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.

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.

The researchers found two alternative explanations for how climate change made the extraordinary heat more likely. One possibility is that, while climate change made such an extreme heatwave more likely to happen, it remains a very unusual event in the current climate. Pre-existing drought and unusual atmospheric circulation conditions, known as the ‘heat dome’, combined with climate change to create the very high temperatures. In this explanation, without the influence of climate change the peak temperatures would have been about 2°C (3.6°F) lower. 

Until overall greenhouse gas emissions are halted, global temperatures will continue to increase and events like these will become more frequent. For example, even if global temperature rise is limited to 2°C (3.6°F), which might occur as soon as 2050, a heatwave like this one would occur about once every 5 to 10 years, the scientists found.

An alternative possible explanation is that the climate system has crossed a non-linear threshold where a small amount of overall global warming is now causing a faster rise in extreme temperatures than has been observed so far — a possibility to be explored in future studies. This would mean that record-breaking heatwaves like last week’s event are already more likely to happen than climate models predict. This raises questions about how well current science can capture the behaviour of heat waves under climate change.

The event sends a strong warning that extreme temperatures, far outside the temperature range currently expected, can occur at latitudes as high as 50°N, a range that includes all of the contiguous US, France, parts of Germany, China and Japan. The scientists warn that adaptation plans should be designed for temperatures well above the range already witnessed in the recent past.

The study was conducted by 27 researchers as part of the World Weather Attribution group, including scientists from universities and meteorological agencies in Canada, the US, Germany, the Netherlands, Switzerland, France and the UK.

More information

Study authors

Sjoukje Y. Philip¹, Sarah F. Kew¹, Geert Jan van Oldenborgh^1,19, Wenchang Yang², Gabriel A. Vecchi^2,3, Faron S. Anslow⁴, Sihan Li⁵, Sonia I. Seneviratne⁶,  Linh N. Luu¹, Julie Arrighi^7,8,9, Roop Singh⁷, Maarten van Aalst^7,8,10, Mathias Hauser⁶, Dominik L. Schumacher⁶, Carolina Pereira Marghidan⁸, Kristie L Ebi¹¹, Rémy Bonnet¹², Robert Vautard¹², Jordis Tradowsky^13,14, Dim Coumou^{1, 15}, Flavio Lehner^16,17, Michael Wehner¹⁸, Chris Rodell²⁰, Roland Stull²⁰, Rosie Howard²⁰, Nathan Gillett²¹, Friederike E L Otto⁵.

¹ Royal Netherlands Meteorological Institute (KNMI), De Bilt, The Netherlands
² Department of Geosciences, Princeton University, Princeton, 08544, USA
³ The High Meadows Environmental Institute, Princeton University, Princeton, 08544, USA
⁴ Pacific Climate Impacts Consortium, University of VIctoria, Victoria, V8R4J1, Canada
⁵ School of Geography and the Environment, University of Oxford, UK
⁶ Institute for Atmospheric and Climate Science, Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland
⁷ Red Cross Red Crescent Climate Centre, The Hague, the Netherlands
⁸ Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Enschede, the Netherlands
⁹ Global Disaster Preparedness Center, American Red Cross, Washington DC, USA
¹⁰ International Research Institute for Climate and Society, Columbia University, New York, USA
¹¹ Center for Health and the Global Environment, University of Washington, Seattle WA USA
¹² Institut Pierre-Simon Laplace (IPSL), CNRS, Sorbonne Université, Paris, France
¹³ Deutscher Wetterdienst, Regionales Klimabüro Potsdam, Potsdam, Germany
¹⁴ Bodeker Scientific, Alexandra, New Zealand
¹⁵ Institute for Environmental Studies (IVM), VU Amsterdam, The Netherlands
¹⁶ Department of Earth and Atmospheric Sciences, Cornell University, USA
¹⁷ Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, USA
¹⁸ Lawrence Berkeley National Laboratory, Berkeley, California USA
¹⁹ Atmospheric, Oceanic and Planetary Physics, University of Oxford, UK
²⁰ Department of Earth, Ocean, and Atmospheric Sciences, The University of British Columbia, Vancouver, V6T1Z4, Canada
²¹ Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, Victoria, BC, Canada.

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.