My New Favorite Conference Title

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My New Favorite Conference Title

Julianne at 1:00 pm, May 9th, 2008

I just received the following workshop announcement:

11th Birmingham-Nottingham Extragalactic Workshop - 1st Announcement

“Semi-analytic models - are we kidding ourselves?“

Refeshingly honest conference title aside, this is a terrific topic for a workshop. Semi-analytic galaxy formation models are extremely useful tools, which consist of (1) an underlying prescription for the growth of dark matter halos and (2) a set of knobs for grafting complicated baryonic physics onto those halos. The first step is well-understood analytically, and has been reliably calibrated with N-body simulations. The second step, however, contains a lot of crafty juju (how much gas winds up inside the dark matter halos? At what rate does that gas cool? How and when does that gas convert into stars? How does the formation of stars and the subsequent supernovae affect the surrounding gas? How do mergers between dark matter halos change the spatial distribution of the stars and the temperature structure of the gas?). The developers of these models are smart folks, and make reasonably well-informed assumptions about all of these baryonic processes, leading to results that are decent matches to the ensemble properties of the galaxy populations. (Note that I didn’t say “predict results” — these models are ex post facto in most usages.) However, just because the models can be tuned to produce a rough statistical match to observations in no way means that the input assumptions are correct or unique descriptions of what actually happens. Moreover, there are serious discrepancies between the models and the observed properties of very low mass galaxies — when the models are tuned to match the properties of relatively massive galaxies, they predict that the low mass galaxies are red and gas poor, whereas the observations say they’re blue and gas rich. It’s great that the community is looking at these issues head on, given the usefulness the semi-analytic galaxy models.

25 Comments on “My New Favorite Conference Title” rss feed

Sam Gralla on May 10th, 2008 at 9:13 pm

Hi Julianne,

I hesitate to bring this up, but have you looked in to the science of global warming at all? The case for “alarmist” global warming has its basis in simulations which are even more complicated and tuned than the ones you mention here. I have long thought that physics/astonomy people with expertise in such areas should provide an independent review of the science of global warming, at least as far as simulations are concerned. But when I mention it to such people, they generally don’t seem interested. Personally, I see this as only secondarily related to the issue of whether Kyoto etc. is a good idea. Mainly it is an integrity of science issue. In my opinion, climate scientists, by expressing overwhelming confidence in their simulation-based claims, are quite simply misleading the public. But I’m no expert… so I would love it if one would look in to it. Have you?

-Sam
Dave on May 10th, 2008 at 10:24 pm

Dear Sam,

As an astronomer who was originally trained as a physicist, I can tell you that it would not help one iota to have “physics/astronomy people” provide an independent review of climate science. The climate science community has already done this themselves, and they are appropriately conservative when reporting their results. Astronomers tend to make bolder claims based on much more dubious computer calculations.

The problem with climate science is that there is a well funded fringe of pseudo-scientists who make widely publicized attacks on mainstream climate science. We don’t have this in astronomy, because there isn’t anyone who stands to make billions by delaying action to slow global warming.

A good clue to indicate that that what you are reading is pseudo-science BS and not actual science is the term “alarmist”. A real scientific paper would never use such a term, and would focus on various technical details of particular simulations or assumptions. The term “alarmist” classifies calculations based upon their final result and not on the assumptions and calculational methods used.
Dave on May 10th, 2008 at 10:26 pm

Julianne - I would expect a very short meeting.

The first speaker would have only a one-word slide, “yes”, and then everyone could go home.
Aaron Bergman on May 10th, 2008 at 10:44 pm

In my opinion, climate scientists, by expressing overwhelming confidence in their simulation-based claims, are quite simply misleading the public. But I’m no expert…

I’m trying to find a way to say this without being too caustic, but that’s a pretty offensive pair of sentences right there, I have to say.
Sam Gralla on May 10th, 2008 at 11:40 pm

Well Dave and Aaron, your instant attacks are exactly why I didn’t want to bring this up. Hopefully Julianne will give me her expert opinion, rather than attacking me for daring to use the word “alarmist” or simply expressing my opinion (respectively). Julianne, please don’t be turned off by the immediate degeneration of the tone of discussion…
Dave on May 11th, 2008 at 12:15 am

Dear Sam,

I’m not attacking you for using the word “alarmist”. I only point out that this term does not appear in scientific papers - only in pseudo-scientific political writings. If you could find a real science paper that uses this term, I’d be very interested to see it.

Also, your claim of “overwhelming confidence in their simulation-based claims” is also pretty far off the mark. If you read any of the relevant NAS or IPCC reports, you will see a careful discussion of the uncertainties and rather cautious statements of the results. The climate scientists have argued about this for many years, and have reached a consensus. They also have a lot more data with which to check their computer models than astronomers generally do.
Aaron Bergman on May 11th, 2008 at 12:21 am

Do you understand the difference between saying something like “I wonder what sorts of independent checks are done on climate models” and “I think climate scientists don’t what they are doing and are lying to the public, but I’m not, you know, an expert”?
Julianne on May 11th, 2008 at 12:23 am

Sam — The semi-analytic models I’m describing are vastly less sophisticated than the full hydrodynamical climate models run by atmospheric scientists. Climate scientists also have the benefit of many testable time points — you can take your model, start it at 1980 and “extrapolate” out the next ~30 years to the present day, then compare to reality to see how well you did. If you did well, you might feel reasonably secure in extrapolating it out 30 years from the present. These sorts of exercises are exactly the sorts of tests that physicists and astronomers would imagine running before feeling confident in the results. Independent groups of climate scientists have already done this exercise, many many times, which has led to their confidence that there might be reasons to be alarmed. At this point, I think the odds of widespread errors or delusions are pretty small, and the “overwhelming confidence” has been well earned.
Mike M on May 11th, 2008 at 3:24 am

The first speaker would have only a one-word slide, “yes”, and then everyone could go home.

That rather depends on who ends up being selected to be first speaker at our workshop! And which “we” is being referred to in the title. There are certainly those who are deluded into thinking that semi-analytics provides a “real” view of the Universe, but there are also those who use it as an appropriate tool.

At least in my view, semi-analytics is at its most useful when it doesn’t work. When you take a very generic parametrized model of the Universe and try to twiddle all the knobs and dials and switches to reproduce the real thing, it is pretty much uninteresting when you succeed. But when you cannot reproduce the full set of observations then that is telling you that even your generalized fully-tunable model is missing some fundamental ingredient.

Unfortunately, most proponents of semi-analytics are more interested in advertising the successes of their codes than their failures.
Lab Lemming on May 11th, 2008 at 8:15 am

Sam,
Julia Hargreaves and James Annan are, respectively, an astrophysicist-turned climatologist and a mathematician-turned climatologist. They have a blog at:
http://julesandjames.blogspot.com/

One of James’ pet professional peeves is the shape of the probability density function of climate sensitivity (the global temperature change caused by a doubling of CO2).

A variety of methods give a climate sensitivity of around 3 degrees. But how fast this tails off as you get substantially higher or lower than this average is one of his areas of research.

A good place to start is his post “Climate sensitivity is 3C”:
http://julesandjames.blogspot.com/2006/03/climate-sensitivity-is-3c.html

In climatology, “Alarmist” can be defined as, “Anyone whose preferred sensitivity value is substantially higher than my own.” Similarly, the epithet “denialist” means “Anyone whose preferred sensitivity value is substantially lower than my own.”

Since we don’t know what your preferred value for climate sensitivity is, we have no way of knowing who you refer to when you use the “A-word”. But the links above (and papers referenced therein) explain why the actual value is unlikely to be less than 2 or greater than 4 degrees per doubling of CO2.
Sam Gralla on May 11th, 2008 at 11:16 am

Hi Julianne,

Thanks for the response. Do you think that if the galaxy-formation models became (with money, time, and effort) as sophisticated as the climate ones, then they too would be useful? I think that by “sophisticated” you probably mean either “take in to account more processes” or “do a better job of modelling these processes”. You mention full hydrodynamics, which presumably falls under the category of “do a better job”. But the models also contain–and are very sensitive to–many elements that are just as crudely modelled as what you mention from galaxy formulation (your “crafty juju”). For example, one of the big models changed its long-term prediction by a few degrees when the model for earth’s vegetation was changed from “one-leaf” to a supposedly more accurate regional cover model.

But I don’t think it’s worth focusing on climate in this forum. What would be most helpful to me is if you could point out the similarities and differences between the respective models, using your expertise from astronomy. The name of the game in climate simulations is to parameterize your ignorance of the details of all the physical processes you think should play a role, and then determine the free parameters by fitting to past data (the period from ~1980 you mention). As I understand it there are typically about 100 free parameters that are tuned in this way. Is there an anlogous step in semi-analytic galaxy formation, and how many free parameters are involved?

Also, is falsification of these models ever discussed (or they entirely post-dictive as you suggest)? The model-based IPCC predictions have been falsified at least twice, but only for “secondary” predictions–the artic ice melt rate and the warming of the oceans. Is there anything analogous in the galaxy-formation case?

Your response is eagerly awaited…

-Sam
Sam Gralla on May 11th, 2008 at 11:27 am

To others,

I presented my opinion of (much of) climate science and (many) climate scientists without justification. I qualified it as an opinion of a non-expert in the hopes that people wouldn’t get mad. Maybe I should have also said “Lots of people know more than I do, but I’m still allowed to have an opinion.” It seems obvious to me, but you can never be too careful in this world. In any case please understand that I wasn’t hoping to convince anybody to agree with me based on this one-sentence non-argument. I guess are should have made that clearer. I’m happy to try to convince you (or be convinced myself), but this isn’t the right forum. If anybody wants to discuss it with me, you are welcome to email me at capook@gmail.com, or stop by physics if anybody is in Chicago.

-Sam

Lab Lemming–Thanks for the pointer.
Ben on May 12th, 2008 at 2:40 am

Suppose we commissioned a bunch of climatologists to review the phenomenology and simulation of, for example, the CDF or D0 experiments. Or the WMAP data pipeline. So they pore over it for a year, trying to learn high energy particle experiment or microwave radio astronomy in the process. And at the end of the year they report “It looks great!” or “Everything you did was wrong!” Who’s going to believe them? This is why what Sam proposes, even if it were feasible (and even if it didn’t irritate climatologists, who would presumably see it as patronizing), would be an exercise that is not useful; it wouldn’t persuade anyone.

I’ll simplify the climate change issues under debate into two questions: (1) is anthropogenic global warming happening? (2) for a given rate of CO2 production, how large is the temperature rise going to be N years in the future? Basically, (1) is a question that can now be answered by looking at the record, since the models are actually constrained by the data, and (2) is extrapolation. The uncertainties in complex climate models mostly affect (2) now. However, many climate change skeptics persist in attacking (1), although virtually all serious climatologists now accept it as established. Testing the models, then, only affects the question of the future prospects: whether it looks sort of bad, bad, or really bad.

Sorry for continuing the threadjacking. I could try to make analogies to some issues in semi-analytic modeling, but for safety’s sake I better stop now.
Vagueofgodalming on May 12th, 2008 at 4:44 am

Here via Dynamics of Cats.

I’m intrigued. What is the alternative, apart from throwing up your hands and saying we won’t understand this until fresh observational or experimental evidence (driven by looking at other problems) comes along?

Is it the analytic or the semi- part that’s the issue? You’d think the more you can make something analytic the better, if only because it’s easier to explain what’s going on.

David
Sam Gralla on May 12th, 2008 at 11:28 am

Hi Ben,

The difference is that climate scientists don’t nearly have the expertise required to learn about particle physics experiment and theory, whereas physcists who already do simulations do (easily) have the expertise to learn about, and critically evaluate, climate models. They’re just fancier versions of the same old stuff physcists do all the time (I think including these semi-analytic models; I hope Julianne will respond to clarify that). I agree that it would irritate climate scientists, and convince few of them, but in my mind, as I’ve said, it’s about the integrity of science. The only real “practical” application would be to ensure that scientists maintain the trust of the public. The public (especially the right) is already quite mistrustful. If global warming doesn’t continue as the climate scientists tell congress it is going to, why will they ever believe us again? Why trust us when we say we’re sure of the big bang, or of evolution, if we were so confident, and yet so wrong, about climate change?

-Sam
Scott H. on May 12th, 2008 at 12:53 pm

climate scientists don’t nearly have the expertise required to learn about particle physics experiment and theory, whereas physcists who already do simulations do (easily) have the expertise to learn about, and critically evaluate, climate models

On what possible basis can you claim that physicists have the expertise to learn about the (highly coupled and nonlinear) dynamics of climate, yet climate experts don’t have the expertise to learn about physics experiments???

Sheesh; exhibit A for the public perception of physicist arrogance!
Count Iblis on May 12th, 2008 at 12:57 pm

If we physicists are well trained in something then it is being able to look at a complex problem and come up with a rough approximate model to estimate the quantities of interest. In fact, if some complicated computer simulation were to deviate substantially from a back of the envelope computation our first reaction would be to distrust the computer simulation.

We would only be convinced that there are no errors in the simulation if we could pin down why the simulation deviates so much from the rough estimate. But that is effectively upgrading your back of the envelope model.

Now, we already understand climate at the “back of the envelope level” and computer simulations agree with such rough estimates. The climate skeptics, who mostly lack physics training, are the ones who argue in favor of unphysically low values for climate sensitivity.
Sam Gralla on May 12th, 2008 at 3:50 pm

Hi Scott,

The expertise of physicists that I refer to is a familiarity with what level of quantitative significance should be ascribed to results from complicated models, given what tests on those models are applied. For example, when a graduate student mentioned to me the sensitivity of that model to a change in the way vegetation is handled, my immediate reaction was “why should I believe anything that has ever come out of these models, or anything that will come out of them in the future, until they demonstrate that their results converge on some value as they add more and more physical processes?” Isn’t that your reaction, too?

All I hear in physics and astronomy colloquia and discussions is how (complicated) simulations are sometimes nice to have around but can’t be trusted quantitatively. I can’t count how many times somebody starts a sentence about their work or idea with “if you believe simulations…”. So, we don’t trust simulations. Then I hear the climate pepole talking about their simulations, which are immeasurably more coupled, non-linear, and tuned/made up, so I suggest that maybe we shouldn’t believe those, either. Maybe we should even be purportionaly more distrustful of those. At this point, somebody starts talking about Kyoto, or the oil companies, or tells me what some other “skeptic” once told them, as in this thread. But I just want to know why we trust simple N-body simulations and the most complicated non-linear systems ever attempted, but nothing in between. It doesn’t make any sense to me. Do you at least see where I’m coming from?

(By the way, I’d love to continue this in private some time, if you’re interested. It can be very frustrating in a group or online, but I was hoping to get Julianne’s opinion, so I posted.)

-Sam
Sam Gralla on May 12th, 2008 at 4:18 pm

Now, we already understand climate at the “back of the envelope level” and computer simulations agree with such rough estimates.

I don’t think this is correct, but it would certainly make me feel better about the whole field if it were. My understanding is that the temperature increase from man-made c02 would be negligible without the many poorly understood feedbacks, which must be simulated (and some still can’t be, such as cloud cover changes).

However I should say that if your sentence does turn out to be correct, then, as honest scientists, that is what we should tell Congress. Instead, we have Hansen presenting his model predictions with dramatic errorbarless increases out a hundred years, or giant international bodies putting uncertainties on numbers from simulations that, in line with your above sentence, we know just aren’t useful.

-Sam
Scott H. on May 12th, 2008 at 4:44 pm

Hi Sam —

This is a much more nuanced explanation of your concerns than what you wrote above. The comment which pushed my button really reeked of “Me physicist so smart I figure it ALL out myself!” This is an attitude which some physicists certainly give off, and drives my non-physicist science colleagues absolutely crazy.

scott
Sam Gralla on May 12th, 2008 at 8:24 pm

Hi Scott,

Makes sense… I suppose I should always be explicit in sensitive matters. Let me mention for the sake of further harmony and agreement that should an engineer (or even a biologist!) also have experience with complicated simulations, I’d want that person’s opinion, too. I’m not sure about a social scientist, however (me physicist so smart definitely no need whatever they do).

BTW Julianne (I think) wrote a really funny post on this a while back… the punch line was how we love to talk about bioligists, enginners, chemists… but we never mention mathematicians because we secretly fear they are smarter than us :).

-Sam
Ben on May 12th, 2008 at 10:53 pm

The difference is that climate scientists don’t nearly have the expertise required to learn about particle physics experiment and theory, whereas physcists who already do simulations do (easily) have the expertise to learn about, and critically evaluate, climate models. They’re just fancier versions of the same old stuff physcists do all the time

This is very optimistic. At some level, all simulation codes are similar: you have a bunch of PDEs and are approximating them with a finite difference scheme. On the other hand, the devil is in the details. Have you ever written a CFD simulation code or tried to use, test, and verify one? I did for my thesis and it took me an entire year to get to the point of trusting it, even though I started by adopting almost wholly a code someone had already written, and even though the code was primitive by modern standards and solved a highly simplified CFD problem. I would not expect to be able to vet someone else’s code in a different field without a huge effort. People in different fields need to make entirely different sets of approximations, so for example the techniques used in CFD in astrophysics can be very different from those used in atmospheric studies or oceanography. Also, particle physicists, bless their hearts, are not all competent to teach fluid dynamics, which is widely regarded as a scary nonlinear subject, and worst of all, applied science.

when a graduate student mentioned to me the sensitivity of that model to a change in the way vegetation is handled

I think if you want to cast doubt on people’s work and demand some kind of high level review, it would help to do some more homework and not rely on anecdotal evidence.

Some useful, non-technical summaries of the history of climate modeling can be found at
http://www.aip.org/history/climate/simple.htm
http://www.aip.org/history/climate/GCM.htm
I strongly recommend the whole site
http://www.aip.org/history/climate/
Sam Gralla on May 13th, 2008 at 11:04 am

Hi Ben,

I think if you want to cast doubt on people’s work and demand some kind of high level review, it would help to do some more homework and not rely on anecdotal evidence.

I totally agree. Like I’ve said before I didn’t expect to convince anybody by the tidbits I mentioned on this thread. I only included them to give people a feel for the types of things I was worried about, to see if there was anything parallel in this galaxy formation case. This whole thread is part of doing my homework.

-Sam
Lab Lemming on May 13th, 2008 at 9:26 pm

Sam,
If you want to get a climate model and tinker with it, check out the edgcm site:
http://edgcm.columbia.edu/
This is the GISS model 2 (Hansen et al. 1983) code, originally written for old mainframes but now compiled to run on a modern desktop. Obviously it is a lot simpler than modern models, but you can run it overnight, or while you are doing the dishes, and have a play that way.

Note that since the American people own the source code, you can freely dl that and really get into it, should you so desire:
http://www.giss.nasa.gov/tools/

And most climate modellers are physicists. That’s what makes them so insufferable.

Hansen, J., G. Russell, D. Rind, P. Stone, A. Lacis, S. Lebedeff, R. Ruedy, and L. Travis, 1983: Efficient three-dimensional global models for climate studies: Models I and II. M. Weather Rev., 111, 609-662
Lab Lemming on May 13th, 2008 at 11:34 pm

Ahhh!
I tried to post links to the GISS model source code and desktop model 2 versions, but I think the spam filter much have thought it had too many links.

But all the government stuff is public domain, so you can look it up yourself if you like.

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