Large Divergence in Tropical Hydrological Projections Caused by Model Spread in Elevated CO2 Responses
We identify the processes accounting for model differences in tropical hydrological changes using multiple experiments in the Coupled Model Intercomparison Project. We show that differences mainly arise from model representations of vegetation responses to elevated CO2, and associated changes in atmospheric moisture and circulation.
Vegetation responses to elevated CO2 and associated atmosphere feedbacks are of huge importance. Our results underscore the need to improve representations of the vegetation physiological response to rising CO2, to provide reliable tropical hydrological projections.
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 CO2 quadrupling, before slower sea surface warming effect comes into play. Our results identify differences in the representation of fast vegetation physiological responses to elevated CO2 and associated atmospheric moisture and circulation changes, as the primary cause of divergent long-term hydrological projections over the tropical regions.