The importance of flexible hydropower in providing electricity stability during China’s coal phase-out

•higher emission intensities of coal power plants running at below optimal efficiency to provide flexibility increases emissions from 3.4% to 11.1% depending on the region.•The coal phase-out lowers flexibility indicators, with loss-of-load increasing in the full phase-out scenario in the absence of mitigating technologies such as electricity storage.•Flexible hydropower eases peak demand issues significantly and the operating range of hydro-units has no significant effect on system flexibility or emissions.