Lethal heat waves will triple in size even if climate agreements kept, study finds

Heat waves that are impossible to survive without air-conditioning could soon afflict far more of the globe than they do now, a new study has found.

That change could happen even if climate change is kept within the limits of international agreements, according to findings published on Tuesday in Nature Reviews Earth and Environment.

Heat waves have killed more than 260,000 people since 1990, and a large body of research shows that human burning of fossil fuels makes these events more common.

The coming rise in temperaturas means an urgent need for hot-weather countries to adapt to prepare for “unsurvivable heat thresholds,” lead author Tom Matthews, who teaches environmental geography at Kings College of London, said in a statement.

Those are conditions where “prolonged outdoor exposure – even for those if in the shade, subject to a strong breeze, and well hydrated – would be expected to cause lethal heatstroke” even in the young, Matthews said.

Under current levels of warming, such heat events currently afflict about 2 percent of the Earth’s landmass, or about 14 times the size of Texas, the researchers found. For adults over 60, the regions currently at risk for lethal heat waves are ten times higher: 20 percent of the earth’s surface.

But if average temperatures rise to 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels — or just another .5 degrees Celsius (0.9 degrees Fahrenheit) of warming — then the regions affected by lethal heat waves will spread until they have enclosed a total global area about the same size as the United States as a whole, the study found.

Those impacts will be worst for parts of densely populated equatorial countries in Sub-Saharan Africa and South Asia, where billions of people could be at risk — even if planetary heating is kept at bay by significant shifts in how the world’s nations use energy and limits on the burning of fossil fuels, according to the study.

If those shifts away from burning coal, oil and gas don’t happen, projections suggest that warming could exceed 4 degrees Celsius (7.2 degrees Fahrenheit). In that scenario, the study found, 40 percent of the planet’s land area could feel the impacts of heat waves that adults can’t survive.

In that scenario, Matthews said, only regions furthest from the equator — or at high elevation — would be unaffected.

In all cases, he said, “as more of the planet experiences outdoor conditions too hot for our physiology, it will be essential that people have reliable access to cooler environments to shelter from the heat.”

Such protections may require more than simply access to air conditioning. While air conditioning is nearly ubiquitous across the cities of the Southern U.S., these protections are vulnerable to disaster. During post-hurricane blackouts last July, millions of residents of Houston, Texas, sweated in the heat and dark for more than a week — underscoring the overlapping dangers posed by a heating planet.

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