Snow increases the albedo of the ground it covers. Forests are estimated to have an albedo of 0.1. The albedo of snow on bare ground is estimated at 0.8-0.9, and on forested areas, 0.3 (the presence of trees dramatically lowers the snow's continuity). So in wintertime, where snow is on the ground, far more solar energy is reflected back into space than in summertime. (Though in their respective wintertimes, no solar energy reaches the polar latitudes. It might be a several-months' twilight near 66.5°N/S, but closer to the pole it's dark.)
Snow, like ice, transfers heat very slowly, so it serves as
an insulator for the ground beneath. It also prevents evaporation of moisture
in the soil. These are less a factor in global warming but are extremely
important for the ecosystem, protecting animals, plants and other life from dying
due to extreme cold (even when they can survive freezing). And where large
numbers of plants exist, the insulative effect of snow is important to their
life cycle.
Snow is also a critical part of the water cycle, since springtime thaw feeds streams, rivers, ponds and lakes. In this way snow is less a driver of climate change than, as with glaciers, a valuable, if noisy (i.e. large year-to-year fluctuations which can obscure a longer trend) indicator of the state of the climate. It also plays a feedback role. Decreased snowfall leads to drier soil in spring, as there is less snow to thaw. Over time this diminishes the health of forests and, combined with other influences, leads to die-offs and fewer photosynthesizing trees.
Northern hemisphere snow cover, winter (left)/summer (right).
To the extent that seasons are the functions of earth's
orbital behavior, snow cover does not drive climate change. Like hurricanes,
snow cover forms a variable but essential and broadly predictable component of
the global climate system.
Tomorrow: sea ice.
Be well!
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