Overcoming the Northern Hemisphere snow-water resources paradox

Although Earth system models (ESMs) overestimate historical land surface warming, paradoxically, they also overestimate snow amounts in the Northern Hemisphere (NH). This apparent contradiction constitutes a snow-water resources paradox, but the underlying mechanisms of this paradox remain unclear. Combining ground-based datasets and ESMs, we found that the snow-water resources paradox can be explained by the overestimation of the frequency of light snow by ESMs. Using spatially-distributed emergent constraints, we show that this paradox persists in the mid- (2041–2060) and long-term (2081–2100) projections over more than half of the NH’s land surface, with the frequency of freezing days underestimated by 12%–19% and snow water equivalent (SWE) overestimated by 41%–47%. The constrained projections indicate that the raw ESMs overestimate future NH snowmelt runoff by 17%–24% (over 33%–35% of the NH’s land surface). This extensive and long-standing snow-water resources paradox poses a serious adaptation planning risk in that the degree of future snowmelt water available for agriculture, industry, ecosystems and domestic life may well be less than suggested by unadjusted ESM projections.