Changes in lengths of the four seasons over the drylands in the northern hemisphere midlatitudes

Abstract

As a result of global warming, the lengths of the four seasons, which are always taken as constant values, have experienced significant variations with rising temperature. Such changes play different roles with regard to regional climate change, with the most significant effect on drylands. To guarantee local crop yields and preserve ecosystems, identification of the changes of the four seasons in drylands is important. Our results show that, relative to humid lands, changing trends in lengths of spring, summer, and autumn were particularly enhanced in drylands of the Northern Hemisphere midlatitudes during 1951-2020. In this period, summer length has increased by 0.51 days per year, while spring and autumn lengths have both contracted by 0.14 days per year. However, the enhanced changes in drylands did not appear in winter length. The winter has shortened by 0.23 days per year in drylands. Such changes of spring, summer, and autumn in drylands are dominated by internal variability over the entire study period, with a stronger external forcing effect on drylands than on humid lands. In drylands, the external forcing contributed to the lengths of spring, summer, and autumn by 30.1%, 42.2%, and 29.4%, respectively. The external forcing has become an increasingly important component since 1990, with the ability to dominate all seasons in drylands after 2010. Nevertheless, only 1 of the 16 models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) used in this study can capture the enhanced changes in the lengths of spring, summer, and autumn in drylands. Further investigation on the local effects of changes in seasons on agriculture and ecosystem would be needed, especially for the fragile regions.