Thursday, May 2, 2013

Temperatures may be flat – but there’s no free lunch

The most commonly used measurement of global warming is the Global Annual Mean Surface Air Temperature Change (see graph at right and note below). When you read about “rising temperatures,” this is the number they’re talking about. If you look closely at the far right of the graph you can see that this temperature measurement, after shooting up for the previous couple of decades, leveled off over the last 10 years or so. This despite the fact that we’re still pumping out prodigious amounts of greenhouse gasses.
Several attempts have been made to explain this very short-term pattern in such a way as to show we might be out of the woods on global warming, mainly by weakening the “sensitivity” of our climate models. Hey – those sensitivity factors were inferred from a lot of real data covering centuries and millennia – it makes no sense to me to start fiddling with them based on 10 years worth. I don’t believe in free lunches.
In fact, the recent (and very short by ecosystem timeframes) leveling of the mean global air temperature is likely a case of borrowing from Peter now, only to have to pay Paul back later. I like this explanation because (a) it makes sense, and (b) doesn’t involve a free lunch.
According to a paper in the American Geophysical Union's Geophysical Research Letters, the reason the air temps aren't rising is that the global warming energy has gone into heating the deeper ocean waters. Normally, that doesn't happen - the surface water's warm first and since warm water floats over cold water, the deeper ocean is slow to heat. But - enter the Pacific Decadal Oscillation (PDO), which during the past decade has been pushing colder water up from the depths, and circulating the warm water down. With the cold water on top, the ocean has been absorbing more of the heat energy of global warming, leaving the surface air temperatures unchanged.
Trouble is, when the next PDO cycle starts, that pattern will reverse, and much of that absorbed energy will re-enter the atmosphere, and we'll pay for this decade of "flat" temperatures with a decade of rapidly increasing temperatures. In fact, say the scientists, this is exactly what happened in the late 80s and 90s (1998 remaining one of the hottest years on record).
That's the trouble with looking at one number (mean global surface temperature) as a proxy for the complex phenomenon of climate change. We have to remember that “Global Warming” refers to the entire 3D globe, not just the surface.
We're coasting now, but hang on to your hat!

Note 1: The graph from NOAA (see the original at Temperatures are shown as a departure (“anomaly”) from the 1951-1980 average. So over the last 10 years or so, the mean surface temperature has been holding steady at 0.6 degrees Celsius (1.08 degrees Fahrenheit) over the 1951-1980 average.

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