Climate Change in the Sierra Nevada: California's Water Future
From UCLA’s Center for Climate Science, this report describes the findings on how climate change will impact the Sierra Nevada snowpack, the water cycle in the Sierra, and the larger implications for California's water resources. The project primarily focused on changes in climate averages: average springtime temperatures, snowpack at April 1st, and timing of runoff.
“California’s Sierra Nevada is a critical natural resource, providing more than 60% of the water used by communities, agriculture, and industry across the state. The mountain snowpack accounts for about half of this resource.”
Downscaling methods were used to create higher-resolution simulations from global climate model information for the Sierra Nevada region. The UCLA climate scientists (Dr. Alex Hall and team) established high-resolution projections of future climate in the Sierra - that they created using a special technique they developed called “hybrid” downscaling. Some of the downscaling methods are described as “dynamical,” meaning they use a regional climate model (a high-resolution version of a global climate model) to simulate future physical climate impacts on a local scale. The hybrid approach combines dynamical with statistical downscaling methods.
The research is also unique in taking into account snow albedo feedback - a cycle of amplified warming and snowmelt due to loss of reflectivity - on the melting of of snowpack. They found that Sierra Nevada elevations most vulnerable to climate change are 5,000 - 8,000 feet, which is correlated with where snow albedo feedback is occurring.
The first part of this report focuses on future climate projections for 2081–2100 under a “Business as Usual” scenario in which GHGs continue to rise. A comparative analysis was done on the same climate projections within a scenario where GHG mitigation does occur. Findings suggest that at end-of-century projections under the mitigation scenario, warming still occurs but is less severe.
Future projections under the Business as Usual scenario and the Mitigation scenarios focus on temperature and change in snowpack. Here are some of the Key Points/ findings as described in the report.
- By 2081–2100 under the Business as Usual scenario, temperatures across the Sierra increase by as much as 10 degrees Fahrenheit, depending on the month and elevation, compared with 1981–2000.
- The most severe warming occurs at elevations of 5,000–8,000 feet. This is where snow albedo feedback is occurring.
- Warming sets the stage for snow loss by causing more precipitation to fall as rain instead of snow, and snow to melt faster.
Change in snowpack
- Warming increases the ratio of rainfall to snowfall, and rain runs off right away.
- By 2081–2100 under the Business as Usual scenario, the midpoint of runoff occurs 50 days earlier, on average, than in 1981–2000.
- Earlier, flashier runoff is harder to capture and store than a steady, dependable flow from gradual snowmelt.
- At 2081–2100, marked differences are seen between Business as Usual and a Mitigation scenario of global greenhouse gas emissions cuts.
- In the Mitigation scenario, changes to temperature, snowpack, and runoff timing are half of what they are under Business as Usual.
Publication Date: April 4, 2018
- UCLA Center for Climate Science
- Climate science