The Black Swan and the Bell Curve

Many developments in the last few years have suggested that general public concern about global warming is much shallower than one might have supposed earlier in the decade, most recently Germany's decisions to end reliance on nuclear energy entirely and accelerate the phase-out of currently operating reactors. That means much more dependence in the next decade on domestic coal and natural gas mainly imported from the Russian sphere, and probably somewhat greater imports of nuclear-generated electricity from France. And it means that Germany is very unlikely to meet its ambitious greenhouse gas reduction goal--a cut of 40 percent by 2020, vis-a-vis the 1990 level.

Deutsche Bank now predicts that greenhouse gas emissions from Germany's power sector will more than double in the next decade as a result of the country's new course, an astonishing prospect for a country that until now had been a leader in global efforts to address climate change.

The retreat of Republican senators John McCain and Lindsay Graham, who earlier had cosponsored cap-and-trade greenhouse gas reduction bills; the refusal of Congress to take up such a bill and the Obama administration's decision not to fight for one; the global breakdown of the Kyoto process and the acquiescence of the once-activist European nations to a program of purely voluntary carbon cuts; the relative indifference of general publics to the whole subject of global warming and climate change, despite a plague of violent and weird weather that has afflicted the United States, western Europe, Russia, China, and Pakistan in the last few years--what's going on?

"Planet Earth Doesn't Know How to Make It Any Clearer It Wants Everybody to Leave," led the satirical weekly The Onion last week. In this week's opener to The New Yorker, Elizabeth Kolbert muses in a more serious vein about why people aren't getting the message, despite the consistency of extreme weather events with climate model predictions. Why did the president brood about such events being "beyond our power to control," she wonders, in his recent visit to tornado-ravaged Joplin, Missouri?

Of course the president is not exactly wrong. Unusually violent events will take place occasionally under any circumstances. And--the public probably grasps--the likelihood of such events will continue to increase even if we all start sharply cutting greenhouse gas emissions tomorrow, because of the warming "inertia" already injected into the system.

What's easily lost sight of in that kind of reflection, however, is the simple fact that we can work to reduce the probability of ever-more extreme climate events, or not. Right now, we have drifted into zone where global greenhouse gas concentrations are much higher than they have been any time in the history of homo sapiens and close to 50 percent higher than they were at the dawn of the industrial revolution, 250 years ago. We are in uncharted waters and getting deeper into those waters all the time. The rational decision is to reverse course and start navigating our way out of those waters as fast as can be prudently done.

How fast is that? There are those--a lot of those--who argue, in light of the Fukushima and other worse-than-worst case nuclear accidents, that we don't need to do anything so desperate as resort to more reactor construction just to mitigate the risks of climate change. Respectfully, I would suggest that the risks associated with global warming are of a completely different order than the risks attached to atomic energy. For the weird weather we've seen so far may be no more than a nasty foretaste of what's to come.

Suppose, to take a different kind of worse-than-worst case, some catastrophic combination of flood, drought, and pest conspired to completely shut down one of the world's major breadbaskets for three or four years--say Illinois-Iowa, the Yangtze or Yellow River basin, the Mekong, Ganges, Indus, or Nile. The effect would be a a global food shortage, mass starvation, and global convulsions the like of which have never  been seen before.

To focus for a moment on the Illinois-Iowa scenario, standard business as usual projections say that this part of the Midwest will have, by the end of the century, a climate like that in East Texas or Alabama--a temperate climate zone will have changed to a subtropical one characteristic of the deep South.

That's the mainstream scenario. But what if things were even worse than that, much worse? "Black swan" scenarios associated with the mathematicians Nassim Nicholas Taleb and Benoit Mandelbrot postulate that seemingly very improbable events take place because the probability distribution turns out (mainly in hindsight!) to have "fat tails"--that is, instead of conforming to the "normal" Bell Shape form, the distribution has a shape in which the probability of extreme events is greater than normal.

We don't actually have to resort to black swan theory, even if geoscience is one of its more obvious applications. The politically conservative Chicago jurist Richard A. Posner, in Catastrophe: Risk and Response (2004), has pointed out that the more uncertain we are about mainstream climate projections, the more we should be worrying about worse-than-worst case scenarios. Thus, he suggests, climate skeptics are shooting themselves in the foot when they emphasizeuncertainties. By the symmetrical logic of the Bell curve, if climate scientists are underestimating the possibility things may be not nearly as bad as they predict, they also are underestimating the probability they will be much worse.

How much worse could they be? In an agricultural collapse scenario, we could be talking about millions, hundreds of millions or even billions of deaths--not the "mere" 20,000-30,000 premature deaths that have followed from the Chernobyl accident.

POSTSCRIPT, 6/30/11L

The Financial Times reported yesterday that 46 percent of the United States has been either abnormally wet or abnormally dry this spring; 21 percent is normal. "It is highly improbable that the remarkable extreme weather events of 2010 and 2011 could have all happened in such a short period of time without some powerful climate-altering force at work," said Southern Methodist University business professor Bernard Weinstein, quoted in the FT. "I expect that by 20 to 30 years from now, extreme weather years like we witnessed in 2010 will become the new normal."

The re-insurance company Munich Re has arrived at a similar conclusion: "The only plausible explanation for the rise in weather-related catastrophes is climate change. The view that weather extremes are more frequent and intense due to global warming coincides with the current state of scientific knowledge."
 

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