I am (at least by degree) a scientist. By that same degree, however, I am a biologst, or perhaps an ecologist, rather than a meteorologist. Compounding my error, my following musings are based on bits and pieces of random information I accumulated in haphazard fashion and then attempted to relocate and double-check, not on long and sober perusal of the literature. That said, I tried to make sure it held together coherently, so it should at least be a sensible discussion. So, on with the reasoning...
Let's start by laying out the facts.
Fact the first: Dark surfaces absorb heat and warm up their surroundings; pale surfaces reflect heat and cool their surroundings. So: Snow and cloud cover reflect heat and cool us down, tarmac and sea water absorb heat and warm us up. So far so good.
Fact the second: Europe and most of North America consist of First World nations. Europe is also fairly densely populated (we've had umpteen thousand years to fill the space).
Collorary to fact the second: First World nations like to live in a certain way, i.e. they like cities. And roads. Lots of roads. For a few hundred million people, that means large tracts of road surface. (Not easy to find exact figures, but I found estimates that between 0.77 to 1.6 percent of North America is covered by road surface. Let's unscientfically split the difference and call it about 1.2%.)
Fact the third: Roads are made of tarmac.
Fact the fourth: Large parts of North America and Europe recieve snow during the winter months. (Or at least they should, were the weather well-behaved; but alas, humanity has ill-treated the poor climate enourmously over the last few decades, and it is now temperamental and inclined to sulk.)
Fact the fifth: First World people, accustomed to a life where they can do many things freely, including travel, do not see why they should be restricted in their movements merely because the clouds have the temerity to dump frozen water all over the street. They therefore protest strenously to the authorities, who grit the roads, or shovel the snow.
Reiteration of fact the first: Snow reflects heat. Tarmac absorbs heat.
So, logically, this means that across the (approximate) 1.2% of snowbound North America and Europe that is covered by road surface, the ground is warmer than it should be.
Winter is warmer than it should be.
1.2% may not sound like a lot, but continents are huge. A hundredth of that is big. And thin trails of reflection or adsorption can have huge effects: Some people think that aeroplane contrails keep the U.S. 1-2 degrees cooler than it would be if we stopped all air traffic, due to the temperature rise when flights were cancelled after 9/11.
I am now going to veer into unscientific sources and somewhat unsound hypotheses: In his book A Brief Histry of Nearly Everything, Bill Bryson quotes the idea that Ice Ages are not started by brutal winters, but by poor summers. I am going to suggest that the opposite also holds: Global warming is accelerated not by brutal summers (though those can't help), but by lousy winters. Winters with not enough snow cover, so that heat continues to be absorbed even through months when it should be shed.
Not nearly researched an formal enough for a scientific paper, but I thought it was an interesting idea to play with. I hope you enjoyed it.