Anthropogenic Radiative Forcing Has Accelerated the Melting Rates of Winter and Spring Snow Depth in the Eastern Qinghai–Xizang Plateau during Recent Decades

The winter and spring snow depths on the eastern Qinghai–Xizang Plateau (QXP) and surrounding areas exhibit interdecadal variability, demonstrating a significantly increased trend approximately before 1990 and a decreased trend thereafter. In this paper, we first analyze the trends of the winter and spring snow depths over the eastern QXP during 1960–1989 and 1990–2014 and their relationships with the trends of temperature, precipitation, and other atmospheric circulation conditions. Three sets of outputs from the Community Earth System Model Version 2 (CESM2) model, including (1) the total external radiative forcing, (2) the well-mixed greenhouse gas radiative forcing, and (3) the anthropogenic aerosol radiative forcing, are then used to assess the respective contributions of radiative forcing and North Atlantic Oscillation (NAO) to the snow depth trends in the eastern QXP in the winter and spring during 1960–1989 and 1990–2014. Observational analyses indicate that a snow reduction post-1990 was mainly caused by increasing surface air temperature and decreasing snowfall, while spring snow depth was mainly caused by increasing surface air temperature. During 1960–1989, all radiative forcing, greenhouse gas forcing only, or aerosol forcing only contributed little to the increased trends of the winter and spring snow depths over the eastern QXP. The NAO instead contributed to approximately 49% of the increased trends of the winter snow depth over the eastern QXP, but it had little impact on the increased spring snow depth trends over the eastern QXP. Comparison of observed and modeled trends for other atmospheric variables suggests that all radiative forcing led to significant warming and reduced snowfall on the eastern QXP from 1990 to 2014. This warming contributed significantly to the continued reduction in the winter and spring snow depths in the later period, explaining the 29% and 82% of the observed winter snow depth reduction in the later period. Greenhouse gas forcing and aerosol radiative forcing contributed to plateau warming significantly and the reduction in the snow depth in the eastern QXP in winter and spring during 1990–2014. The decreasing trend observed in the NAO index can explain the 22% reduction in the snow depth noted in the eastern QXP in winter from 1990–2014, but it contributed slightly to the reduction in the snow depth in spring. With future increases in anthropogenic greenhouse gases and decreases in aerosol emissions, warming in the eastern QXP is expected to intensify further, and the snow depth in this area will continue to decrease.