Lengthening of summer season over the Northern Hemisphere under 1.5 °C and 2.0 °C global warming

Summer season has lengthened substantially across Northern Hemisphere (NH) land over the past decades, which has been attributed to anthropogenic greenhouse gas increases. This study examines additional future changes in summer season onset and withdrawal under 1.5 °C and 2.0 °C global warming conditions using multiple atmospheric global climate model (AGCM) large-ensemble simulations from the Half a degree Additional warming, Prognosis and Projected Impacts project. Five AGCMs provide more than 100 runs of 10 year length for three experiments: All-Hist (current decade: 2006–2015), Plus15, and Plus20 (1.5 °C and 2.0 °C above pre-industrial condition, respectively). Results show that with 1.5 °C and 2.0 °C warmer conditions summer season will become longer by a few days to weeks over entire NH extratropical lands, with slightly larger contributions by delay in withdrawal due to stronger warming in late summer. Stronger changes are observed more in lower latitudes than higher latitudes and largest expansion (up to three weeks) is found over East Asia and the Mediterranean. Associated changes in summer-like day frequency is further analyzed focusing on the extended summer edges. The hot days occur more frequently in lower latitudes including East Asia, USA and Mediterranean, in accord with largest summer season lengthening. Further, difference between Plus15 and Plus20 experiments indicates that summer season lengthening and associated increases in hot days can be reduced significantly if warming is limited to 1.5 °C. Overall, similar results are obtained from Coupled Model Intercomparison Project phase 5 coupled GCM simulations (based on RCP8.5 scenario experiments), suggesting a weak influence of air-sea coupling on summer season timing changes.