Researchers warn extreme heat in U.S. and Canada “virtually impossible” without human-caused climate change
- July 9, 2021
- Posted by: Elaine Coles
- Category: Environmental, Global, Research Papers, North America
Researchers from the World Weather Attribution network are warning that the recent extreme heat in the U.S. and Canada was “virtually impossible” without human-caused climate change
During the last days of June 2021, Pacific northwest areas of the U.S. and Canada experienced temperatures never previously observed, with records broken in many places by several degrees Celsius.
Scientists from the US, Canada, the UK, the Netherlands, France, Germany and Switzerland have collaborated to assess to what extent human-induced climate change made this heatwave hotter and more likely.
Multiple cities in the U.S. states of Oregon and Washington and the western provinces of Canada recorded temperatures far above 40ºC (104 ºF), including setting a new all-time Canadian temperature record of 49.6ºC in the village of Lytton. Shortly after setting the record, Lytton was largely destroyed in a wildfire.
The exceptionally high temperatures led to spikes in sudden deaths, and sharp increases in hospital visits for heat-related illnesses and emergency calls. Heatwaves are one of the deadliest natural hazards and this heatwave affected a population unaccustomed and unprepared for such extreme temperatures, for instance with most homes lacking air-conditioning.
Using published peer-reviewed methods, the research analysed how human-induced climate change affected the maximum temperatures in the region where most people have been affected by the heat (45–52 ºN, 119–123 ºW) including the cities of Seattle, Portland, and Vancouver (with well over 9 million people in their combined metropolitan areas).
Currently available mortality estimates of at least several hundred additional deaths are almost certainly an underestimate. The full extent of the impact of this exceptional heat on population health will not be known for several months. Describing the latest heat-related death numbers as alarming, the researchers say they are likely a severe undercount and the real toll will only become clear after mortality statistics are reviewed for the role of heat in exacerbating underlying conditions.
Main findings
Based on observations and modeling, the occurrence of a heatwave with maximum daily temperatures (TXx) as observed in the area 45–52 ºN, 119–123 ºW, was virtually impossible without human-caused climate change.
The observed temperatures were so extreme that they lie far outside the range of historically observed temperatures. This makes it hard to quantify with confidence how rare the event was. In the most realistic statistical analysis the event is estimated to be about a 1 in 1000 year event in today’s climate.
The scientists are suggesting two possible sources of this extreme jump in peak temperatures.
The first is that this is a very low probability event, even in the current climate which already includes about 1.2°C of global warming — the statistical equivalent of really bad luck, albeit aggravated by climate change.
The second option is that nonlinear interactions in the climate have substantially increased the probability of such extreme heat, much beyond the gradual increase in heat extremes that has been observed up to now.
The researchers say they now need to investigate the second possibility further, although they note the climate models do not show it. All numbers below assume that the heatwave was a very low probability event that was not caused by new nonlinearities.
However, in either case, the future will be characterized by more frequent, more severe, and longer heatwaves, highlighting the importance of significantly reducing greenhouse gas emissions to reduce the amount of additional warming.
Event would have been at least 150 times rarer without human-induced climate change
With this assumption and combining the results from the analysis of climate models and weather observations, the research study concludes that an event, defined as daily maximum temperatures (TXx) in the heatwave region, as rare as 1 in a 1000 years would have been at least 150 times rarer without human-induced climate change.
In addition, the heatwave was about 2°C hotter than it would have been if it had occurred at the beginning of the industrial revolution (when global mean temperatures were 1.2°C cooler than today).
Looking into the future, in a world with 2°C of global warming (0.8°C warmer than today which at current emission levels would be reached as early as the 2040s), the event would have been another degree hotter. An event like this – currently estimated to occur only once every 1000 years, would occur roughly every 5 to 10 years in that future world with 2°C of global warming.
“Our results provide a strong warning: our rapidly warming climate is bringing us into uncharted territory that has significant consequences”
In summary, the researchers conclude that an event such as the Pacific Northwest 2021 heatwave is still rare or extremely rare in today’s climate, yet would be virtually impossible without human-caused climate change.
The WWA members commented:
“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 around the world, including surprises such as this unexpected extreme.
“Deaths from extreme heat can be dramatically reduced with adequate preparedness action. Heat action plans that incorporate heatwave early warning systems can strengthen the resilience of cities and people.
“In addition, longer-term plans are needed to modify our built environments to be more adequate for the hotter climate that we already experience today and the additional warming that we expect in future. In addition, greenhouse gas mitigation goals should take into account the increasing risks associated with unprecedented climate conditions if warming would be allowed to continue.”
Siberian heatwave of 2020 also “almost impossible” without climate change
In July a World Weather Attribution study similarly concluded that the Siberian heatwave of 2020 would have been almost impossible without climate change
In the first six months of 2020, Siberia experienced a period of unusually high temperatures, including a record-breaking 38 degrees C in the town of Verkhoyansk on 20 June, causing wide-scale impacts including wildfires, loss of permafrost, and an invasion of pests.
WWA is a partnership of:
- Environmental Change Institute, University of Oxford
- Royal Netherlands Meteorological Institute
- Laboratoire des Sciences du Climat et de l’Environment
- University of Princeton
- National Center for Atmospheric Research
- Red Cross Red Crescent Climate Centre
Click here to download the full study NW US extreme heat 2021 scientific report