The Changes Of The Climatic Four Seasons In The Midlatitudes Under Global Warming

Under global warming,the increasing evidence suggests that the onset,withdrawal and length of the four seasons have changed significantly,which have serious impacts on phenology,agricultural production,human health,social economy and so on.Therefore,the cycles of the Earth's temperature in the midlatitudes with four distinct seasons are no longer appropriately defined by dividing a year into four seasons with constant length based on traditional astronomical or meteorological definitions.However,existing studies on seasons' variation have mainly focused on the Northern Hemisphere and have mostly focused on changes of a single season or in regional scales over historical periods.A global-midlatitudes response of onset and length of the four seasons in the past and future remains unknown.In our study,based on near-surface air temperature from ERA5 during 1951-2020 and outputs of 16 CMIP6 models under historical simulations and future scenarios,we first used the temperature threshold method to redivide the four seasons in the Northern Hemisphere midlatitudes,and quantify the past(1951-2020)and future(2021-2100)changes of the onset and length of the four seasons.In addition,considering the difference in warming between arid and humid regions during global warming,we compared the changes in length of the four seasons in drylands and humid lands in the Northern Hemisphere midlatitudes.Meanwhile,compared with the Northern Hemisphere midlatitudes,the Southern Hemisphere midlatitudes are dominated by arid regions with fragile ecological environments,and seasons' changes can cause serious hazards to local agriculture and increase the risk of wildfire disasters.Moreover,the seasonality of the annual temperature cycle in this region is weaker,and the changes of the four seasons may be different from those in the Northern Hemisphere midlatitudes.Therefore,we have also quantified the past and future evolution of the onset and length of the four seasons in the Southern Hemisphere midlatitudes,with the aim of supplementing a clear examination of the four seasons timing and lengths in global scale and providing scientific basis for decision-making and management of season-related activities.The main conclusions are as follows:(1)The length of the four seasons in the Northern Hemisphere midlatitudes was characterized by longer summer and shorter spring,autumn and winter,accompanied by earlier spring and summer and later autumn and winter.The seasons' changes in the Mediterranean region,the Qinghai-Tibet Plateau and its western regions were particularly prominent.The changes in timing and length can be mainly attributed to the increase in greenhouse gases.The changes in the four seasons will be exacerbated in the future with radiative forcing rising.Longer and longer summer and shorter and shorter winter will be the new normal in the 21 st century.Under the high emission scenario(SSP5-8.5),summer is projected to last half a year,but winter less than one month by the end of this century.(2)In the Northern Hemisphere midlatitudes,the lengthening variations in summer and the shortening variations in spring and autumn were greater in drylands than that in the humid regions,while the shortening variations in winter were greater in the humid regions.During the period of 1951-2020,the contribution of drylands to the length trends of the four seasons for the entire Northern Hemisphere midlatitudes was 71.7%,57.5%,64.5%,and 48.1%,respectively.By comparing the contributions of internal variability and external forcing to the length of the four seasons,it was found that internal variability dominated the length of the four seasons,but the contribution of external forcing cannot be ignored.The contribution of external forcing to the length of spring,autumn and winter in drylands was higher than that in humid lands.In addition,all 16 models from CMIP6 failed to capture the enhanced changes in the lengths of spring,summer,and autumn and greater contribution of external forcing in drylands.(3)Compared with the Northern Hemisphere midlatitudes,the Southern Hemisphere midlatitudes showed slower warming trend and weaker seasonality.The four seasons in the Southern Hemisphere midlatitudes were mainly characterized by changes in spring and summer lengthening and winter shortening,while the trend in autumn length is not significant.Consistent with the Northern Hemisphere,spring and summer started earlier,while autumn and winter started later.The changes of seasons will be further intensified in the future,especially in summer and winter.By the end of this century,summer will last more than half a year covering a period from October to April under SSP5-8.5.Even under SSP2-4.5,summer will last nearly half a year in 2100.More seriously,winter is disappearing gradually in the future,which means the traditional temperature cycle with the four seasons in some areas need reconstructing.