As the world works to eliminate harmful aerosol pollution, heatwaves are becoming more common, exposing the intensifying effects of greenhouse gases in the atmosphere. This warming trend is particularly pronounced in densely populated areas where air pollution typically rises.

“Mitigating aerosol pollution is imperative for public health,” states Geeta Persad from the University of Texas at Austin. “However, we must acknowledge that this reveals specific risks that become magnified in populated regions.”

Aerosol pollution, primarily stemming from fossil fuel combustion, has effects that generally counteract those of greenhouse gases. While gases like carbon dioxide trap heat, aerosols tend to cool the atmosphere by reflecting sunlight either directly or by altering cloud behavior. Some estimates suggest that aerosol pollution masks half of the global warming effect of greenhouse gases.

This interplay means that cleaning up air pollution can inadvertently amplify climate warming effects. However, the specific ways in which aerosol variations impact heat in populous regions have remained unclear until now.

To analyze this more precisely, Persad and her team utilized climate models to evaluate how aerosol levels influence the occurrence of land heatwaves, examining both historical data and future projections. They defined a heatwave as three consecutive days where temperatures would rank among the hottest 10% for that time of year in a pre-industrial context.

Throughout most of the 20th century, the team discovered that aerosols mitigated the rise in heat wave occurrence driven by increasing greenhouse gases. However, since 2005, this trend has shifted as aerosol reductions have accelerated the growth of global heatwaves by nearly two days each decade.

The researchers also found that aerosols exert a more significant influence on heatwave frequency in densely populated regions compared to less populated areas. In certain regions, the reduction of aerosol levels has proven to be more than twice as impactful as the increase in greenhouse gas concentrations. “If you examine the spatial distribution of aerosol concentrations, they correlate strongly with areas of high population,” remarks Persad.

In a scenario where greenhouse gas emissions rise significantly while aerosol levels gradually decrease, the team predicts a notable escalation in the frequency of heatwaves. By 2080, the number of heatwave days in densely populated regions could surge from about 40 to over 110 days annually.

“What sets this study apart is its focus on daily timescale data. You can genuinely perceive the decrease in aerosol levels across different areas of the globe,” says Shiv Priyam Raghuraman at the University of Illinois, Urbana-Champaign, who was not involved in the study. He highlights that these results arise from a single model under the most severe greenhouse gas emission scenario.

“These findings are compelling and enhance existing literature on the significant role aerosols play in climate extremes,” states Daniel Westerbert from Columbia University in New York. “It will be fascinating to see how other models might differ in their findings, and whether past observations support these results.”

Another significant uncertainty lies in the future trajectory of aerosol concentrations in the coming years, adds Persad. “Current emissions scenarios could determine aerosol trends over the next three decades,” she remarks.