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Using the Community Earth System Model Large Ensemble experiments, we investigate future heatwaves under the Representative Concentration Pathway 8.5 scenario, separating the relative roles of greenhouse gas increases and aerosol reductions. We show that there will be more severe heatwaves (in terms of intensity, duration, and frequency) due to mean warming, with minor contributions from future temperature variability changes. While these changes come primarily from greenhouse gas increases, aerosol reductions contribute significantly over the Northern Hemisphere. Furthermore, per degree of global warming, aerosol reductions induce a significantly stronger response in heatwave metrics relative to greenhouse gas increases. The stronger response to aerosols is associated with aerosol-cloud interactions, which are still poorly understood and constrained in current climate models. This suggests that there may exist large uncertainties in future heatwave projections, highlighting the critical significance of reducing uncertainties in aerosol-cloud interactions for reliable projection of climate extremes and effective risk management.

Plain Language Summary The past few years have seen record heatwaves worldwide, primarily driven by human activities. We used a state-of-the-art earth system model to investigate future changes in heatwave characteristics under the Representative Concentration Pathway 8.5 scenario and seek to separate the roles of projected changes in anthropogenic greenhouse gases and aerosols. The model shows that there will be more severe heatwaves (in terms of intensity, duration, and frequency) primarily because of global warming, while the internal variability of the climate system does not change much by 2100 and hence has limited influences. Also, these changes are mainly associated with greenhouse gas increases. However, anthropogenic aerosol changes have important influences, through their effects on clouds and radiation, and produce larger impacts comparing to greenhouse gases per unit of warming. Effects of aerosols on clouds such as changes in cloudiness and other rapid adjustments (e.g., changes in vertical temperature profiles), however, are still poorly represented in present generation climate models, leading to large uncertainties in future heatwave projections. Therefore, we call the attention of the community to prioritize efforts into reducing uncertainties involved in aerosol-cloud interactions, in order to get reliable projections of future climate extremes, as well as effective strategies for climate risk management.