Prepared by the UCLA IoES Center for Climate Science. Key Points

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1 Summary of Significant and Inevitable End-of-21 st -Century Advances in Surface Runoff Timing in California s Sierra Nevada by Marla Schwartz, Alex Hall, Fengpeng Sun, Daniel Walton, and Neil Berg Prepared by the UCLA IoES Center for Climate Science Key Points At the end of this century in California s Sierra Nevada, climate change leads to earlier surface runoff timing. This is due to warmer temperatures, which cause more precipitation to fall as rain instead of snow and snow to melt earlier. Changes in runoff timing are statistically significant in all of the plausible scenarios of future greenhouse gas emissions that the UCLA team assessed. In other words, significant changes in runoff timing are inevitable. Changes are greatest under a business as usual scenario in which greenhouse gas emissions are unchecked and continue to increase. In this scenario, the midpoint of total runoff is projected to occur 50 days earlier, on average, at than it did at In some locations, the advance is as great as 90 days. In a mitigation scenario similar to the outcome expected if the world adheres to the 2015 Paris Agreement, runoff timing changes are still significant but less severe. In this scenario, the midpoint of total runoff occurs 25 days earlier, on average. Changes in runoff timing are important because the Sierra snowpack acts as part of California s water storage system. That water will run off sooner, and won t stay stored as snowpack as long as we re used to, poses a major challenge to water management. Context The Sierra Nevada provides California with more than half of the freshwater we use in agriculture, industry, and our homes. Sierra snowpack is an important part of the State s water system because it stores water in frozen form throughout the winter and early spring, and then releases it through snowmelt. Snowmelt runs off, flows downstream in creeks and rivers, and replenishes our reservoirs throughout the late spring and summer months, when we need water most. But as human emissions of greenhouse gases such as carbon dioxide continue to change the climate, warmer temperatures threaten the status quo. Not only does warming cause a greater share of precipitation to fall as rain instead of snow, but it also causes snow to melt faster and run off earlier. Past studies have found that Sierra Nevada rivers already show earlier pulses of streamflow than they used to because of springtime warming. But to date, studies projecting 1

2 changes to future Sierra runoff timing have been limited, especially at the level of spatial detail water managers need to plan for the future. In this study, researchers at the UCLA IoES Center for Climate Science set out to close this knowledge gap. Methods The best tools for making projections of future climate are global climate models computer models that simulate the climate system. But global climate models are too coarse in resolution to simulate what happens in topographically complex places like the Sierra Nevada. Dr. Hall and his team at the UCLA IoES Center for Climate Science have used an innovative technique to bring global climate model information to very high spatial resolution, creating detailed projections of future climate that can be used to plan for the future. This technique, called hybrid downscaling, involves using a regional climate model, a cousin of global climate models designed to run at high spatial resolution, to run a limited number of simulations representing a few global climate models. From these dynamical simulations, the team developed a statistical model that mimics the regional climate model, and created simulations representing the rest of the latest-generation global climate models. The team then analyzed the set of projections and compared the climate at the end of the 21 st century to that of the end of the 20 th century. They looked at future climate under four different scenarios of greenhouse gas emissions, the same ones used in the Intergovernmental Panel on Climate Change s 5 th Assessment Report. Findings In this study, the focus of the team s analysis was the runoff midpoint (a metric they called R50 ). The runoff midpoint is the point in time by which half of the total water that runs off in a given year has done so. The researchers compared the timing of the runoff midpoint in the future period to that of the historical period, and found that in the future, the runoff midpoint is projected to occur earlier. Under a scenario of increasing greenhouse gas emissions, a business as usual scenario, the runoff midpoint occurs about 50 days earlier at on average across the mountain range, compared with the average. In some locations, the runoff midpoint advances by as much as 90 days. Under a scenario in which global greenhouse gas emissions are curtailed in the coming decades, or a mitigation scenario, the average advance in runoff timing is 25 days and as much as 45 days in some locations. The mitigation scenario is similar to what would occur if the world hews to the 2015 Paris Agreement. How much earlier the runoff midpoint occurs depends on the amount of warming at the location in question. Areas with strong local warming, such as mid-elevation areas ( feet) and the Western side of the Sierra, are projected to have greater advances in runoff timing. This is because warming causes more precipitation to fall as rain instead of snow and causes snow to melt earlier. The highest elevations are less affected; they remain cold enough that changes in runoff are relatively small. 2

3 Warming and advances in runoff timing are greatest at mid-elevation locations because of a phenomenon called snow albedo feedback. Snow has a high albedo, meaning it reflects most sunlight. As the climate warms and melts snow, the snowpack retreats and uncovers land surfaces with a lower albedo, which reflect less sunlight and absorb more heat. This exacerbates local warming, which in turn causes more snow to melt and more heat-absorbing land to be uncovered, which causes more local warming, and so on. This study is the first study of future runoff timing changes in the Sierra Nevada to take snow albedo feedback into account. Key figures Figure 1 illustrates the concept of a shift in the timing of runoff. The circles shown are sized according to the portion of total annual runoff that occurs in each month. During the historical period ( ), half the year s total runoff occurred by early May (see Runoff Midpoint ), on average. At the end of this century under business as usual greenhouse gas emissions, the runoff midpoint is projected to shift to early March, on average. Figure 1: At the end of this century under a business as usual greenhouse gas pathway, the midpoint of total runoff shifts earlier in the year, compared with the end of the 20 th century. 3

4 Figure 2 (adapted from Figure 8 in the published study) shows the change in runoff timing across the mountain range. Runoff timing in mid-elevation areas ( feet) is projected to advance days. Figure 2: End-of-21 st -century change in runoff midpoint under the business as usual scenario, shown on a map of the Sierra Nevada. Implications California s water resources are currently managed assuming the Sierra snowpack will store water until a certain time of year, then gradually release it. This study shows the pattern of snowpack water storage and release will change drastically by the end of this century, particularly if the world stays on its current greenhouse gas emissions pathway. Earlier runoff means earlier flow of water into the state s reservoirs, which is problematic because reservoirs have a finite storage capacity. If reservoirs fill up earlier, it s unclear whether they can maintain enough water to provide water in late summer months when the state needs it most. This question is complicated by the fact that water storage isn t reservoirs only purpose. They also play a role in flood control, holding big pulses of rain and snowmelt back so communities downstream aren t flooded. When runoff due to rain or snowmelt occurs earlier and reservoirs fill up faster, their ability to prevent floods may be compromised. The implications of these findings for water supply and flood control pose great challenges for water management. Next steps This study is the third in a series investigating many aspects of future climate in the Sierra Nevada. The UCLA team recently analyzed warming in the Sierra and the impact of climate change on snowpack during drought, and is currently analyzing changes to snowpack and soil 4

5 moisture. These studies are forthcoming, and will provide insight into climate change impacts not only on water resources but also on wildfire, ecosystems, and recreation. Visit our website to learn more about the Climate Change in the Sierra Nevada project. To stay updated, sign up for our mailing list. This study was made possible by a grant from the Metabolic Studio in association with the Annenberg Foundation, and by funding from the US Department of Energy. Study citation Schwartz M, A Hall, F Sun, DB Walton, and N Berg, 2017: Significant and inevitable end-of-21stcentury advances in surface runoff timing in California's Sierra Nevada. Journal of Hydrometeorology, 18(12): DOI: /JHM-D