Climate change made North America’s deadly heatwave 150 times more likely

Last week’s record-breaking heatwave in parts of the US and Canada would have been virtually impossible without the influence of human-caused climate change, according to a rapid attribution analysis by an international team of leading climate scientists. Climate change, caused by greenhouse gas emissions, made the heatwave at least 150 times more likely to happen, the scientists found. The co-author of this article is Maarten van Aalst, professor at the Faculty of Geo-Information Science and Earth Observation at the University of Twente.

Pacific Northwest areas of the US and Canada saw temperatures that broke records by several degrees, including a new all-time Canadian temperature record of 49.6ºC (121.3ºF) in the village of Lytton - well above the previous national record of 45ºC (113ºF). Shortly after setting the record, Lytton was largely destroyed in a wildfire. Every heatwave occurring today is made more likely and more intense by climate change. To quantify the effect of climate change on these high temperatures, the scientists analysed the observations and computer simulations to compare the climate as it is today, after about 1.2°C (2.2ºF) of global warming since the late 1800s, with the climate of the past, following peer-reviewed methods.

Prof. dr. Maarten van Aalst
“Heatwaves topped the global charts of deadliest disasters in both 2019 and 2020. Here we have another
terrible example - sadly no longer a surprise but part of a very worrying global trend. Many of these deaths
can be prevented by adaptation to the hotter heat waves that we are confronting in this region and around the
Prof. dr. Maarten van Aalst

Human-caused climate change

A group of 27 scientists with the World Weather Attribution (WWA) project rushed to analyse whether global warming had influenced the likelihood of such an intense heatwave occurring in the region. The group consists of scientists from universities and meteorological agencies in Canada, the US, Germany, the Netherlands, Switzerland, France and the UK. Their analysis reveals an unambiguous footprint of human-caused climate change. The team compared the observed heat with maximum daily temperatures predicted by climate models, including simulations of temperatures in an atmosphere unaltered by the effect of rising greenhouse-gas concentrations. They concluded that the global average temperature increase of 1.2 °C since pre-industrial times made the extreme heatwave at least 150 times more likely to happen.


In the Canadian province of British Columbia, hundreds of people died last week because of the heat. In the town of Lytton, about 200 kilometers northeast of Vancouver, the temperature reached almost 50 degrees Celsius (49.6) on 29 June. A scorching forest fire reduced Lytton almost to ashes.

“This heatwave would have been virtually impossible without the influence of human-caused climate change,” says Sjoukje Philip, a climate scientist at the Royal Netherlands Meteorological Institute (KNMI) in De Bilt and a co-author of the analysis. “It was probably still a rare event, but if global warming might exceed two degrees, it might occur every five to ten years in the future.” City planners and emergency workers worldwide need to prepare more effectively for the impacts of more frequent heatwaves on human health, agriculture and infrastructure, says co-author prof.dr. Maarten van Aalst.

The extreme temperatures experienced were far outside the range of past observed temperatures, making it difficult to quantify exactly how rare the event is in the current climate and would have been without human-caused climate change — but the researchers concluded that it would have been “virtually impossible” without human influence. 

More information on this study

Visite this WWA website to read more on this study. World Weather Attribution (WWA) is an international collaboration that analyses and communicates the possible influence of climate change on extreme weather events, such as storms, extreme rainfall, heatwaves, cold spells, and droughts. More than 400 studies have examined whether climate change made particular weather events more likely. A study by the same group that conducted today’s analysis found that climate change made both last year’s heatwave in Siberia and the 2019/20 Australia bushfires more likely. It also recently found that the loss of the French grape harvest following a frost was made more likely by climate change.

R. Kwakman MSc (Robin)
Communication Advisor (Faculty ITC)