Detection and attribution of the summer length changes in the Northern Hemisphere

In recent decades, summer has become hotter and longer in many regions. However, no optimal fingerprinting detection study has been performed in the effect of human activities on summer length. Here we conduct a detection and attribution analysis of the external forcing factors affecting the summer length in the Northern Hemisphere during 1961-2014 based on Berkeley Earth observations and the Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations. The summer length increases by 15.06 days in 2000-2014 relative to 1961-1975. From the decadal change trend, the change in summer length is mainly affected by the Greenhouse gas, aerosol and natural forcing in the 1960s. After the 1970s, the greenhouse gas forcing then gradually become the dominant factor. The regularized optimal fingerprinting method is used to detect and attribute different external forcing factors affecting the summer length changes. Results show that the effect of anthropogenic forcing can be robustly detected and separated from the response to the natural forcing in a two-signal analysis. The increase in summer length can be largely attributed to anthropogenic forcing, while natural forcing has little contribution. In a three-signal analysis, the effect of greenhouse gas can be also clearly detected and successfully attributed. Greenhouse gas forcing is the dominant anthropogenic factor and causes an approximate 15-day increase in the summer length.