If it walks like climate change, quacks like climate change, and brings terrible heatwaves like climate change… it’s climate change.

Scientists calculated that climate change dramatically increased the chances of this type of extreme heat happening. The study, not peer-reviewed yet, found that before the industrial era, this type of heatwave just wouldn’t have happened. Even in today’s warming world, the heat was a once-in-a-millennium event, the researchers say.

Cities in western provinces of Canada and in the US states of Oregon and Washington saw temperatures above 40ºC (104 ºF) on the last few days, with a new all-time Canadian temperature record of 49.6ºC (120 ºF) in the town of Lytton — the record had been broken on three consecutive days. The record temperatures led to spikes in sudden deaths and sharp increases in hospital visits.

Heatwaves are not only a nuisance but can be very dangerous, leading to illness and death, especially among older adults and the very young. Because of climate change, they are occurring more frequently, are more intense, and are lasting longer than they did in the past. Global temperatures have already increased by 1ºC compared to pre-industrial times, and show little sign of slowing down. 

Scientists from the US, Canada, the UK, the Netherlands, France, Germany, and Switzerland collaborated to assess to what extent human-induced climate change made this heatwave hotter and more likely. They analyzed how human-induced climate change affected the maximum temperatures in the cities most affected by the heat. 

Using well-established climate models, the scientists concluded that the occurrence of a heatwave with maximum daily temperatures as observed in some areas of the US and Canada was “virtually impossible” without climate change. The temperatures were so extreme that they lie far outside the range of historically observed temperatures. 

“Without the additional greenhouse gases in the atmosphere, in the statistics that we have available with our models, and also the statistical models based on observations, such an event just does not occur,” Friederike Otto, one of the researches behind the study, told BBC. “Or if an event like this occurs, it occurs once in a million times, which is the statistical equivalent of never,” she told a news briefing.

There are two possibilities for the extreme jump in temperatures seen in the region, according to the researchers. The first is that it’s just an extremely rare event, made worse by climate change. The other possibility is that the climate may have crossed a threshold that would make the type of heatwaves that recently occurred much more likely. In other words, it’s either that climate change is happening and it’s bad, or it’s happening and it’s very bad.

Limiting the damage

Until now, researchers had seen a gradual increase in heat extremes due to climate change. But what happened in the last few days in Canada and the US has altered that idea of a gradual change. According to the study, if the world warms by 2ºC, the chances of having a similar heatwave to last week could drop from once every 1,000 years to once in every 5-10 years. 

The researchers, grouped under the World Weather Attribution organization, had done similar quick analyses in the past, which are later published in peer-reviewed journals. In the past, they have found climate change in heat waves such as in Europe and Siberia. But in a few cases they found climate change wasn’t a factor, as they did in a Brazilian drought. 

“Our results provide a strong warning: our rapidly warming climate is bringing us into uncharted territory that has significant consequences for health, well-being, and livelihoods. Adaptation and mitigation are urgently needed to prepare societies for a very different future. Adaptation measures need to be much more ambitious and take account of the rising risk of heatwaves,” the researchers wrote. 

Our best chance of limiting the effects of climate change is by reducing our greenhouse gas emissions. While we’re making some progress on this front, it’s far too underwhelming to avoid problems like this heatwave (and even more catastrophic effects down the road). Keeping the planet within 2ºC of warming will require a quick decarbonization of our economy and a transition to renewable, sustainable energy and fuel — as soon as possible.

The study can be found here.