By Tim Radford

LONDON, 2 March, 2016 – Heatwaves that used to arrive once every 20 years or so could become annual events by 2075 across almost two-thirds of the planet’s land surface – if humans go on burning ever more fossil fuels and releasing ever more greenhouse gases.

Claudia Tebaldi, visiting scientist at the US National Centre for Atmospheric Research, and Michael Wehner, senior staff scientist at the Lawrence Berkeley National Laboratory, report in Climatic Change journal that stringent emissions reductions could reduce the risk of such extreme heat events.

But, even so, by 2075, an estimated 18% of the Earth’s surface could still experience those once-rare extreme heat events every year.

Sizeable benefits

“The study shows that aggressive cuts in greenhouse gas emissions will translate into sizeable benefits, starting in the middle of the century, for both the number and the intensity of extreme heat events,” Dr Tebaldi says. “Even though heatwaves are on the rise, we still have time to avoid a large portion of the impacts.”

Climate researchers around the world have repeatedly warned of the increasing incidence of extremes of heat. A heatwave – defined as three consecutive days of unusual heat – can be lethal.

A recent study by the UN Office for Disaster Risk Reduction reported that of the 164,000 who perished when the thermometer dropped or climbed to catastrophic levels, in the last 20 years, 148,000 died during heatwaves, and 90% of these deaths were in Europe.

“That’s going to have a dangerous impact on
the poor, the old, and the very young,
who are typically the ones dying in heatwaves”

Another study looked at levels of humidity that might accompany these extremes and pronounced some parts of the world would potentially be uninhabitable under such conditions.

The two US statisticians looked at two questions: how frequently in future would today’s once-in-20-years heatwaves happen, and how intense would these become? The answer is no surprise: if the once-in-20-years event arrives every year, then in future such heat extremes will be more extreme than anything experienced today.

For 60% of the globe a sequence of days 3°C hotter than normal might happen once every 20 years by 2050. Around 10% of the planet might experience 20-year extremes of 5°C or more.

Drastically cut

By 2075, assuming humans go on burning fossil fuels at current rates, those regions subjected to ever-greater extremes will be larger. An estimated 54% of the globe will experience extremes 5°C hotter every 20 years.

Even if emissions are drastically cut – and 195 nations promised to do just that at the Paris climate conference last December − the dangers from extreme heat remain high. A quarter of the land area of the planet could experience a 5°C heatwave every two decades or so.

“Even with such dramatic reductions in carbon dioxide emissions, future heatwaves will be far more dangerous than they are now,” Dr Wehner says.

“It’s extreme weather that impacts human health. This week could be 2°C hotter than last week, and that doesn’t matter. Now, imagine the hottest day you can remember and, instead of 42°C, it’s now 45°C. That’s going to have a dangerous impact on the poor, the old, and the very young, who are typically the ones dying in heatwaves.” – Climate News Network

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