Sunday, May 09, 2010

Climate change priors and posteriors

I recommend this nice discussion of climate change on Andrew Gelman's blog. Physicist Phil, the guest-author of the post, gives his prior and posterior probability distribution for temperature sensitivity as a function of CO2 density. I guess I'm somewhere between Skeptic and Phil Prior.

As an aside, I think it is worth distinguishing between a situation where one has a high confidence level about a probability distribution (e.g., at an honest casino game like roulette or blackjack) versus in the real world, where even the pdf itself isn't known with any confidence (Knightian uncertainty). Personally, I am in the latter situation with climate science.

Here is an excerpt from a skeptic's comment on the post:

... So where are we on global climate change? We have some basic physics that predicts some warming caused by CO2, but a lot of positive and negative feedbacks that could amplify and attenuate temperature increases. We have computer models we can't trust for a variety of reasons. We have temperature station data that might have been corrupted by arbitrary "adjustments" to produce a warming trend. We have the north polar ice area decreasing, while the south polar ice area is constant or increasing. Next year an earth satellite will launch that should give us good measurements of polar ice thickness using radar. Let's hope that data doesn't get corrupted. We have some alternate theories to explain temperature increases such as cosmic ray flux. All this adds up to a confused and uncertain picture. The science is hardly "settled."

Finally the public is not buying AGW. Anyone with common sense can see that the big funding governments have poured into climate science has corrupted it. Until this whole thing gets an independent review from trustworthy people, it will not enjoy general acceptance. You can look for that at the ballot box next year.

For a dose of (justified?) certitude, see this angry letter, signed by numerous National Academy of Science members, that appeared in Science last week. See here for a systematic study of the record of expert predictions about complex systems. Scientists are only slightly less susceptible than others to group think.


Max said...

Anyone with common sense can see that the big funding governments have poured into climate science has corrupted it.

Well anyone using this angle frankly comes to me as either mentally retarded (I do consider iq 100 pretty close to retarded :) ) or hopelessly biased . The money side of climate deniers is entire world economy. Because 85% of world energy production comes from fossil fuels. Energy sector alone is trillion of dollars. And then you have transportation and then everything else.- Non coal and oil dependent sectors practically do not exist . And then you have "common people" whose jobs depend on ultimately on the energy. Everyone is scared of change

I did not research who really killed nuclear industry but willing to bet the current energy giants had stake in it , not very direct maybe ,but in term of more insidious media propaganda.And lobbying of course (nuclear powers plants are regulated to death while coal firing are allowed to throw radioactive ash straight into atmosphere at no cost, that without even taking into account environmental destruction caused by mining)

The global warming ultimately is an issue on whether we can control run away barbaric methods of manufacturing and production driven by mindless consumerism. That question was unfortunately answered negatively long time ago. No -we cant. Deniers are ultimately seeking justification and approval of their way of life, which is inconsistent with any sane responsible resource use, deep down they know it and its the reason they are so angry

klem said...

"Scientists are only slightly less susceptible than others to groupthink"

This comment brings to mind something which has bothered me for months now but no one has addressed to my knowledge; I refer to the 'errors' in the UN IPCC 2007 AR4 Report. For example the ridiculous claim that the Himalayan glaciers would be gone in 25 years. Why was this and other statements allowed to stand for 3 years without criticism? This report was met with silence from the scientific community; it wasn’t until Copenhagen was safely dead before anyone said anything. The failure of Copenhagen opened the floodgates, but why, what was different before Copenhagen that kept everyone silent? In my experience, a hugely important paper like the AR4 would not go 3 months without being cut to ribbons. I have great faith in the scientific community to speak up when there are questions about a published paper but I have found no satisfactory explanation for the silence. You’re a smart man, was this a 3 year long example of groupthink?

Guest said...

Anyone with common sense can see that the big funding governments have poured into particle physics has corrupted it. I mean come one, for 30 years these government funded scientists have been taking our money and telling us a Higgs or a superparticle is just around the corner. How long can they keep this going? Any "discoveries" coming from that massive tax boondogle in Switzerland will not enjoy general acceptance until they have been confirmed by trustworthy people.

oarobin said...

what i would like is the analysis that informs your prior?
also accusation of groupthink are easy to make especially if no evidence is provided. can you support your insinuation of groupthink?
do you have any reason to think that the five conclusion articulated in the Science are not robust and supported by faulty analysis?

steve hsu said...

I have observed strong tendencies toward groupthink in *every* context that I have observed closely: finance professionals (see last 2 bubbles), technology investors and technologists (Silicon Valley), educators (political correctness), economists (efficient markets), and scientists (even particle physicists -- recall the strong expectation that the top quark mass was around 40 GeV or the current strong expectation that we will find SUSY at the LHC or string theory itself).

This, together with Tetlock's and other similar studies (see Expert Predictions link), suggests to me that "experts" are almost always overconfident about predictions concerning complex systems. You never hear from the guys who are appropriately skeptical, because it is boring (or not instrumental) to hear the statement "we just don't know".

If particle physicists were asking you to bet 1% of global GDP on their conclusions concerning the Higgs, I would think it very wise to find some outside experts to check the conclusions before proceeding. I say this despite the fact that Higgs physics has far fewer uncertainties than modeling a complex system with limited time series data. If you don't understand the last sentence I suggest you do some homework.

oarobin said...

so you will agree that you have no strong evidence for group think in climate science only a general expectation that it is there?

also does this expectation of group think invalidate the science, i.e. start with the conclusions of the 255 person in the Science Article.

is it only time series analysis on which the conclusion of AGW is based or are there convergent lines of evidence from paleoclimatology and physics of the cryosphere?

what would an "outside expert" look like if not a working climatologist? ie. when one looks at the qualification and knowlegebase required to perform such an evaluation can you come up with persons who are experts but not working in the field?

finally if you think that there is insufficient data to model the climate to extract useful robust conclusion like those in the science article, then there are plenty of professional papers and blogs of working climate scientist that have discussed this:

i would greatly appreciate your engagement with them.

steve hsu said...

As I've said before on the blog, I'm not an expert in this area, and don't have the time to become one. Therefore I do not assign a high weight to my own pdf -- did you not understand the reference to Knightian Uncertainty?

But I do think you should take much more seriously the observation that "experts" predicting the behavior of complex systems almost always underestimate the uncertainties.

From an email exchange with a colleague who has spent significant time studying climate change:

> What is your confidence level in the following statements?

> 1) we know the sign of anthropogenic effects on global temperature

--> high

> 2) current models get the size of the temperature change in the next
> 50 years to within a factor of 2

--> medium

> 3) there are nearby nonlinear tipping points in the climate system
> (i.e., that anthropogenic effects can push us to within 50 years)

--> low (except for the methane channel which I am seriously concerned with)

> 4) climate modelers are clear-headed scientists with a mature grasp of
> the uncertainties in data analysis and modeling complex nonlinear
> systems, and NOT ideologically motivated people whose main skill is
> writing simple computer programs and press releases

--> highly variable - the real problem is "how do you know you have produced the best model "- there recently has been a lot of Markov chain analysis associated with trying to determine this.

Nanonymous said...

The letter is signed by NAS members without regard to their field of specialization. As such, many of them have no more formal credentials in climate science than any random Joe the plumber. I recognize only about 1/3 of the names but willing to bet that the majority are not even physicists, much less earth/climate scientists. This should be kept in mind when reading statements like "There is compelling, comprehensive, and consistent objective evidence that humans are changing the climate in ways that threaten our societies and the ecosystems on which we depend".

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