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 in the village of Lytton - well above the previous national record of 45ºC. Shortly after setting the record, Lytton was largely destroyed in a wildfire. Climate change, caused by greenhouse gas emissions, made the heatwave at least 150 times more likely to happen, the scientists found.
Virtually impossible without human influence
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 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.
Heat dome combined with climate change
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 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, which might occur as soon as 2050, a heatwave like this one would occur about once every 5 to 10 years, the scientists found.
Record-breaking heatwaves more likely to happen
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.
A strong warning
Dim Coumou: “Currently we do not understand the mechanisms well that led to such exceptionally high temperatures. 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.” 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.