Attribution study shows Pacific Northwest heatwave almost impossible without human-caused climate change

The heatwave broke temperature records in US and Canada last month and led to several sudden deaths and cases of heat-related illnesses

Temperatures in the Pacific Northwest areas of US and Canada almost reached highs of 50°C in June as a result of a deadly heatwave. While the US states of Portland and Oregon observed temperatures far greater than 40°C, the village of Lytton, Canada, recorded an all-time high Canadian temperature of 49.6°C. A subsequent wildfire destroyed the village. Several of the affected areas reported sudden deaths and heat-related illnesses. Although the preliminary toll has been estimated to be several hundreds, the study World Weather Attribution (WWA) stated this is still an underestimation and the real impact will only be known after a few months.

What the study says

The WWA study used published peer-reviewed methods and climate models to understand how human-induced climate change could have played a part in this extreme weather event. Researchers found that chances of such an event taking place would have been almost impossible if it weren’t for climate change. In today’s climate, the study estimated that this kind of heatwave is a 1 in 1,000-year event.

Possible reasons for the extreme heatwave

The study pointed to two possible reasons why such an extreme jump in peak temperatures occurred. With global warming still at 1.2°C, the study stated that the probability of such an event occurring was low. Therefore, the event, in this scenario, can be chalked down to just pure bad luck, which was still aggravated by climate change.

A second reason, according to the study, is that “nonlinear interactions in the climate” substantially pushed up the possibility of such extreme heat. This reason, however, needs further investigation, the study stated, because climate models do not reflect this possibility. According to the researchers, current numbers point to this being a low-probability event.   

What the study found

After combining the assumption that this was a 1 in 1,000 years event along with data from climate models and weather observations, the study found that such an event would be 150 times rarer without human-induced climate change.     

It stated that this heatwave was 2°C hotter than it would have been if it had occurred at the start of the industrial revolution (when it was 1.2° cooler than today). In the future (as early as the 2040s), where global warming is expected to be at 2°C (0.8°C hotter than today), such an event would be a degree hotter, the study stated. Not only that, heatwaves such as this one would be a lot less rare – occurring once every five to 10 years – in a 2°C warming scenario, compared to the 1 in 1,000 years frequency that has been estimated in the current climate.

“What we are seeing is unprecedented. You’re not supposed to break records by four or five degrees Celsius (seven to nine degrees Fahrenheit). This is such an exceptional event that we can’t rule out the possibility that we’re experiencing heat extremes today that we only expected to come at higher levels of global warming” said Friederike Otto, associate director at the Environmental Change Institute, Oxford University.

 A strong warning to act now

The world needs to urgently adapt and mitigate climate change if it wants to correct the course that it is currently on, the study warned. Measures such as adequate preparedness action plans, heat action plans that include early warning systems, will help reduce the deaths from extreme heat, it stated. It urged that greenhouse gas mitigation goals include the increasing risks from unprecedented climate conditions in a business-as-usual scenario.

“This event should be a big warning. Currently we do not understand the mechanisms well that led to such exceptionally high temperatures. We may have crossed a threshold in the climate system where a small amount of additional global warming causes a faster rise in extreme temperatures” said Dim Coumou, an extreme weather scietist at the Institute for Environmental Studies (VU Amsterdam), Royal Netherlands Meteorological Institute (KNMI).

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