Impacts of climate change on global total and urban runoff

Climate models consistently project that frequency, severity, and duration of hydroclimatic extremes will increase over this century under climate change. Urban flooding and runoff in general have become prominent issues for many cities and regions, arising from a combination of altered precipitation patterns, urban growth, development in floodplains, and increases in impervious surfaces. In this study, we first validate total (grid cell-level) runoff from the fully coupled Community Earth System Model (CESM) historical simulations against one observed-runoff/streamflow-based dataset and one reanalysis dataset, and further analyze both grid cell-level runoff and urban subgrid runoff under future climate change scenarios. We calculated global annual average of monthly runoff from the period 1986-1995 for the validation and calculated bias and correlation coefficients between CESM and each of the datasets. Additionally, we analyzed future grid cell and urban runoff across three CMIP6 coupled Shared Socioeconomic Pathways and Representative Concentration Pathways - 2- 4.5, 3-7.0, and 5-8.5 - and evaluated changes between the future period of 2041-2050 and the same past period of 1986-1995 for each scenario. Results show spatial consistency and robustness between the CESM simulations and both datasets. However, there is some spatial inconsistency in the areas highlighted as major runoff producers, such as the Amazon basin and Southeast Asia, as well as mountainous regions outside the United States. Grid cell-level runoff and urban runoff projections suggest that future hydroclimatic conditions will vary depending on the climate scenario. However, certain locations, such as Madagascar, Indonesia, and the Himalayan mountain range, consistently see decreases in both grid cell-level runoff and urban runoff across all scenarios, and locations such as Nigeria and Ecuador consistently see increases in both grid cell runoff and urban runoff across all scenarios. Our findings provide quantitative insights on hydrology representation in the global Earth system model and advance the understanding of the impacts of large-scale climate change on future local-scale urban runoff.