For nine days in the summer of 2021, an extreme heat wave weighed on western North America. In late June and early July, average temperatures were ten degrees Celsius above normal; on some days the air even heated up more than 30 degrees Celsius above the usual mean.
And just a day after Lytton, British Columbia, set a new national record of 49.6 degrees Celsius, a bush fire completely devastated the town.
In “Nature Climate Change”, a team led by Samuel Bartusek from the Lamont-Doherty Earth Observatory presents the most comprehensive study to date of how this catastrophe could have happened.
The work confirms the hitherto unique character of the heat wave and the role that climate change played in it. Bartusek and Co. analyzed climate data since the 1950s and compared them with daily weather observations from the weeks before and during the heat wave.
Their central conclusion: without human-caused global warming, such an event would have been possible only over an extremely long time scale – or in human terms: practically impossible.
Since then, however, atmospheric heating has meant that such an event can occur once every 200 years. The probability of occurrence had therefore increased significantly.
In addition to this long-term trend, there were various other climate and weather-related influences, the team writes. First and foremost, the scientists name the jet stream, which was virtually blocked at the time in question.
Instead of meandering slightly, it bulged sharply north and south in the summer of 2021 and remained in this pattern for extended periods of time, rather than these waves moving rapidly from west to east.
As a result, western North America such as Scandinavia and Eastern Europe as well as parts of Siberia came under the influence of high pressure with an intensive supply of hot air masses from south to north. As a result, so-called heat domes formed over these areas and temperatures continued to rise.
Western North America was hit the hardest, due to a regional peculiarity. A series of smaller atmospheric waves originating in the western Pacific moved eastward and merged with the larger jet stream wave over land, intensifying and solidifying it.
Meteorologists were able to recognize this pattern about ten days in advance and therefore warned of the heat wave in good time.
In addition to these factors, another long-term trend exacerbated the situation, which, however, is also related to the climate. The west of the USA and Canada has become increasingly dry in recent decades, which has reduced soil moisture, for example.
As a result, less water evaporated from vegetation and the ground during the heat wave, which earlier would have helped to reduce the heating of the air near the ground. Accordingly, the surface and the air above it heated up more. In fact, the researchers found that the heatwave was most extreme in the regions with the driest soils.
“As a result of global warming, the Pacific Northwest is gradually becoming drier,” says Mingfang Ting, co-author of the study. In the long term, the probability that such extremes will occur more frequently increases. October 2022 also indicated this: in the middle of the month, many daily temperature records were broken with peak values that are more typical of midsummer than autumn.
The hot and again too dry weather triggered such intense and widespread forest fires that on October 20 Seattle had the worst air quality of all major cities in the world because of the smoke – even ahead of the usual candidates like Beijing or Delhi.
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The original for this article “Nearly 50 degrees Celsius in Canada: What led to the Lytton disaster” comes from Spektrum.de.