Multimodel detection and attribution of changes in warm and cold spell durations

The duration of warm and cold spells is measured by the persistence of instances when daily temperatures are greatly above or below their normal values. These spells represent a prolonged period of times when daily temperatures are extreme and can potentially be connected to climate impacts in the agricultural, health, energy and other sectors. This study aims to determine evidence of responses in the durations of warm (WSDI) and cold (CSDI) spells to forcings external to the climate system. We consider the globe, the six continents and China during the period from 1958-2010 in this analysis. Here we compare the observed duration indices with those derived from simulated daily temperature by climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5), using an optimal fingerprint method. The results show that, averaged over the global lands, the WSDI has substantially increased by 15 days while the CSDI has decreased by 3 days over the 53 year period. The simulated changes are generally consistent with the observations; models, however, overestimate the observed changes in the WSDI in five of the six continental regions. We consider a signal is detected if it is significant at the 10% level. At the global scale, human influences on both warm and cold spell durations are detected. At the continental scale, an anthropogenic signal is detected in the warm spell durations of most continents. The human influence on cold spell duration is detectable only in the Asian and European continents. In China, human influence can clearly be detected in both the warm and cold spell durations. Responses to natural forcings are generally not detected at the continental or smaller spatial scales.