The winter and spring snow depths on the eastern Tibetan Plateau (TP) are characterized by interdecadal variability, with a significantly increased trend approximately before 1990 and a decreased trend after 1990. In this paper, we first analyze trends of winter and spring snow depth over the eastern TP in 1960-1989 and 1990-2014, and its relationships with the trends of temperature, precipitation and other atmospheric circulation conditions. Three sets of outputs from the CESM2 model, including (1) total external radiative forcing, (2) well-mixed greenhouse gas radiative forcing, and (3) anthropogenic aerosol radiative forcing, are then used to assess the respective contributions of radiative forcing and North Atlantic Oscillation (NAO) to the trends of snow depth in the eastern TP during the winter-spring period of 1960-1989 and 1990-2014. Observational analyses indicate that the post-1990 snow reduction 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 increased trends of winter and spring snow depth over the eastern TP. The NAO instead contributed about 49% of increased trends of winter snow depth over the eastern TP, but little of increased trends of spring snow depth over the eastern TP. Comparison of observed and modelled trends in other atmospheric variables suggests that all radiative forcing led to significant warming and reduced snowfall on the eastern TP from 1990-2014, and contributed significantly to the continued reduction in both winter and spring snow depths in the later period, explaining 29% and 82% of the observed winter snow depth reduction in the later period. Both greenhouse gas forcing and aerosol radiative forcing contribute significantly to plateau warming and together contribute to the reduction in snow depth in the eastern TP in winter and spring during 1990-2014. The decreasing trend in the NAO index can explain 22% of the reduction in snow depth in the eastern TP in winter from 1990-2014, but contributes slightly to the reduction in snow depth in spring. With future increases in anthropogenic greenhouse gases and decreases in aerosol emissions, warming in the eastern TP is expected to intensify further and snow depth in this area will continue to decrease.