As winter begins to tighten its grip on much of the United States, air conditioning doesn’t seem like much of a survival strategy. But a new study has found that home air conditioning played a key role in reducing American death rates over the past half-century, by keeping people cool on extremely hot days.

The installation of air conditioning in American homes is the reason why the chances of dying on an extremely hot day fell 80 percent over the past half-century, according to an analysis by a team of American researchers.

The findings, based on a comprehensive analysis of U.S. mortality records dating from 1900, suggests the spread of air conditioning in the developing world could play a major role in preventing future heat-related deaths linked to climate change. Very few U.S. homes had air conditioning before 1960; by 2004, that figure had climbed to 85 percent.

A team of researchers from Tulane University, Carnegie Mellon University, the National Bureau of Economic Research and the Massachusetts Institute of Technology examined patterns in heat-related deaths between 1900 and 2004. The group found that days on which temperatures rose above 90 degrees Fahrenheit accounted for about 600 premature deaths annually between 1960 and 2004, one-sixth as many as would have occurred under pre-1960 conditions.

“It’s all due to air conditioning,” said MIT environmental economics professor Michael Greenstone, one of the paper’s co-authors, adding that factors including increased electrification and health-care access did not affect heat-related mortality.

The likelihood of a premature death on an extremely hot day between 1929 and 1959 was 2.5 percent, the academics found, dropping to less than 0.5 percent after 1960. The paper, which is under review at an academic journal, compared days on which temperatures exceeded 90 degrees Fahrenheit with days when they ranged between 60 and 69 degrees Fahrenheit.

Matthew E. Kahn, an economics and public policy professor at UCLA’s Institute of Environment, called the study “a very strong paper” that could show one strategy for adapting to increasingly frequent bouts of warmer weather. The U.N. Intergovernmental Panel on Climate Change issued a report this year linking the increase in heat waves to human-generated greenhouse gas emissions, predicting the frequency of these events will increase in the coming decades.

“We have to begin to wake up to the new normal,” Kahn said. “Rational people have to learn how to duck and take action so we don’t get rolled by Mother Nature.”

The study’s results could be particularly important for nations such as India, where only a small portion of the population has residential air conditioning. The typical person in India experiences 33 days per year where the temperature rises above 90 degrees Fahrenheit; that could increase by as much as 100 days by the end of the century, according to some climate projections.

Anand Patwardhan, a visiting professor at the School of Public Policy at the University of Maryland in College Park, said he expects home air conditioning to become more common in India, but not as a conscious response to global warming.

“While it is certainly the case that residential air-conditioning helps in reducing mortality due to temperature extremes, the rapid growth of air-conditioning in the past is perhaps more due to rising incomes and increasing affordability of air-conditioning,” he wrote in an e-mail.

The spread of air conditioning has one obvious problem, Greenstone noted, since many of these units will likely be powered by fossil fuels and will therefore increase the world’s carbon output.
“The painful part of that is the solution involves more energy consumption,” he said. “And that is going to exacerbate the problem of increased temperatures.”

Andrew Steer, president of the World Resources Institute, said that although there is no question about “air conditioning growing in leaps and bounds in developing countries with rising temperatures,” policymakers also need to explore “ecological” adaptation strategies that yield environmental benefits instead.

Indian Institute of Technology professor Ambuj Sagar wrote in an e-mail that the world should focus on improving appliance efficiency in the face of warmer weather.

“To me, if there is any policy relevance of this study, it is that the developing countries, in their drive for a comfortable life (which will also help adapt to hotter temperatures) are following the same pathway that their industrialized-country counterparts because they don’t have any other pathway available,” Sagar wrote.

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