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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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SkS Analogy 21 - Snow on a Hot Tin Roof

Posted on 17 December 2019 by Evan, jg

With many feet of snow on our shed roof and with another winter storm approaching, I knew I had to get some of the snow off the roof to prevent its collapse. So, I worked for a couple of days pulling off some of the snow. When I finished there was a thin layer of remaining snow over most of the roof, and the sheet metal at the edge of the roof was exposed, bare metal.

In the following days I noticed that the amount of bare metal kept getting bigger and bigger, moving up from the edge of the roof towards the peak of the roof. This did not happen with a nice, uniform layer of snow on the roof, all the way down to the edge. For months the snow had not retreated, but once I exposed the edge of the tin roof, the snow started retreating, exposing the tin roof as the snow line proceeded on its upward march towards the peak of the roof.

Of course, you know what was happening. White snow reflects most of the sunlight, keeping the snow cool. But when even a sliver of metal roof is cleared of snow, the sunlight hits it, warming it up, because the tin roof absorbs most of the sunlight. Now that the tin is warm, the snow next to the tin melts, causing more tin to be exposed, causing the larger amount of exposed tin to absorb more heat, etc. What started as only a sliver of metal exposed on the edges and a small amount of melting, soon turned into a rapidly increasing fraction of the roof being cleared of snow, accelerating the local warming and the melting of snow. Before long the snow on the shed roof had melted all the way to the peak of the roof and cleared itself off.

Snow on a hot tin roof


This phenomenon is called the ice-albedo feedback. “Albedo” is a fancy term that essentially means how white something is. Snow has a high albedo, and blacktop has an albedo near 0. Surfaces with high albedo reflect most of the sunlight, whereas surfaces with low albedo absorb most of the sunlight. The idea of the ice-albedo feedback is that when snow is sitting on top of something that has a low albedo, like the snow on our tin roof or like the ice on top of the Arctic ocean, the dark surface near the ice melts more ice or snow, which exposes more dark surface underneath, which causes more warming, which melts more ice or snow, and so on and so forth until soon a large area of snow or ice is melted. One of the only things that interrupts this process is that at some point the sun sets, allowing the environment to cool. This interrupts the ice-albedo feedback because the sunlight that drives the warming is not available during polar night. In Minnesota the sun rises and sets once a day, but in the Arctic there is a period when the sun sets and does not rise again for many months, just as there is also a period when the sun never sets, allowing the ice-albedo feedback to proceed unchecked for months at a time.

Why does this snow-clearing effect not keep our shed roof clear all winter long, and why did this ice-albedo feedback not cause massive ice loss in the Arctic 100 years ago? Sun shining on dark waters or on a shed roof causes heating, but if there are other processes causing more cooling than warming, then the overall system will cool. Ultimately whether ice melts or water freezes depends on the balance of many competing heating and cooling processes. The ice-albedo feedback mechanism simply describes an acceleration once melting has begun. But the overall heat transfer processes must allow melting to begin before the ice-albedo feedback process can accelerate the melting.

Because of this ice-albedo feedback mechanism the Arctic is warming at least twice as fast as elsewhere on Earth, because where ice is removed to expose a dark surface underneath, the environment goes from stable cooling to stable heating. This dramatic shift from cooling to heating is much of what is causing the Arctic to rapidly warm. Read here, here, and here.

This is a problem partly because the Arctic provides a source of global warming on steroids, and therefore a strong, localized source of intense warming. This extreme Arctic warming is also a problem because maintaining stable weather patterns in the Northern Hemisphere requires maintaining reliably cold temperatures in the Arctic. As the Arctic warms faster than elsewhere on Earth, the global circulation patterns are disrupted, affecting the motion of the jet stream. Because the jet stream controls weather systems in the Northern Hemisphere, Arctic warming is at the source of much of the recent disruption of our weather patterns according to the following effects.

  • The jet stream may shed high-pressure systems that act like blocking highs. This is part of what it believed to have caused SuperStorm Sandy and Hurricane Florence to turn into the East coast of the US rather than assuming a northward trajectory that would have kept them away from the East Coast of the US. Read here and here.

Although the ice-albedo feedback primarily occurs in the Arctic, because of its influence on the jet stream, its effects are felt all over the Earth.

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Comments 1 to 6:

  1. The warming arctic is also increasing carbon emissions from  the permafrost tundra, so this is another feedback: "Rising emissions are turning arctic permafrost into a carbon source, research shows"

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  2. Evan,  I enjoyed your use of a hot tin roof imagery to explain your thoughts about the urban heat-island effect.

    Anyhow, what answers can we speculate on as far as how the northern hemisphere, having a larger amount of heat absorbing land-mass, having faster melting occur today than the Antartic region?   Does the albeto feedback seem to hold its integrity by the presence of the giant continent of ice - to where the North Pole is nothing but ocean water?

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  3. Darin, the basically understood idea by the world Government departments is that, yes, the big ice block of the antipodes acts as the worlds airconditioner... and when it's hot we all know that we would prefer to be as close as possible- give or take a few hundred thousand miles lol! Merry Christmas Everyone.... 

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  4. Darinscoop@2 The ice-Albedo feedback describes a spontaneous change in Albedo with time, whereas the heat-island effect describes a contrast of Albedo (i.e. cities have lower Albedo than surroundings areas), which does not spontaneously change with time, unless we deliberately cause a change through development or other urban planning.

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  5. speaking of the albedo effect then, all other things being equal, would a tan and dust-covered planet be warmer or cooler than a dark blue and green colored planet of the same size at the same distance from the sun?  My point is that I think a planet covered with trees will actually get warmer than a bare planet, at least until those trees can soak up a lot of the CO2 and change the atmosphere's ability to trap heat.  My point is that planting trees will actually make it least in the short run.

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  6. Fred@5 Good points. A quick Google search turns up the following papers on this subject that might be of interest (here and here). So yes, it appears that trees have an initial warming effect until they soak up enough carbon to offset the warming they cause by reducing albedo with respect to an open field. I think it safe to say that burning down rainforests has a warming effect, because the release of carbon has a much higher warming effect that the increased albedo caused by replacing mature trees with open fields.

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