Sunday, April 25, 2010

Oceans of Climate Change

Climate change is all about energy --- energy in --- energy out, the balance of energy coming to the Earth and leaving the Earth. The incoming energy is light from the Sun, the outgoing energy is earthshine, infrared radiation from the Earth. To the extent that greenhouse gases exist in our atmosphere, the incoming sunlight arrives unimpeded, but the outgoing infrared light is absorbed by those same gases, which in turn gain energy and increase in temperature, and themselves emit infrared light. In this way, the planet gains energy, and all things being equal and stable, eventually reaches a new, higher equilibrium temperature where the outgoing infrared energy matches the incoming sunlight. But the trouble is, all things are not equal. The concentrations of those greenhouse gases keeps going up, and more and more infrared light is being intercepted on its way out, and the planet's overall energy content continues rising.

How do we know all this is happening? Well, the measurement of the concentration of greenhouse gases is rather straightforward. And we can see from satellite measurements that less infrared light escapes the atmosphere as time passes. We can even estimate Earth's average temperature --- and this is where the oceans come in.

Air temperatures get lots of attention in measuring global warming, but the atmosphere is the least dense, and least massive, of the parts of the Earth that gain energy due to an increased greenhouse effect. Of much greater density, and much greater capacity to absorb energy, are the world's oceans. It will take much more energy to warm the oceans than to warm the atmosphere.

The oceans cover 70% of the planet and average 3,790 meters in depth. Up until recently, we have only measured sea surface temperatures. Limited past measurements of ocean temperatures below the surface exist, but are few in number and geographic extent. It is only since the beginning of this century that a network of bouys have been deployed that measure the heat content of the oceans down a significant 2000 meters in depth. That network is called Argo, and as of today it includes 3,254 bouys spread around the world's oceans.

So what do those bouys tell us about the ocean's heat content? If you can manage to read the smallish reproduction of a graph from a recent paper published using the Argo database (by J. von Schuckmann et al. in the Journal of Geophysical Research, Vol. 114, 2009), note that the line shows a heat content increasing from 2003 through 2008. There are ups and downs, but the trend is in one direction, increasing heat content. The average over the period is plus 0.77 Watts per square meter of ocean surface.

Doesn't sound like much, less than one Watt per square meter of the ocean. Imagine your typical neighborhood swimming pool, 25 meters long with six lanes each 5 meters wide. There would be 602 billion such pools needed to equal all the world's oceans, and for the six years between 2003 and 2008 each of those pools, assuming they were each 2000 meters deep (!) would experience an increased heat content equivalent to the heat from six 100 watt light bulbs. That adds up to 3.6 trillion 100 watt light bulbs in all the world's oceans, a lot of extra heat to evaporate more water, power more storms, and change climate in all sorts of energetic and surprising ways.

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