The Climate.gov article Climate Change: Spring Snow Cover by LuAnn Dahlman and Rebecca Lindsey (2/14/2020) provides the answer as seen in their graph here.
This change is another example of a feedback loop.
About one-third of Earth’s land surface is covered by snow for some part of the year. The bright white covering affects global conditions by reflecting solar energy away from surfaces that would otherwise absorb it. Therefore, the earlier decrease in snow cover increases the amount of sunlight absorbed by Earth, and in turn, surface temperatures.
The article includes the data for the graph shown here.
