Heatwave risk in US may increase by over 30 per cent in coming years

Study finds three atmospheric circulation patterns co-occurred during the 2021 North American heatwave

The risk of having extreme heatwave events will increase by over 30 per cent in the US in the coming years, according to a new analysis of atmospheric circulation patterns and human-caused emissions in North America.

Greenhouse gas (GHG) emissions are the primary reason for increased temperatures seen during heatwaves in 2021, said scientists, including those from the Chinese Academy of Sciences.

This includes the one in June 2021 that led to the death of hundreds of people in the US and Canada.

The research, published last week in the journal Advances in Atmospheric Sciences, said carbon emissions could also explain the current heatwave in the UK.

“An extraordinary and unprecedented heatwave swept western North America in late June of 2021, resulting in hundreds of deaths and a massive die-off of sea creatures off the coast as well as horrific wildfires,” Chunzai Wang, lead author of the study from CAS, said in a statement.

“In this paper, we studied the physical processes of internal variability, such as atmospheric circulation patterns, and external forcing, such as anthropogenic greenhouse gases,” Dr Wang explained.

Researchers found carbon emissions will continue to be the main contributing factor behind such extreme heat events, with simulations showing such heatwaves will increase by over 30 per cent in the coming years.

“The probability of this extreme event increases by 32.18 per cent under the ALL simulation (considering all external forcing) and 20.42 per cent under the GHG simulation in the future,” scientists wrote in the study.

“It is likely that global warming associated with an increase in greenhouse gases influences these three atmospheric circulation pattern variabilities, thus affecting the occurrence of heatwaves in western North America,” they added.

The atmospheric circulation patterns assessed in the study describe how air flows and influences surface air temperatures around the planet.

These factors can change based on natural warming from the Sun and atmospheric internal variability, as well as Earth’s rotation.

Daily weather, and long-term patterns comprising climate depend on these configurations, they said.

In the study, researchers identified that three atmospheric circulation patterns co-occurred during the 2021 heatwave – the North Pacific pattern, the Arctic-Pacific Canada pattern and the North America pattern.

“The North Pacific pattern and the Arctic-Pacific Canada pattern co-occurred with the development and mature phases of the heatwave, whereas the North America pattern coincided with the decaying and eastward movements of the heatwave,” Dr Wang said.

“This suggests the heatwave originated from the North Pacific and the Arctic, while the North America pattern ushered the heatwave out,” he added.

To understand how much of the 2021 extreme heat event was influenced by human activities, scientists used internationally curated, tested and assessed climate models, specifically the Detection Attribution Model Comparison models of the Coupled Model Intercomparison Project Phase 6 (CMIP6).

“From the CMIP6 models, we found that it is likely that global warming associated with greenhouse gases influences these three atmospheric circulation pattern variabilities, which, in turn, led to a more extreme heatwave event,” Dr Wang said.

“If appropriate measures are not taken, the occurrence probability of extreme heatwaves will increase and further impact the ecological balance, as well as sustainable social and economic development,” he added.

Scientists, however, said the regional and short-timescale variations in climate, such as heatwaves, need to be studied further.