Longer summers in the Northern Hemisphere under global warming

Summers have become hotter in recent decades, with earlier onsets in many regions. Here, we explore changes in the summer length under global warming based on the observations and CMIP6 models, identify the influences of external forcing and internal variability, and use CMIP6 models to project variations of the future summer length. Summer is defined as when the daily mean temperature is above the 1961-1990 75th percentile for 5 consecutive days. The summer length significantly increases, and the observed trends show marked regional differences. External forcing is the main contributor to the lengthening of summer, while internal variability may explain the regional differences. Our results reveal that a 1 celcius global surface mean temperature increase is associated with 15 days of the summer length increase during 1961-2014 in the observations, while a 1 celcius temperature increase corresponds to 10 days of the summer length increase in CMIP6 models. CMIP6 models are also used to project the change of the summer length in the future, and it is found that the summer length will continue to increase in the future. Summer will last 142 days (175 days) under SSP2-4.5 (SSP5-8.5) scenario of global warming by the end of the twenty-first century, equivalent to an approximate 1.2 (1.5)-fold increase relative to that of 2014.