Snow accumulation is critical for water availability in the Northern Hemisphere(1,2), raising concern that global warming could have important impacts on natural and human systems in snow-dependent regions(1,3). Although regional hydrologic changes have been observed (for example, refs 1,3-5), the time of emergence of extreme changes in snow accumulation and melt remains a key unknown for assessing climate-change impacts(3,6,7). We find that the CMIP5 global climate model ensemble exhibits an imminent shift towards low snow years in the Northern Hemisphere, with areas of western North America, northeastern Europe and the Greater Himalaya showing the strongest emergence during the near-term decades and at 2 degrees C global warming. The occurrence of extremely low snow years becomes widespread by the late twenty-first century, as do the occurrences of extremely high early-season snowmelt and runoff (implying increasing flood risk), and extremely low late-season snowmelt and runoff (implying increasing water stress). Our results suggest that many snow-dependent regions of the Northern Hemisphere are likely to experience increasing stress from low snow years within the next three decades, and from extreme changes in snow-dominated water resources if global warming exceeds 2 degrees C above the pre-industrial baseline.