Human activity increases likelihood of more extreme heatwaves

July 19 was the hottest day ever recorded in the United Kingdom, with temperatures surpassing 40 degrees Celsius (about 104 degrees Fahrenheit). The heat continues across Europe today, as well as in the United States, where more than a third of the country is under heat warnings.

Will temperatures continue to rise and lead to more frequent extreme heat events?

The answer is yes, according to the latest analysis of atmospheric circulation patterns and human-caused emissions that led to the 2021 heatwave in North America. The findings, published on July 22 in Advances in Atmospheric Sciences, may also explain the current heatwave in the U.K.

The research team found that greenhouse gases are the primary reason for increased temperatures in the past and will likely continue to be the main cause of hotter temperatures in the future. The researchers' simulations showed that extreme heatwave events will increase by more than 30 percentage points in the coming years. Of this increase, almost two-thirds will be caused by greenhouse gases, according to their results.

"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," said leading author WANG Chunzai from the South China Sea Institute of Oceanology (SCSIO), Chinese Academy of Sciences (CAS).

To understand the roots of this devastating heatwave, the researchers studied internal variabilities such as atmospheric circulation patterns along with external forces such as anthropogenic greenhouse gases.

Atmospheric circulation patterns describe how air flows and how it affects surface air temperatures around the planet. Both air flows and surface air temperatures can change based on natural warming from the Sun, atmospheric internal variability, and Earth's rotation. These configurations are responsible for daily weather as well as the long-term patterns comprising climate.

Using observational data and climate models, the researchers found that the heat dome, which is responsible for this heatwave, is related to three atmospheric circulation patterns originating from the North Pacific and the Arctic.

"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," said Prof. WANG. "This suggests the heatwave originated from the North Pacific and the Arctic, while the North America pattern ushered the heatwave out."

But atmospheric circulation patterns can co-occur—and have before—without triggering an extreme heatwave. So how much was the 2021 event influenced by human activity? Prof. WANG and the team used internationally curated, tested, and assessed models from the World Climate Research Programme, specifically, models from the Detection Attribution Model Intercomparison Project, which is part of the Coupled Model Intercomparison Project Phase 6 (CMIP6).

From the CMIP6 models, they found that it is likely that global warming associated with greenhouse gases influenced the variability of these three atmospheric circulation patterns, which, in turn, led to a more extreme heatwave event.

"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," said Prof. WANG.