The start of frozen dates over northern permafrost regions with the changing climate - Li, Jialing (Nanjing University. International Institute for Earth System Sciences) ; Wu, Chaoyang (Chinese Academy of Sciences. Institute of Geographic Sciences and Natural Resources Research) ; Peñuelas, Josep (Centre de Recerca Ecològica i d'Aplicacions Forestals) ; Ran, Youhua (Chinese Academy of Sciences. Northwest Institute of Eco-Environment and Resources) ; Zhang, Yongguang (Nanjing University. International Institute for Earth System Science)