By Adrienne Marshall
As an environmental scientist, I’ve done plenty of hiking in the western U.S., always with a map, water bottle and list of water sources. In dry areas it’s always smart to ration water until you get to a new source. Sometimes a stream has dried up for the season, or a pond is too scummy to drink from, so your supply has to stretch further than planned. On one memorable hike, I found that a water source was dry. The next one, three miles later, was dry too. And the one after that had a dead bear carcass in it. While one dry water source was tolerable, several in a row created a serious problem.
Something similar is happening to snow resources in the western United States. Scientists have long known that the warming temperatures associated with climate change are diminishing the region’s snowpack, with more precipitation falling as rain, rather than snow. That’s a problem because snowpack is a critical resource, acting as a natural reservoir that stores winter precipitation. Are we likely to face several low snowpack years in a row?
In a study published last year, my colleagues John Abatzoglou, Timothy Link, Christopher Tennant and I analyze year-to-year variations of future snowpack to see how frequently western states can expect multiple years in a row of snow drought, or very low snow.
Multiyear snow droughts are akin to drawing down a bank account. What could this mean for agricultural production, or for natural resources in our region? For example, lower snow years typically have longer summer periods with low soil moisture. Trees and other plants may be able to survive these stresses for one year, but longer stretches could lead to increases in forest mortality. These periods also test western reservoirs, many of which are managed for dual purposes: storing spring runoff for times of high water demand, like the irrigation season, and holding space for potential floodwaters. The amount of space allocated to storage versus flood control varies by time of year. Water managers may need to update their rules to account for higher chances of snow drought or changes in the timing of snowmelt runoff.
In our study we defined snow droughts as years with snowpack low enough to have historically occurred only one out of every four years or less. Such events occurred recently in the Cascades in 2014-2015. To project future snows we used a publicly available dataset (described in more detail by our colleagues at the Climate Impacts Research Consortium). In this dataset, future projections from 10 global climate models were downscaled and used as inputs to a hydrologic model that estimates daily snow accumulation and melt. We used the results from this hydrologic model to calculate changes in snowpack in future conditions, relative to historical conditions.
Today, back-to-back snow droughts in the western U.S. occur around 7% of the time. By mid-century, if greenhouse gas emissions continue to increase as described in the high emissions scenario, our results predict that multiyear snow droughts will occur in 42% of years, on average. If you focus in on the Pacific Northwest (Washington, Oregon, and Idaho; Figure 1) specifically, back-to-back snow droughts also occurred about 7% of the time historically, but increase to 56% of years in the mid-century high emissions scenario. We also found that peak snowpack is projected to decline and become less variable in a warming climate across much of the mountainous West. This will mean there will be fewer very high-snow years to offset the impacts of low-snow years.
We also explored what would happen to the timing of peak snowpack. As the climate warms snow is melting earlier, which leads to earlier spring runoff and less water available in summer. Our results suggest that in many places the date of peak snowpack will also become more variable from year to year. For example, on average across the Pacific Northwest, the middle 50% of the years (that is, neither the years with earliest date of peak snowpack, nor the years with the latest date of peak snowpack) historically hit their peak across a range of 25 days in March to early April. This increases to a range of 30 days, spanning early February to early March, in the future scenario we considered. Remember that these numbers are averages across the Pacific Northwest; if you’d like to know more about how these results differ for different regions, climate models, or variables of interest, we developed an interactive tool that allows you to explore these data on your own. It’s important to note that our results are based on a future in which the world continues to rely on fossil fuels. Reducing greenhouse gas emissions would limit the impacts on western snowpack that we project.
On the hike where all of my water sources were dry, I was saved by a kind stranger. The trail intersected a road, and a passing driver gave me some water. Global climate change won’t be solved so easily: addressing these issues will require major coordinated efforts to limit future warming and manage Earth’s natural resources strategically to provide for society’s needs and environmental conservation.
This article originally appeared in the Conversation in August 2019, titled “Climate change will mean more multiyear snow droughts in the West.” It has been modified, with permission, for AgClimate.net.
More information and the downscaled future projections we used, developed as part of the Integrated Scenarios project, are available here: https://climate.northwestknowledge.net/IntegratedScenarios/
Marshall, A. M., Abatzoglou, J. T., Link, T. E., & Tennant, C. J. (2019). Projected changes in interannual variability of peak snowpack amount and timing in the Western United States. Geophysical Research Letters, 46, 8882–8892. https://doi.org/10.1029/2019GL083770