Illuminating snow droughts: The future of Western United States snowpack
in the SPEAR large ensemble
Abstract
Seasonal snowpack in the Western United States (WUS) is vital for
meeting summer hydrological demands, reducing the intensity and
frequency of wildfires, and supporting snow-tourism economies. While the
frequency and severity of snow droughts (SD) are expected to increase
under continued global warming, the uncertainty from internal climate
variability remains challenging to quantify. Using a 30-member large
ensemble from a state-of-the-art global climate model, the Seamless
System for Prediction and EArth System Research (SPEAR), and an
observations-based dataset, we find WUS SD changes are already
significant. By 2100, SPEAR projects SDs to be nearly 9 times more
frequent under shared socioeconomic pathway 5-8.5 (SSP5-8.5) and 5 times
more frequent under SSP2-4.5. By investigating the influence of the two
primary drivers of SD, temperature and precipitation amount, we find the
average WUS SD will become warmer and wetter. To assess how these
changes affect future summer water availability, we track April 15th
snowpack across WUS watersheds, finding differences in the onset time of
a “no-snow” threshold between regions and large internal variability
within the ensemble that are both on the order of decades. For example,
under SSP5-8.5, SPEAR projects California could experience no-snow
anywhere between 2058 and 2096, while in the Pacific Northwest, the
earliest transition happens in 2091. We attribute the inter-regional
uncertainty to differences in the regions’ mean winter temperature and
the intra-regional uncertainty to irreducible internal climate
variability. This analysis indicates that internal climate variability
will remain a significant source of uncertainty for WUS hydrology
through 2100.