UCLA

Abstract Increasing atmospheric CO 2 and associated global warming are expected to alter the global hydrological cycle, thereby posing widespread threats to freshwater availability. However, future hydrological projections differ greatly between models, particularly over the tropical regions. The large difference between model projections directly limits policy planning efforts, and the responsible modeling processes remain unclear. Here, we identify the primary processes accounting for model differences in tropical hydrological changes using multiple CO 2 sensitivity experiments in the Coupled Model Intercomparison Project. We show that differences in projected changes to tropical evapotranspiration, precipitation, and surface water availability mainly arise from model representations of vegetation cover and stomatal conductance responses to elevated CO 2 and associated changes in atmospheric moisture and circulation. Atmospheric responses to sea surface warming contribute additionally to the divergence in hydrological projections. Given the importance of vegetation responses to elevated CO 2 and associated atmosphere feedbacks, our results underscore the need to improve representations of the vegetation physiological response to rising CO 2 and its coupling to the atmosphere, to provide reliable tropical hydrological projections. Plain Language Summary Future hydrological changes pose great challenges to global freshwater and food security. However, hydrological projections diverge greatly between models, especially over tropical lands, which directly impedes planning for adaptation to climate change. Projection divergence is largely thought to be caused by differences in the representation of sea surface warming effect. Our study shows, however, that large divergence in tropical hydrological projections occurs immediately, in response to abrupt CO 2 quadrupling, before slower sea surface warming effect comes into play. Our results identify differences in the representation of fast vegetation physiological responses to elevated CO 2 and associated atmospheric moisture and circulation changes, as the primary cause of divergent long‐term hydrological projections over the tropical regions. Key Points Future hydrological projections diverge greatly between models, especially over tropical lands The divergence mainly arises from fast vegetation physiological responses to elevated CO 2 and associated atmosphere feedbacks Atmospheric responses to slow sea surface warming play a second role in the divergence of hydrological projections