Question of the Week

Does snow cool the world by reflecting light?

Sun, 21st Feb 2010

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Jacob, USA asked:

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?


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.  A snow covered Bodträsk railway station, Kalix kommun, SwedenSo, 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.


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This is John Berger writing from Canada, not far from the warmest winter Olympics ever.
I'm in the home repair business, and one way of quickly determining whether a home's insulation is adequate or not is to see how quickly snow melts from its roof. If the attic insulation is inadequate, snow will melt more quickly on that roof than from those of the neighbouring homes, thus leaking heat to the environment.
Similarly, snow is an excellent insulator, being an icy equivalent of fibre-glass batts we press into walls and ceilings. Therefore, removing snow from the roof is similar to removing insulation from the attic. With flat roofs, some people haul up a snow-blower machine to speed the job. Most snow-blowers are gasoline fueled, and thus add to the green house gas volume.
Finally, if the snow on the roof is heavy enough to cause the roof to collapse, and you consider the energy needed to rebuild the roof, then green-house gases will be generated via transporting new building materials and manufacturing asphalt shingles, etc.
Hope this answers the question.
John Berger
Nanaimo BC Canada diverjohn, Wed, 17th Feb 2010

→ Floods reduce source of anthropogenic warming  → equilibrium is restored RD, Thu, 18th Feb 2010

Yes, the impact of ice and snow reflections have been appreciated for a long while. This is why an ice-age is a positive feedback loop: more snow and ice means more energy reflected back into space, which means a cooler planet, which means more snow and ice and so on. What arrests - and reverses - these processes though, I'm not sure.

Milankovitch cycles, which are long-term (120,000 year cyclical) periodic warming and cooling episodes, are linked to a range of planetary factors including precession, tilt wobble and orbit eccentricity. These changes alters solar energy input, affecting global temperatures. The huge amount of energy already stored in the system means that there is inertia, so changes occur gradually.

Chris chris, Thu, 18th Feb 2010

Hello Guys,

Yes absolutely snow can not pass light to pass through it and reflect it back.

Mark. Mark Lehman, Thu, 29th Apr 2010

The way I understand it is, snow reflects sunlight off the Earth's surface, which by itself would help cool the Earth by preventing some of the sun's energy from being absorbed into the Earth's surface; however, an increasing layer of carbon dioxide traps the reflected enegy, heating the earth up more, melting the polar icecaps, killing us all. BechtelEngineer, Mon, 5th May 2014

Suntan lotion is a good idea for spring skiing as one gets a "double dose" of sunlight when out on the slopes. 

Light is both reflected, as well as absorbed, then re-emitted as IR by the warmth on the planet, with the sum total being about the same as the inbound sunlight.

The advantage of reflected light is that it is at a wavelength that isn't particularly affected by CO2, although it may be blocked somewhat by water vapor and clouds (which would be the same for the inbound sunlight). 

As far as the winter, there is little or no sunlight in the Arctic regions during the winter, and thus the ice may act more as an insulator than a reflector.  However, it would be important during the long summer days. 

One thing that hasn't bee discussed much.  The Northern sea ice has been lower than average during the relatively short recorded period.  However, the southern sea ice has on average been slowly increasing.  As of today, we are at the 57th daily record high for the sea ice in Antarctica (records from 1978 to 2014).  We are currently at about a half a million sq km of sea ice above the Record, and over 1.5 million sq km above the mean. 

2014 Sea Ice Notes
May 5th Sea Ice Graphs
Antarctic Sea Ice graphs, last 2 years
Total Global Sea ice, 1979 to present
NOAA Snow Cover Anomalies

When looking at the total global sea ice (Northern + Southern), the trends are much less clear.  We are currently above average for global sea ice.  We were slightly below average for global sea ice in the middle of last decade, but it has completely recovered, and the overall trend may well be flat. 

Likewise, the trend for snow cover is more ambiguous with the global (northern hemisphere) snow cover being greater than average over the last few years. 

The loss of northern Arctic Sea Ice over the last few decades, as well as the retreat of many glaciers is disturbing.  However, good records are only available for a relatively short period.  Snow cover may well be fairly robust as slightly warmer ocean temperatures may bring more moisture into the atmosphere, and thus more snow.  It certainly would be premature to predict the end of the world based on changes of snow and ice. CliffordK, Tue, 6th May 2014

I don't know how good the scientific evidence for snowball Earth actually is, but it's often claimed that the whole world once froze over and would have remained locked into that state ever since if it wasn't for the mechanism of CO2 from volcanoes building up to the point where the whole lot rapidly melted from equator to poles. David Cooper, Wed, 7th May 2014

      Hi, my name is Thomas Ackerman, I would like to add some information to this page on common logic and viewed experience.  I like in Saskatchewan, Canada and from my experiences with many many winters is that when the sun is shining the lack of cloud coverage allows all the warmth from the snow to escape. Let me explain....    The ice the snow consists of can only reach 0 degrees celcius at which point it freezes and expands and becomes locked. Meaning it will get no colder than that.  The snow actually staying at 0 keeps the air above and the earth below to be regulated.  Cloud cover holds the warmth in so to speak. ANYBODY living in this climate sees this clear as day.  As for the snow reflecting radiation, thus, heat back out into space I must conclude from my observations of winter climate over 35 years that on years where there is less snowfall and ground coverage the average temperature outside is actually lower and I hypothesize that this is the reason. Energy reflected by snow and ice is immiscule to the actual amount of heat energy that becomes licked in the ice and insulated from the elements which on a sunny day can reach -70 degrees. But on cloudy days the snow warms the environment and stabalizes ground freeze.  As many scientist have suggested snow and ice in larger than usual accumulations do result from previous overheating but a worst case senario that covered the whole planet and left us without any warm zone would not last very long because it naintains that 0 degrees which is not that bad and anything actually exposed to open air and the sun becomes way colder than 0 degrees celcius.  If anything snow and ice coverage during the winter contributes to global warming by way of insulating the surface.  KeizerPaPa, Sun, 20th Jul 2014

You would think that it actually warms it because sense the light is being reflected that the UV rays are going to go somewhere in the air.. I heard that you can get sun burned from the snow.. if you expose yourself to it Oceanbliss, Tue, 16th Dec 2014

    Snow being subject to its own limitations only reaches 0 degrees Celcius.
  That being said...  exposed earth during winter months is exposed to the
atmosphere whch quickly can bring temoeratures of matter other than water
to -30 to -85 degrees Celcius.  When the sun is shining in the winter it is
generally alot colder than when it is cloudy because the clouds hold in the
warmer air just as the snow on the ground insulates it from the sub-zero
temperature air.  So in contrast to what you might think snow (which always
by rule of thumb falls in greater amounts during warmer winter seasons) is
actually a contributor to global warming.  I am from Saskatchewan where we
see some of the coldest temperatures anywhere in the world.  The coldest day
every year for the last 20 years has been bright and sunny.  As well, the years
with the least snow cover are always the coldest.  Arctic high pressure systems
bring the coldest air and low pressure systems bring clouds and snow. So, no,
snow reflecting Sunlight actually doesn't at all cool the Earth.  Because no matter
what, under the snow the ground stays close to Zero unless exposed, then it gets
colder. KeizerPaPa, Wed, 29th Jul 2015

It's important not to confuse several mechanisms at work here.

1. Snow is a good reflector

2. and a good insulator, preventing heat exchange between the soil and the sky

3.Melting snow requires a very large heat input (333400 joules/kilogram) at 0 deg C compared with raising the temperature of soil by 1 degree (about 1000 joule/ so it adds thermal inertia. Evaporating water from the surface (wet soil or standing water) requires another 2,260,000 joules per kilograpm.

4. Bare soil is a better absorber of radiation than snow

5. Good absorbers are also good emitters, so bare soil, concrete, etc., heat up quickly during the day and lose heat quickly at night

6. Air is a poor absorber of radiation (which is why we can see through it, and get warmed by the sun). The troposphere, the layer in which we live, is principally heated and cooled by (a) convection from contact with the ground and (b) heat transfer to and from water as it evaporates, condenses, freezes, and falls out of the sky.

7. Nearly all the infrared that is absorbed by the atmosphere, is absorbed by water vapour. It is the principal greenhouse gas.

All of which makes meteorology very complicated - it's an exact observational science with only the merest whiff of mathematically predictive theory. The world's greatest megacomputers are about as accurate as your local air traffic service or a ship's navigator's handwaving guesstimate.  alancalverd, Thu, 30th Jul 2015

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