How is climate change affecting the West’s snowpack? What are the projections for the future?
Numerous scientific reports and studies in recent years have tried to answer these questions and others.
Below is an annotated bibliography of key scientific works related to climate change and the snowpack, focusing on the American West. At the bottom of the post is a list of recent journalism covering the issue.
Have a suggestion for a new study or story to add? Please contact me by email.
Reports: global, national and regional
Publications from the IPCC, the United Nations body that assesses climate science, are a great place to start for a global overview of climate change. If you’re looking for insights into the cryosphere, check out this special report from the IPCC, which also covers the oceans.
Below are some of the (verbatim) conclusions from the summary for policymakers (see footnote 4 of this report for an explanation of the confidence-related terms).
- “Over the last decades, global warming has led to widespread shrinking of the cryosphere, with mass loss from ice sheets and glaciers (very high confidence), reductions in snow cover (high confidence) and Arctic sea ice extent and thickness (very high confidence), and increased permafrost temperature (very high confidence).”
- “Cryospheric and associated hydrological changes have impacted terrestrial and freshwater species and ecosystems in high mountain and polar regions through the appearance of land previously covered by ice, changes in snow cover, and thawing permafrost. These changes have contributed to changing the seasonal activities, abundance and distribution of ecologically, culturally, and economically important plant and animal species, ecological disturbances, and ecosystem functioning. (high confidence)”
- “Since the mid-20th century, the shrinking cryosphere in the Arctic and high-mountain areas has led to predominantly negative impacts on food security, water resources, water quality, livelihoods, health and well-being, infrastructure, transportation, tourism and recreation, as well as culture of human societies, particularly for Indigenous peoples (high confidence).”
- “Global-scale glacier mass loss, permafrost thaw, and decline in snow cover and Arctic sea ice extent are projected to continue in the near-term (2031–2050) due to surface air temperature increases (high confidence), with unavoidable consequences for river runoff and local hazards (high confidence).”
- “Future land cryosphere changes will continue to alter terrestrial and freshwater ecosystems in high mountain and polar regions with major shifts in species distributions resulting in changes in ecosystem structure and functioning, and eventual loss of globally unique biodiversity (medium confidence).”
- “Future cryosphere changes on land are projected to affect water resources and their uses, such as hydropower (high confidence) and irrigated agriculture in and downstream of high mountain areas (medium confidence), as well as livelihoods in the Arctic (medium confidence). Changes in floods, avalanches, landslides, and ground destabilization are projected to increase risk for infrastructure, cultural, tourism, and recreational assets (medium confidence).”
The special report has a chapter on high mountain areas, which includes the graphic below that summarizes the changes in regions around the world.
Fifth National Climate Assessment, U.S. Global Change Research Program (2023)
This congressionally mandated synthesis of research, known as NCA5, concludes there is “widespread consensus” that warming will “decrease the proportion of US precipitation that falls as snow, decrease snow extents, advance the timing of snowmelt rates and pulses, increase the prevalence of rain-on-snow events,” and transform the runoff that is vital for farms, cities and ecosystems.
The report concludes that climate change has already diminished the West’s snowpack, with warming global temperatures leading to earlier peaks and shorter seasons, especially at lower elevations and in areas closer to the coast. In areas where snow is the dominant source of runoff, the volume of water stored in the snowpack may decrease by more than 24% by 2050 under some emissions scenarios, with “persistent low-snow conditions emerging within the next 60 years,” according to the report.
NCA5 stresses that climate change’s reshaping of the water cycle and other impacts will exacerbate inequalities in U.S. society and pose a special threat to some marginalized communities.
Map “a” shows changes in the volume of the snowpack on April 1, a key date for water managers as they plan for the runoff season. About 93% of sites have experienced a decrease in April 1 snowpack since the 1950s, with the decline averaging about 23%. Map “b” concerns the timing of the snowpack’s peak, which has come nearly eight days earlier on average since 1982. Map “c” presents data on the length of the snow season, which has decreased by 18 days on average over the last four decades.
Colorado River Basin Climate and Hydrology: State of the Science, Western Water Assessment (2020)
Researchers analyzed nearly 800 peer-reviewed studies, agency reports and other sources to assess the state of the science related to climate change and the hydrology of the Colorado River. Some of the key (verbatim) findings include:
- “The period since 2000 has been unusually drought-prone, but even more severe and sustained droughts occurred before 1900.”
- “There has been a substantial warming trend over the past 40 years; the period since 2000 has been about 2°F warmer than the 20th-century average, and likely warmer than at any time in the past 2000 years.”
- “Decreases in spring snowpack and shifts to earlier runoff timing in many parts of the Upper Basin, as well as decreases in annual Colorado River flows at Lees Ferry, Arizona, have occurred in recent decades. These changes in hydrology can be linked, at least in part, to the warming trend.”
- “There is still considerable uncertainty in the quantification of the relative roles of temperature, precipitation, antecedent soil moisture, dust-on-snow, and vegetation change in recent and ongoing variability and change in Upper Basin snowpack and streamflow.”
- “Mainly due to the pervasive effects of warming temperatures on the water cycle, nearly all of the many datasets of climate change-informed hydrology and related studies show a strong tendency toward lower annual runoff volumes in the Upper Basin and the Lower Basin, as well as reduced spring snowpack and earlier runoff.”
Peer-reviewed scientific papers
Barnhart, T.B., N.P. Molotch, B. Livneh, A.A. Harpold, J.F. Knowles, and D. Schneider, 2016: Snowmelt rate dictates streamflow. Geophysical Research Letters, 43 (15).
Researchers examined how the speed at which snow melts affects streamflow and found that faster snowmelt causes higher and quicker peaks. “Earlier, slower snowmelt usually produces less streamflow than more rapid melt,” according to the paper. Ecoregions in the American West have varying sensitives to the change in snowmelt rate.
Belmecheri, S., F. Babst, E.R. Wahl, D.W. Stahle, and V. Trouet, 2016: Multi-century evaluation of Sierra Nevada snowpack. Nature Climate Change, 6 (1).
This paper uses tree-ring data to examine the Sierra Nevada snowpack over the past five centuries and concludes that “the 2015 low is unprecedented in the context of the past 500 years.” The scientists found the record-low snowpack corresponded with record-high temperatures in California from January to March 2015, noting that “the exacerbating effect of warm winter temperatures is stronger at low than at high Sierra Nevada elevations.”
Davenport, F.V., J.E. Herrera-Estrada, M. Burke, and N.S. Diffenbaugh, 2020: Flood size increases nonlinearly across the western United States in response to lower snow-precipitation ratios. Water Resources Research, 56 (1).
This study finds that as more precipitation falls as rain rather than snow, flood sizes increase in a nonlinear fashion. Researchers found that the largest streamflow peaks driven by rainfall are more than 2.5 times the size of peaks driven by snowmelt. “Overall, as a higher percentage of precipitation falls as rain, increases in the size of rainfall-driven and ‘rain-on-snow’-driven floods have the potential to more than offset decreases in the size of snowmelt-driven floods,” according to the paper, which also notes there is a “large potential for continued regional warming to increase flood risk, even without changes in precipitation frequency, magnitude, or timing.”
Dudley, R.W., G.A. Hodgkins, M.R. McHale, M.J. Kolian, and B. Renard, 2017: Trends in snowmelt-related streamflow timing in the conterminous United States. Journal of Hydrology, 547.
Scientists found “widespread trends toward earlier snowmelt runoff related to warmer air temperatures,” with the timing significantly correlated with February to May air temperatures. In high-elevation basins in the West, streamflow timing was related to both temperature and precipitation.
Evan, A. and I. Eisenman, 2021: A mechanism for regional variations in snowpack melt under rising temperature. Nature Climate Change, 11 (4).
The timing of snowpack melting varies greatly across the West, and this study seeks to understand why there are significant regional differences. “For 1 °C of warming, snowpack disappears 30 days earlier in some regions, whereas there is almost no change in others,” according to the study. Elevation, geographic location, precipitation patterns and the annual temperature cycle of a location impact the timing. Looking around the world, the researchers conclude that “the timing of snowpack disappearance will change most rapidly in coastal regions, the Arctic, the western United States, Central Europe and South America, with much smaller changes in the northern interiors of North America and Eurasia.”
Gergel, D.R., B. Nijssen, J.T. Abatzoglou, D.P. Lettenmaier, and M.R. Stumbaugh, 2017: Effects of climate change on snowpack and fire potential in the western USA. Climatic Change, 141 (2).
This study examines 10 climate scenarios for the 21st century for snow, soil moisture and fuel moisture in the West. “A decline in mountain snowpack, an advance in the timing of spring melt, and a reduction in snow season are projected for five mountain ranges in the region,” according to the paper, which found “April 1 SWE losses by the 2080s of up to 81% for the Cascades and 76% for the Sierra Nevada mountains.” The diminished snowpack, combined with drier soils and fuels, is projected to increase wildfire potential across much of the region.
Gottlieb, Alexander R., and Justin S. Mankin, 2024: Evidence of Human Influence on Northern Hemisphere Snow Loss. Nature, 625 (7994).
Researchers concluded that the snowpack shrunk in the U.S. and other places around the Northern Hemisphere from 1981 to 2020, but not everywhere on the planet. The researchers say warming is causing many watersheds to approach a tipping point they call a “snow-loss cliff,” where relatively small temperature rises could accelerate the shrinking of the snowpack in a “highly nonlinear” fashion.
Hale, Katherine E., Keith S. Jennings, Keith N. Musselman, Ben Livneh, and Noah P. Molotch, 2023: Recent Decreases in Snow Water Storage in Western North America. Communications Earth & Environment 4 (1).
This paper introduces a new measure—the Snow Storage Index—to analyze the changes to the hydrologic cycle. In Western North America, the annual snow storage index has decreased from 1950 to 2013 in 25% of mountainous areas due to “substantially earlier snowmelt and rainfall in spring months, with additional declines in winter precipitation.” The study projects further decreases in the index as warming causes earlier snowmelt and a shift from snowfall to rainfall.
Harpold, A.A. and P.D. Brooks, 2018: Humidity determines snowpack ablation under a warming climate. Proceedings of the National Academy of Sciences, 115 (6).
Scientists found that atmospheric humidity plays a big role in controlling how the snowpack responds to warming temperatures, with the frequency and magnitude of winter melt events rising under higher-humidity conditions. “Increased winter melt in humid areas will require enhanced storage capabilities (reservoir, groundwater, etc.) to compensate for the decrease in snow storage and safeguard against increased winter flooding events,” the authors write. “Conversely, earlier and slower snowmelt in less humid areas could lower annual water yields due to sublimation losses and increased evapotranspiration, requiring updated water management strategies to conserve water in dry years.”
Huning, Laurie S., and Amir AghaKouchak, 2020:. Global Snow Drought Hot Spots and Characteristics. Proceedings of the National Academy of Sciences, 117 (33).
This paper identifies hotspots for “snow droughts” and shows that “eastern Russia, Europe, and the western United States experienced longer, more intense snow droughts in the second half of the period 1980 to 2018.” “Natural and human-driven factors (e.g., atmospheric circulation patterns, polar vortex movement, and Arctic warming) likely contribute to snow droughts,” according to the study.
Huning, Laurie S., and Amir AghaKouchak. Mountain Snowpack Response to Different Levels of Warming. Proceedings of the National Academy of Sciences, 115 (43).
In this study of the Sierra Nevada, the authors show that “even a 1.0 or 2.0 °C increase in average temperature leads to approximately a 20 to 40% increase in the likelihood of below average SWE.” The paper also found that the snowpack in the northern Sierra Nevada is more vulnerable to warming than in the southern part of the range.
Il Jeong, D. and L. Sushama, 2018: Rain-on-snow events over North America based on two Canadian regional climate models. Climate Dynamics, 50 (1).
Scientists examined both historical data and future projections for rain-on-snow events, which can cause severe flooding. The researchers conclude that rain-on-snow events will generally increase during November to March for most regions of Canada and the northwestern U.S., but southern regions may see a decrease due to reduced snow cover. The results also show a general increase in rain-on-snow events at higher elevations and a decrease at lower elevations.
Klos, P. Z., T. E. Link, and J. T. Abatzoglou, 2014: Extent of the rain–snow transition zone in the western U.S. under historic and projected climate. Geophysical Research Letters, 41.
This study investigates the rain-snow transition zone across the American West for both the late 20th-century climate and the projected climate in the middle of the 21st century. “At broad scales, these projections indicate an average 30% decrease in areal extent of winter wet-day temperatures conducive to snowfall over the western United States,” according to the study. The findings suggest that “many mountainous areas will be characterized by a mixed rain-snow regime in November, in contrast to the historic strongly snow-dominated precipitation regime.” The researchers also project that the likelihood of rain falling instead of snow will increase in March, April and May.
Knowles, N., M. D. Dettinger, and D. R. Cayan, 2006: Trends in snowfall versus rainfall in the western United States. Journal of Climate, 19, 4545–4559.
This analysis of long-term precipitation patterns in the American West finds an overall decrease in snowfall and an increase in rainfall from 1949 to 2004. The trend, which is due to rising temperatures, affects the volume of water stored in the snowpack, alters the timing of snowmelt and increases the risk of flooding in winter and spring. The trend was most pronounced across the region in March and in January near the West Coast. “Temperatures have warmed during winter and early spring storms, and, consequently, the fraction of precipitation that fell as snow declined while the fraction that fell as rain increased,” according to the paper.
Li, D., M.L. Wrzesien, M. Durand, J. Adam, and D.P. Lettenmaier, 2017: How much runoff originates as snow in the western United States, and how will that change in the future? Geophysical Research Letters, 44 (12), 6163–6172.
This study (covered in a prior post) quantifies the contribution of snowmelt to runoff and projects how that will be altered by climate change. About 53% of runoff in the West originates as snowmelt, but warming will cause a shift from snow to rain, so the contribution of snowmelt to runoff is projected to decline to between 30.4% and 39.5%, under intermediate and high emissions scenarios. “Future runoff will be driven more by rainfall than snowmelt,” according to the study. “Since the western U.S. heavily relies on snowmelt stored in reservoirs to meet demands for water in the low flow season, reduced spring snowpack and earlier melt onset will likely put significant pressure on water supply in the late summer and fall.”
Livneh, B. and A.M. Badger, 2020: Drought less predictable under declining future snowpack. Nature Climate Change, 10 (5).
Researchers found that the shift from snow to rain will make it harder to predict droughts in the West. “By mid-century (2036–2065), 69% of historically snowmelt-dominated areas of the western United States see a decline in the ability of snow to predict seasonal drought, increasing to 83% by late century (2070–2099),” according to the study. Lower-elevation coastal areas will be most impacted by warming and generate more uncertainty in drought forecasts.
Milly, P.C.D. and K.A. Dunne, 2020: Colorado River flow dwindles as warming-driven loss of reflective snow energizes evaporation. Science, 367 (6483).
This study concludes that annual mean discharge for the Colorado River has been falling by 9.3% per degree Celsius of warming. The researchers say the decline is primarily driven by increasing evapotranspiration as the loss of snow cover increases the absorption of solar radiation. The snow cover acts like a “protective shield” that limits evaporative losses, but continued warming is projected to shrink the snowpack.
Mote, P.W., S. Li, D.P. Lettenmaier, M. Xiao, and R. Engel, 2018: Dramatic declines in snowpack in the western US. Npj Climate and Atmospheric Science, 1 (1).
Mote, P. W., A. F. Hamlet, M. P. Clark, and D. P. Lettenmaier, 2005: Declining mountain snowpack in western North America. Bulletin of the American Meteorological Society, 86.
The 2018 paper updates the 2005 study, which found substantial declines in the West’s snowpack due to warmer temperatures. The more recent paper found that “over 90% of snow monitoring sites with long records across the western US now show declines, of which 33% are significant (vs. 5% expected by chance).” The researchers also report that “declining trends are observed across all months, states, and climates, but are largest in spring, in the Pacific states, and in locations with mild winter climate.” Average April 1 readings for SWE have declined 15% to 30% since the middle of the 20th century, which is comparable to the volume of Lake Mead, the West’s biggest reservoir.
Musselman, K.N., N. Addor, J.A. Vano, and N.P. Molotch, 2021: Winter melt trends portend widespread declines in snow water resources. Nature Climate Change, 11 (5).
Researchers found that 34% of monitoring stations in Western North America exhibited increasing winter snowmelt trends, a rate three times as large as the 11% of stations showing declines in SWE. Snowmelt trends are very sensitive to temperature and ongoing warming, while SWE trends are more sensitive to precipitation variability. The scientists also found that “the percentage of annual melt that occurs before 1 April is increasing by 3.5% per decade at 42% of the available stations” and argue that “this substantial and widespread rate of change implies a loss of seasonal storage of snow water resources in North American mountain water towers.”
Musselman, K.N., F. Lehner, K. Ikeda, M.P. Clark, A.F. Prein, C. Liu, M. Barlage, and R. Rasmussen, 2018: Projected increases and shifts in rain-on-snow flood risk over western North America. Nature Climate Change, 8 (9).
In this paper, scientists look at how climate change will impact rain-on-snow events and associated flood risks in Western North America. In a warming climate, rain-on-snow events are projected to become less frequent at lower elevations because of snowpack declines, especially in warmer areas like the Pacific maritime region. At higher elevations, however, these events are expected to become more common due to the shift from snow to rain. The greatest increase in flooding risk is projected in the Sierra Nevada, the Colorado River headwaters and the Canadian Rocky Mountains.
Musselman, K.N., M.P. Clark, C. Liu, K. Ikeda, and R. Rasmussen, 2017: Slower snowmelt in a warmer world. Nature Climate Change, 7 (3).
Researchers conclude that a “shallower snowpack melts earlier, and at lower rates, than deeper, later-lying snow-cover” and find that “the fraction of meltwater volume produced at high snowmelt rates is greatly reduced in a warmer climate.” The findings have implications for “soil moisture deficits, vegetation stress, and streamflow declines,” according to the study.
Pierce, D. W., T. P. Barnett, H. G. Hidalgo, T. Das, C. Bonfils, B. D. Santer, G. Bala, M. D. Dettinger, D. R. Cayan, A. Mirin, A. W. Wood, and T. Nozawa, 2008: Attribution of declining western US snowpack to human effects. Journal of Climate, 21.
This study examines the decline in the West’s snowpack from 1950 to 1999 and finds that about half of the reduction is “the result of climate changes forced by anthropogenic greenhouse gases, ozone, and aerosols.” The study used 1,600 years of simulations to account for natural variability in the climate and also ruled out solar or volcanic activity causing changes to the snowpack.
Qin, Y., J.T. Abatzoglou, S. Siebert, L.S. Huning, A. AghaKouchak, J.S. Mankin, C. Hong, D. Tong, S.J. Davis, and N.D. Mueller, 2020: Agricultural risks from changing snowmelt. Nature Climate Change, 10 (5).
This global study identifies the regions and crops that are most dependent on snowmelt for irrigation and finds that the American West is one of the hotspots, along with the Tibetan Plateau, Central Asia, Western Russia and the Southern Andes. Under a 4°C warming scenario, reduced snowmelt will require some basins to find up to 40% of their irrigation demand in alternative sources.
Rauscher, S.A., J.S. Pal, N.S. Diffenbaugh, and M.M. Benedetti, 2008: Future changes in snowmelt-driven runoff timing over the western US. Geophysical Research Letters, 35 (16).
This study projects future changes in the timing of snowmelt-driven runoff in the Western United States. Rising greenhouse gas emissions could lead to 3°C to 5°C increases in seasonal temperatures that cause snowmelt-driven runoff to occur as much as two months earlier. “These large changes result from an amplified snow-albedo feedback associated with the topographic complexity of the region,” the authors write.
Siirila-Woodburn, E.R., A.M. Rhoades, B.J. Hatchett, L.S. Huning, J. Szinai, C. Tague, P.S. Nico, D.R. Feldman, A.D. Jones, W.D. Collins, and L. Kaatz, 2021: A low-to-no snow future and its impacts on water resources in the western United States. Nature Reviews Earth & Environment, 2 (11).
This review paper discusses how climate change is decreasing snowpacks around the world and warns of “potentially catastrophic consequences on water resources, given the long-held reliance on snowpack in water management.” Across the West, SWE declines of about 25% are expected by 2050, and it may be “~35–60 years before low-to-no snow becomes persistent if greenhouse gas emissions continue unabated.”
Recent journalism
- Snowpack in Colorado, across the West is melting earlier than it did in the 20th century. Lucy Haggard, The Colorado Sun, 4/6/2021.
- In the West, Signs in the Snow Warn That a 20-Year Drought Will Persist and Intensify. Bob Berwyn and Judy Fahys, Inside Climate News, 4/9/2021.
- The winnowing of winter. Heather Hansman, High Country News, 11/1/2021.
- Western U.S. could become nearly snowless due to climate change. Diana Leonard, The Washington Post, 12/3/2021.
- Life Was Built Around Snow. What Happens When It Vanishes? Ruth Fremson and Kirk Johnson, The New York Times, 10/21/2022.
- Pink snow is a red flag for the West’s water. Kylie Mohr, High Country News, 11/29/2022.
- Warmer winters spell trouble for snow plow business. Mara Hoplamazian. National Public Radio, 1/20/2023.
- Snow eases California drought and restores water levels but more needed. Joshua Partlow, The Washington Post, 3/4/2023, (video here).
- Warming to push snowline higher in California, study finds. Ian James, Los Angeles Times, 5/25/2023.
- Study shows how warming climate is sapping the Colorado River. Ian James, Los Angeles Times, 7/30/2023.
The Water Desk’s mission is to increase the volume, depth and power of journalism connected to Western water issues. We’re an editorially independent initiative of the Center for Environmental Journalism at the University of Colorado Boulder.
