August 24, 2006

A Sea Change in Global Warming?

For years now, we have been deluged with the news that the earth’s oceans are heating up as a result of changes in atmospheric composition resulting from the combustion of fossil fuels.

Typical of these was a 2005 story titled “Where’s The Heat? Think Deep Blue,” from United Press International, describing a recent paper in Science by NASA climate modeler James Hansen. UPI wrote that “Over the past ten years, the heat content of the ocean has grown dramatically.” This study covered more than just the ocean surface temperature, which can fluctuate considerably from year to year. Rather, by considering a much deeper layer (the top 2,500 feet), Hansen could actually calculate the increasing amount of heat that is being stored. According to UPI’s story, this provided “a match” with computer model projections of global warming.

The ocean is a huge flywheel that integrates and stores long-term climate changes. Consequently, when computer models are driven with ever-increasing atmospheric carbon dioxide, the deep oceans warm, warm, and warm. But it takes time to start up, just like a big pot of water heated by a small match. Once the process starts, though, nothing should be able to stop it.

That’s the current wisdom of our climate models, but like current wisdom on so many other aspects of life, nature has behaved differently.

Within weeks, a paper is going to appear in the refereed journal Geophysical Research Letters, by John Lyman of the National Oceanic and Atmospheric Administration, showing that, globally, the top 2500 feet of the ocean lost a tremendous amount of heat between from 2003 through 2005—about 20% of all the heat gained in the last half-century.

Needless to say, this has climate scientists scratching their heads. No computer model predicts such behavior. And further, the changes in surface temperatures haven’t corresponded (yet?) to the average change at depth, although they, too, have dropped some. Nor has the sea level dropped an amount requisite with the cooling (water volume varies slightly with temperature).

This last observation has led to speculation that much more ice must be melting into the ocean than is normally assumed—but no one can find it, and the searching has been intense.

There’s another hypothesis out there that has received very little attention. It has to do with the amount of carbon dioxide accumulating in the atmosphere.

If carbon dioxide increases at a constant rate, basic physics—known since the 1860s—says that surface temperature will go up, but that the rate of heating will become less and less. In other words, the sensitivity of temperature to carbon dioxide becomes less for each equivalent change in the gas.

So, in order for temperatures to simply increase at a constant rate, as has been observed since 1975, carbon dioxide has to go up at an ever-increasing rate, much like compound interest.

But the ocean is so vast and slow to change that it takes several decades to realize the heating caused by carbon dioxide. Consequently, if there is any change in the rate of carbon dioxide accumulation in the atmosphere, that won’t be noticed for 30 to 60 years, depending upon whose calculations one believes.

About 30 years ago, something very peculiar began to occur with atmospheric carbon dioxide. Between when it was first directly measured, at Mauna Loa, Hawaii, in 1957, and 1975, the increase was clearly of the compound-interest variety. And, not surprisingly, once the temperature response was initiated, it was at a constant rate, just as predicted.

Since 1975, changes in atmospheric carbon dioxide have are of a different nature. It is now impossible to tell whether the increases are maintaining the compound-interest structure or are simply constant.

Because of the lag time required for the oceans to notice this, it may not be a surprise that an interval of cooling has been detected. The timing is about right—around 30 years.

But that’s just another climate change hypothesis that time will test. Be forewarned, though, as shown by the completely unexpected cooling of the deep ocean that began in 2003, we know a lot less about climate change than we (or some of us) think.




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