Reframing the Rarity of Extreme Low-Snow Years in the Upper Colorado River Basin

Sydney Carr

University of Colorado Boulder

Tuesday, Oct 06, 2026, 2:00 pm MT
DSRC Room 1D403

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Abstract

Mountain snowpack reflects both anthropogenic warming and substantial internal climate variability, complicating efforts to determine when forced changes emerge from the historical range of variability and become consequential to water resources. In the Upper Colorado River Basin (UCRB), this challenge extends beyond detecting long-term declines in mean snowpack to understanding how warming is reshaping the occurrence of extreme low-snow years. Water year (WY) 2026 produced the lowest basin-wide snowpack on record, providing a timely benchmark for evaluating this changing risk.

We combine observations with dynamically downscaled CESM2 large-ensemble simulations and a 100-member reconstruction of basin-scale snow water equivalent (SWE) to examine anthropogenic change across mean and extreme conditions. Time of Emergence identifies climate change signals in UCRB seasonal snowpack, while two historically consequential low-snow years—stakeholder-informed WY 2002 and the unprecedented WY 2026 extreme—characterize changes in the recurrence of rare events through the end of this century. Increases in the frequency and persistence of low-snow extremes outpace gradual declines in average snowpack conditions and represent an important source of hydrologic, ecological, and socioeconomic risk. Large ensembles further reveal how strongly estimates of extreme-event rarity depend on the extent to which internal climate variability is sampled.

Because adaptation is often informed by the most extreme events in living memory, observed low-snow extremes provide physically plausible storylines of future risk. Our large-ensemble framework places these events in a probabilistic context by quantifying how their recurrence changes under continued warming. As hydroclimatic baselines shift and historical observations become increasingly unreliable guides to future risk, our results reframe the rarity of low-snow extremes as an evolving probability governed jointly by internal climate variability and anthropogenic warming.

Bio

Sydney Carr is a Ph.D. student in Geography at the University of Colorado Boulder and a graduate researcher in the Musselman Lab at the Institute of Arctic and Alpine Research (INSTAAR). Her research examines how anthropogenic climate change superimposed on internal climate variability is reshaping mountain snowpack and water resources in the Upper Colorado River Basin. Using downscaled large-ensemble climate simulations, she investigates emerging changes in snowpack and extreme low-snow conditions to characterize future hydroclimatic risk in snow-dominated basins.