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Thursday, December 13, 2012

Rapid Arctic Freeze

Climate change deniers are taking great encouragement from the rapid increase in ice extent following the Sept15, 2012 record low ice cover in the Arctic ocean.  Not so fast guys.  This is just about what one would expect.

Heat moves by three basic physics phenomenon.  One is radiation.  This is how we get our heat from the sun.  The sun gives out masses of electro-magnetic energy and we intercept a small portion of it.  Some of it is absorbed by earth materials and converted into heat.  Depending on the temperature of the source, large amounts of heat can be transferred by radiation.

The second is convection or 'mass transfer'  This is by far the strongest of the three.  A heat pipe is a good example of a device using convection to transfer massive amount of heat.  A little closer to home, you have a furnace in your basement.  It heats air which is transferred to upper rooms through ducting with cold air returning to the furnace by other ducting.  Huge amounts of heat can be transferred this way.

The third and weakest is conduction and this is the one that concerns us here.  On one side of a material is a source of heat which sets the atoms in the material vibrating.  They pass on the vibrations (heat) to the adjacent molecules and so forth until the heat reaches the other side and heats up whatever is on the cold side of the material.  Even with such good conductors as silver and copper, the amount of heat transferred compared to, say, a heat pipe that uses convection, to transfer heat, is tiny.  In an insulator such as ice, heat transfer is indeed minuscule.

As a mind exercise, consider a time, say, during the little ice age which froze the Thames River.  On Sept 15, the Arctic ocean was probably completely covered with ice.  As the sun left the Arctic and the air temperature plunged to minus 50 degrees, there was a temperature gradient across the ice from minus one or two degrees (the lowest temperature at which sea water is liquid) to minus 50 in the air;  A great temperature gradient to help heat escape from the sea but a thick layer of ice is slowing down the flow of heat.  Of course to freeze more ice on to the bottom of the floating ice, heat has to escape by conduction.

Now consider the present situation.  Well over half of the Arctic was ice free on Sept 15 2012 and the rest of the ocean was covered with much thinner ice than in previous years.  The sun leaves the Arctic and the freeze commences.  Even after there is a complete ice cover, the ice is far thinner than in previous times so there is more heat transfer.  Remember that heat transfer under the influence of a given delta T across a substance is inversely proportional to the thickness of the substance.  Of course as the ice gets thicker, heat transfer slows.  There are a few other little wrinkles in the story.

As water freezes, it gives out 80cal of heat per gram which tends to keep the air over the Arctic warmer.  The freezing water is "trying" to keep the temperature at zero.  Of course this reduces the temperature gradient across the ice and reduces freezing.  Not to take too much comfort from the fast freeze, though.  Consider a time in the future when the Arctic becomes ice free in, say, June.

Now the Arctic ocean can really begin to accumulate heat.  Not only is the whole surface of the ocean turned into a giant solar collector but there is no ice to keep the water cool as it melts.  worse still, hurricanes, such as the one we saw on Aug 6 ff, 2012 are much more likely and will mix deep and shallow water, storing up great amounts of heat in the depth of the Arctic ocean.  Incidentally, there is a huge heat store in the deep water of the Arctic ocean already, kept there by a salinity gradient.  Without the melting of ice freshening the surface water, storms of a given magnitude will cause much greater mixing than when there was a strong density gradient.

This may explain fossil records of a much warmer Arctic Ocean even though the ocean was at the North pole when the fossils were laid down.  We could be heading for a totally unrecognizable climate regime in the not too distant future.

Just one last comment.  No fun if you don't put your whatsit on the block.  We are nearing the peak of a fairly weak solar maximum and it will probably arrive next year.  This means that the small effect of the solar cycles will be greater than last year.  It also appears (dec14, 2012) that we are heading into an El Nino.  This is also said to increase warming in the Arctic.  Now, of course, these weather phenomenon seem to be subject to random (another word for "we don't yet understand them") variations but it seems very likely that 2013 will break this year's record low ice extent.  We will just have to wait and see*.

*Note, the ice extent returned to the trend line in 2013 and looks to be about to do the same in 2014.  No El Nino occurred and ice extent did not drop down in an exceptional manner.   

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