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The Deadly Combination Of Heat And Humidity

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The most deadly weather-related disasters aren’t necessarily caused by floods, droughts or hurricanes. They can be caused by heat waves, like the sweltering blanket that’s taken over 2,500 lives in India in recent weeks.

Temperatures broke 118 degrees in parts of the country. The death toll is still being tallied, and many heat-related deaths will be recognized only after the fact.

Yet it’s already the deadliest heat wave to hit India since at least 1998 and, by some accounts, the fourth- or fifth-deadliest worldwide since 1900.

These heat waves will only become more common as the planet continues to warm.

They don’t just affect tropical, developing countries; they’re a threat throughout the world. The July 1995 heat wave in the Midwest caused over 700 deaths in Chicago.

The August 2003 heat wave in western Europe led to about 45,000 deaths. The July-August 2010 heat wave in western Russia killed about 54,000 people.

But as anyone who’s spent a summer in the eastern United States knows, it’s not just the heat; it’s also the humidity. Together, they can be lethal, even if the heat doesn’t seem quite so extreme.

Scientists measure the combination using a metric known as wet-bulb temperature. It’s called that because it can be measured with a thermometer wrapped in a wet cloth, distinguishing it from the commonly reported dry-bulb temperature, measured in open air.

Wet-bulb temperature can also be calculated from relative humidity, surface pressure and air temperature.

But this fate is not yet locked in. Moderate reductions in emissions of heat-trapping gases sufficient to stop global emissions growth by 2040 and bring emissions down to half their current levels by the 2070s.

This can avoid those paralyzing extremes and limit the expected late-century experience of the average American to about 18 dangerously humid days a year.

And strong reductions — bringing global emissions to zero by the 2080s — can cap the growth of humidity extremes by the midcentury.

Climate change is increasing the risks to our health, our economy and our environment.

Communities need to prepare. But as world leaders get ready for the United Nations climate change conference in Paris this December, it’s also important to recognize that shifting to carbon-free energy will reduce the risks we will face from extreme heat and humidity.

As India’s tragic heat wave shows, these risks cannot be ignored.

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