The only thing we can be sure of about the future is that it will be absolutely fantastic. So if what I say now seems to you to be very reasonable, then I have failed completely. Only if what I tell you appears absolutely unbelievable, have we any chance of visualizing the future as it really will happen.

- Arthur C. Clarke (h/t Brin)

Friday, May 2, 2008

The Falsifiability Question

OK, the new meme among the denialists is that the tide is with them, that their evidence is overwhelming, that we must be religious zealots not to be able to see the overwhelming evidence that, um, that "very not what the IPCC says". I think we need to talk about the balance of evidence, someday, but I'd like to address the "falsifiability question", i.e., what evidence would it take to shake me of my firm "belief" in AGW.

There is some implication that there is an "AGW theory" and that there is an argument in its support, and that said argument is a cohesive thread starting with Fourier and ending at the dreaded-extremist-boogeyman-Gore, and that failure of any chain in said argument necessarily implies "see, so no carbon policy is necessary". (I'm missing a few steps in their reasoning here, too, but that's another topic still.)

I claim there is no "AGW theory" in the sense that there is an argument that four colors suffice, or more fairly, that stars follow an evolutionary path based on their mass. AGW is not an organizing principle of climate theory at all.

Hypotheses, organizing principles, of this sort emerge from the fabric of a science as a consequence of a search for unifying principles. The organizing principles of climatology come from various threads, but I'd mention the oceanographic sysyntheses of Sverdrup and Stommel, the atmospheric syntheses of Charney and Lorenz, paleoclimatological studies from ice and mud core field work, and computational work starting with no less than Johnny von Neumann.

The expectation of AGW does not organize this work. It emerges from this work. It's not a theory, it's a consequence of the theory.

Admittedly it's a pretty important consequence, and that's why the governments of the world have tried to sort out what the science says with the IPCC and its predecessors. That tends to color which work gets done and which doesn't, and I think it should. As Andy Revkin pointed out, it may be time to move toward a service-oriented climatology, or what I have called applied climatology. The point is that this amounts to application of a theory that emerged and reached mathematical and conceptual maturity entirely independent of worry about climate change.

So attacks on climate change as if it were a "theory" make very little sense. Greenhouse gas accumulation is a fact. Radiative properties of greenhouse gases are factual. The climate is not going to stay the same. It can't stay the same. Staying the same would violate physics; specifically it would violate the law of energy conservation. Something has to change.

The simplest consequence is that the surface will warm up. That this is indeed most of what happens is validated pretty much in observations, in paleodata, in theory and in simulation. Further, all those lines of evidence converge pretty much about how much warming: about 2.5 C to 3C for each doubling of CO2. (It's logarithmic in total CO2, not in emitted CO2, guys, by the way.) There's no single line of reasoning for this. There are multiple lines of evidence.

If you want to convince me that the sensitivity is less than 2 or more than 4, you will have to provide quite a good deal of evidence, but I don;t think this is what the denialists have in mind when they ask me what would "falsify the hypothesis". In fact, though, they haven't defined their terms. If the sensititivity is less than 1, is the supposed hypothesis falsified? What if it is more than 6? If the onset time is a hundred years rather than ten?

They want to know what it would take to pry my free of my "beliefs", but they are not beliefs, they are estimates. Estimates of the sensitivity (2.8 C per doubling). Estimates of the built-in delays (about twenty years for full effect of current concentrations). Estimates of the threshold of excessively high social risk (some range here but I go with 2 C ~ 450 ppmv).

What would it take to change my opinion of the threshhold to 451 ppmv? A nice dinner at Fonda San Miguel, margaritas included, would surely do it. If that constitutes a falsification, bring it on.

Really, though, I don't understand the question. If these numbers wobble around a bit that might shift the optimum policy a bit, but we're so far from the optimum now that it's not worth putting much thought into it yet. The numbers, however, are never going away. There will always be a sensitivity, a response curve, a risk threshold. If you are asking what evidence could make me believe that there are no such numbers, I can't actually imagine it.

That's not because I have a blind attachment to some theory. It's because the numbers must exist, and we have lots of evidence as to what they might be.

What would it take to change my estimate of the numbers? That depends by how much. What would it take to convince me that the meteorology and oceanography I have learned is wrong? I don't know. What would it take to falsify any mature quantitative science?

14 comments:

apa tycka, apa skriva said...
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Jon said...

Great post. You articulated a lot of thoughts running around in my head regarding the 'falsification' meme clearly and concisely. If this isn't cross-posted at your Wired blog, it should be.

Michael Tobis said...

Thanks!

KCET owns everything I write over there, so I can't cross-post.

Also this really isn't the most accessible thing I could write on the subject. Maybe I'll try writing something similar for them, though.

Chris Colose said...

Good post. I'm going to add this blog to my blogroll.

From basic physical principles, AGW just follows from physics, such as radiative transfer, fluid dynamics, etc. I think that from a purely rigorous view, a falsification would require something absurd like showing that CO2 does not absorb infrared radiation, but since that's just not going to happen, AGW does carry some baggage to it along with just that...such as

- climate sensitivity is not negligible (I'll say greater than 2 C). Societal impacts will also be non-negligible

- Water Vapor increases with temperature to increase the absolute humidity but keep relative humidty pretty constant, and is thus a positive feedback to the GHG-induced warming.

- Expectations in a warmer climate, such as polar amplification and expectations of AGW, such as stratospheric cooling will be borne out

There's not much to convince me that any of this is wrong. If "skeptics" (not exactly the best word) want to make an argument, then they'll need to stop the nonsensical arguments such as "CO2 lags temperature" and "but it cooled in mid-century!" and boasting their arguments on fraudulent foundations of OISM, Inhofe's gang, etc. I would say that an appropriate rebuttal to AGW would also include something coherent from "the other side."

Michael Tobis said...

Some discussion about this post here on an anti-superstition site: http://forums.randi.org/showthread.php?p=3892650

more or less as an indirect consequence of the word "skeptic" being appropriated by the delay squad.

Ian said...
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Anonymous said...

Hi
Would you explain, please, why "CO2 lags temperature " is a "ridiculous" argument. At least it means that historically CO2 did not cause increases in temperature

Michael Tobis said...

I don't see the word 'ridiculous' anywhere on this posting or the comments to it, and I doubt I would have used in this context, so I don't see why I should feel obligated to defend or clarify anything.

To answer your question, CO2 is not the only influence on climate.

It appears to lag temperature in the onset of the last glacial cycle especially in the southern hemisphere. The relationship between geochemistry and CO2 on that time scale in nature is not a solved problem, but it is pretty much conventional wisdom that the deglacitations are kicked off by solar forcing at the edge of the expanded ice sheets and not by CO2.

If CO2 has no great effect, explaining the 100,000 year cycle becomes dramatically more difficult, not less so.

That said, the local warming before the onset of northern hemisphere deglaciation in Antarctica is indeed peculiar and not well explained, but then again neither are most of the other bumps and wiggles in the record. It may be coincidental.

Nobody has made a significant scientific case about this event, as far as I know. The denialists have picked it as one of their favorite refutations but it really doesn't refute much of anything.

This peculiarity in the Antarctic ice cores just shows that CO2 is not the only influence. It doesn't show that CO2 has no influence. Of course, it can't do that, because reality has the highly useful property of being consistent.

climatesight said...

Very nice post, it sort of clarified some topics which were swirling around in my head, the "how could it not change climate?" idea is very true. The burden of proof is really on the skeptics to explain how we couldn't change the climate.

If some new explanation came up which proved Arrhenius wrong (and really proved, not Watts-says-so proved), something which turned our understanding of radiative physics on its head, then I would accept that we weren't changing the climate. But until then, the prospect is too illogical.

Michael Tobis said...

Kate, yes, thanks.

You should blame Fourier, not Arrhenius, for the fundamental physics. Arrhenius was first to work out the arithmetic, but the principles go back to Fourier around 1820.

See http://www.aip.org/history/climate/simple.htm

And of course, if we don't understand radiative physics, we also don't have any satellite measurements of the earth, or really any idea at all about planets and stars. That is a lot of apparently successful work to contemplate throwing away!

georgesdelatour said...

Hi Michael

Sorry if I'm in the slow class here.

I thought that, without any feedbacks, a doubling of CO2 would result in 1°C of global warming. The IPCC estimates that positive feedbacks will multiply this to produce around 3ºC of global warming.

This is crucial, because a warming of the planet by 1ºC is barely newsworthy, whereas 3ºC is a real problem.

So what are the feedbacks? Mainly water vapor, ice-albedo & clouds. How well do we understand them? Are they all positive? Might clouds, for instance, have a mixture of positive and negative feedback effects?

I think the 3ºC figure has a lot going for it, and is probably right. But the assumptions this figure is based on could be proved false, if the future climate diverges significantly from the IPCC predictions.

Michael Tobis said...

Georges,

That is essentially correct, yes. Multiple lines of evidence converge on the net closed-loop amplification being close to 3 C. But if the earth had no water and simpler surface processes, I believe the number comes out to 1.2 C if I recall correctly.

There is little doubt that water vapor feedback is an amplifying effect; the phenomenon is straightforward. Quantifying it is more difficult.

Should the closed loop factor be different from 3.0 C, it will not constitute a falsification. If it turns out less than 1.5 or more than 5, it will be very surprising, and it will be an indication that something important has been missed, most likely. Some phenomenon we would have missed. It's very unlikely.

But for the climate not to change at all under such forcing, that is essentially impossible. So when people ask what it would take to falsify, they're not asking a well-posed question.

llewelly said...


"I thought that, without any feedbacks, a doubling of CO2 would result in 1°C of global warming. The IPCC estimates that positive feedbacks will multiply this to produce around 3ºC of global warming."

The figure of 3 C for a doubling of CO2 is the Charney sensitivity. It leaves out several aspects of climate because they were (or are) poorly understood or hard to simulate. These include ice sheets and the carbon cycle. Melting ice sheets reduces the planetary albedo, which in turn increases warming. Melting of ice sheets is a positive feedback not included in the 3C figure. Warming of arctic and subarctic soils melts permafrost. Melting permafrost releases CO2. Another positive feedback not in the Charney sensitivity. Warming enables beetle infestations in boreal forests, which kill trees, releasing CO2. Large expanses of dead trees increase fire risk, resulting further carbon emissions. More exotic positive feedbacks may also be significant, and yet not in the Charney sensitivity. Please read Real climate for an in-depth discussion.

tullfrog said...

I think you are committing something of a straw man on this one and will endeavour to provide you with a more cogent argument against AGW concerning Falsifiability so that your refutations have more credence.
As Sir Karl Popper originally argued, falsification, not validation, is the criterion of a scientific theory. That is, in order for a theory to be scientific it must make risky (the more so the better) testable predictions.

Einstein's theory of relativity is a great example of this. Eddington, in accordance with Einstein's theory of relativity predicted that during a total eclipse, some light from the sun would still be seen, as the moon would bend the light so it could be seen, even though the moon was directly between the sun and earth. In this way, he was able to test Einstein's theory of relativity, either the predicted phenomena would be observed and the theory would be strengthened by a genuine failure to falsify it, or it wouldn't be and the theory would be discarded.

The criticism of AGW is that it does not make risky, testable predictions, so is not falsifiable. Following from my previous premise, if AGW is not falsifiable, it is not scientific.

You claim that AGW is a consequence of a theory rather than a theory itself. I think that from this we can say that AGW isn't all that scientific then. It's like NASA trying to predict where one of their spacecraft will land, it's too applied, there are too many extraneous details for anything like a reasonable prediction.