Limiting global warming requires rapid and sustained reductions in greenhouse gas emissions. Yet, our ability to predict the pace and regional pattern of climate change remains dominated by uncertainties linked to aerosols and their interactions with clouds. These tiny airborne particles are often emitted together with greenhouse gases, and influence how sunlight and heat are reflected or absorbed in the atmosphere, shaping cloud formation, rainfall, and temperature extremes. Uncertainties in how air pollution interacts with clouds (so-called aerosol–cloud interactions, or ACI) are now the largest source of uncertainty in projecting near-term climate evolution and risk. Because aerosols respond quickly to emission changes, their effects unfold over years to decades, the timescales most relevant for adaptation and mitigation planning. Reducing these uncertainties is therefore essential to guide effective climate action, improve confidence in regional projections, and ensure that mitigation and clean-air policies deliver their intended climate benefits.