MadSci Network: Environment & Ecology
Query:

Re: could heat of fusion contribute to rapid arctic melt?

Date: Wed Mar 9 20:49:02 2011
Posted By: Edward Hyer, Post-doc/Fellow, Aerosol Group, Marine Meteorology Division, Naval Research Lab
Area of science: Environment & Ecology
ID: 1296070367.En
Message:

will newly exposed arctic water equivalent to the mass of the melted sea ice rise in temp some 80 degrees C if exposed to the same radiant energy over the same amount of time?


Great question!
The answer is yes, it will. Except it's more extreme than that even-- remember that ice reflects sunlight (it's white), while sea water does not. So the amount of radiant energy that is applied to heating the sea water will be much more than that applied to melting the ice, under the same conditions!
Now, obviously the ocean is not boiling, and it not at risk of boiling, while ice all over the Arctic and Antarctic melts and re-freezes every year. This can only be because the water has a mass far greater than the ice, which of course it does: the maximum depth of sea ice in the Arctic Ocean is about 5 meters (16 and a half feet), while the average depth of the water (which, remember, is more dense than ice) is more than 1,000 meters.
So, let's try a simple calculation with these figures. If sunlight (which is 80% reflected back into the atmosphere by sea ice) melts away 1m of sea ice (which has a density around 80% that of sea water) how much water (which absorbs 95% of sunlight) could we raise by 1 degree C?

  1. 1m ice melted *
  2. 0.8 density of sea ice vs sea water *
  3. 4.75 times as much sunlight absorbed *
  4. 80 (latent heat of melting / specific heat of water)
  5. ----------------
  6. 304m of sea water raised by 1 degree

Now, if the Arctic Ocean was well-mixed all the way to the bottom, then we would see a 1.3 degree warming throughout its depth. But the transfer of heat through layers of the ocean is actually quite slow, most of the heating occurs at the surface, and below about 200m, the ocean isn't very sensitive to what's happening above it (though it can change over decades or centuries).
It's worth running backwards through this calculation: consider the amount of sunlight needed to raise 200m of sea water by 5 degrees. That's a lot of water, and that's a lot of sunlight! But that sunlight could only melt 3.3m of sea ice!
So what we actually observe is that the temperature of the water at the surface of the Arctic Ocean changes very little, not more than a couple of degrees C, during the course of the year, because during the summer, the heat of the air and the sun is directed to melting ice, while during the winter, the water cannot cool below -2C without freezing, which of course releases lots of energy.
But what if the temperatures rose to a point where all the ice went away? In that case, we could expect that there would be much larger fluctuations in the sea water temperature from summer to winter.
The Arctic is a fascinating place, and what makes it even more interesting for scientists is how strong changes in weather and ocean currents are driven by the simplest physical facts, such as, the latent heat of freezing!
Thanks for the question!


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