Dramatic thinning of Alaskan river ice and its climatic controls

River ice thickness(RIT)directly influences human activities,such as rural transportation and subsistence activities,in addition to ecosystem and hydrology processes in the Arctic.Knowledge of RIT response to the rapid Arctic warming is very limited or essentially lacking.The scientific objective of this study is to investigate changes and variations in RIT and their response to rapid Arctic warming.We used ground-based measurements of 45 river gauge sites from 1961 through 2015 spanning 12 river basins across Alaska.The results indicate that the long-term mean maximum river ice thickness(MRIT)ranged from 40.3±12.7 cm in the southeast to 187.3±31.9 cm in northwest Alaska.MRIT decreased dramatically from 1961 to 2015,on average,at a rate of−0.26±0.17 cm per year,and RIT declined significantly in all months from October through March,and more rapidly in winter than in autumn and spring.The impacts of air temperature and snowfall on MRIT change were analysed,and their relative influences were 74%and 26%,respectively.Specifically,an increase in air temperature was the primary factor contributing to MRIT decrease,while increasing snowfall,and snow on river ice enhanced MRIT decline.Seasonally,snowfall was the primary regulator for thickness change and higher air temperature resulted in RIT declining in autumn,while ice thickness decrease was mostly driven by warming in spring.However,neither air temperature nor snowfall is the primary control factor for declining RIT in winter,and further work needs to be done to detect the reason.