About one half of the freshwater supply to humanity originates in cold regions—the circumpolar and high mountain regions around the world where the hydrology is dominated by precipitation phase, snowpacks, glaciers, frozen soils and energy-driven water cycling. Here, amplified global warming is causing rapid declines in snow and glacier cover, shifting snowfall to rainfall, thawing permafrost, and increasing extremes of floods, drought, and wildfire, resulting in greatly altered hydrological regimes. Prediction of future hydrology requires new and improved physics-based modelling tools that capture the complex processes and their interactions, as the trajectories of hydrological systems can have unanticipated and sometime surprising outcomes that empirical models or extrapolations will fail to capture. A synthesis of future projected changes in climate, snowpack, glaciers, and hydrology, based on physically based modelling applications from around the world, show regional contrasts in the impact of climate change. The results consistently show dramatic and accelerating loss of snow and ice, greater extremes of low and high flows, and earlier peak flows, but varying sensitivity of streamflow regimes and seasonality of flows depending on elevation, physiography, glacier coverage and pre-existing climate.