Accelerated hydrological cycle over the Sanjiangyuan region induces more streamflow extremes at different global warming levels

Serving source water for the Yellow, Yangtze and Lancang-Mekong rivers, the Sanjiangyuan region affects 700 million people over its downstream areas. Recent research suggests that the Sanjiangyuan region will become wetter in a warming future, but future changes of streamflow extremes remain unclear due to the complex hydrological processes over high-land areas and limited knowledge of the influences of land cover change and CO2 physiological forcing. Based on high-resolution land surface modeling during 1979-2100 driven by the climate and ecological projections from 11 newly released Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models, we show that different accelerating rates of precipitation and evapotranspiration at 1.5 degrees C global warming level induce 55% more dry extremes over Yellow River and 138% more wet extremes over Yangtze River headwaters compared with the reference period (1985-2014). An additional 0.5 degrees C warming leads to a further nonlinear and more significant increase for both dry extremes over Yellow River (22 %) and wet extremes over Yangtze River (64 %). The combined role of CO2 physiological forcing and vegetation greening, which used to be neglected in hydrological projections, is found to alleviate dry extremes at 1.5 and 2.0 degrees C warming levels but to intensify dry extremes at 3.0 degrees C warming level. Moreover, vegetation greening contributes half of the differences between 1.5 and 3.0 degrees C warming levels. This study emphasizes the impor-tance of ecological processes in determining future changes in streamflow extremes and suggests a "dry gets drier, wet gets wetter" condition over the warming headwaters.