Western heat wave influenced by climate change, analysis finds


The extraordinary heat wave that scorched the Pacific Northwest last week almost certainly would not have happened without global warming, an international team of climate researchers said on Wednesday.

Temperatures were so extreme – including readings of 116 degrees Fahrenheit in Portland, Oregon, and a Canadian record of 121 in British Columbia – that researchers struggled to say how rare the heat wave was. But they estimated that in any given year, there was only a 0.1% chance that such an intense heat wave would occur.

“Although this is a rare event, it would have been virtually impossible without climate change,” said Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute, who conducted the study with 26 other scientists, part of a collaborative group called World Weather Attribution.

If the world warms another 1.5 degrees Fahrenheit, which could happen this century unless there are drastic reductions in greenhouse gas emissions, similar events would not be so rare, the researchers found. The chances of such a severe heat wave happening somewhere in the world would increase by up to 20% in any given year.

“For heat waves, climate change is an absolute game changer,” said Friederike Otto, from the University of Oxford in England, one of the researchers.

Alexander Gershunov, a research meteorologist at the Scripps Institution of Oceanography in San Diego, said the results were consistent with what is known about the effects of global warming on heat waves.

“These are the extreme weather conditions most affected by climate change,” said Dr Gershunov, who was not involved in the study. As for the Pacific Northwest event, he said, “climate change has obviously made it stronger.”

Temperature records for towns and villages in the region were broken, and by a much larger margin than researchers had ever seen in a heat wave. In view of this, they also raised the possibility that the world is witnessing a change in the behavior of global warming. Perhaps, they said, the climate was crossing a threshold to a point where a relatively small increase in global temperatures could dramatically increase the likelihood of a big, extreme heat jump.

“We are concerned,” said Dr van Oldenborgh. “We are much less certain of the behavior of the heat waves than two weeks ago.”

He stressed that this idea was only a hypothesis. A lot of research is needed to try to determine if and how this change might happen, which this quick study failed to address, he said.

The heat wave in the Pacific Northwest occurred in late June when a large expanse of high-pressure air, called a thermal dome, stalled over the area. In four days, temperatures have skyrocketed, as have heat-related deaths in places where air conditioning is not as prevalent as in other parts of North America.

Several hundred people are estimated to have died, a total that is expected to increase dramatically in the coming months as death certificates and mortality data are analyzed. The heat contributed to poor harvests and helped spawn forest fires, one of which destroyed the town of Lytton, British Columbia, where the Canadian heat record was set the day before.

The study is the latest in a growing body of research called “rapid attribution analysis”, which aims to establish whether there is a link between climate change and specific extreme events such as heat waves, strong torrential rains and flooding. The goal is to quickly publicize any climate connection, in part to thwart climate deniers who might claim that global warming has had no impact on a particular event.

The study, which lasted just over a week, has yet to be peer-reviewed or published in a scientific journal. But it uses techniques that were peer reviewed before the decade this type of study was done. World Weather Attribution itself has done around 30 since 2015.

Essentially, the research uses computer simulations, 21 in all for this analysis, to compare what’s going on in the existing world, which has warmed by about 2 degrees Fahrenheit since the industry boom and emissions that accompany him, to a hypothetical world in which humans had never pumped any greenhouse gases into the atmosphere.

Climatologists are certain that global warming has made heat waves worse, as the baseline temperatures from which they start are higher than they were decades ago. Quick Attribution Analysis attempts to answer two questions about a specific heat event: How much worse and more likely has climate change made it?

For the Pacific Northwest heat wave, analysis showed that, although rare, it was much more likely to occur in today’s warmed world than in a non-warming world. And if the heat wave had occurred in such a hypothetical world, it wouldn’t have been so hot, with maximum temperatures around 3.5 degrees lower.

But the extreme nature of this heat wave got scientists thinking. Maximum temperatures in many places were 7 to 9 degrees higher than previous records, about double the increase seen in other heat waves.

“It was by far the biggest leap in records,” said Dr Otto. “We have seen some pretty big increases, but never so big. “

There were two possible explanations for this, said Dr van Oldenborgh. One is that the Pacific Northwest has been hit by an extremely rare combination of factors – that the impact of climate change on the heat wave has been compounded by the recent severe drought that has gripped the west, may -be, or by changes in the jet stream, or both.

In this explanation, “the people over there were extremely unlucky and had this extreme heat,” he said.

Dr van Oldenborgh said there is an urgent need to determine whether the other explanation, that some sort of climatic threshold has been exceeded, has merit, and whether there will be other extreme heat waves similar to the ‘to come up.

“This is something that no one saw coming,” he said. “Could this happen in other places? At the moment, we just don’t know.


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