Volcanic Effects on Climate
Our error estimation indicates that there is a possibility that a few (primarily sixteenth century) years exceeded the 2015 low, but the estimated return interval for the 2015 SWE value — as calculated based on a generalized extreme value (GEV) distribution (Supplementary Information) — is 3,100 years and confirms its exceptional character. GEV-estimated return intervals can have large confidence intervals (Supplementary Fig. 2), but the 2015 SWE value exceeds the 95% confidence interval for a 500-year return period (Supplementary Fig. 3). In comparison, the previous lowest SWE reading (in 1977) exceeds the 95% confidence interval for only a 60-year return period. We also find that the 2015 SWE value is strongly exceptional — exceeding the 95% confidence interval for a 1,000-year return period — at low-elevation Sierra Nevada sites where winter temperature has strong control over SWE9 , but less so at high-elevation sites, where it exceeds the 95% confidence interval for only a 95-year return period (Supplementary Information and Supplementary Fig. 2). The 2015 record low snowpack coincides with record high California January–March temperatures10 and highlights the modulating role of temperature extremes in Californian drought severity. Snowpack lows, among other drought metrics, are driven by the co-occurrence of precipitation deficits and high temperature extremes11, and we find that the exacerbating effect of warm winter temperatures12 is stronger at low than at high Sierra Nevada elevations. Anthropogenic warming is projected to further increase the probability of severe drought events13, advance the timing of spring snowmelt and increase rain-to-snow ratios14. The ongoing and projected role of temperature in the amount and duration of California’s primary natural water storage system thus foreshadows major future impacts on the state’s water supplies.