Regional anthropogenic aerosol reductions temporarily increase probability of record-breaking heat extremes

Abstract 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 all-forcing and single-forcing large ensemble simulations, Regional Aerosol Model Intercomparison Project (RAMIP) future aerosol scenarios, and ERA5/MERRA-2 reanalysis data. In reanalysis data, there is a statistically significant correlation between changes in sulfate aerosol optical depth (SO$_4$AOD) and the occurrence of record-breaking heat extremes 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 SO$_4$AOD. For context, the global land-mean annual record-breaking probability during 2010--2025 is 6.7\%. Consistent with this empirical relationship, model simulations reproduce the negative correlation of record-breaking heat extremes and aerosol loads. In CESM2, present-day record-breaking probabilities are higher across most regions in the all-forcing simulations than in simulations driven by greenhouse gases alone. Using Generalized Extreme Value (GEV) distribution estimates, they are 59\% higher in Central Europe, where aerosol forcing weakens, but 51\% lower in South Asia, where aerosol forcing strengthens. 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 14.1\% instead of 9.2\% 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 elevate record-breaking heat probabilities by unmasking greenhouse-gas-induced warming, although greenhouse gas forcing remains the dominant driver of long-term increases.