I'm not unworried about it, I'm just more worried about other things. There's a chapter in Superfreakonomics on global warming that changed my thinking on it. Warming can be halted. Doing so requires interventions that would have unanticipated consequences--throwing sulfur into the stratosphere, seeding clouds over the oceans. But if disaster were at our door, we could do these things within a few years and halt the problem until we figured out a more permanent solution. The people who want us to worry about global warming don't just want to solve the problem; they want to use it as an excuse to solve other, more difficult environmental problems that some people don't agree are problems at all.
Man, the rule that we have to pay karma to reply to down-voted comments is not just stupid--on a blog that is specifically about rationality, it's hard for me to believe it isn't a deliberate attempt to enforce groupthink.
ADDED: I'm going to add further discussion into this comment, instead of paying 5 karma each time I reply.
It's odd to talk about possible irreparable harm to ecosystems, when we've already pretty much destroyed most ecosystems on Earth. 12% of the US is national Forest, but most of that is either mountains or desert. Everything between the Mississippi and the Rocky Mountains is gone. Oceans around the world are fished nearly empty, etc. That train has left the station.
I wonder if the shortness of my comment made people think it was some sort of snark rather than a legitimate question.
My understanding is that the experts are predicting irreparable harm to many ecosystems if action isn't taken soon (already? http://en.wikipedia.org/wiki/Avoiding_dangerous_climate_change). It seems to me that by the time policy-makers decide to take some of those interventions, it may be too late.
FOLLOWING YOUR EXAMPLE: Just because we've done SOME irreparable damage doesn't mean we should just heap on more. You don't junk your car after getting into a fender bender.
(Mathematicians may find this post painfully obvious.)
I read an interesting puzzle on Stephen Landsburg's blog that generated a lot of disagreement. Stephen offered to bet anyone $15,000 that the average results of a computer simulation, run 1 million times, would be close to his solution's prediction of the expected value.
Landsburg's solution is in fact correct. But the problem involves a probabilistic infinite series, a kind used often on less wrong in a context where one is offered some utility every time one flips a coin and it comes up heads, but loses everything if it ever comes up tails. Landsburg didn't justify the claim that a simulation could indicate the true expected outcome of this particular problem. Can we find similar-looking problems for which simulations give the wrong answer? Yes.
Here's Perl code to estimate by simulation the expected value of the series of terms 2^k / k from k = 1 to infinity, with a 50% chance of stopping after each term.
(If anyone knows how to enter a code block on this site, let me know. I used the "pre" tag, but the site stripped out my spaces anyway.)
Running it 5 times, we get the answers
ave sum=7.6035709716983
ave sum=8.47543819631431
ave sum=7.2618950097739
ave sum=8.26159741956747
ave sum=7.75774577340324
So the expected value is somewhere around 8?
No; the expected value is given by the sum of the harmonic series, which diverges, so it's infinite. Later terms in the series are exponentially larger, but exponentially less likely to appear.
Some of you are saying, "Of course the expected value of a divergent series can't be computed by simulation! Give me back my minute!" But many things we might simulate with computers, like the weather, the economy, or existential risk, are full of power law distributions that might not have a convergent expected value. People have observed before that this can cause problems for simulations (see The Black Swan). What I find interesting is that the output of the program above doesn't look like something inside it diverges. It looks almost normal. So you could run your simulation many times and believe that you had a grip on its expected outcome, yet be completely mistaken.
In real-life simulations (that sounds wrong, doesn't it?), there's often some system property that drifts slowly, and some critical value of that system property above which some distribution within the simulation diverges. Moving above that critical value doesn't suddenly change the output of the simulation in a way that gives an obvious warning. But the expected value of keeping that property below that critical value in the real-life system being simulated can be very high (or even infinite), with very little cost.
Is there a way to look at a simulation's outputs, and guess whether a particular property is near some such critical threshold? Better yet, is there a way to guess whether there exists some property in the system nearing some such threshold, even if you don't know what it is?
The October 19, 2012 issue of Science contains an article on just that question: "Anticipating critical transitions", Marten Scheffer et al., p. 344. It reviews 28 papers on systems and simulations, and lists about a dozen mathematical approaches used to estimate nearness to a critical point. These include:
So if you're modeling global warming, running your simulation a dozen times and averaging the results may be misleading. [1] Global temperature has sudden [2] dramatic transitions, and an exceptionally large and sudden one (15C in one million years) neatly spans the Earth's greatest extinction event so far on the Permian-Triassic boundary [3]. It's more important to figure out what the critical parameter is and where its critical point is than to try and estimate how many years it will be before Manhattan is underwater. The "expected rise in water level per year" may not be easily-answerable by simulation [4].
And if you're thinking about betting Stephen Landsburg $15,000 on the outcome of a simulation, make sure his series converges first. [5]
[1] Not that I'm particularly worried about global warming.
[2] Geologically sudden.
[3] Sun et al., "Lethally hot temperatures during the early Triassic greenhouse", Science 338 (Oct. 19 2012) p.366, see p. 368. Having just pointed out that an increase of .000015C/yr counts as a "sudden" global warming event, I feel obligated to also point out that the current increase is about .02C/yr.
[4] It will be answerable by simulation, since rise in water level can't be infinite. But you may need a lot more simulations than you think.
[5] Better yet, don't bet against Stephen Landsburg.