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We investigated the nonlinearity of runoff response to global mean temperature (GMT) change in the Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models at the river basin scale globally. Results show that changes in long-term mean annual runoff are nonlinear with GMT rise over most extended subtropical basins, suggesting that estimation of future runoff change derived from the linear scaling relations would be biased. As for the interannual variability, nonlinearities are apparent mainly in central and western Asia, southern and western Africa, most of Europe, and Australia when GMT increases beyond 1.5 degrees C. This suggests that impacts of climate change under 1.5 degrees C GMT rise on runoff variability should not be simply scaled from that under a 2 degrees C warming world. Our results highlight the contrasting response of areal runoff to GMT rise across global major river basins and reveal the threshold of GMT increment at which the nonlinear runoff response is projected to emerge.

Plain Language Summary This paper addresses the nonlinear response of areal runoff to global warming at river basin scale globally and reveals the specific basins and the target global mean temperature rise where and when the nonlinearity is projected to occur. Furthermore, we highlight the possible mechanisms behind the revealed nonlinear response of river basin mean runoff to global mean temperature rise. These findings greatly differ from the global-scale linear relation concluded from previous analyses. This will provide great scientific implications for the ongoing impact assessment of limiting global warming to 1.5 degrees advocated by 2015 Paris agreement, and thus benefit local climate impact assessment, mitigation, and adaptation.