CICERO - Center for International Climate Research

A wet, hot future

Climate news - News and opinions about climate science

Published 11.08.2017

There are heat waves, and then there are humid heat waves. A warmer atmosphere can hold more moisture. This is going to make tomorrow’s heat waves dangerously hot.

– Large parts of the world will get annual heatwaves with perceived temperatures above 40 degrees within the century, says Jana Sillmann, senior Researcher at CICERO Center for International Climate Research.

Sillmann recently contributed to a study showing that the frequency of high humidity heatwaves will increase dramatically over the next century.

She points at a map showing the most affected areas: The US Gulf coast, South America, North Africa, the Middle East, India, eastern China, Southeast Asia and Australia. Some of these areas have large populations. India alone is 1.3 billion; urban areas such as Houston and Shanghai have populations in the millions.

The maps to the left show the probability of an annual humid heatwave with a apparent temperature of more than 40 degrees C under different scenarios, the maps on the right show the probability heat waves of over 55 degrees. 

– At 40 degrees working outside becomes dangerous for your health. That might not be a problem for people who commute from their air-conditioned home to their air-conditioned office in their air-conditioned cars. But it is a problem for those who have to be outside, such as farmers, construction workers, homeless and poor people, and those with existing health issues who will be most affected, says Sillmann.

The difference between a regular heat wave and one with high humidity is that the latter feels a lot warmer. During this years heat wave in Europe, the Italian village Grazzanise experienced perceived temperatures above 50 degrees, 20 degrees higher than the measured temperature. 

High levels of humidity prevents us from sweating, which is nature’s way of cooling us down when we get too hot. As sweat evaporates from our bodies, we cool down. However, when the air is humid the sweat does not evaporate, and we stay warm. Working or simply being outside becomes difficult and dangerous.

With global warming, heatwaves are expected to become more frequent and hotter, but also the warmer atmosphere’s ability to hold more humidity will increase the chances of more intense humid heatwaves.

The researchers behind the study have calculated the probability of these heatwaves under different scenarios of global warming: 1.5 degrees, 2 degrees and 4 degrees.

– 1.5 degrees we’ll pass around 2020 so we are practically there, 2 degrees we still may have a chance of avoiding, and 4 degrees... Well, we will just have to avoid, says Sillmann.

If the world warms 4 degrees compared to pre-industrial times, large parts of world, including parts of southern Europe will experience annual humid heatwaves of 40 degrees. Some areas of the world, such as urban areas in India, China and the US could experience heat waves with a perceived temperature of 55 degrees Celsius!

At 55 degrees being outside is not just unhealthy. It is deadly.

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