However, Patz says, "their estimate is extremely conservative." Not only are the underlying assumptions conservative, but the analysis only concerns a few of the relatively better-understood health risks of climate change.

Climate change might have a few pluses for our species--for example, warmer winters probably mean fewer cold-related deaths in North America and Europe, while in some parts of the tropics hotter and drier conditions could reduce the survival of disease-carrying mosquitoes. But most of the effects of climate change are likely to be harmful ones: declining agricultural production and more hungry people, increased spread of infectious diseases, dangerous heat waves and floods. Although no region of the globe will be entirely spared, the negative effects are likely to fall most heavily on poor nations in tropical and subtropical regions. In other words, the people most vulnerable to the effects of climate change are precisely those who are least responsible for causing it--and those who have the least resources with which to adapt to it.

FOOD

"Malnutrition will very likely be one of the biggest impacts in low-income countries," says Kristie Ebi, an environmental consultant who has served on the Intergovernmental Panel on Climate Change (IPCC) and several other climate-change-related scientific bodies. Globally, food production is likely to decrease only modestly at worst, but this overall pattern hides what many researchers see as a growing inequality between the haves and have-nots of the world.

Some relatively wealthy countries in temperate regions will likely see crop yields rise, mainly due to longer, warmer growing seasons. Even the excess carbon dioxide in the air that is the underlying cause of climate change can theoretically be a boon for agriculture, acting as a fertilizer when other conditions for plant growth are favorable. Though it's not yet clear whether or how this effect of carbon dioxide will play out in the real world, any beneficial effects are most likely to be seen at middle and high latitudes. The prospect of these changes causes skeptics of climate-change doom and gloom to envision vast stretches of northern tundra transformed into a future breadbasket.

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Meanwhile, however, crop yields are likely to fall in the tropical and subtropical world, latitudes with many poorer countries where most of the world's hungry and malnourished live today. There, many crops are already growing near the upper bound of their temperature tolerance, so further warming would push them beyond their limits. In some areas, precipitation may increase, causing crops to rot; elsewhere, rainfall may diminish and become more...