We know climate change made the heatwave that swept the northern hemisphere last year more likely, but is it possible to say that it actually caused it?
In a bold claim, researchers are suggesting the extent of the event would have been impossible without the carbon dioxide humanity has pumped into the atmosphere. Global warming appears to be caught red handed.
From record temperatures in Japan to wildfires in Sweden, many regions were hit by extreme heat between May and July 2018. A 5 million square kilometre area was affected by hot days over the period 鈥� that鈥檚 22 per cent of populated and agricultural areas in the northern hemisphere.
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鈥淭he area affected could not have occurred without climate change,鈥� says Martha Vogel of ETH University, who presented the findings at the Earth Geosciences Union conference in Vienna. Other scientists have cautioned against unequivocally pinning the blame on climate change.
Vogel鈥檚 team modelled the extent of areas concurrently affected by heat in a world without the 1掳C of warming that humanity has caused since the industrial revolution.
When they compared this with the size of observed heatwave areas in the past, they found the two were largely in line.
But their simulations could not replicate the size of the area affected in 2018. The highest they could reach was 20 per cent of the area.
When the team added our warming impact back in, they found comparable heatwaves could happen every six years. 鈥淪o it is not unlikely to have such an event like last year,鈥� she says.
Vogel鈥檚 research suggested that if temperatures rise to 2掳C in the future, as they are on track to exceed, heatwaves like 2018 could occur every year.
Cautious on climate
Other efforts to attribute specific extreme weather events to climate change look at how much more likely an event was made by the warming we have caused. For example, an assessment last year found the 2018 heatwave had been five times more probable because of climate change.
鈥淐limate models are not good at simulating heat waves, so people are usually cautious about reporting the results as having been impossible,鈥� says Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute.
The research also set a relatively low threshold for what for what constitutes a hot day: temperatures above the 90th percentile of the historical long-term average. A higher bar, closer to what most people would define as a hot day, would yield a different result, he says.
The timeframe of May-July also leaves out August, meaning around half the heatwaves of a typical year are left out.
鈥淭here are many uncertainties involved in attribution studies which makes concrete statements very difficult to defend,鈥� says Hannah Cloke at Reading University.
It is reasonable to measure the area affected by hot days but is not the only way of looking at heatwaves. Equally there are many different ways of defining what extreme heat is, she says.
Vogel and the team has submitted the work to the Earth鈥檚 Future journal for publication.
