1. Illuminating Snow Droughts: The Future of Western United States Snowpack in the SPEAR Large Ensemble.
- Author
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Schmitt, Julian, Tseng, Kai‐Chih, Hughes, Mimi, and Johnson, Nathaniel C.
- Subjects
CLIMATE change models ,WATER shortages ,ATMOSPHERIC models ,DROUGHTS ,SPRING ,SUMMER - 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), that is, anomalously low snowpacks, are expected to increase under continued global warming, the uncertainty from internal climate variability remains challenging to quantify with observations alone. 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 data set, 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, compared to a 1921–2011 average. 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 late winter and spring 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. We attribute the inter‐regional variability to differences in the regions' mean winter temperature and the intra‐regional variability to irreducible internal climate variability which is not well‐explained by temperature variations alone. Despite strong scenario forcing, internal climate variability will continue to drive variations in SD and no‐snow conditions through 2100. Plain Language Summary: Snow drought (SD) occurs when there is significantly less snow on the ground than normal. SDs can intensify water shortages, accelerate wildfires, and harm snow‐based tourism economies. For the Western United States, whose water supply is already limited, a recent increase in snow drought frequency is particularly concerning. Here, we use observational data and a new climate model to examine snow drought changes across the region between 1921 and 2100. We find SDs are already more common and could increase almost nine times under a high emissions scenario or five times under moderate emissions cuts by 2100. To better understand the increase, we tracked the evolution of the two main snow drought drivers: warmer temperatures and decreased precipitation. We find the average snow drought will become warmer and wetter, indicating warming temperatures are driving the increase. As the model consists of multiple simulations of future climate, or ensemble members, that differ only in the realization of random climate variability, we can determine when Western regions are expected to lose most of their spring snowpack. We find that loss timing varies dramatically between regions and ensemble members, suggesting random climate variability will shape the West's future water availability. Key Points: Excluding the Pacific Northwest, observed increases of Western U.S. severe snow drought by 4%–46% are supported by the Seamless System for Prediction and EArth System Research (SPEAR) climate model21st century increases in Western U.S. severe snow drought in SPEAR are driven by rising temperatures and not decreased precipitationBy 2100 under SSP5‐8.5, SPEAR projects persistent Western U.S. spring no‐snow with timing sensitive to internal climate variability [ABSTRACT FROM AUTHOR]
- Published
- 2024
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