Historical evolution and future trend of Northern Hemisphere snow cover in CMIP5 and CMIP6 models

Historical snow cover over the Northern Hemisphere was examined in the satellite-based NOAA-CDR data for the period of 1970-2019. Observed annual snow cover fraction (SNF) has reduced over most areas by up to 2%/decade, while annual snow cover area (SCA) has reduced by 2 x 10(5) km(2)/decade. However, SCA in the October-December season has increased by about 5 x 10(5) km(2)/decade. CMIP5 and CMIP6 historical experiments were validated against the NOAA-CDR data. Snow cover was generally well simulated in both CMIPs, with CMIP6 models performing better. The biases in SCA reduction were larger and smaller during summer and winter, respectively. The observed increase of October-November-December SCA in the 2000s was not reproduced. Climate projections of future snow cover were evaluated in CMIP6. SNF is projected to decrease in the next 80 years, under all four scenarios evaluated (SSP126, SSP245, SSP370 and SSP585). The higher the greenhouse emissions in the shared socio-economic pathways, the faster the reduction. Under the SSP585 scenario, the rate of SCA reduction is projected to exceed -1.2 x 10(6) km(2)/decade. By 2081-2100, annual (January-March) SCA is projected to decrease by more than 30% (20%). Under the SSP126 scenario, annual (January-March) SCA is projected to only reduce by about 10% (5%) relative 1995-2014 values. The reduction of Greenhouse gas emissions is critical to controlling the loss of snow cover; future snow cover only stabilizes under the SSP126 scenario, but continue to decrease under the other three scenarios.