John Cross

Absorbing ideas

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* Warning - arithmetic ahead *

seaice.GIF

If you have been following the arctic sea ice extent, you see that it has smashed the old minimum record and shows no sign of slowing. In fact, not only does it show a new minimum extent, but it is also exhibiting the steepest decline for this time ever recorded. Not good!

While it is interesting to discuss the statistics much more important is what this actually means. To understand this you need to understand that snow and ice are good reflectors of sunlight (not all bands, but most). From my days in the remote sensing industry, a rule of thumb we used was that snow and ice in the arctic will reflect about 90% of the sunlight that strikes it. On the other hand water is a fairly weak reflector of light - I am going to use a reflection of about 15%.

Now, keep in mind that the arctic is a highly nonlinear place. Curvature of the earth’s surface, angle of incident sunlight, time of daylight, angle that the sun strikes the ice or water, clouds; all these factors play their part. I won’t bore you with the details, but using some WAGs I get a maximum number for solar energy striking the surface of about 420 Watts/square meter.

So if we have ice and snow, the surface will absorb 10% of this number or 42 watts/meter square. If we have water the surface will absorb 357 watts / square meter. So if we replace snow and ice with water we will have a net absorption of 315 watts / square meter.

That number probably doesn’t mean much to most, but lets put it like this: it is similar to having 3 one hundred watt bulbs turned on. I am talking about the old incandescent ones and I am sure you are familiar with how much heat they put out.

As of today the sea ice is about 700,000 square kilometers below where it has historically been. If you consider all this area is now absorbing energy as opposed to reflecting it, it is as if you had 2,200,000,000,000 (2.2 trillion) 100 watt lightbulbs sitting in the ocean and turned on. That is several orders of magnitude more light bulbs than we have in Canada. Granted that is under good conditions so it is not as if they were turned on all day - probably only 5 or 6 hours a day.

Compared to the overall energy budget of the earth this is a very small amount, but you cannot expect to keep adding this much energy without some consequences.

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This page contains a single entry by John Cross published on August 29, 2012 12:36 PM.

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