Lengthening Summer in the Northern Hemisphere with the Declining Arctic Sea Ice

Satellite records show that autumn Arctic sea ice extent has a downward trend since 1979, along with the frequent occurrence of heat wave events over the Northern Hemisphere. The highest anomalies of maximum and mean air temperature (AT) reach 13.83°C (in summer 2023) and 15.65°C (in 2024). By defining the length of summer using daily surface air temperature, the summer length increases in both polar regions (33 days) and mid-latitude regions (43 days) from 1982 to 2024, particularly in the Black Sea, Eastern Asia, central North America, and the Arctic. The beginning (end) date of summer in 2024 is 17 (16), 29 (15) days ahead (late) of the date in 1982 in the Arctic and mid-latitude, respectively. The highest spatial correlation coefficient of summer length and sea ice reaches −0.92 in the Kara Sea. Summer length increases by 6.8 days for every 1 Mkm2 decrease in Arctic sea ice extent. The regression and correlation coefficients of AT, specific humidity, and geopotential height with the autumn Arctic sea ice extent are also calculated from spring to autumn, which are all negatively correlated, and the lowest value reaches −0.94. The changing of the atmospheric variables is consistent with the increasing summer length. By the 17 models from CMIP6, the trends of summer length are showing in different values at SSP1.2.6, 0.0048; SSP2.4.5, 0.0143; and SSP5.8.5, 0.0301 during 2015–2100, which are low of Global (0.0298) and polar (0.0392) temperature from 1948 to 2024.