Quantifying human contribution to the 2017 earliest summer onset in Korea

Korea experienced the hottest temperature during May 2017 with 1.5°C warmer than the climatology (1981-2010), which was the culmination of four consecutive years of record-breaking May temperature. The hottest May coincides with the earliest summer onset about 8 day earlier than climatology, exerting huge societal impacts. To examine the human contribution to the 2017 extreme May temperature and the earliest summer onset, this study analyzes extreme events under real world and counterfactual world conditions using high-resolution large-ensemble regional climate model (RCM, w@h) and atmospheric global climate model (GCM, CAM5.1) simulations. Results are also compared with those from CMIP5 multiple coupled GCMs with a coarse resolution. The anthropogenic contribution is quantified by using fraction of attributable risk (FAR) and risk ratio (RR). All models consistently show that the probability of occurrence of the extreme events like the 2017 May case increases by two-three times when including anthropogenic forcing (mainly due to greenhouse gas increases). Further, it is suggested that differences in the attribution results among different boundary SSTs (or GCMs) are in part due to the inter-model differences in aerosol cooling effects, supporting previous findings. Our multi-model assessment provides a convincing evidence that human influence has contributed to the stronger and earlier heat wave by better considering inter-model uncertainties. Physical mechanisms responsible for the observed advance of summer onset and its uncertainty need to be further investigated.