Friday, December 24, 2010

conceptualizing the cold

--climate happenings are at blister data --

How come global warming involves winter cooling? Last winter, complicated explanations were making the rounds. Because warming is wiggly, said some. Because there's a third wiggle, said others. That's all true, but now we know better.

In the past winter, J. Hansen et al. were saying that warming wiggles up but the zigzag of the seasons still out-wiggles the warming-wiggle for now. Hence cold winters happen in the shift to a hot world and can still be expected for a few more years.

Others were pointing to another culprit, a wiggle between the warming wiggle and the season wiggle, the Arctic Roll or Arctic Oscillation (AO), a multi-decadal climate wave. Arctic sea level pressure (SLP) oscillates around a mean. For forty years the Roll is in the low index of the mean with negative values, and then it surfs to the high index with positive values. When the AO is rolling low, SLP is above normal and we see hard winters. When the AO rolls high, SLP is below normal, and the northern hemisphere enjoys mild winters. The string of hard winters 1899-1939 was followed by mild winters 1940-1988. Now we are in a string of hard winters again, presumably until 2029. So there's a third wiggle overlaying seasonal zigzag and the warming wiggle. This AO-wiggle is giving us one cold winter after another.

A year ago, we had these two accounts: wiggles happen, and we're in a cold phase. Both are empirically true, and neither of them contradicts global warming. But as they are independent events, they make for a complicated picture. The picture was that global warming is compatible with cold winters, or it can be cold when the planet is heating up. That was then. Now we know that global warming entails cold winters, or it must be cold when the planet is heating up. The simulation Petoukhov ran in "A link between reduced Barents-Kara sea ice and cold winter extremes over northern continents" J of Geophysical Research 115 (2010) D21111 (pdf), suggests an explanation that's as powerful as it is elegant. It's beautiful, tight, and it is not paradoxical. Mother Nature operates perfectly rationally.

This is how it works. Because of all these Americans with their SUVs and the Chinese following suit etc., the planet heats up, and the ice floating on the Arctic sea is shrinking. It wiggles, like Nature is wont to do, but its summer thaws and winter freezes wiggle along a death spiral of no more summer ice pretty soon. What satellite pictures of the ice pulse show is that the annual maximum is getting smaller. What the pics don't show is that it's also getting thinner. So each winter there's less ice on the ocean; the frozen area is shrinking, and what's left is thin ice.

The winter blanket is smaller and threadbare. Less ice insulates less. The ocean used to be insulated by a large sheet of thick ice but is now exposed to the elements. The evil bloodsucking Republicans etc. etc. have pulled the lid off the sea, exposing warm water to the raw winds. The warmth rises from the water into the cold air. The cold air warms up. The warmed-up air expands outward.

Shrinking ice exposes water whose rising warmth creates a high pressure zone, hence air is moving outward and southbound. The Arctic winter air, warmer than it used to be but still searingly cold, travels south, arrives here, and makes our temperatures plummet. And here we are, shivering.

We learned from Ockham that great insights are not complicated, and Petoukhov's findings fit in one sentence. Amerigenic climate change peels ice off the Arctic ocean, and the sea reacts by belching up its inner warmth, yielding a widening high pressure disk over the Pole that drives frigid air ahead of itself. Air that would otherwise stay at the Pole is pushed south to Atlanta and Paris, and Tampa and Berlin.

That's why it's so cold on a heated-up planet.

Seventy-two months left.

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