Regional anthropogenic aerosol reductions amplify probability of record-breaking heat extremes

Florian Kraulich, Peter Pfleiderer, Sharar Ahmadi, Robert J. Allen, Pierre Nabat, Geeta G Persad, Bjørn H. Samset, Laura J. Wilcox, Sebastian Sippel

Record-breaking heat extremes are becoming more likely due to anthropogenic climate change, with their probability depending on the regional warming rate. Anthropogenic aerosol forcing modulates these warming rates, and aerosols are declining globally. However, the influence of aerosols on the probability of record-breaking heat extremes remains unclear. Here, we assess how aerosol trends alter record-breaking heat probabilities by combining idealized simulations, CESM2 large ensemble and single-forcing large ensemble simulations, Regional Aerosol Model Intercomparison Project (RAMIP) future aerosol scenarios, and ERA5/MERRA-2 reanalysis data. Reanalysis data provide empirical evidence for a link between changes in aerosol concentrations and record-breaking heat extremes. There is a statistically significant correlation between changes in sulfate aerosol optical depth (SO4AOD) and record occurrence after accounting for latitudinal differences in record probability, with a 0.5 percentage points increase in annual record-breaking probability for every 10% decrease in SO4AOD. For context, the global land-mean annual record-breaking probability during 2010--2025 is 6.7%. Consistent with this empirical relationship, idealized simulations show that increased warming rates can lead to higher record-breaking probabilities, with a lagged response. In CESM2, present-day record-breaking probabilities are amplified relative to GHG-only across most of the world, e.g. by 67% in Central Europe where aerosol forcing declines, whereas they are damped by 47% in South Asia where aerosol forcing increases. RAMIP simulations show that aerosol reductions would increase record-breaking probabilities, especially in regions with high remaining aerosol concentrations. For example, reducing global anthropogenic aerosol emissions from SSP3-7.0 to SSP1-2.6 results in an annual record-breaking heat probability of 11.4% instead of 6.7% in parts of South Asia in the 2040s. These results show that regional aerosol trends can substantially modulate record-breaking heat extremes by altering regional warming rates. Aerosol reductions can therefore temporarily amplify record-breaking heat probabilities, although greenhouse gas forcing remains the dominant driver of long-term increases.