9 graphics that visualize EPA’s climate change indicators for snow
The downward trajectory for snow will impact water supplies, wildlife habitat, wildfire activity and outdoor recreation.
Here’s a story I just wrote for The Water Desk, a program at the University of Colorado Boulder’s Center for Environmental Journalism that I co-direct.
I can’t vouch for its shelf life in the Trump administration, but the U.S. Environmental Protection Agency continues to publish a revealing set of indicators of climate change impacts, including 14 connected to snow and ice.
These data sets, many of them visualized with simple maps and time-series charts, show the unmistakable effects of warming and cover a wide range of subjects, including public health, ecosystems and oceans.
Below I share and describe nine graphics that focus on snowfall, snow cover and the American West’s snowpack. All of these measures document concerning trends about this corner of the cryosphere—the frozen portion of the Earth’s surface.
The downward trajectory for snow carries serious consequences, including reduced water supplies, increased wildfire activity, imperilment of species and harm to outdoor recreation.
I’ve been meaning to write about these indicators for a while, but the task took on added urgency when I started to read about scientists and others scrambling to download data and other resources from federal websites before the information was removed by the Trump administration (see stories here and here for more).
On February 26, Trump said during a cabinet meeting that he planned to slash EPA’s staff by 65%, with aides later clarifying that this number referred to budget cuts.
Curious about whether these indicators will continue to be published, let alone updated, I emailed EPA’s press office, but the agency declined to comment for this story.
These climate change indicators have gone through layers of scientific peer review and involve partnerships with more than 50 data contributors, including government agencies, academic institutions and other organizations (see this FAQ on the EPA website for more). I’ve listed sources at the bottom of the post.
“EPA's indicators are designed to help readers understand observed long-term trends related to the causes and effects of climate change. In other words, they provide important evidence of ‘what climate change looks like,’” EPA says. “Together, these indicators present compelling evidence that climate change is happening now in the United States and globally.”
Snowfall
This indicator looks at snow in the contiguous 48 states using two measures: the total amount of snowfall and the fraction of precipitation that falls as snow rather than rain.
EPA notes the many ways in which snowfall is critical, both economically and ecologically: snowmelt provides the bulk of the water supply in many Western communities, snowfall underlies winter recreation activities and snow keeps some species alive. Snowfall is the major driver of the two indicators discussed below: snow cover and the snowpack.
Overall, warming leads to increased evaporation of moisture into the sky and more resulting precipitation, but higher temperatures are causing more of this precipitation to fall as rain, rather than as snow. “Some places, however, could see more snowfall if temperatures rise but still remain below the freezing point, or if storm tracks change,” according to EPA. “Areas near large lakes might also experience more snowfall as lakes remain unfrozen for longer periods, allowing more water to evaporate. In contrast, other areas might experience less snowfall as a result of wintertime droughts.”
As shown in the map below, snowfall from October to May decreased in many parts of the contiguous 48 states from 1930 to 2007, with 57% of stations declining. More than 400 stations are included in the data set, and their average change was a decrease of 0.19% per year. EPA says the stations were selected for their high-quality, long-term data, but it’s not clear why the data ends in 2007; it would be interesting to see updated figures.
The graphic below shows a pronounced shift in the rain/snow mix for precipitation: more than 80% of the stations saw a decrease in the percentage of precipitation falling as snow from 1949 to 2024. This data set runs from November through March, but in some regions, that time period doesn’t capture the entire snow season.
EPA highlights some regional differences in the snowfall trends. In the Pacific Northwest, there has been a decline in both total snowfall and the fraction of precipitation falling as snow. Some areas in the Midwest have seen a decrease primarily due to changes in the snow-to-precipitation ratio, but other locations, such as those near the Great Lakes, have received more snow than in the past.
The process of measuring snow depth is familiar to anyone who has used a yardstick in their backyard, but EPA notes that precisely measuring snowfall is challenging because it’s subject to human error, and snowfall can vary dramatically across short distances due to wind, trees and other factors. Snow gauges may catch less snow than rain because of the wind, and many of the stations in mountainous regions are in lower-elevation valley towns that may not reflect conditions higher up.
Snow cover
One important measure of snow’s prevalence is the amount of land it covers. With this indicator, the depth or water content of the snow doesn’t matter: this metric only concerns whether there is snow or not. Thanks to satellite imagery, scientists can look back many decades to study trends in snow cover; in this case, the time series extends back to 1972.
Changes in both precipitation and temperature affect snow cover. Dry times mean less snow on the ground, but even with normal precipitation levels, the snow cover may be reduced if it’s too warm to snow, causing rain to fall instead.
Climate change is influencing snow cover around the world, and the reverse is also true: the fraction of land covered by snow affects the Earth’s climate because snow is so much more reflective than bare ground or open water. Snow’s high “albedo” means that it exerts a cooling effect, but if snow cover is reduced, the planet’s surface absorbs more energy from the sun.
“On a more local scale, snow cover is important for many plants and animals. For example, some plants and animals rely on a protective blanket of snow to insulate them from sub-freezing winter temperatures,” according to EPA. “Snow cover also keeps the soil moist, so if the snow melts away earlier in the spring, the soil may dry out sooner, which can stress plants and increase the risk of wildfire.”
The chart below shows the average area in North America (minus Greenland) that was covered by snow each year, based on an analysis of weekly maps.
Although the line in the graphic above looks flat, snow cover slightly decreased at a rate of about 2,083 square miles per year.
In the most recent decade (2014-2023), the annual average area covered by snow was 3.25 million square miles, which was about 3% less than during the first 10 years of the time series (1972-1981). That’s nearly 93,000 square miles less, or an area slightly smaller than Michigan, according to EPA.
The graphic below shows how snow cover has changed during each of the four seasons. “Decreases in snow cover have largely occurred in spring and summer, whereas winter snow cover has remained fairly steady over the time period studied and fall snow cover has increased,” according to EPA. “Spring and summer snow cover can have a particularly important influence on water supplies.”
EPA’s final indicator for snow cover concerns the length of the season, as shown in the chart below. This measure ends in 2013 and only covers the contiguous 48 states and Alaska, rather than all of North America.
“Between 1972 and 2013, the U.S. snow cover season became shorter by nearly two weeks, on average,” EPA says. “By far the largest change has taken place in the spring, with the last day of snow shifting earlier by 19 days since 1972. In contrast, the first date of snow cover in the fall has remained relatively unchanged.”
Snowpack
The snowpack—the seasonal accumulation of snowfall—plays a critical role in the West’s water supply and ecosystems. The annual melting of the West’s snowpack fills rivers, reservoirs and irrigation canals, providing vital water to crops, residents and wildlife while also generating hydropower at dams. “In most western river basins, snowpack is a larger component of water storage than human-constructed reservoirs,” EPA notes.
This indicator is based on snow water equivalent (SWE), the key measure of the snowpack’s water content. The SWE at a location is equivalent to the depth of water you’d get by melting a column of snow.
Some trees rely on the snowpack for insulation from freezing temperatures, and EPA says that “fish spawning could be disrupted if changes in snowpack or snowmelt alter the timing and abundance of streamflows” (see EPA’s streamflow indicator for more on this issue). A diminished snowpack can also “accelerate the start of the wildfire season and promote more wildfire activity in the western United States and Alaska,” according to EPA.
The map below shows trends in the American West’s snowpack from 1955 to 2023: red circles indicate declines, blue circles show increases and the circles are sized according to the magnitude of the change. Overall, April SWE declined at 81% of the sites, with an average decrease of about 18%. “Large and consistent decreases in April snowpack have been observed throughout the western United States,” according to EPA. “Decreases have been especially prominent in Washington, Oregon, northern California, and the northern Rockies.”
Although SWE increased at some stations, the overall trend was downward in all 12 states included in the indicator. In the Pacific Northwest region (Idaho, Oregon and Washington), all but four stations saw decreases in the snowpack.
The map above is based on nearly 700 measuring sites, but the graphics below are based on a smaller subset of 340 stations that have daily data stretching back to 1982.
One metric examines changes in the timing of the West’s peak snowpack from 1982 to 2023, as shown in the map below. Red triangles indicate earlier peaks, blue triangles show later peaks and the triangles are sized according to the size of the change.
“Almost 80 percent of sites have experienced a shift toward earlier peak snowpack,” according to EPA. “This earlier trend is especially pronounced in southwestern states like Colorado, New Mexico, and Utah.”
EPA also reports the date at which the West’s snowpack peaked from 1982 to 2023, as illustrated in the chart below. There is considerable year-to-year variability in this measure, but based on the long-term average rate of change, peak snowpack has come earlier by an average of nearly seven days since 1982.
Finally, EPA reports how the snowpack season’s length has changed from 1982 to 2023, as shown in the map below. At about 80% of the sites, the snowpack season decreased (red circles), with an average decline of about 15 days.
Data sources and studies
Snowfall
Kunkel, et al. (2009). Trends in twentieth-century U.S. snowfall using a quality-controlled dataset. Journal of Atmospheric and Oceanic Technology, 26(1), 33–44.
Feng, S., & Hu, Q. (2007). Changes in winter snowfall/precipitation ratio in the contiguous United States. Journal of Geophysical Research: Atmospheres, 112(D15), D15109.
Snow cover
National Oceanic and Atmospheric Administration’s National Environmental Satellite, Data, and Information Service.
Snowpack
Natural Resources Conservation Service’s Water and Climate Center.
Mote et al. (2005). Declining mountain snowpack in western North America. Bulletin of the American Meteorological Society, 86(1), 39–50.
Mote et al. (2018). Dramatic declines in snowpack in the western US. Climate and Atmospheric Science, 1, 2.
Evan, A. T. (2019). A new method to characterize changes in the seasonal cycle of snowpack. Journal of Applied Meteorology and Climatology, 58(1), 131–143.
Petersky, R. and Harpold, A. (2018). Now you see it, now you don’t: A case study of ephemeral snowpacks and soil moisture response in the Great Basin, USA. Hydrology and Earth System Sciences, 22(9), 4891–4906.
Well, yeah. We’re coming out of an Ice Age. Hello???