Friday, August 5, 2011

Us and Them

Any model for global warming has to explain the following figures, which show significantly more warming in the northern hemisphere than the southern hemisphere:

 (Bottom figure from

One obvious difference between the northern hemisphere and the southern hemisphere is that there is a much larger fraction of ocean in the southern hemisphere than the northern hemisphere, and the ocean surely acts to moderate the air temperature. Whatever mechanism is heating the atmosphere, this heat may be being transferred to the ocean at an appreciable rate, despite the very slow mixing between surface water and deep water. 

Let’s do a rough estimation of this thermal buffering capacity using Google and our mad arithmetic skillz. If we suddenly spiked the temperature of the atmosphere by 10 degrees, how much would this increase the temperature of the deep ocean, assuming all that heat was transferred across?

8.50 km = height of atmosphere if it were all at 1 atm pressure
3.80 km = average depth of ocean

So volume of atmosphere : volume of ocean (assuming ocean covers 70% of Earth)
8.50 : 2.66

90% of the volume of the ocean is below the thermocline and will have at temperature between 0 and 3, saith the interwebz. Let’s assume at equilibrium we have just heated up this cold water, and the air and surface water have returned to the same temperatures they are at now.

So volume of hot atmosphere : volume of cold ocean
8.50 : 2.40

Volumetric heat capacity of dry air: 1.3
Volumetric heat capacity of water: 4180

So if the heat change in the air is
10 K x 8.50 V x 1.3 = 110.5

Then the ocean will increase by a temperature T, where
T K x 2.40 V x 4180 = 110.5

T = 110.5/(10042) K

My mad mental arithmetic skillz have deserted me, but instead of opening another window and doing it in Excel I will round it off to 100/10,000 K – near enough to 0.01 degrees. So the net effect of whatever cockamamie stuff we have done to increase the temperature of the atmosphere ten degrees has been to increase the temperature of the deep ocean from 1.5±1.5 C to 1.51±1.5 C.

Just something to think about.

But, maybe the difference between hemispheres is not due to the effectiveness of heat transfer to this ubersink. Since there is another obvious difference between the hemispheres. About 90% of the human population – and all that other stuff associated with humans – is in the northern hemisphere.
Most of the things we do to the atmosphere change what is going on near the surface, where there are big fluxes for all sorts of other reasons.  But we have made big changes to parts of the atmosphere where not much usually happens. Pumping carbon dioxide and water vapour into the upper atmosphere with our big jet planes, f’rinstance.

I’ve stolen this picture of a place in the northern hemisphere where there is a lot of air traffic. It often looks like this, though not quite as bad, where I live, which is a relatively air-traffic-crowded part of the southern hemisphere. The picture is at sunset because, while I don’t know much about clouds at all, it is obvious from standing around under them that they have a cooling effect during the day and a warming effect at night. And the Cloudspotter’s Guide by Gavin Pretor-Pinney, which I can recommend without reservation, assures me that the warming effect wins.

Just another thing to think about.


September 13th:

Of course, if you pick a different zero point, the two curves don't look all that different:

So probably there is nothing in it after all.

September 10th, 2013:

The sea-ice graph looks even better now, a few years on: