USS Clueless              Voyages of a restless mind

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Stardate 20010831.0051 (On Screen): Global climate is an extremely difficult problem to analyze. It's not fully understood at all. For instance, it turns out that the movement of the continents has drastically altered global climates, in ways not obvious. There is a theory in climatology which originally was considered a bit heretical when proposed, but which has over time gained credence and has become part of the orthodoxy. It talks about something called the Oceanic conveyer belt. Everyone knows about the Gulf Stream, a more-or-less circular ocean current in the Atlantic which hauls warmth up from the tropics and heats Northern Europe and gives it a mild climate. Part of that sheers off and keeps going north, keeping the coast of Norway clear of ice and much warmer than equivalent latitudes in the middle of Canada or Siberia. But it's what happens to that part of the current next which is the most interesting: it sinks. That's because the ice cap there takes freshwater out of circulation and increases the salinity of the water. It also is cooled, and both of those make the water more dense. The Gulf Stream moves quite rapidly because it's pumped by winds, but the water which sinks creates an ocean current of particularly cold and highly saline water which moves quite slowly along the bottom of the ocean. It follows an extremely long path and ends up, believe it or not, in the Pacific Ocean (via the Indian Ocean) where it eventually mixes with the warmer water and vanishes some two thousand years after visiting the ocean west of Norway. But while it runs, it keeps Northern Europe very warm because it carries cold away deep in the ocean, and pulls warm water north on the surface.

Ice cores were taken in Greenland extending down through its glaciers. The ice cores are layered, with individual lines representing years, and each layer contains trapped gas. It is possible to do analysis on this to determine the temperatore in that part of Greenland each year extending back the a quarter of a million years, and this has been done. The result is rather surprising: the last ten thousand years are an anomaly. Not only are they far warmer than average, but also far more stable in temperature. It appears that this current is the reason why; it doesn't run all the time but when it does it helps warm the planet overall and keep it stable.

Why is this interesting? Because it suggests a surprisingly different potential outcome for increases in atmospheric CO₂. I didn't completely understand the sequence of events, but it more or less went like this: A slight rise in temperature would cause the northern icepack to melt a bit. This decreases the local salinity of the water, making it so that the water doesn't sink any longer, which makes it so that it doesn't pull water up from the Gulf Stream. The north branch would stop happening and the Gulf Stream would settle into a new equilibrium without that branch, never extending north of Ireland. That, in turn, ceases to warm up that area which then begins to freeze. The icepack on water and land would grow dramatically and increase the albedo of the Earth, reflecting more light off into space and causing further cooling. The effect, taken to its extreme, is to set off a new ice age. In other words, the ultimate effect of global warming would be global cooling. Ain't complex-feedback systems fun?

I'm not saying this would necessarily be the outcome, although that's what the theory suggests. What I'm saying is that climate is a very complicated problem. That chart about the temperature over the last quarter million years makes blatantly clear that it isn't possible to deduce anything about future climatalogical changes from a baseline of 300 years. To a human, that's a long time, but to the planet it's a wink of an eye. It is true that over the course of the last 300 years that the amount of CO₂ in the atmosphere has risen and also true that the global temperature has risen during that time. But are the two related? Unknown. The temperature of the planet fluxuates massively anyway; and it may be that other things entirely caused that temperature rise. It may be that temperature would have risen in that period even if humans had never discovered fire (or if humans hadn't existed at all). There has been a lot of pronouncements about the coming crisis of Global Warming caused by release of greenhouse gases, based in some cases on computer predictions made by climatologists. But I never heard of one of those models predicting an ice age as a consequence of increasing levels of CO₂. (Broecker, discoverer of the conveyer, predicts substantial global cooling to set in within fifty years as a result of partial shutdown in the conveyer.) What else is missing from those models? They appear to have been based on very reductionist (i.e. simplistic) models of the Earth, and as such may be wildly wrong in their predictions. Why should we base extremely expensive public policy on science which is so tentative? (discuss)