Warming Mountains, Earlier runoff and Declining Snowpack in Western North America

Abstract:

Climate changes, which will very likely mount over the next several decades, would add to a set of shifts that have appeared in snow and runoff during recent decades over western North America. Water from mountain snow has historically provided a vital portion of the water supply for the region, but warming since the mid-1970’s has produced more rain, less snow, diminished snow accumulation and earlier runoff over mountain catchments that historically have accumulated significant snow cover. The observed warming and the hydrologic shifts have, with a high degree of likelihood, been boosted by additional anthropogenic warming. A many-member ensemble of CMIP5 GCM simulations indicates that even under moderate scenarios of rising greenhouse gases, the warming, relative to historical climatology, could approach +2°C by mid-21^st Century. This amount of warming would produce substantial changes in watersheds which have historically been dominated by snowmelt. In the California Sierra Nevada, this warming would advance spring runoff timing by more than two weeks. Combined with changes towards increased precipitation amounts in the region’s largest storms, higher than present-day freezing levels would increase flood flows in Sierran rivers. Historical observations and hydrological model simulations indicate that California’s total spring snow water storage will decline by more than half by 2100. Heavy snow years would still occur, but not as often as today, and springs with highly depleted mountain snow pack would occur with increasing frequency. These changes would produce a cascade of impacts to natural landscapes and resources, requiring multifold adaptation strategies by the humans who use and are affected by them.