Evidence (and global warming)

by Chris Bertram on October 7, 2003

Suppose there are two possible states of the world, S1 and S2, and we don’t know which of the two states the world is in. An event E occurs which is consistent with the world being in either S1 or S2, but is more likely in S1 than it is in S2. We should surely say that, given E, the world is more likely to be in S1 than in S2, and that _to that extent_ E (though consistent with both possible states) is evidence for the world’s being in S1.

Such evidence isn’t, of course, conclusive. After all, by hypothesis, E is _consistent_ with both possible states. But evidence doesn’t need to be conclusive evidence to count as evidence.

That sensible view of what evidence is “doesn’t appear to be shared by new enviroblogger Professor Philip Stott”:http://greenspin.blogspot.com/2003_10_01_greenspin_archive.html#106545283636725804 , whom I welcome to the blogosphere in the traditional way – by arguing with him.

Stott writes:

bq. I have just been contemplating two recent polls on ‘global warming’. They are fascinating and unexpectedly consistent, though quite different questions. First, my ever-vigilent younger daughter has just let me have the position of a current poll on Discovery Europe (choose UK option):

bq. Does global warming concern you?
No: 48%
A little: 12%
Very much: 39%
[As at this posting]

bq. Secondly, here are the results of a poll held by The Scientific Alliance (see my Links):

bq. Do you believe that this summer’s exceptionally hot temperatures were evidence of climate change?
Yes: 40%
No: 48%
May be: 12%

bq. These are surprisingly mature results – public opinion might be changing.

I take it (by implication) that Stott believes that the 48 per cent who rejected the view that the exceptionally hot temperatures were evidence of climate change were giving the right answer. But surely they weren’t. No-one but a fool would think that Europe’s hot summer was _conclusive_ evidence for climate change. But , as noted above, evidence can be evidence without being conclusive. Indeed it can be evidence even if it just raises our confidence in a proposition by a tiny degree. Europe’s summer should have made believers in climate change a bit more confident in their beliefs and should have made sceptics a bit less sceptical. Typically, that’s what evidence does.



Vance Maverick 10.07.03 at 12:00 pm

What strikes me about these poll results (assuming we take them at all seriously) is the small number of votes for “A little” and “May be”. Seems very polarized — which might be maturity, I suppose, or self-selection.


Brian Weatherson 10.07.03 at 12:02 pm

I love it when blogs start like this:

The purpose is not to take up a particular position on a given subject (e.g., ‘global warming’), but to assess whether the topic is being fairly covered by press, radio, and television.

Do you think we’ll see many (any?) articles complaining about (parts of) the press understating the dangers of global warming? Do you think that would probable if it was a fair coverage?

There’s obviously nothing wrong with pushing a line on a blog, but I always get annoyed by partisans who try and pretend they’re the balanced one and everyone else has biases.


Marc 10.07.03 at 12:43 pm

The overall tone of the global warming sections does not inspire confidence. There is a great deal
of evidence that human-induced changes in the atmosphere are causing
climate change. There is real debate about how much warming there will be, and what the consequences are. If the bloggers over there were concentrating on a critique of exaggerations of the problem – that’s a service. When they post denialist rhetoric, such as their dismissal of sea level rises, they are simply pushing a weak scientific case.



Jonathan Ichikawa 10.07.03 at 1:30 pm

The principle you site isn’t obvious to me. Suppose you were trying to decide whether or not I’m a Martian, diguised as Jonathan. You happen to know that Martians require orange juice every morning to survive. So if I’m a Martian, there is, say, a 99% chance of my drinking orange juice in the morning. But you know from past experience that Jonathan drinks orange juice 75% of all mornings.

Surely if you see me drinking orange juice, this isn’t evidence that I’m a Martian?

(Of course, I’m making no claim as to whether the global warming statistics look anything like these, or whether the conclusion about global warming you suggest is correct.)


Nabakov 10.07.03 at 1:32 pm

I followed Stoat’s link through to his article at the “The Scientific Alliance” and, as someone who has helped set up more than one or two upmarket/trade astroturf groups m’self, my nostrils were twitching.

I think I can recognise when a $800 an hour PR firm has been giving neckrubs to copy and presentation. The pale blue color scheme is a nice touch. So natural and calming and yet so…not green.

And this line really caught my eye…”establishment of nuclear power, the only realistic medium-term alternative to hydrocarbons.”

Can you think of any other industry with a shitpot of potential investors (especially those with a vested interest in the construction and regional planning industries), so desperate to rehabiliate its public image.

Disclaimer: I have shares in and deal in power companies moving heavily into wind and tidal power – One hundredth the setup costs per megawatt compared to nukes and the worst scenerio is a gannet or lobster gets occasionally clipped.


Nabakov 10.07.03 at 1:39 pm

Yo, Jonathan I.

After reading your post – yep you’re a martian.

And it’s “cites” not “sites”


tim 10.07.03 at 2:02 pm

I’m not sure I understand the point of the poll.
The interesting question is whether the hot summer was the consequence of global climate change – and more specifically, a global climate change triggered by increased CO2 production.
It either is, or it isn’t. No poll of a scientifically illiterate population is going to settle the score.

Furthermore there are some obvious problems with your S1 and S2 example. That is, after all, exactly the tactic of the creationists. S1 – species evolved from a common ancestor. S2 – God created us all, species do not evolve into other species. The creationist can pick any bit of evidence for which science does not have an explanation, and it can be explained well by omnipotence and omniscience. Here is some material from a creation science web site:
There are no transitional links and intermediate forms in either the fossil record or the modern world. Therefore, there is no actual evidence that evolution has occurred either in the past or the present.
Natural selection (the supposed evolution mechanism, along with mutations) is incapable of advancing an organism to a “higher-order”.
Although evolutionists state that life resulted from non-life, matter resulted from nothing, and humans resulted from animals, each of these is an impossibility of science and the natural world.
Natural selection can be seen to have insurmountable social and practical inconsistencies.
Natural selection has severe logical inconsistencies.
The rock strata finds (layers of buried fossils) are better explained by a universal flood than by evolution.
[each of these items is elaborated upon on the website.]
Is this evidence for creationism? It certainly is if your S1 and S2 hypothetical is your model for evidence.
What we really require of a scientific theory is coherence. (There is an excellent American Scientist article of a few years past which turns the tables on the creationists, and shows that creationism is incoherent and filled with contradictions. Well worth reading if you find yourself encountering creationists.) Vague anecdotal evidence is not enough.
Is the hot European summer (or the rainy northeast American summer) evidence of global climate change? It is if it is part of a good coherent theory of the effects of increased CO2 on global climate and is not part of a good coherent theory of climate in which increased atmospheric CO2 content does not lead to dramatic changes in the climate. But that’s not the same at all as saying that it is consistent with the idea that the climate was different this year than it was in recent memory, therefore man-made emissions are changing the climate.
Climate change is not my specialty, and I haven’t made any effort to keep up with the field, so I don’t know if the European and northeast American summers fit one particular coherent model better than another. It may be a little involved for a blog, but could you make the case for those of us who are genuinely interested in the question?


Nabakov 10.07.03 at 2:16 pm

Although I’d think we’d all agree current climate patterns are incoherent to say the least.

And here’s the gonzo response to Stoat and his “Scientific Alliance’.



Matt McIrvin 10.07.03 at 2:17 pm

E is evidence for S1, but whether it means that S1 is more likely than S2 depends on the conditional probabilities of E given S1 and S2, and what the prior probabilities of S1 and S2 were. It’s Bayes’ Theorem again.

A single hot summer is evidence, but not strong evidence, since its probability in the case of S1 and S2 is almost equal. Indeed, it may be such weak evidence that it’s not worth citing as evidence, especially if much better evidence already exists (such as the long-term trend of rising measured temperatures).


Matt McIrvin 10.07.03 at 2:20 pm

…Now, you could argue that although the evidential decisiveness of a single hot summer is small, it might still be politically useful. But that is a dangerous game to play, since you’ll get clobbered the next time there’s an unusually cold winter. Better to play to your argument’s strengths.


Chris 10.07.03 at 2:25 pm

Matt, indeed. But I’m not clear about the connection between your first and second paras (in the first of your comments). Suppose our theory were to tell us that in case S1 such a hot summer would happen once a decade, but in case S2 it would occur once in a thousand years (extreme figures picked for illustrative purposes only ). In that case, a single such hot summer would make me feel pretty confident that we were in S1 … don’t you agree?


Dick Thompson 10.07.03 at 2:36 pm

The evidence of a hot summer is not evidence for global warming, since hot summers have always happened. A few years ago we had a very hot summer in the US midwest. You may remember, almost 100 people died in Chicago. And all the global warming proponents were saying see! But the hot summer has succeeded by cold winters, and cool springs,… and cool summers.

The way out of this useless anecdotal barrage is climatic science. But climatic science doesn’t seem to be strong enough to do the job, and the anti warming folks are perfectly right to point out that many of the scientists are interested parties.


Paul 10.07.03 at 2:43 pm

EnviroSpin supersedes Stott’s earlierAntiEcohype blog. ‘Greenspin’ sounds fairer and more balanced than ‘probiotech,’ I suppose. The Scientific Alliance is your basic pro-business lobby masquerading as an educational organization.

Of course, none of that necessarily means that Stott’s conclusions are wrong, but I agree with Nabokov; lack of full disclosure makes me less likely to trust anything Stott says.


Chris 10.07.03 at 2:53 pm

Dick, the thread is not, basically, about global warming at all, but about the nature of evidence: i.e. bits of information that give people reason to revise their set of beliefs. I quite agree that we need good science on global warming as on everything else. But that doesn’t mean that human beings trying to decide what they believe about it are irrational if they allow themselves to be guided by their everyday experience (including of the weather). The hot summer in Chicago was certainly evidence that people could reasonably invoke in the sense I just outlined, but so were the cool periods that followed (for the contrary proposition.


Bungo 10.07.03 at 3:20 pm

Isn’t this just the question of whether you are a Bayesian or not? If there is a 90 per cent chance that that you will say yes to cake only one time in ten and a 10 per cent chance that you will say yes to cake nine times in ten and then I find that you do say yes to cake it would be 50/50 wether you liked cake or not. This depends upon how much you are willing ot leave to chance and knowledge of a priori probabilities.
Also, to put numbers to Jonathan’s example, if the probability in the preceding is changed to 99 per cent and 1 per cent, it is still more likely that you say no most often.


tim 10.07.03 at 3:27 pm

That’s exactly what’s wrong with the hot summer as evidence of global warming: “The hot summer in Chicago was certainly evidence that people could reasonably invoke in the sense I just outlined, but so were the cool periods that followed (for the contrary proposition.”

The sense you outlined is nonsense. We’ve had a couple of extra cool days where I live. Is that evidence for the onset of an ice age? If tomorrow is unseasonably hot, should I change my mind again? or only if I get more unseasonably hot days this week than cold days?

If you don’t know what the climate theories are, or how the data fits those theories, the data doesn’t tell you anything.



a different chris 10.07.03 at 3:41 pm

>the anti warming folks are perfectly right to point out that many of the scientists are interested parties.

“Interested” in what? I’m curious??

Tim, good point. “Warming” refers to an overall trend, but those that “know what the climate theories are” know that events outside normal statistical distribution are expected. The fact that one or the other summers may not be terribly abnormal doesn’t bother the model, because it’s predictions are matching up with the fact that most of the warmest summers ever recorded have occured in the past decade.

As have most of the worst snowstorms.

So thanks for supporting this opportunity to enlighten the scoffers.

This is a different chris, reporting from Pennsylvania where we have just set the all-time record for precipitation.


Chris 10.07.03 at 3:52 pm

Tim, writing of “changing your mind” suggests switching from believing P, to not-P, to P again. But that’s most unlikely. Rather, the thought is that people experience tiny, perhaps imperceptible, shifts in their set of beliefs such that they think P a bit more or a bit less likely than they did previously. (Though sometimes tiny shifts can set off avalanches!).

Obviously, I agree that a single hot (or cold) day shouldn’t make a difference. But then again, lots and lots of them might….

But generally, I think your approach is much too austere. Citizens, as opposed to scientists, have to make their minds up about something like climate change in the face of many competing arguments, articles in the media, spin from lobbyists etc etc. Perhaps you think that they shouldn’t have an opinion at all and should leave it to the scientists? I’m happy with the idea that in coming to decide whether to believe in climate change or not, whether to believe these scientists or those scientists, this lobbyist or that lobbyists, citizens may rationally give _some small amount of weight_ to an event like Europe’s hot summer.


dsquared 10.07.03 at 4:04 pm

If you don’t know what the climate theories are, or how the data fits those theories, the data doesn’t tell you anything.

Decent couple of theorems in Bayesian statistics say it does …


Ted W. 10.07.03 at 4:07 pm

Bayes’s rule is fine for the Martian example, but because it requires you to have an estimate of P{S1} and P{S2} it begs the question when it comes to global warming. (That is, you have P{E|S1} but want P{S1|E}. To get that, you use P{S1|E}=P{E|S1}xP{S1}/P{E}. Same for P{S2|E}: P{S2|E}=P{E|S2}xP{S2}/P{E}.) If you are completely knowledgeless about whether global warming is happening, then P{S1}=P{S2} and it’s easy to see from P{E|S1}>P{E|S2} that P{S1|E}>P{S2|E}. That’s Chris’s reasoning, as I take it.

The problem is that P{S1} shouldn’t be thought of as equal to P{S2}. It makes for tidy logic, but it dismisses too much hard scientific work in the field. I see it as a capitulation to the anti-environmentalists’ (and also the creationists’) demands for phony balance in debate.

I claim that P{S1}>P{S2}. A global-warming naysayer would claim that P{S1}


Ian 10.07.03 at 4:47 pm

There is also the arguemt that global warming/increases in CO2 are natural phenomena and therfore no amount of cutting back on CO2 emissions will make any difference. These arguments however ignore the trends in hot summers/cold winters (remember the issue is really climate change not just summer heat) and say nothing about how we should respond – especially since there are one or two more humans around than the last time this sort of climatic shift is said to have occurred.

As for Prof. Stott’s site, something smelt wrong when I read it – having read the comments here I think I now know what it was – the smell of undercover corporate funding.


Kramer 10.07.03 at 4:51 pm

One technical point which hasn’t been made yet, and which might help to clarify the dialogue, is that global warming (as it is used by climate scientists) implies globally averaged warming. This means that even trends (of a decade to several decades in length) in temperature in a particular locale should not be taken as evidence for or against global warming.

In recognition of this point there is a good deal of work focused on the consequences of a globally averaged warming for regional climates (I don’t have the links for the relevant reports handy – sorry about that). Most often what is found is that in a globally averaged climate that is warming regional climates will eventually warm (you have to in order to get the global average after all) but in the short term (where short is ~1-20 years) most regional climates change by having a lot more extreme weather events (more really hot summers, more really cold winters, maybe more hurricanes if you live somewhere like the eastern US).

Taken in this light it seems entirely reasonable (and indeed even well informed) to take an especially hot summer (but perhaps even an especially snowy winter) as evidence of global warming.


Chris 10.07.03 at 4:58 pm

Ted W. You are clearly more competent in this area than I am, but I’m not sure that you’re right that I need the assumption that S1 and S2 are equiprobable. All I need is that given E the probability of S1 increases from what it was before (and that P{S2} decreases). So, suppose I’m a dyed-in-the wool envirosceptic, who believes that P{S2} is very high (say .9), the occurence of a (on my view very unlikely hot summer) ought at least to diminish the probability I assign to P{S2} somewhat, even if I continue to believe that S2 is overwhelmingly likely.


Ryan 10.07.03 at 5:15 pm

It’s a shame he doesn’t have comments on the blog. His latest post is about how a recent story about organic food was not reported in the British press.

“Ten maize meal products have been voluntarily withdrawn from sale in the UK after tests showed that they contained high levels of toxins called fumonisins. …
High levels of fumonisins have been shown to cause liver and kidney damage in animals after they have eaten them over a long period….
It is possible that they could have the same effect on people if they eat high levels of fumonisins for long periods….
However, there is unlikely to be any significant risk to health if these products are consumed.”

“Why were these two stories not covered by the British media, I wonder?” he writes. Well, it’s hardly the BSE crisis, is it?


Ted W. 10.07.03 at 5:41 pm

Chris, I wouldn’t say more competent, just that I knew which chapter in the textbook to look the stuff up in. It’s not that P{S1} is bigger given E, but that P{S1|E}>P{S1}. In words, the probability of S1 given that E is the case is bigger than the probability of S1 when you know nothing about E. (The way these things are usually defined in this kind of model, P{S1} and P{S2} implicitly include your ignorance of E, and your knowledge of E is dealt with by using the conditional probability. Just a tidier way of expressing your notion, which I basically agree with.) You would also say that P{S2|E}averages are busy at work instead of blogreading.) With this E, P{S1|E}P{S2}. More important, P{E|S1}enough bigger than P{S2} for E not to change your mind. In fact, you can figure out how much bigger P{S1} has to be: something like this (and I’m not sure here) must be true: P{S1}/P{S2}>P{E|S2}/P{E|S1}. That is, the ratio of your opinion of the two situations has to be at least as big as the degree to which a non-warming world is likelier to produce cool days.

But yes, the cool stretch does make you less sure about S1.


Ted W. 10.07.03 at 5:43 pm

(But I’m not a probability expert. Someone please correct me if I’m doing this stuff wrong.)


bungo 10.07.03 at 5:45 pm

If Prof Stott starts with his mind made up P(S2)>>P(S1) and takes the view that temperatures fluctuate a lot and remembers running around in short pants in 1976 he may not give very different conditional proabilities to the hot summer event. In that case he still ends up with P(S2) >> P(S1) and I suggest that most people can’t tell the difference between a 96 per cent proability and a 95 per cent proability over any extended period.

In short, if Prof Stott is behaving like a Bayesian learning machine he should end up assigning a marginally hihger probability to the possibility of global warming but he won’t be as confirmed as someone with a less determined view. If he is not behaving rationally and like many bloggers just concerned with whether or not there is any viable way of supporting his view, he will have taken on nothing. Even if he didn’t also believe that a very hot summer was not unlikely, starting with a decided enough view could still leave him pretty decided.

In short, you don’t need the equiproability but it does make a difference to the size of the effect, as will his determination of the relevant conditional proabilities.

Where he might be vulnerable is in his interpretation of the event. There is a difference between is this possible given what I believe and is this likely. That is, if prof Stott was asked what is the proability that the temperature will reach 100F in the UK last January, given his environmental assumptions what would he have said? Without such a clear assumption the goalposts can be moved all over the place.


Prof. Ted W. 10.07.03 at 6:04 pm

Also: P{C|T}>P{C}, where C is that a blogger reaches for sketchy intellectual crutches and T is that the blogger makes the point of prominently calling himself Professor.


Philip Stott 10.07.03 at 6:11 pm

Hi Everyone, I am delighted by your discussion of my new Weblog. Thanks for finding time to visit. Can I kill off some of the personal speculation though? I am an entirely independent academic (Professor Emeritus of Biogeography) with no links to the fossil fuel or agricultural industries whatsoever. I have no corporate funding, and I would not accept any (I value my independence too much). I am mildy left wing (both Old and New Labour, for their sins), passionately anti-smoking, and primarily interested in the science that isn’t reported. I think the real question at the moment is why the British media are dead quiet about the very fine work of Veizer and Shaviv (scroll down), which frankly could sink the Good Ship ‘Global Warming’. [The poll stuff was just an idle blog on something that was brought to my attention.] I am also particularly concerned about the way in which environmental constructs developed in the rich North impinge on the developing world.

Thanks again for such interesting comments. Philip.


tim 10.07.03 at 6:37 pm

Chris thinks my approach “is much too austere. Citizens, as opposed to scientists, have to make their minds up about something like climate change in the face of many competing arguments, articles in the media, spin from lobbyists etc etc.”

So, is it better to have a firm opinion, or is it better to be correct? Citizens have to make up their minds about all kinds of issues – like the alleged increased incidence of leukemia due to electromagnetic fields from power lines. And there is plenty of anecdotal evidence out there to support it.

If you know someone who contracted leukemia, and you know there are high tension power lines nearby, then shouldn’t you (I’m using your sense of evidence), believe that the power lines cause leukemia. After all, as a citizen, it is important that you have a belief about this!

Or, take a more absurd case. Someone has discovered a bomb. There are two wires sticking out, one defuses it and one explodes it. That would be a pretty important thing to have an opinion about, especially since there is a timer on it that is ticking down. As citizens, should we leave this to the experts? Or isn’t it vital that we form an opinion (perhaps based on movies, perhaps based on standard wiring color conventions, perhaps based on “warning colors”), take a poll and cut a wire.

Certainly my position is austere, but if the truth value of the belief matters, then it seems that’s what you are stuck with.


ogged 10.07.03 at 6:44 pm

Chris, let me come at this in a slightly different way (though I may be restating some previous comments). You seem to say that a single hot summer is an event that conveys some–albeit very little–information. But it’s not clear to me that, given what’s at issue, a single hot summer should count as an “event.”

Imagine that you are in charge of quality control at a bottling plant. Generally, one bottle in every hundred contains an impurity. Would it be reasonable for you to test the first bottle after it was one-fifth full, find it pure, and declare that your confidence in a clean batch of one hundred had increased? Strictly speaking, yes. But (and here we get into the criteria for calling something “evidence”) I don’t think you would be reasonable to draw any conclusions from your pure one-fifth bottle. It tells you so little that nothing follows from it; it is, if I may, “theoretical data.”

So, your “sensible view” is correct. But it’s deployment in a discussion of global warming seems inapposite.


jam 10.07.03 at 6:53 pm

If E is within three sigmas for both S1 and S2, then it’s meaningless; it shouldn’t be taken as evidence either way. If E is outside 3 sigmas for S1 and inside for S2, then you can take it as evidence for S2.


msg 10.07.03 at 7:12 pm

a. ‘Global warming’ sounds like a great big round sweater.
Global fever might be a more accurate description.
Chills, sweating, cell death, delirium, inanition, convulsions, all seem consistently parallel.
b. There’s an awful lot of first-hand information around, including glacier melt, Arctic ice thinning prematurely, seasonal boundary movement, freak weather occurrences proliferating… Enough that it makes serious argument seem more a mental health problem than a scientific one.
c. The moral implications are interesting, in that the scope of the crime is so vast and the reality of it so unthinkably horrible. And conclusive proof will only be available after it’s much too late to matter.
Convicting, as opposed to accusing, someone of destroying the mammalian branch of the tree of life, doesn’t have an appropriate place on the schedule of events.
d. The individuals most responsible, most culpable, have all along been in rigid control of the primary means of research, and the media for the dissemination of the results of that research. Which is why this ‘topic’ is still seen as debatable by so many.


dsquared 10.07.03 at 7:15 pm

If E is within three sigmas for both S1 and S2, then it’s meaningless; it shouldn’t be taken as evidence either way. If E is outside 3 sigmas for S1 and inside for S2, then you can take it as evidence for S2.

This is not true at all if one is using a rigorous definition and only good as a rule of thumb under quite restrictive assumptions about the distribution of E.

In any case, “within three sigmas” of what? You can only make sense of this sentence if you have an accurate estimate of the mean (more properly, the entire pdf or distribution) of E, which is exactly what we’re trying to estimate here.


markus 10.07.03 at 7:18 pm


S. Baum 10.07.03 at 7:51 pm

Yep, nothing signals a dispassionate observer just trying to get to the real facts like, e.g.

“Ouch! If Shaviv and Veizer are correct, then the Good Ships ‘Global Warming’ and ‘Kyoto Protocol’ are severely holed below the water line and sinking fast.”

I really need to start a collection of such quotes from all the “dispassionate” observers who’ve danced jigs on the grave of global warming over the last decade with the release of each and every paper containing even the most tenuous of evidence ostensibly proving the CO2/global warming hypothesis incorrect.

The Canuck and I have discussed becoming turncoats for a few years, getting filthy stinking rich from all the oil company money that’d pour in, and then writing a tell-all expose upon retiring to an exotic island to drink ourselves to death. The subtle flaw to the plan is – given that our laziness far outstrips our desire to give such folks a black eye – our livers would give out long before we got the first paragraph on paper. Well, that and our fear of being publicly labeled as “disgruntled former employees.”


Bob 10.07.03 at 7:56 pm

Yeah, that’s interesting. Could you use papers as events, and estimate P{warming|Shaviv&Veizer}? And then with all the other papers out there?


Philip Stott 10.07.03 at 7:56 pm

Facts, facts! Not just me running around in shorts in 1976, Bungo! Just a few Gradgrind facts about the summer, folks! It wasn’t quite that hot! In the Scotland Mean Temperature Series, begun in 1961, August was only the fourth warmest; in the England and Wales Series (same date), it was again only the fourth warmest.

The highest recorded European temperature was in Seville, Spain, August 4, 1881 (50 degrees Celsius). By continent, the highest recorded temperatures were: Australia, 1889; Latin America, 1905; Oceania, 1912; North America, 1913; Africa, 1922; Asia, 1942; Antarctica, 1972. Not much pattern there! Also, bear in mind that modern temperature measurements using thermistors are likely to record higher temperatures than the old ‘mercury-in-glass’ thermometers because they pick up every momentary fluctuation. An old high occurred at 36.1 degrees Celsius in 1911 at Camden Square, London. Also, some argue that temperatures during the Medieval Warm Period (1100-1300) were 0.7 – 1.6 degrees higher than today.

Sorry to bring a little reality into the debate, folks. Philip.


Chris 10.07.03 at 8:14 pm

Whatever the mainstream media have done, at least this blog hasn’t ignored Shaviv and Veizer. “An earlier post”:https://www.crookedtimber.org/archives/000470.html linked to “Shaviv’s page”:http://www.fiz.huji.ac.il/~shaviv/Ice-ages/ice-main.html and quoted him as follows:

bq. Some of the global warming is still because of us humans (probably about 1/3 to 1/2 of the warming).

As I remarked then, 1/3 to 1/2 is quite a lot, and if you are up to your neck in water then a few extra inches can make all the difference whatever the cause of the underlying few feet.


Philip Stott 10.07.03 at 8:23 pm

Now with regard to the two states, S1 and S2: climate changes every second, every minute, every hour, every month, every year, every decade, every century, every millennium, and onwards. Temperatures are always either rising or falling, although they can do both regionally. It is a truism, therefore, that there is always either ‘global warming’ or ‘global cooling’.

Within these changes are the hidden and linked, and possibly chaotic, influences of many drivers or cycles working at different time scales. A brief change towards what appears to be one state, S1, may simply be a brief mask for a longer move to another state, S2. It is currently arguable that the most probable move in the near future is to another Ice Age. Over the last 700,000 years, the warm periods (like our own Holocene period) have lasted, on average, only 10,000 -12,000 years. The Holocene is now well over 10,000 years old. On purely statistical grounds, we are due to enter a new Ice Age, although there is some evidence that the present pattern of the so-called Milankovitch cycles may delay this a little. My guess ultimately is the Ice Age – from the climate optimum of 8000 years ago, the smoothed-out curve remains resolutely downwards.

Chilly evening. Philip.


mgl 10.07.03 at 8:37 pm

In response to the statement:

“…the anti warming folks are perfectly right to point out that many of the scientists are interested parties.”

A Different Chris asks:

“‘Interested’ in what? I’m curious??”

A better question might be why we should assume that scientists are not interested parties. Scientists are economic actors like any other; competition for research money is fierce at the best of times, and more so when there are several tens of millions of dollars sloshing around out there for scientists who decide to investigate climate change-related matters.

I work at a research institute at a Canadian university, and I can tell you first-hand that climate change is sustaining a great many academic careers right now (including mine, in an indirect way). I’m not saying that’s wrong, but we should keep in mind that individual scientists are just as interested in the outcome of the climate change debate as corporations.

I’ll illustrate with an anecdote: a climate scientist at my university has become quite well-known for his work with the IPCC. Ten years ago he was a relatively obscure scientist at a small Canadian university; today he’s much sought after by the media for soundbites and information on climate change. He travels around the world to speak to conferences on his climate work, and his opinion is actively solicited by many people in government and the business community. His research work is thriving, with lots of government money to develop his work further. In a lot of ways, he’s living the scientist’s dream life. According to people who know him, he greatly enjoys his position as Well-Known Expert. Some say he enjoys it a little too much, but they’re probably jealous.

Now, if the climate change edifice were to collapse (and I’m not saying it will, or it should), he’d go back to being a talented but essentially ordinary research scientist. It’s not much of a stretch to think that he may regard this as an undesirable outcome, perhaps a very undesirable one. So, yes. Of course he has an interest.

It’s irritating to see people (including many in the comments above) who seem to think that any hint of corporate influence renders an argument null and void, but fail to acknowledge that the IPCC is–whatever its virtues–an enormous multi-national gravy train for a host of scientists, bureaucrats, and other hangers-on.

Money corrupts, sure, but so does power, so does influence, and so does fame–for some. Scientists are never disinterested, unemotional observers of natural phenomena, and we do a disservice to the cause of knowledge when we pretend that they are.


tim 10.07.03 at 8:40 pm

Chris, writing on citizens coming to conclusions about things scientific: “I quite agree that we need good science on global warming as on everything else. But that doesn?t mean that human beings trying to decide what they believe about it are irrational if they allow themselves to be guided by their everyday experience (including of the weather).”

The problem is that there is lots of science in which the intuition of human beings is wrong, and in which the everyday experience of human beings is also wrong. Example: Laws of motion for the former, and Relativity/Quantum Mechanics for the latter.

And please note that evolution as the origin of the species is not grounded in our everyday human experience, either. Our everyday experience is one in which no new species emerge – the only change to the count is through extinction.

The world is full of people who doubt that Einstein’s Special Relativity is true because their everyday human experience suggests to them that the world operates on principles of Galilean Relativity.


Chris 10.07.03 at 9:05 pm

Hey Tim, sure, we have to make our minds up about things under circumstances and with equipment that’s less that optimal. No disagreement there.


back40 10.07.03 at 9:07 pm

You might find this debate at the Boston Review New Democracy Forum relevant. Charles Sabel, Archon Fung, and Bradley Karkkainen have an interesting and controversial essay Beyond Backyard Environmentalism: How communities are quietly refashioning environmental regulation that sees NIMBY instincts as the nucleus of a more sophisticated, localized and democratic style of environmentalism. It seems that Chris is arguing for something similar to this, the value of contextual intelligence which when aggregated makes a useful contribution to policy decisions.


tim 10.07.03 at 9:32 pm

Chris: “sure, we have to make our minds up about things under circumstances and with equipment that?s less that optimal. No disagreement there.”

Look, it is as simple as this: given only the vaguest sense of a theory of climate change (which is all most lay people have: humans are or aren’t messing their nest), assessing the truth of various theories of climate change from points of data like “This was the hottest summer in Europe that I can remember” is a particularly unreliable. Just as is making an assessment about the relationship between leukemia and power lines based on a friend’s contact with both. Or deciding which wire to cut based on some vague, incomplete, or wrong theory about how a bomb is wired.

What is important is not having beliefs. Everyone has beliefs. What is important is having correct beliefs. And sometimes that means deferring to the experts. (How many of you who believe that Einstein’s theories of relativity were correct have actual personal proof of the same?) And sometimes, in particular, that means ignoring one’s everyday experience.

In the case of global climate change, what is important is not coming to a belief about it, what is important is coming to a *true* belief about it.

And it is far from obvious that our “everyday experience (including of the weather)” is going to be a reliable guide. Even when compiled over a decade of weather watching. And especially when believer doesn’t have a clear sense of the theories of global climate change or how they fit with the current, broader scientific consensus about things geophysical or chemical.

The right answer to a poll about whether the European summer is evidence of global warming is, for the vast majority of people, “I don’t know.”

The experts have their own opinions, about which I make no judgement here for all the reasons expressed above – it’s not my field, nor have I followed it as an interested scientist.


S. Baum 10.07.03 at 9:55 pm

Given that damned near anybody who studies anything is an “interested party” – including those who attempt to debunk the anthropogenically-induced global warming theories advanced by “interested parties” – is it possible to come up with a more disingenuous and inane attempt at debunking such theories? Until recently, I was part of the great global warming conspiracy research community, but my desire for money outweighed my obsessive need to further the aims of the conspiracy so I switched from climatology to physical oceanography. But golly, if I’d just known at the time that my research would have been lent more credibility had I taken a poverty vow and crawled through dumpsters to satisfy my material needs while I was researching and writing papers, you can bet I’d be quite a few pounds lighter today. I really did try to apply sufficient world-weariness and apathy to the research such that I wouldn’t be corrupted by it, though, but that had to stop when I started coming in at noon and leaving at four, leading my overly interested employer to express extreme disapproval and threaten to terminate my interest in the project altogether.

I’d suggest to those that choose to continue promulgating this particularly empty canard to call a plumber the next time they’re feeling unwell. After all, we can’t have an interested party mucking things up when we’re trying to find the Truth.


Chris 10.07.03 at 9:57 pm

Tim, I don’t especially disagree with you! I’ve certainly never suggested that people make up their minds entirely (or even mostly) on the basis of one hot summer.

You write:

bq. What is important is having correct beliefs. And sometimes that means deferring to the experts.

Agreed. Unfortunately that requires that people decide _which_ of the many self-proclaimed experts to defer to, which are real, which are bogus. Other things being equal people are likely to grant authority to experts whose testimony is supported by other elements in their experience (including their experience of the weather). Hopefully, though, there are other and better things to go on too.

Having said that, if a person gives some weight to that fact that the weather is unusually hot in deciding whether to trust expert A or expert B, I’d qualify that as less epistemically vicious than deciding on the basis of which expert’s story they’d most like to be true – unfortunately a much commoner method of selection!


mgl 10.07.03 at 10:02 pm

s. baum,

I don’t know what gives you the impression that I’m a global warming debunker, or that I think there’s any sort of conspiracy taking place, but the intemperate and sarcastic tone of your reply speaks for itself, I think.

Scientists are interested parties, as much so as any other player in the debate. There’s nothing wrong with that at all. I apologize if I misled you into thinking I meant anything else.


Russell L. Carter 10.07.03 at 10:04 pm

Philip Stott, with regard to “The Center for the Study of Carbon Dioxide and Global Change”, the source for your “scientific” commentary on the Shaviv and Veizer paper you so enthusiastically endorse, I have a question: where do they get their funding? I note that the linked commentary is authored by the top three officers, (who appear to be related) of that org; the Chairman is an employee of Peabody Energy, and the outfit is advised mostly by people “associated” with Arizona State University. They did manage to rope in Donald Hodel.

Given the outright political propaganda function of this organization, how does this square with your quest to “assess whether a subject is being fairly covered by press, radio, and television?”

Just curious, thanks, the important issue here is where does that organization get its money.

You weren’t duped, were you? These people clearly don’t value *their* independence. What good is yours, if you just go off and borrow their conflicted opinion?


greg 10.07.03 at 10:15 pm

Fascinating thread! From the nature of evidence; it’s impact on the perception and belief systems of individuals; the implications on policy making, motive “analysis” (ad hominem), emotional tensions, gerrymandered facts hurled across fences, and even theorems in Bayesian statistics (which are way beyond me), etc. I can’t quite figure out if I want to laugh or cry — both I guess.

The total system of what we call climate — can any of you really comprehend something so vast? Can anyone? My experience suggests that when systems of such magnitude are discussed, the discussion eventually deteriorates into entrenched positions. Especially in the limited format of the comments section of a blog!

Even still, Global Worming Fact or Fiction? Certainly a question of momentous import? Let’s say, for the sake of argument, we wait another 20 years until we gather more data (assuming it is gathered and analyzed in good faith – which personally I doubt could happen). In addition, at that time in the future we agree that it is more likely than not that the climate is changing due in large part to human activity related to fossil fuel consumption. How fast do you think we can change direction? I refer you to an article from a different perspective – System Dynamics.

While I recognize, as Chris has stated, “the thread is not, basically, about global warming at all, but about the nature of evidence: i.e. bits of information that give people reason to revise their set of beliefs.” I do believe global warming warrants serious investigation and the efforts of all to keep the common good as a guiding principle throughout the debate.


mgl 10.07.03 at 10:22 pm


Can an opinion be conflicted? A person certainly can, and I’ll accept (solely for the sake of argument) that Shaviv, Veizer et al. are up to their eyeballs in evil corporate funding.

But their article appeared in a respected journal, and the authors are working academics at the Hebrew University of Jerusalem and Ruhr Universität, so perhaps we can’t dismiss their work out of hand, eh? Why not judge it on its own merits, regardless of the funding or the affiliations of its authors?

Or is it simply that the only conceivably “interested” scientists are those whose work questions the IPCC consensus?


Russell L. Carter 10.07.03 at 10:39 pm

I ought to put my own opinion on GW out there, just so it’s clear that my first post is not just a knee jerk reaction against evidence conflicting with my internal belief system. I personally don’t know with any certainty what the effects are of the facts on the ground. The effects are all that matter. And as greg suggests, I doubt anybody else knows either. In the face of this uncertainty, a measure of prudence would seem to be a small price to pay until we understand things better. That of course runs smack into anti-statist ideology, and the whole debate gets mired down and rendered useless.

So I don’t know, maybe that paper pointed to by Stott means we’re saved! But he’ll need to find better vetters to keep the audience from laughing him off the stage.


mgl 10.07.03 at 10:57 pm

Well put, Russell, though I guess we define prudence in different ways. My perception is that the balance of evidence seems to be quite favourable to the IPCC’s climate change hypothesis. But I’m not sure that the policy path is quite so clear-cut.

You seem to be advocating some variety of the precautionary principle, in which we should act for fear of the potential consequences of not acting. But action also carries the risk of undesirable consequences, and it is not at all clear to me that the policy remedies on offer (i.e. Kyoto) will not be disastrous wastes of scarce resources. The IPCC scientist I used as an example, above, acknowledges that Kyoto will have little to no effect on climate, even if fully implemented by all Annex I nations, including the US. He thinks we should ratify anyway, “to send the right message” but that seems to me an extraordinarily foolish rationale for such a far-reaching and potentially expensive policy.


dsquared 10.07.03 at 10:58 pm

If you want to know what genuinely “interested” (in the Hayekian sense) parties to the global warming debate think, when they have a lot of their own money riding on the issue and no axe to grind one way or another, try to buy long-dated reinsurance against catastrophic flood damage some time.

I also think that anyone who gives it some of that (I’m making the “rabbit rabbit yak yak” gesture with my right hand now) about how scientists are “interested parties” because they really want to be famous and their research grants process is politically biased, has to be absolutely scrupulous in noting the oil affiliations of the other side if I am to take his claims to objectivity at all seriously.


mgl 10.07.03 at 11:19 pm

Well, that’s precisely the point, Daniel. I find myself irritated by arguments that rely heavily (as many of the comments here do) on the political leanings, institutional affiliations, or perceived “interests” of parties to the climate change debate. My original post was pointing out that if you want to play that game, no-one is disinterested. I’d rather see people play the ball, rather than the man.

Look, it’s hardly a secret that scientists compete for research funding, and that climate change has opened up many new such funding opportunities. It’s also hardly a secret that the answers you get depend on the questions you ask, and there are an awful lot of questions being asked about the projected effects of climate change on ecosystem X, or community Y. The question presumes at the outset that climate change is occurring, and that it will likely have an effect, so it’s no surprise that researchers tend to find effects. None of this means that the researchers are corrupt, or conspiring–it’s just messy reality.

It seems to me that questioning someone’s politics or funding provides an easy (not to say braindead) excuse for ignoring their actual argument. It’s something to be aware of, sure, but surely their arguments stand or fall on their own merits?

My exemplary IPCC climate scientist is, as far as I know, not intentionally dishonest in any way. But (in public forums) he’s awfully quick to pick on someone’s political or institutional ties in an attempt to discredit them. While I trust his science, I am much less sure of his politics, but he pretends that they are one and the same. As such, he is inviting the same criticism.


Gil 10.07.03 at 11:26 pm

There isn’t S1 and S2.

There is S1 and ~S1. This “Evidence” is not known to be more probable in S1 than ~S1 so it’s not evidence at all and shouldn’t change anybody’s confidence.


a different chris 10.07.03 at 11:45 pm

>Now, if the climate change edifice were to collapse (and I’m not saying it will, or it should), he’d go back to being a talented but essentially ordinary research scientist.

But wouldn’t the same thing happen if human induced global warming was accepted as fait accompli?

I don’t mean to demean, but does this guy know how to make a CO2 free automobile? Design livable cities? Can he build a windmill? No, he reads charts for a living.

So I could half-jokingly contend that he’s probably withholding his best evidence, because as soon as everybody says “ok, you’re right” then the spotlight immediately moves from him to somebody who might be able to friggin’ do something about it.

So I don’t see how he has the incentive that Exxon-Mobile does. In fact, I don’t see why he would have risked such a major tilt at that big, er, windmill*, unless he was pretty confident in his data.

Everybody has interests, but some of them are more credible to society in general than others.

*Actually, if Don Quixote was re-written today would he in fact, be charging oil wells?


mgl 10.08.03 at 12:17 am

Man, you’re even more cynical than me, ADC. I was just trying to ask–in a confrontationally oblique way, admittedly–why it’s seemingly de rigeur to question the “interests” of climate change skeptics, but outrageous to question those of proponents (illustrated by s. baum’s convenient reply). My scientist example may not have interests equivalent to those of Exxon-Mobil in absolute terms, but I’d argue that to him they are at least as important, and therefore drive his actions to some degree. I honestly don’t know why this is a controversial position, except to those who automatically associate “interests” with venal and corrupt behaviour.

One can have interests and act with the best intentions, and still get it wrong. Or not. As I’ve said in my other posts, I’d be happiest if we could dispense with the silly interests ad hominem altogether and try to focus on the merits of the arguments.


a different chris 10.08.03 at 12:55 am

mgl- apologies, but this is a pet peeve of mine. I actually *do* basically believe what you are saying. The thing that gets me is, like in this thread and 1000 others, it’s the the guy on the right that first throws that mud. The old “liberal professors want to control your lives” canard vs. “my people are just interested in science, the loss of billions of dollars doesn’t really bother them that much” is just taken as such an article of faith by these people I can’t stand it.

And, in fact, I won’t let it stand. So I’ve vowed to never let it pass unchallenged.

Now, to the erstwhile Philip. Listen dude, here’s another introduction to blogging: read the g’dammed comments. Several of us had already made the point about multiple events, so don’t give us that hoary crap about wearing your shorts and sweaty days in Victorian England. I would take it as just over-enthusiastic quick posting by a newbie, except for this little gem:

>Sorry to bring a little reality into the debate, folks. Philip.

My response to that know-it-all gbg, phrased in a way that I think matches the climatic tone of the thread, is:

“(what a strong wind does) me”


markus 10.08.03 at 1:20 am

the point mgl is, that you seem to be arguing for some sort of equivalence of interests here and I’d say you’re death wrong if that’s your argument. Could you please clarify whether you were arguing for equivalence or merely bringing up the fact that non-corporate researchers have motivations, too?


mgl 10.08.03 at 3:17 am

ADC, I’m not even sure what “right” and “left” have to do with this; you didn’t catch the terms in anything I wrote, since I consider them pretty much meaningless. But our perceptions differ, I guess, if you meant that climate change skeptics are those on the “right” who are the first to sling mud. My perception is that each side is happy enough to claim some special insight into the other’s motives and character, but that climate change proponents are usually the first to imply that those who disagree are Bad People, rather than sincere critics of what they see as misguided science or public policy. For the record once again, I’m only half Bad, since I accept the science (though I expect further big surprises), but am extremely skeptical of the policy. But this is not really an empirical question; I’m tired of seeing the climate change debate shoehorned into a made-for-TV-movie plot, in which the Evil Corporate Minions attempt to thwart the Plucky Blue-Helmeted Idealists. You see it differently. Agree to disagree? And thanks for the civil conversation.


Russell L. Carter 10.08.03 at 4:17 am

“you didn’t catch the terms in anything I wrote, since I consider them pretty much meaningless”

Well said! I agree. No need to reiterate.


bigring55t 10.08.03 at 4:36 am

To be succinct, the reason to question the motivations of enviro skeptics is the same reason one would question the motivations of those scientists that work for the tobacco companies and continue to produce “research” that shows smoking isn’t deleterious to your health.


mgl 10.08.03 at 5:20 am

Thanks for illustrating my point, bigring55t.


s. baum 10.08.03 at 5:57 am

Mum always said that intemperance and sarcasm would be my downfall. I guess I’ll have to retract all those climate research papers now that my betters have taken the bloody piss out of me. Funny thing is, I don’t get intemperate and sarcastic until somebody impugns my motives, e.g.

“But climatic science doesn’t seem to be strong enough to do the job, and the anti warming folks are perfectly right to point out that many of the scientists are interested parties.”

That’s about as empty a statement as one can make, even if one cared to offer an operational definition of “interested” that didn’t cut at least equally both ways. Or, to put it another way, discussing motives is boring and pointless vis a vis whether or not anthropogenic contributions are indeed warming the planet. On the other hand, topics such as the statistics of extreme events, the parallelization of finite difference (and other) numerical methods, the parameterization of turbulent processes, the modeling of radiation processes in the atmosphere, etc. are both interesting and very much to the point. On yet another hand, if that’s still too intemperate and sarcastic for the local delicate (as opposed to climate) sensitivities, then I’ll get back to a nasty PDL script whose last (hopefully) nasty bug still eludes me and leave the important stuff to the medieval scholars.


msg 10.08.03 at 6:11 am

The levels of erudition and mannered discourse here, though fluctuating, are still higher than any I’m used to elsewhere. Which makes me feel like Buddy Hackett somehow.
But the topic is one that a lot of us are vitally concerned with, some to the point of despair.
There is much more than a trickle of first hand evidence of major climate shift, or change, it’s an avalanche now, even through the fog of backspin and intentionally planted hysteria you can hear it.
The circumpolar people were talking about it a few years back, hunters dependent on ice holding, on animals being in a certain phase of their cycle, finding things coming earlier than ever, or later. Though recently I haven’t been hearing much from up there.
Wasn’t the Matterhorn closed to climbing this summer because of the danger of thawing and consequent rock fall?
Didn’t some reputable journal state unequivocally that every glacier on the planet is shrinking, and more rapidly than previously thought?
I’m suggesting that the people on the front lines of this subject, the field researchers themselves, are being driven to hysteria or numb apathy by their findings.
I’m humbly suggesting that the possiblity of climate change on the scale it’s proposed, and what it implies, is so enormous, and horrifying, that it’s causing neurotic reactions in almost all of us.
Denial and an excessive interest in the dissection of minutiae, the pathologies of shock.
And I’d like to suggest that intellectual prowess is a lot like wealth, an equivalent responsibility attends its gifts.


mgl 10.08.03 at 6:39 am

Sorry, s. baum. I thought you were responding to my post. It seems you weren’t. On this:

…discussing motives is boring and pointless vis a vis whether or not anthropogenic contributions are indeed warming the planet.

…I couldn’t agree more. Thanks.


Jack 10.08.03 at 7:44 am

I think it is interesting to discover from exploring Chris’s point that, because Bayesian learning depends upon initial assumptions, the same piece of information might not draw two bayesian learners closer together, even if they are not disingenuous, at least in the short run.

I also wonder if people really do engage in Bayesian learing on this sort of issue. It actually seems more plausible that they engage in some kind of dynamic Lakatosian (urk, is that a word?) monster barring exercise. That is instead of deciding between a number of static theories in the light of subsequent events and initial assumptions, they instead recalibrate and rebuild their theory to make it work with the known history of events as a central proposition.

If that is normal human operating procedure, what does prof Stott have to do to demonstrate the good faith necessary for dialogue?

I think it unlikely that people carry fixed or accurate probabilities for certain events around in their heads with any degree of reliability. In particular people are quite bad at tweaking their understanding — big changes stick but marginal ones are hard to track.


Ian 10.08.03 at 10:40 am

Are there not two issues in the climate change debate?
First is climate change actually taking place?
Second – if it is what are the causes?

The evidence appears to be stacking up in favour of a yes to the first question but the answer to the second is much less clear cut.

A third question then arises – assuming the answer to Q1 is yes and to Q2 is don’t know – what do we do as a race? I think there are one or two people in Bangladesh (and Holland and East Anglia and …) who might be interested in an answer


Ray 10.08.03 at 11:43 am

I think, with regard to Bayesian aspects, the important thing here is the Bayes factor: the conditional probability of the observed datum given the hypothesis is true divided by the conditional probability of the observed datum given that the hypothesis is false.

If the Bayes factor is sufficiently small, I think, for the purposes of human beings with heuristic minds, it’s safer to call something “no evidence” than evidence. I believe the term of the art is “statistically insignificant,” much as we’d regard a coin flipping heads 5 times in a row as unusual, but not a sign that the coin is asymmetric in any serious way.


dsquared 10.08.03 at 3:03 pm

Jack: Nonconvergence of Bayesian learners isn’t an issue when the two hypotheses are A and ‘A, as they are here. If it’s just a matter of a piece of evidence which is more likely under A than not-A, then convergence is strictly monotonic, IIRC (which I may not). You need more complicated decisions to get the local nonconvergence result.


Brian Weatherson 10.08.03 at 3:45 pm

I thought under a really subjective Bayesianism, you could have any kind of divergence you wanted under any conditions. Most of the convergence results I’ve seen assume that all parties have some particular attitude towards the possible pieces of evidence. (Exchangability is the most common assumption.) But there’s nothing in Bayesianism per se that requires exchangability.

(I think the more complicated results dsquared mentions are cases where you make assumptions like exchangability and you still don’t get convergence. But I could be misremembering.)

To put the point another way, for any logically independent H and E, and any x in [0,1] Pr(H|E) can be x consistent with Bayesian principles, so any amount of divergence with respect to H is possible after any evidence E.


S. Baum 10.08.03 at 4:03 pm

The theory being advanced by some as evidence of the sinking of “the Good Ships ‘Global Warming’ and ‘Kyoto Protocol'”, i.e. the ostensible refutation of the theory that anthropogenic increases in atmospheric CO2 concentration over the last hundred years are causing the measured increases in global average temperature, is well summarized in The Milky Way’s Spiral Arms and Ice Age Epochs on Earth on Nir Shaviv’s home page. The first paragraph of Shaviv’s summary reads:

There are indications that Cosmic Rays affect climate on Earth. If this is true, then one should expect climatic variations while we roam the galaxy. This is because the density of cosmic ray sources in the galaxy is not uniform. In fact, it is concentrated in the galactic spiral arms (it arises from supernovae, which in our galaxy arise predominantly from the death of massive stars, which in turn form and die predominantly in spiral arms). Thus, each time we cross a galactic arm, we should expect a colder climate. Current data for the spiral arm passages gives a crossing once every 135+/-25 Million years.

That is, he’s discovered a plausible and interesting quasi-periodic mechanism for explaining changes in global average temperature, with a periodicity of 135+/-25 million years. Inspection of Figure 2 of his summary gives a generous estimate of a maximum change of 5 deg. C over 100 million years, or 0.00000005 deg./yr. If we look at IPCC Fig. 5.16, we see that the measured global average temperature change over the last 140 years is, being conservative, about 0.7 deg. C, or 0.005 deg./yr.

Those interested (in the good way, of course) in the scientific debate might want to attempt to explain how a mechanism that changes global temperatures at a rate 1 million times more slowly than has been observed over the last 140 years can be used to supposedly “sink” another proposed mechanism that predicts that same order of magnitude of yearly temperature change that is observed.

Shaviv’s work provides another piece that will undoubtedly help to solve several puzzling questions about the climate during the
, but the million-fold difference in time scales doesn’t make it a realistic candidate for explaining global temperature increases over the last century.


dsquared 10.08.03 at 4:11 pm

Just to clear up, I was referring to “Bayesian statistics”, which is a bit of a bastard child of Bayesian probability theory, being the branch of mathematics aimed at making tractable the estimation of statistical parameters of interest through iterative use of Bayes’ rule. Which does assume exchangeability in this way. Although note above, Chris was stating ex hypothesi that P(E|S1) > P(E|S2)


Jack 10.08.03 at 5:12 pm

d-squared — I wasn’t really getting at something as subtle as a system with multiple attractors. I just wanted to point out that Chris and Prof. Stott say may start out with such different estimations of the probablities involved that even if they pay scrupulous attention to the event of record temperatures, the same single event may not bring their views closer together.

For over elaborate example if someone believed in a sunspot driven cycle they might even violate Chris’ tacit hypothesis andin any case they will likely judge recent events similar events as not that likely. Combined with an overwhelming initial probability the result might shift rather less than those of someone initially undecided who thinks that a hot summer is far more likely given global warming than not. The enviroskeptic may actually have a probability more different from undecided’s after a single iteration, even if both get pushed in the same direction.

With repeated samples they will both converge to the same result (ergodicity assumed etc.) but the convinced skeptic may well be a slow learner, even if they do accept some kind of Bayesian imperative. Reading some apologists for the Whitehouse over the Plame affair, I’m not sure that’s a mild assumption.


dsquared 10.08.03 at 6:04 pm

If I didn’t occasionally monstrously overestimate the importance of marginal results and apply them to non-marginal contexts, I think they’d take my economists’ licence away …


Nabakov 10.09.03 at 11:43 am

The very length of this thread (shurely shome record): highlights Mark Twain’s comment “Everyone talks about the weather but no one ever does anything about it”.

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