Question of the Week

Does snow cool the world by reflecting light?

I've read that painting rooftops white can help to cool cities. So does ploughing away the snow warm the earth up and add to global warming? Jacob, USA

We put this question to John King from the British Antarctic Survey in Cambridge:

John -   Well, the thing about snow is that it’s quite reflective compared to bare ground.  A good thick snow cover will reflect back up to 80 percent or even more of the sunlight that’s falling on it.  Whereas bare ground or grasslands might only reflect 10 or 20 percent of the sunlight falling on it and so, the sunlight warms it up considerably.  So, if you replace that bare ground by snow cover, then a lot of the sunlight that would’ve heated the ground just gets reflected back into space.  So, if you remove a snow cover by ploughing it up or sweeping it away or whatever, revealing the bare ground underneath, then the ground is going to absorb a lot more sunlight, and will warm up a lot more quickly than if the snow were there.  We are having an effect on the reflectivity, the albedo of the planet by changing land use for instance; cutting down forests and replacing them with grasslands.  But that generally has the opposite effect, forests absorb quite a lot of sunlight, grassland is less reflective.  People have suggested that we could partially offset global warming by painting the roofs of all of our buildings white.  I think some calculations have been done that have showed that this will be a good thing, but it wouldn’t have a very large effect because you're only talking about a rather small area of the planet that you'll be changing the reflectivity of.

February 2010