Click the link to read the article on The Water Desk website (Mitch Tobin):

April 1, 2024

The latest National Climate Assessment warns of a shrinking snowpack and serious downstream consequences

A recent federal synthesis of climate change research paints a grim portrait of snow’s future in the American West and warns that the fast-growing region’s water supply is vulnerable.

“Climate change will continue to cause profound changes in the water cycle, increasing the risk of flooding, drought, and degraded water supplies for both people and ecosystems,” according to the Fifth National Climate Assessment (NCA5) released in November.

The congressionally mandated report 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. 

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,” the report said.

“When we have less snow in the West, it can strain our water supplies,” said report co-author Steph McAfee, regional administrator of the U.S. Geological Survey’s Southwest Climate Adaptation Science Center. “We’ve tended to rely on the snowpack as a reservoir that didn’t need to be built and it doesn’t need to be maintained, so it’s been a key place for storing water. Having less snow directly means less water stored for use in the summer.”

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.

“All communities will be affected,” the report said, “but in particular those on the frontline of climate change—including many Black, Hispanic, Tribal, Indigenous, and socioeconomically disadvantaged communities—face growing risks from changes to water quantity and quality due to the proximity of their homes and workplaces to hazards and limited access to resources and infrastructure.” 

NCA5 describes itself as the federal government’s “preeminent report on climate change impacts, risks, and responses,” though it is required to steer clear of policy prescriptions. 

The report is based on the latest science, but it is produced for decision-makers and the general public, so it is written in relatively accessible language, and data visualizations play a leading role in communicating the findings. 

Below I use 10 visuals from NCA5—mostly maps but also charts, an infographic and a photo—to help summarize the report’s conclusions about climate, snow and water in the West, focusing on the more arid parts of the region. 

Climate, snow and water

At one level, the story of snow and climate change is simple: in order for snow to fall and stick around, it has to be cold enough, so the warming of the planet is generally bad news for snow. 

“I think the changes to snow and snowpack are changes that we have more confidence in than just about any other water parameter because of the direct effect of warming on snowpack and snow precipitation,” said Elizabeth Payton, NCA5’s Water Chapter Lead and a water resources specialist at the Western Water Assessment at the University of Colorado Boulder.

Co-author Ben Harding, senior water resources engineer at Lynker, summed up the report’s findings on snow this way: 

“We’re going to see shorter periods of time with snow on the ground, the snow will start to accumulate later and it’ll start to melt earlier,” he said.

A smaller snowpack, a curtailed snow season and a new runoff regime will test the region’s complex water infrastructure of dams, aqueducts and canals, many of which were built in the early to mid-20^th century, before climate change was recognized as a peril. The altered snowpack will also strain the West’s water laws and policies, many of which emerged in the 19^th century, before some Western states were even admitted to the union. 

But while climate change has already shrunk the snowpack in most parts of the world and will continue to take a toll as temperatures climb, there are exceptions that buck the trend. Total global precipitation is expected to increase due to warming, including in places where the snowpack shrivels. NCA5 predicts there will be worse droughts and floods. 

For example, atmospheric rivers, which are pivotal for the West’s snowpack and water supply, are expected to strengthen in the years ahead. But beyond a certain point, warming makes it more likely that rain will fall instead of snow, even high in the mountains, raising the risk of flooding and a subpar snowpack. 

As temperatures keep rising, increasing rates of melting and evaporation will play a key role. Another critical factor is how much moisture gets sucked up by plants and then transpired into the atmosphere. Some snow never becomes snowmelt and is “lost” to the atmosphere through sublimation, moving directly from the solid to the gaseous phase. Soil moisture is yet another essential element of the water cycle, impacting drought, flooding, agriculture and ecosystems. 

But that’s not all. In Colorado, for example, dust-on-snow events are a big deal because the darker material reduces the snow’s reflectivity and causes it to absorb more heat, accelerating the meltout. Climate change threatens to worsen the dust problem as it continues to aridify parts of the West.

Warming is adjusting the dials on all of these factors, and the magnitude of these changes matter, but there’s yet another crucial dimension: timing. In spring, farmers, water managers and dam operators not only care deeply about the volume of the snowpack that will fill reservoirs, canals, ditches and pipes, but also are keenly interested in when that water will be entering the system. 

“Having a pulse of snowmelt at the beginning of the growing season has been helpful to farmers and ranchers, and the timing of the snowmelt has been something that ecosystems have evolved to adapt to,” Payton said. “The timing is going to be shifting dramatically.”

Warming has already taken a toll on the West’s snowpack

While much of NCA5 focuses on the future, the report also looks back at how climate change has already transformed the nation. The graphic below depicts how the West’s snowpack has shifted in recent decades, with red circles indicating declines, blue circles showing increases and the circle scaled to the size of the change. 

The figure’s title says it all: “Western snowpack is declining, peak snowpack is occurring earlier, and the snowpack season is shortening in length.”

While the vast majority of circles in the figure are red, there are also some blue locations, such as in north-central Colorado. When I asked NCA5 co-authors about those sites, several noted that many of them lie at higher elevations—like those along the Continental Divide in Colorado—and the naturally colder conditions there can help preserve their snowpack in a warming world, up to a point. 

“There are some parts of Alaska or some very high elevations that might have more snow when the snowpack is at its largest,” McAfee said. “They’re starting out really cold, so if it warms up some, it’s still cold enough to snow. If it warms up enough, then there’s the possibility for snow melting earlier or more of those storms bringing rain than snow.”

While some high-elevation locations may see their snowpack increase in coming years, it’s “by and large definitely not enough to compensate or offset the widespread losses in snow that are occurring everywhere else,” said co-author Justin Pflug, a scientist at the University of Maryland and NASA Goddard Space Flight Center.

How much warmer it gets will be crucial for the snowpack (and much else)

One of the challenges in producing a report like NCA5 is the uncertainty surrounding future greenhouse gas emissions. Innovation, geopolitics, consumer preferences and more make it hard to predict how rapidly the economy will decarbonize. As a result, scientists must use varying emissions scenarios, and it remains to be seen just how much temperatures will rise at a global level.

While the rate of future warming is uncertain, one thing that’s clear is that some parts of the planet will warm more than others and have already experienced much steeper temperature increases. 

The graphic below, which maps the projected change in temperatures at various levels of global warming, shows that the effects are expected to be uneven across the United States. For example, at 2°C of global warming, parts of the Interior West would be more than 5°C warmer. Across the globe, researchers have found “growing evidence that the rate of warming is amplified with elevation,” according to a 2015 paper in Nature Climate Change.

Locations in Alaska would be even hotter than that, mirroring a global trend of much more rapid warming in the Arctic. A 2022 study in Communications Earth & Environment is titled “The Arctic has warmed nearly four times faster than the globe since 1979.”

“One of the key messages for us in the water chapter is that temperature really matters for water,” McAfee said. “Temperature influences whether or not we get rain or snow. It influences when the snowpack melts. It influences how big a sip the atmosphere takes from the water and all of that. So we can’t think about precipitation and we can’t think about our water systems separate from temperature.”

When people hear about droughts and water shortages, they naturally think of a lack of precipitation, which remains the primary driver of such dry times. But as NCA5 notes, “higher temperatures can cause drought to develop or become more intense than would be expected from precipitation deficits alone.”

In a “hot drought,” the atmosphere demands more moisture and desiccates the landscape. Warmer temperatures also contribute to “snow droughts” (discussed below), “flash droughts” that develop in a matter of weeks and “megadroughts” that can extend over decades. 

NCA5 also emphasizes two other messages related to temperature: the degree of change matters greatly, and how hot the planet gets depends on the choices society makes now. [ed. emphasis mine]

“The more the planet warms, the greater the impacts—and the greater the risk of unforeseen consequences,” according to the report. “While there are still uncertainties about how the planet will react to rapid warming and catastrophic future scenarios that cannot be ruled out, the future is largely in human hands.”

Climate change is projected to increase global precipitation, but not necessarily in the Southwest

Scientists and their models can paint a much clearer picture of how temperatures will change compared to the projections for precipitation. That said, global warming is expected to increase overall precipitation on the planet because there will be higher evaporation rates and warmer air can hold more moisture. 

The figure below shows projected changes in annual precipitation according to four different levels of warming, with greens indicating increases and browns depicting decreases. The hatching shows areas where 80% or more of the models agree on whether precipitation will increase or decrease. 

Most of the country is expected to see more precipitation overall, with higher levels of warming generally leading to wetter conditions and more certainty about those changes. But in all of the maps, precipitation is expected to decrease in Southern California, much of Arizona, New Mexico and Texas, plus portions of Colorado.

“Precipitation changes also scale with global warming, but these projections vary by location and are less certain than temperature changes,” according to NCA5. 

Payton said “there’s not a very strong signal” for total precipitation changes for the Southwest. “The atmosphere can hold more moisture when it’s warmer,” she said, “but that moisture has to come from somewhere, so over the Southwest, where it’s already dry, is it going to be able to suck up that additional amount of moisture that it can hold?”

While precipitation projections are cloudier, Westerners should expect a shift from snowflakes toward raindrops in many parts of the region: “it is virtually certain that less precipitation will fall as snow, leading to large reductions in mountain snowpack and decreases in spring runoff in the mountain West,” according to NCA5.

Overall, NCA5 concludes that “changes in future precipitation and temperature are expected to exacerbate drought across large portions of the US,” with projections showing “the strongest drying signal occurring in the Southwest.”

While drought and water scarcity are dominant themes in more arid parts of the West, these areas also contend with floods that can turn dry washes into raging torrents in a flash and threaten both lives and property.

“Warmer air is thirstier air, and that really raises the risk of higher-severity precipitation events,” Pflug said. 

Flooding can also be caused by snowmelt, especially in years with a big snowpack, rapid thawing in spring or when it rains on top of snow. 

“Due to climate change, snowmelt-driven flooding is expected to occur earlier in the year due to earlier runoff,” the report said. “Moreover, atmospheric rivers, which have driven much of historical flooding in the region, are expected to intensify under a warming climate.” 

The graphic below shows the importance of atmospheric rivers to extreme precipitation in the Pacific Northwest, especially in winter (see my previous post for more on climate change and atmospheric rivers).

The West’s snowpack will store less water and runoff will change

The maps below depict how warming temperatures and changing precipitation patterns are expected to influence three crucial variables in the Southwest’s water cycle, with the top row of maps showing projections for 2036-2065 and the bottom row showing 2070 to 2099, both relative to the 1991-2020 period.

The leftmost maps show projected changes in soil moisture, a critical factor for agriculture and a host of ecological processes. While drier soils are expected in many parts of the Southwest, and especially in portions of the Four Corners states, other areas are expected to see increases in soil moisture.

The center maps depict projected changes in the maximum volume of snow water equivalent, a measure of the snowpack’s water content. Whereas the soil moisture picture is somewhat muddled, the story for snow is crystal clear: steep declines throughout the region, and especially in California’s mountains. 

The rightmost maps show expected changes to runoff—the water that reaches streams, rivers, lakes, reservoirs and taps. As with soil moisture, the projections vary by location but many of the highest-elevation areas, such as the Sierra Nevada, the Southern Rockies and Utah’s Wasatch Range, are expected to see decreases in runoff.

The report’s co-authors stressed that the interactions between soil moisture, snowpack and runoff are complicated, and there is still considerable uncertainty about future precipitation patterns. With soil moisture, for instance, earlier snowmelt may lead to wetter conditions in spring but drier conditions later in the summer.  

Because the changes will vary across the country, people should “look at results and data and projections for their own region and not necessarily take a message from elsewhere and assume that’s what’s happening where they live,” McAfee said. “Climate change will have different impacts in different places. So the fact that we might be concerned about reduced water supplies in the Colorado River doesn’t necessarily mean we have the same concerns in every river basin.”

In the Colorado River Basin, research has shown that “less snow means more evaporation, and this is because snow is really reflective,” McAfee said. “Anyone who’s ever been out skiing knows this: you can get that reflection up and the nose and chin sunburn, and if the snowpack melts early, the land gets more energy, which makes it possible to evaporate more water from the soils and streams and for the plants to get going earlier.”

One challenge for scientists and water managers is that it’s tough to calculate how much snow is out there. Snow accumulation can vary dramatically on a single run at a ski resort, not only from top to bottom due to thousands of feet of elevation difference, but even from one side of the run to the other due to trees, shading, rocks and wind.  

Another vexing problem is tracing what happens to all those H₂0 molecules after they’ve fallen to earth. 

“There’s still some uncertainties about where the snow is going hydrologically,” Pflug said. 

In recent years, peak snowpack levels in the Rockies that were around normal have translated into below-average streamflows. Some scientists have pointed to deficits in soil moisture as the culprit for the disparity. Others are researching how warming temperatures are impacting sublimation, when snow converts directly into water vapor. A 2023 paper from Colorado State University scientists argued that spring and summer precipitation was important for explaining the discrepancy between snowpack levels and subsequent runoff. 

Here’s how NCA5 sums up the situation for the Colorado River, which supplies some 40 million people in seven U.S. states and Mexico while also irrigating millions of acres of crops:

“Colorado River streamflow over the period 2000–2014 was 19% lower than the 20th-century average, largely due to a reduction in snowfall, less reflected sunlight, and increased evaporation. The period 2000–2021 in the Southwest had the driest soil moisture of any period of the same length in at least the past 1,200 years. While this drought is partially linked to natural climate variability, there is evidence that climate change exacerbated it, because warmer temperatures increase atmospheric ‘thirst’ and dry the soil. Droughts in the region are lasting longer and reflect not a temporary extreme event but a long-term aridification trend—a drier ‘new normal’ occasionally punctuated by periods of extreme wetness consistent with expected increases in precipitation volatility in a warming world.”

Some rural and Indigenous communities are especially vulnerable to the changing water cycle

The consequences of a thinner, less reliable snowpack and changing runoff patterns will be far-reaching, but they will be especially problematic for some rural communities dependent on farming and snow-related recreation. 

The infographic below illustrates some of the downstream effects on agriculture, with snow droughts contributing to the stresses facing the sector and its workers. Reduced snowmelt for irrigation may cause farmers to lose money, generate more dust that harms both farmworkers and the snowpack, and lead to increasing use of dwindling underground aquifers as agriculture shifts from surface water to groundwater.

While the graphic above focuses on agriculture, climate change will also affect the water supply for cities, suburbs and businesses, plus the innumerable species that have evolved to depend on the snowpack and snowmelt. 

Farmers who rely on direct flows from the river may have very senior water rights, but often they lack reservoirs to store the water, so as climate change shifts precipitation from snow to rain and starts the runoff season earlier, these water users—plus fish and other wildlife—face a growing risk of shortages later in the year. 

“For communities that have storage rights, they’re less sensitive to the loss of snowpack if you still are getting precipitation in some form or another,” Payton said. “There are a lot of people and communities in the West who are just living on the edge, and they don’t have the storage, they don’t have the infrastructure to take advantage of when it’s there and are very much dependent on the regime that they’ve been used to.”

NCA5 highlights that “community-based snow-fed irrigation systems in high-elevation watersheds of New Mexico and Colorado, known as acequias, are particularly exposed to the shortfalls in annual snowpack.”

While building more reservoir storage is a potential solution, that strategy has three problems, Harding said. “One is people don’t like reservoirs, except for the people that are going to benefit and use the water. Two is they’re really expensive. And three is we’ve used up most of the really good reservoir sites, so that seems unlikely,” he said. 

Even without the influence of climate change, many Indigenous communities in the West confront major hurdles in securing safe and adequate water supplies (see this 2021 paper for more on incomplete plumbing and poor water quality in U.S. homes). 

The map below shows that many American Indian and Alaska Native homes already face serious problems with their water and sewer systems. At deficiency level 2, a water and sanitation system is in place but it needs upgrades or maintenance, while at level 5, the worst category, “there’s absolutely no water supply, no sanitation system in at all,” said co-author Heather Tanana, a visiting professor of law at the University of California-Irvine, in a webinar.

“As we’re experiencing increased changes in the water cycle, the water quality and quantity impacts are further being exacerbated in part because of aging infrastructure,” Tanana said. “So who is being the most affected? Again, it’s our under-resourced frontline communities.” 

There are two types of snow drought to worry about: dry and warm

The report highlights two kinds of “snow drought” that can afflict the West (this page offers updates on the current status of snow droughts). In a “dry” snow drought, a lack of precipitation diminishes the snowpack. That’s what happened in California’s Sierra Nevada in the 2014/2015 winter, “resulting in the shallowest snow volume ever recorded there,” according to NCA5. 

That same winter, but farther north in Oregon and Washington, there was another snow drought, but this one was a “warm” one. Winter precipitation was 77% to 113% of normal, yet because of higher temperatures, the precipitation shifted from snow to rain, leading to a reduction in the snowpack and higher winter snowmelt, but below-normal flows from April to August. 

The graphic below illustrates the streamflow for two locations: Washington’s Ahtanum Creek and California’s Merced River. In each chart, the black line indicates flows during the 2015 water year (which began October 1, 2014), the gray lines show data from 1952 to 2021 and the dashed line plots the median for that period. The top chart shows that runoff spiked in February and again in March but was then mostly below average during the subsequent warmer months. By contrast, the Merced River’s flow was below normal for nearly the entire runoff season. 

“In Oregon and Washington, irrigated crops—including valuable orchard crops—that depend on direct streamflow diversion water rights failed, but municipal water supplies that relied on storage rights that allow reservoirs to capture winter runoff were sufficient,” according to NCA5. “In California, total water supply was limited, resulting in severe or complete cutbacks to junior water rights and contract holders.” 

The September 2015 photo below from NCA5 shows an apple orchard in the Roza Irrigation District, near Yakima, Washington, suffering the effects of the warm snow drought and reduced irrigation.

Warming will make the landscape “thirstier” in many locations

NCA5’s water chapter discusses a measure known as the “annual climatic water deficit.” In simple language, this metric describes the thirstiness of the landscape. 

“This is a measure that I advocated for because I think it integrates the effects of everything,” said Harding, who defined the deficit as “how much water we’d have to add to the system to fully satisfy the needs of the plants.”

As shown in the maps below, the climatic water deficit is expected to increase by midcentury across much of the nation—and especially in the Southwest. Map “a” shows the average of the projections, while maps “b” and “c” report the average of the wettest and driest 20% of projections. 

The region’s increasing dryness threatens to reinforce snow loss by increasing the amount of dust that lands on the snowpack, thereby accelerating its melting. As a result, NCA5 cautions that “under increasing aridity, agricultural practices such as fallowing and grazing on rangelands will need careful management to avoid increased wind erosion and dust production from exposed soils.”

Adding insult to injury, NCA5 warns that those soils will be more susceptible to blowing around because hotter summers will “degrade protective desert soil crusts formed by communities of algae, bacteria, lichens, fungi, or mosses.” 

Learn more

• Fifth National Climate Assessment
• NCA5 water chapter
• NCA5 Southwest chapter
• NCA5 Northwest chapter
• Interactive Atlas of the Fifth National Climate Assessment
• Webinar on water chapter

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.