In its 2001 Third Assessment Report, the Intergovernmental Panel on Climate Change (IPCC) foresaw that global warming would lead to increasingly deadly heatwaves. “More hot days and heatwaves are very likely over nearly all land areas,” the world’s top climate scientists warned. “These increases are projected to be largest mainly in areas where soil moisture decreases occur.” 

 “The greatest increases in thermal stress are forecast for mid- to high-latitude (temperate) cities, especially in populations with non-adapted architecture and limited air conditioning,” they wrote at the time. “A number of U.S. cities would experience, on average, several hundred extra deaths each summer.”

Sound prescient? And familiar?  All too much so.

Twenty years later, it seems as though these climate scientists were gazing into a crystal ball rather than computer monitors. At the end of June 2021, the normally temperate Pacific Northwest experienced a record-shattering heatwave. The village of Lytton, in British Columbia,  set a new all-time Canadian temperature record of 49.6 degrees Celsius (121.3 degrees Fahrenheit) and was largely destroyed by a wildfire soon thereafter. Quillayute in the northwest corner of Washington, shattered its previous high temperature record by a full 11°F.

Researchers: Heat event ‘virtually impossible’ without human-caused climate change.

At least 800 deaths have so far been attributed to the extreme heat, and experts say they expect the final mortality tally to be considerably higher. Because of the region’s historically temperate weather, many homes lack air conditioning; residents were enveloped by temperatures well in excess of 100°F.

University of British Columbia marine biologist Christopher Harley estimates that the heatwave also caused over a billion marine wildlife deaths, as shells of dead mussels and clams coated rocks along the Pacific seashore.

Climate change made the heatwave more deadly

Contributing to the World Weather Attribution project, 27 scientists worked around the clock for a week immediately after this extreme event to determine the role played by climate change. The team used published peer-reviewed methods, comparing numerous model simulations of two scenarios: the “world as it was” when the event occurred, and a counterfactual “world that might have been” had humans not altered Earth’s climate by burning fossil fuels over the past 150 years.

The results are striking. The authors concluded that a heat event so extreme was “virtually impossible without human-caused climate change.” While it’s difficult to quantify the rarity of such unprecedented weather, their best estimate was that it was a 1-in-1,000-year event. Without human-caused climate change, such an extreme event would be at least 150 times rarer, and the heatwave was about 3.6°F hotter than it would have been naturally.

But continued climate change will make these extreme events much more common. If global warming breaches the Paris Climate Agreement guardrail of 2°C (3.6°F) above pre-industrial temperatures (compared to today’s 1.2°C, or 2.2°F higher), the scientists estimate that an event like this “would occur roughly every 5 to 10 years.” As study co-author Dim Coumou of the Potsdam Institute for Climate Impact Research suggested, “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.”

Heatwave climate connections

In fact, a 2020 analysis by Columbia University climate scientists James Hansen and Makiko Sato found that what was considered extreme Northern Hemisphere summer heat in the 1950s to 1980s is about 200 times more likely to occur today as a result of shifting temperature patterns.

Frequency of occurrence of Northern Hemisphere land temperatures in 2009–2019 relative to the 1951–1980 average in green. From Hansen and Sato (2020).

But one question the new attribution study’s authors were unable to answer involves whether the June 2021 event was simply a case of “really bad luck” – just a natural extreme heatwave amplified by global warming – or whether climate change has loaded the dice in favor of this type of extreme heatwave in other ways.

For example, higher temperatures also draw more moisture out of the soil and vegetation and into the atmosphere, thus tending to exacerbate droughts. The region between southern British Columbia and California has been anomalously dry this year, with below-normal soil moisture since the beginning of April 2021 as a result of low precipitation. With less soil moisture available to evaporate, there is more heatwave energy available to raise air temperatures, as the 2001 IPCC report alluded to.

In short, it’s possible that climate change will worsen droughts in western North America, which will in turn worsen heatwaves.

Another possible factor involves a slowing jet stream resulting from  human-caused changes in the Arctic. The June heatwave was characterized by an atmospheric heat dome trapped by an omega blocking pattern, so-called because the shape of the jet stream current resembles the Greek symbol Ω. Authors of a 2018 study in Science Advances concluded that climate change is making these sorts of jet stream patterns occur more frequently in the Northern Hemisphere summer. But in the June 2021 event, the rapid attribution study concluded that – unlike the record temperatures on the ground – the jet stream circulation pattern was not particularly exceptional.

Those researchers were able to rule out a common suspect often posited by those described as climate deniers – ocean cycles. The El Niño cycle was in a neutral phase leading up to the heatwave, and the authors concluded “it had no influence on the occurrence of the heatwave.” Another ocean cycle called the Pacific Decadal Oscillation was found to “slightly favor cooler conditions for this region” at the time.

What makes extreme heat especially  deadly?

The combination of rising heat and humidity is especially dangerous, as Texas A&M climate scientist Andrew Dessler explained in a Twitter thread. The human body generates heat, and at temperatures above around 82°F, the surrounding air no longer carries away enough heat to keep the body cool. The remaining options to avoid a dangerously overheating body involve air flow across the skin (for example from wind or a fan) or evaporating sweat. And as climate change draws more moisture from the soil into the atmosphere, thus increasing humidity, sweating offers less relief. At 100% relative humidity, the body can’t evaporate any sweat (hence ‘dry heat’ is less uncomfortable because of the body’s ability to cool itself by sweating).

Scientists combine measurements of heat and humidity through what are known as ‘wet bulb’ temperatures. Prolonged wet bulb values close to body temperature (98°F) are not survivable. A 2020 study in Science Advances found that areas near the equator like the Persian Gulf and portions of Central America, India, and Southeast Asia are already very close to this survivability limit, and that the limit will be regularly exceeded if global warming approaches 2.5°C (4.5°F) above pre-industrial temperatures, rendering these regions potentially uninhabitable.

“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,” the authors of the new attribution study concluded. “Greenhouse gas mitigation goals should take into account the increasing risks associated with unprecedented climate conditions if warming would be allowed to continue.”

Also see: Silent calamity: The health impacts of wildfire smoke

Dana Nuccitelli

Dana Nuccitelli, research coordinator for the nonprofit Citizens' Climate Lobby, is an environmental scientist, writer, and author of 'Climatology versus Pseudoscience,' published in 2015. He has published...