Keith Kloor throws a softball my way. He offers up an argument which he attributes to William Pentland at Forbes, quoting Pentland thus:
Uncertainty is intrinsic to complex systems like Earth’s climate, but in the context of catastrophic climate change, this uncertainty is so severe that it is difficult to draw basic conclusions about how fat the fat tail is. According to [Harvard University’s Martin] Weitzman, it “is difficult to infer (or even to model accurately) the probabilities of events far outside the usual range of experience.”
Indeed, ”[r]ather than justifying a lack of response to climate change, the emphasis on uncertainty enlarges the risk and reinforces the responsibility for pursuing successful long-term mitigation policy,” according to a 2010 analysis by researchers at Sandia National Laboratory.
All things considered, alarmism seems like common sense to me.
Right up my alley. See, I have been saying this exact thing for a long time.
The Long Fat Tail
The question amounts to a version of the “long tail”, which leads us into the fraught territory of probabilistic thinking. Let me not stray too far into arrogance when I assert that I frequently see people who should know better and claim to know better blundering hopelessly in this territory. I won’t name names. And surely you, fair reader, are not included among the overconfident…
The point is that there is something peculiar about a probability density of an experiment that can never be repeated. Really, we are not talking about probability but about a Bayesian prior. It resembles frequentist probability in that it gives us a rational foundation for risk assessment. The calculations are identical, and this gives us a reassuring sense. But what we are talking about is not really a probability; it is about how well constrained we believe our beliefs are.
So here’s the really funky thing. People who are forever arguing in the public sphere that climate science is worthless and contemptible are basically arguing for the longest, fattest tails. These people, not believing in the science one little bit, cannot constrain the sensitivity of the system whatsoever. (Judith Curry at least had the decency to admit this – as I recall she once notoriously put the sensitivity between 0C and 10C per doubling.)
What does such a long tail mean to risk assessment? Of course, it matters what your distribution looks like. Is it centered at the IPCC range of 2.5 C per doubling? Perhaps you might argue for the zero-feedback 1.2 C, which apparently means your faith in science doesn’t even extend to established facts about how water evaporates. But if your personal prior doesn’t take much stock in climate science, then you are pretty much hosed, because your long tail extends into the catastrophic.
How It Works
Suppose you roll a pair of conventional dice. Your median roll, as is well known, is a seven. Now let’s play a game. We make a bet wherein you pay me the dollar amount of two raised to the power of your roll. If you roll a two, you pay me two squared dollars, a three, two cubed, etc. How much would I have to pay you up front to make this worth your while? Half the time you will roll less than a seven, and half the time more than a seven, so I propose 2^7 which is $128. You say you are indifferent to that bet, and suggest $256 instead.
You will be ahead 21 times out of 36, behind 10 times, and break even 5 times. A great bet, right? So what is your expected winning? I summon my magic snake and determine
>>> print 256 - reduce(lambda x,y: x+y,[2**(i + j + 2) for i in range(6) for j in range(6)]) / 36
Your expected winning is negative $185, because the cost goes up rapidly in the tail, which is fat enough to matter. So, even though you win more than twice as often as you lose, it is a bad bet.
How Not to Distrust Climate Science
Now, we know that costs go up nonlinearly with global temperature. Clearly, a tenth of a degree is noise, and ten degrees is massive redistribution of population and ecosystems at best. Far more than a hundred times worse. So the less you know, the more scared you should be. Now a couple of prominent people do take that position. Lovelock for one, but more influentially within science, Broecker. These guys believe that we don’t have a good handle on things at all, and so we should be much more scared than IPCC suggests.
Somehow, Dr. Curry is not in that camp. Although she sets herself up as an expert on uncertainty, it is unclear how she manages to be less than enormously alarmed with a 10 C sensitivity in her prior. But in this she is aligned with a lot of people who aren’t thinking rationally at all. Their “gut check” says it’s just a bunch of hooey, just as mine is pretty much unconvinceable on homeopathy. Indeed, the whole thing has a homeopathic smell to them: how can so little CO2 relative to the whole atmosphere make a difference?
But it’s not. The amount of extra CO2 is small relative to the volume of the atmosphere, but it is large compared to the volume of infrared-opaque gases in the atmosphere.
How much ink does it take to change the color of a tank of water, relative to the volume of the tank? It’s exactly the same question. CO2 is infrared-colored ink. It turns out to be not very much. And how does colored water behave in the presence of light? Well, differently than pure water, which is the point. So if there are still any sincere skeptics out there, most of them are operating on a gut check about how much invisible ink there is in a tank, a quantity which is not visible but is easily measured! Others are willing to throw away all of astrophysics along with climate science, and suggest that nothing about radiative transfer is known. How fat should their tails be? There’s no telling.
So let’s presume that the basics are right and we are in “lukewarmer” territory. “Yes it will warm”, they say. “So what? What are you guys panicking about?”
The Long Tale of the Real World
There are a number of components to the answer, but the long, fat tail is part of it. And the tail is suddenly getting fatter on two accounts. First is the astonishing behavior of the Chinese, which appears flatly unsustainable on a very short time frame for purely economic reasons. And while I wish no harm to the Chinese, we’d best hope that they not keep it up. Because if they do, suddenly the emissions picture looks like this:
Why are there three curves for China and only one for everybody else? Because the curves for China have been revised sharply upward over the past decade, while the other major energy consumers remained on track. So what does this mean? If it’s not that either China is in for a hard landing, or the rest of us are, I’d like to know what.
And second is the increasingly palpable collapse of decision making systems in most other countries, especially North America and Europe, and the increasingly confused ownership pattern of the corporations, which are mostly owned by other corporations. In other words, the SkyNet scenario may be upon us already. We are ruled by monstrous artifacts with nothing resembling a conscience. This means that even if we constrain the outcome perfectly given human behavior, human behavior looks increasingly likely to be out of human control.
The Biggest Uncertainty
Eventually the fatness of the tail is the fatness of our heads. What is your prior on the sanity of collective human behavior? I mean, given the Copenhagen fiasco and everything.
The biggets uncertainty by far has nothing at all to do with science. It has to do with the desperate acceleration of unsustainable behavior just as the limits to the behavior are coming into view.
We are behaving like the Easter Islanders in the Jared Diamond scenario, whether that turns scenario out to be true or not. (More on that another day.)
We have gone from vaguely seeing trouble on the horizon to doubling down on the statues.
The politically preferred answer to unsustainability that we perceive is to hurry up and break everything, as seems to have occurred in Easter Island. That makes for some pretty fat tails. And that makes for a drastically losing game. Can we change? How? How?
Images: Alligator via Flickr by Cimexus is in the Creative Commons (CC BY-ND 2.0). Cartoon courtesy Marc Roberts. Graph is a photo by the author of a slide presented by Trevor Houser at the first SXSWEco conference in Austin TX recently at the session ” Texas and China: Non-Obvious Energy and Environmental Bedfellows”.