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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.

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Data related to this paper can be downloaded from the following: Berkeley Earth (; CMIP6 (


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Thanks to Professor Aurelien Ribes of the French National Centre for Meteorological Research for providing the regularized optimal fingerprinting calculation program ( The numerical simulation is supported by the High Performance Computing Division in the South China Sea Institute of Oceanology.


This study is supported by the National Natural Science Foundation of China (42192564, 41731173 and 42005036), the National Key R&D Program of China (2019YFA0606701), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB42000000 and XDA20060502), Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0306), the Innovation Academy of South China Sea Ecology and Environmental Engineering, the Chinese Academy of Sciences (ISEE2021ZD01), the Science and Technology Program of Guangzhou (202102021188), and the Independent Research Project Program of State Key Laboratory of Tropical Oceanography (LTOZZ2102).

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C.W. conceived and supervised the study; W.L. performed the analyses and generated the figures. W.L. and C.W. wrote and reviewed the manuscript.

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Correspondence to Chunzai Wang.

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Lin, W., Wang, C. Detection and attribution of the summer length changes in the Northern Hemisphere. Clim Dyn 60, 3801–3812 (2023).

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