California Snowpack: Annotated Climate Science Bibliography
Fingerprints of Global Warming in California’s Declining Snowpack
Annotated bibliography of peer-reviewed detection and attribution literature
The detection and attribution science on California's declining snowpack has advanced dramatically in the past few years alone. Long-term climate warming is strongly correlated with declining snow pack in the western US. In addition, shifts in stream flow throughout the American West have been driven by decreased spring accumulation and/or increased spring melt. The fingerprint of global warming has been found in these trends at both broad scale and in California in particular.
Below is an annotated listing of the peer-reviewed research detection and attribution reports identifying the fingerprint of global warming in declining snowpack. For more information go to the Climate Signals event page for the 2018 Western snow drought.
Mussleman et al, 2017: Snowmelt response to simulated warming across a large elevation gradient, southern Sierra Nevada, California. The Cryosphere.
Finds that in a warmer climate, the fraction of annual meltwater produced at high melt rates in mountainous areas is projected to decline due to a contraction of the snow-cover season, causing melt to occur earlier and under lower energy conditions. Presents a model sensitivity study of snowmelt response to warming across a 3600 m elevation gradient in the southern Sierra Nevada, USA. A snow model was run for three distinct years and verified against extensive ground observations. The total annual snow water volume exhibited linear reductions (−10 % °C−1) consistent with previous studies. However, the sensitivity of snowmelt rates to successive degrees of warming varied nonlinearly with elevation. Middle elevations and years with more snowfall were prone to the largest reductions in snowmelt rates, with lesser changes simulated at higher elevations. Importantly, simulated warming causes extreme daily snowmelt (99th percentiles) to increase in spatial extent and intensity, and shift from spring to winter.
Hatchett et al, 2017: Winter Snow Level Rise in the Northern Sierra Nevada from 2008 to 2017. Water.
This study reports that warming temperatures have decreased snowpack in the Sierra Nevada and that the snow line—the elevation where rain changes over to snow—in California's northern Sierra Nevada moved uphill by as much as 236 vertical feet per year between 2008 and 2017. Researchers identified a statistically significant positive (negative) trend in winter snow levels (snow fractions) in the northern Sierra Nevada during the winters between WY2008 and 2017.
Berg and Hall, 2017: Anthropogenic warming impacts on California snowpack during drought. Geophysical Research Letters.
Simulates Sierra Nevada climate and snowpack during the period of extreme drought from 2011 to 2015. Compares first simulation to another that is identical except for the removal of the twentieth century anthropogenic warming. Results show that anthropogenic warming reduced average snowpack levels by 25%, with middle-to-low elevations experiencing reductions between 26 and 43%. In terms of event frequency, return periods associated with anomalies in 4 year 1 April snow water equivalent are estimated to have doubled, and possibly quadrupled, due to past warming. Concludes that past human emissions of greenhouse gases are already negatively impacting statewide water resources during drought
Mote et al, 2016: Perspectives on the causes of exceptionally low 2015 snowpack in the western United States. Geophysical Research Letters.
States that over 80 percent of measurement sites west of 115°W experienced record low snowpack in 2015. Estimates a return period of 400–1000 years for California's snowpack under the questionable assumption of stationarity. Hydrologic modeling supports the conclusion that 2015 was the most severe on record by a wide margin. Shows that both human influence and sea surface temperature (SST) anomalies contributed strongly to the risk of snow drought in Oregon and Washington: the contribution of SST anomalies was about twice that of human influence. Finds that SSTs and humans appear to have played a smaller role in creating California's snow drought
Concludes that in all three states, the anthropogenic effect on temperature exacerbated the snow drought.
Diffenbaugh et al, 2017: Quantifying the influence of global warming on unprecedented extreme climate events. Proceedings of the National Academy of Sciences
Analyzes weather stations worldwide and calculates the global trend in warming that contributed to the record for hottest day of the year in at least 82% of the records for hottest day over the 1961-2010 period, including contributing at least 30% of the magnitude over large areas of Europe and eastern Asia, and increasing the probability by at least a factor of 2.5 over most of Europe and parts of western North America and eastern Asia.. Spots climate change's influence 57 percent of the time in records for lowest rainfall in a year and 41 percent of the time in records for most rain in a 5-day period. Formally identifies the fingerprint of anthropogenic warming in the observed trend of increasing heat extremes over the western U.S. and California. Applies four attribution metrics to four climate variables at each available point on a global grid. Results suggest that scientifically durable operational attribution is possible but they also highlight the importance of carefully diagnosing and testing the physical causes of individual events.
Diffenbaugh et al, 2015: Anthropogenic warming has increased drought risk in California. Proceedings of the National Academy of Sciences.
States that the California drought began in 2012 and now includes the lowest calendar-year and 12-mo precipitation, the highest annual temperature, and the most extreme drought indicators on record. Summarizes that extremely warm and dry conditions have led to acute water shortages, groundwater overdraft, critically low streamflow, and enhanced wildfire risk. Analyzes historical climate observations from California and finds that precipitation deficits in California were more than twice as likely to yield drought years if they occurred when conditions were warm. Finds that although there has not been a substantial change in the probability of either negative or moderately negative precipitation anomalies in recent decades, the occurrence of drought years has been greater in the past two decades than in the preceding century. Finds the probability that precipitation deficits co-occur with warm conditions and the probability that precipitation deficits produce drought have both increased. Reveals using climate model experiments with and without anthropogenic forcings that human activities have increased the probability that dry precipitation years are also warm. Reveals using a large ensemble of climate model realizations that additional global warming over the next few decades is very likely to create approximately 100% probability that any annual-scale dry period is also extremely warm. Concludes that anthropogenic warming is increasing the probability of co-occurring warm–dry conditions like those that have created the acute human and ecosystem impacts associated with the “exceptional” 2012–2014 drought in California.
Cordero et al, 2011: The identification of distinct patterns in California temperature trends. Climatic Change.
Analyzes regional changes in California surface temperatures over the last 80 years. Analysis shows a distinctly different spatial and temporal patterns in trends of maximum temperature (Tmax) compared to trends of minimum temperature (Tmin). Finds that for trends computed between 1918 and 2006, the rate of warming in Tmin is greater than that of Tmax. Finds that trends computed since 1970 show an amplified warming rate compared to trends computed from 1918, and the rate of warming is comparable between Tmin and Tmax. Compares annual trends across regions for two time periods, 1918-2006 and 1970-2006 and identifies an accelerated warming trend from 1970-2006. Finds statewide maximum temperature trends between 1970-2006 were +0.27°C per decade—more than three times as large as the trend between 1918-2006, of +0.07°C per decade
Western U.S. Studies
Pierce et al, 2008: Attribution of Declining Western U.S. Snowpack to Human Effects. Journal of Climate.
Observations show snowpack has declined across much of the western United States over the period 1950–99. Performs a formal model-based detection and attribution (D–A) study of these reductions. States that the detection variable is the ratio of 1 April snow water equivalent (SWE) to water-year-to-date precipitation (P), chosen to reduce the effect of P variability on the results. Obtains estimates of natural internal climate variability from 1600 years of two control simulations performed with fully coupled ocean–atmosphere climate models. Takes estimates of the SWE/P response to anthropogenic greenhouse gases, ozone, and some aerosols are taken from multiple-member ensembles of perturbation experiments run with two models. The D–A shows the observations and anthropogenically forced models have greater SWE/P reductions than can be explained by natural internal climate variability alone. Finds that model-estimated effects of changes in solar and volcanic forcing likewise do not explain the SWE/P reductions. Finds that the mean model estimate is that about half of the SWE/P reductions observed in the west from 1950 to 1999 are the result of climate changes forced by anthropogenic greenhouse gases, ozone, and aerosols.
Barnett et al, 2008: Human-Induced Changes in the Hydrology of the Western United States. Science.
Observations have shown that the hydrological cycle of the western United States changed significantly over the last half of the 20th century. Presents a regional, multivariable climate change detection and attribution study, using a high-resolution hydrologic model forced by global climate models, focusing on the changes that have already affected this primarily arid region with a large and growing population. Results show that up to 60% of the climate-related trends of river flow, winter air temperature, and snow pack between 1950 and 1999 are human-induced. These results are robust to perturbation of study variates and methods. Concludes that the results portend, in conjunction with previous work, a coming crisis in water supply for the western United States.
Hildago et al, 2009: Detection and Attribution of Streamflow Timing Changes to Climate Change in the Western United States. Journal of Climate.
Applies formal detection and attribution techniques to investigate the nature of observed shifts in the timing of streamflow in the western United States. States that previous studies have shown that the snow hydrology of the western United States has changed in the second half of the twentieth century, manifesting themselves in the form of more rain and less snow, in reductions in the snow water contents, and in earlier snowmelt and associated advances in streamflow “center” timing (the day in the “water-year” on average when half the water-year flow at a point has passed). Uses observations together with a set of global climate model simulations and a hydrologic model (applied to three major hydrological regions of the western United States—the California region, the upper Colorado River basin, and the Columbia River basin). Finds that the observed trends toward earlier “center” timing of snowmelt-driven streamflows in the western United States since 1950 are detectably different from natural variability (significant at the p < 0.05 level). Finds that the nonnatural parts of these changes can be attributed confidently to climate changes induced by anthropogenic greenhouse gases, aerosols, ozone, and land use. Finds that the signal from the Columbia dominates the analysis, and it is the only basin that showed a detectable signal when the analysis was performed on individual basins. Notes that although climate change is an important signal, other climatic processes have also contributed to the hydrologic variability of large basins in the western United States.