Human activities responsible for extreme heat wave events

Heatwave in the western part of North America that has killed nearly 1,500 people in 2021 was brought about by human activities, a new study has found.

Greenhouse gases played a predominant role in the heatwave, the study published July 22, 2022, in Advances in Atmospheric Sciences stated.

"Greenhouse gases can directly and indirectly warm up the atmosphere," Chunzai Wang, a researcher in the Southern Marine Science and Engineering Guangdong Laboratory, told Down To Earth.

The direct effect is that greenhouse gases help trap heat from the sun and then increase the temperature on earth, said Wang, who is also the head of the state key laboratory of tropical oceanography at the South China Sea Institute of Oceanology.

The indirect effect, he added, is that greenhouse gases change atmospheric motions. This, in turn, increases the atmospheric temperature.

Western North America, including the Pacific Northwest of the United States and Canada, typically witnesses temperatures in the range of 18 degrees celsius to 24°C in June.

But an extreme heatwave took off in late June and early July 2021. On June 30, Lytton in British Columbia broke Canada's record when the temperature topped 49.5°C. The heatwave led to a massive die-off of sea creatures, and a spate of horrific wildfires, according to the study.

Wang and his team analysed surface air temperature and atmospheric circulation pattern data from observations to find the causes of extreme heat events. They also used climate models.

Scientists had previously established that the heatwave was due to a heat dome of high pressure prevailing over western North America.

The heat dome acted as a lid on the atmosphere, trapping the hot air trying to escape and warming it further as it sank, the study noted.

The researchers identified three atmospheric patterns associated with the heat dome: The North Pacific, Arctic-Pacific Canada, and North America.

Atmospheric circulation patterns describe how air flows and influences surface air temperatures around the planet, according to the study.

They are responsible for daily weather, as well as the long-term patterns, according to the researchers.

The North Pacific pattern appears to have helped initialise and develop the heat dome. The Arctic-pacific-Canada pattern may have played a role in the development and mature phases, while the North America pattern is linked with the decay and eastward movement of the heat dome, the study stated.

"This suggests the heatwave originated from the North Pacific and the Arctic, while the North America pattern ushered the heatwave out," Wang explained in a statement.

The researchers, however, stressed that atmospheric patterns could occur together without triggering a heatwave.

However, greenhouse gases influenced these three atmospheric circulation pattern variabilities. This triggered the extreme heatwave events, Wang said.

Global warming associated with greenhouse gases was responsible for the long-term increase of the average daily maximum temperature in the region of western North America during the past, modelling analysis revealed.

What's more, simulations also showed that extreme heatwave events would increase by more than 30 per cent in the future, of which almost two-thirds could be due to greenhouse gases, according to their results.

Heatwaves have swept Europe and the United States this year as well. The researchers said the current atmospheric patterns might be different from those in western North America last year.

"But the dynamics of atmospheric circulations played a role in the heatwaves in Europe and the United States this year," he added.

(Courtesy: Down To Earth)