William M. Briggs

Statistician to the Stars!

Page 395 of 416

CONTEST: Preliminary Discussion of the “Best Internet Conspiracy Theory”

Best Internet Conspiracy Theory
This is the first posting preliminary to the announcement of an Official Contest to find the Best Internet Conspiracy Theory.

The Contest will be officially announced in about one week.

This contest is primarily a public service for those who contribute regularly to sites like Digg.com, Reddit.com, Wikipedia.org, etc. Many of those people are forced to spend an inordinate amount of time concocting theories that neatly explain messy world events. This has led to an enormous increase in carpal tunnel and internet addition syndrome cases worldwide. Thus, we want to provide these overworked souls a handful of ready-made theories to which they can refer. The theories we have in mind are described in the contest rules below.

I will need help in publicizing this Contest, and may need help in judging entries, depending on how many I receive. Volunteers should email me: put “CONTEST” in the subject line.

A sketch of the rules is as follows:

(1) All entries must be shorter than 150 words. Shorter entries will receive more weight than longer ones.

(2) Entries—one per person—must be placed into the Comments Section of the Official Contest Post. No discussion will be allowed on that post; only Contest entries are allowed.

(3) All entries will be judged by the intrinsic awfulness, brevity, completeness of derangement, plausibility, specificity (names named), and potential appeal to the everyday, e.g., Digg reader.

(4) The Contest will last approximately two to three weeks.

(5) A prize, or prizes, to be decided later, will be announced.

(6) An example of an Internet Conspiracy Theory:

Certain scientists discovered a formula, derived from an alien artifact dug up in Area 51, for turning ordinary sea water into limitless, cheap fuel. Green Energies, a subsidiary of MoveOn.org, based in the World Trade Center was about to sell this discovery and eliminate Global Warming, when the Oil Companies learned of it. Big Oil contacted George Bush, who ordered the Twin Towers destroyed before the secret could get out. Ron Paul found out about this and was going to expose the entire matter had he won the Republican Nomination, which he would have done except the Mainstream Media ignored him.

Please do NOT post any conspiracy theories now! Save them for the Contest.

CO2 and Temperature: which predicts which?

Parts of this analysis were suggested by Allan MacRae, who kindly offered comments on the exposition of this article which greatly improved its readability. The article is incomplete, but I wanted to present the style of analysis, which I feel is important, as the method I use eliminates many common errors found in CO2/Temperature studies. Any errors are, of course, entirely my own.

It is an understatement to say that there has been a lot of attention to the relationship of temperature and CO2. Two broad hypotheses are advanced: (Hypothesis 1) As more CO2 is added to the air, through radiative effects, the temperature later rises; and (Hypothesis 2) As temperature increases, through ocean-chemical and biological effects, CO2 is later added to the atmosphere. The two hypotheses have, of course, different consequences which are so well known that I do not repeat them here. Before we begin, however, it is important to emphasize that both or even neither of these hypotheses might be true. More on this below.

The source of monthly temperature data is from The University of Alabama in Huntsville, which starts in January 1980. Temperature is available at different regions: global, Northern Hemisphere, etc. The monthly global CO2 is from NOAA ERSL.

We want to examine the CO2/temperature processes at the finest level allowed by the data, which here is monthly at the time scale, and Northern and Southern Hemisphere and the tropics at the spatial scale. The reason for doing this, and not looking at just yearly global average temperature and CO2, is that any processes that occur at times scales less than a year, or occur only or differently in specific geographic regions, would be lost to us. In particular, it is true that the CO2/temperature process within a year is different in the Northern and Southern hemispheres, because, of course, of the difference in timing of the seasons and changes in land mass. It is also not a priori clear that the CO2/temperature process is the same, even at the yearly scale, across all regions. It will turn out, however, that the difference between the regional and global processes are minimal.

The question we hope to answer is, given the limitations of these data sets, with this small number of years, and ignoring the measurement error of all involved (which might be substantial), does (Hypothesis 1) increasing CO2 now predict positive temperature change later, or does (Hypothesis 2) increasing temperatures now predict positive CO2 change later? Again, this ignores the very real possibility that both of these hypotheses are true (e.g., there is a positive feedback).

During the course of an ordinary year, both Hypotheses 1 and 2 are true at different times, and sometimes neither is true: in the Northern Hemisphere, the temperature and CO2 both increase until about May, after which CO2 falls, though temperature continues to rise. In the Southern Hemisphere, temperature falls in the early months, while CO2 rises, and so on. These well known differences are due to combinations of respiration and changes in orbital forcing.

There are, then, obvious correlations of CO2 and temperature at different monthly lags and in different geographic regions (I use the word “correlation” in its plain English meaning and not in any statistical sense). We are not specifically interested in these correlations, which are well know and expected, and whose role in long-term climate change is minimal. The existence of these correlations present us with a dilemma, however. It might be that, for either Hypothesis 1 or 2, the time at which either CO2 or temperature changes in response to changes in forcing is less than one year, but disentangling this climate forcing with the expected changes due to seasonality, is, while possible, difficult and would require dynamical modeling of some sort (in the language of time series, the seasonal and long-term signals are possibly confounded at time scales less than 1 year).

Therefore, instead of looking at intra-year correlations, we will instead look at inter-year correlations. This introduces a significant limitation: any real, non-seasonal, correlations less than 1 year (or at other non-integer yearly time points) will be lost and it will be possible that we are misled in our conclusions (in the language of time series, the “power” on these non-integer-year lags will be aliased onto the 1 year lag). What is gained by this approach, however, is that there is no chance of misinterpreting lags less than one year as being due to a process other than seasonality. However, the main purpose of this article is not to identify the exact dynamical and physical CO2/temperature relationship, nor to identify the lag that best describes it; we just want to know is Hypothesis 1 or Hypothesis 2 more likely on time scales greater than 1 year?

Most of us have seen pictures like this one, which shows the monthly CO2 for 1980-1984; also shown in the Northern Hemisphere (NH) temperature anomaly (suitably normalized to fit on the same picture).
Co2 through time
You can immediately see the intra-year CO2 “sawtooth”. This sawtooth makes it difficult to find a functional relationship of CO2 and temperature. I do not want to model this sawtooth, because I worry that whatever model I pick will be inadequate, and I do not immediately know how to carry the uncertainty I have in the model through to the final conclusion about our Hypotheses. I also do not want to smooth the sawtooth, or perform any other mathematical operation on the observed CO2 values within a year, because that tends to inflate measures of association.

Instead, let’s look at CO2 in a different way:
Co2 through time by month
This is yearly CO2 measured within each month: each of the 12 months has its own curve through time. It doesn’t really matter which is which, though the two lowest curves are from the winter months (for those in the NH). What’s going on is still obvious: CO2 is increasing year by year and the rate at which it is doing so is roughly constant regardless of which month we examine.

Looking at the data this way show that the sawtooth has effectively been eliminated, as long as we examine year-to-year changes within each month through time.

Suppose we were only interested in Decembers and in no other months. Let us plot the actual December temperature from 1980 to 2006 on the x-axis and on the y-axis plot the increase in CO2 for the years 1981 to 2007. Shown in the thumbnail below is this plot: with black dots for the Southern Hemisphere (SH), red dots for the NH, and green dots for the tropics (redoing the analyses with global or sea surface temperatures instead of separating hemispheres produces nearly indistinguishable results). For example, in one year, the NH temperature anomaly was -0.6: this was followed in the next year by an increase of about 1.5 ppm of CO2 (this is the left-most plot on the figure).
Co2 through time by month

The solid lines estimate the relationship between temperature and the change in CO2 (the dCO2/dt on the graph). These are loess lines and estimate the relationship between the two variables. If the loess lines were perfectly straight (and pointed in any direction), we would say the two measures are linearly correlated. The lines aren’t that straight, so the data does not appear to be that well correlated, linearly or otherwise.

Click on the figure (do this!) to see the same plot for each of the 12 months (right click on it and open it in a new window so you can follow the discussion). Notice anything? Generally, when temperature increases this year CO2 tends to increase in the following year. Hypothesis 2 is more likely to be true given this picture.

The loess lines are not always straight, which means that a straight-line model, i.e. ordinary correlation, is not always the best model. For example, in Januaries, until the temperatures anomalies get to 0 or above, temperature and change in CO2 have almost no relationship; after this point, the relationship becomes positive, i.e., increasing temperatures leads to increases in the change of CO2. The strength of the relationship also depends on the month: the first six months of the year show a strong signal, but the later six show a weakening in the relationship, regardless of where in the world we are.

Coincidence? Now plot the actual December CO2 from 1980 to 2006 on the x-axis and on the y-axis plot the change (increase or decrease) in temperature for the years 1981 to 2007. For example, in one year, the NH CO2 was 340 ppm: this was followed in the next year by a temperature decrease of about -0.5 degrees (this is the bottom left-most plot on the figure). No real signal here:
Co2 through time by month

Again, click on the figure (do this!) to see all twelve months. There does not appear to be any relationship in any month between CO2 and change in temperature, which weakens our belief in Hypothesis 1.

It may be that it takes two years for a change in CO2 or temperature to force a change in the other. Click here for the two-year lag between temperature and change in CO2; and here for the two-year lag between CO2 and change in temperature. No signals are apparent in either scenario.

As mentioned above, what we did not check are all the other possibilities: CO2 might lead or lag temperature by 9.27, or 18.4 months, for example; or, what is more likely, the two variables might describe a non-linear dynamic relationship with each other. All I am confident of saying is, conditional on this data and its limitations etc., that Hypothesis 2 is more probable than Hypothesis 1, but I won’t say how much more probable.

It is also true that, over this period of time and using this data, CO2 always increased. The cause of this increase sometimes was related to temperature increases (rising temperatures led to more CO2 being released) and sometimes not. We cannot say, using only this data, why else CO2 increased, although we know from other sources that CO2 obviously increased because of human-cased activities.

It was bound to happen

Remember how you used to cavalierly ignore those “Keep of the Grass Signs” in your un-enlightened youth?

Well, you brutal, uncaring, beast.

For it has finally been announced—from Europe, naturally, from the Swiss government-appointed Federal Ethics Committee on Non-Human Biotechnology—that plants have feelings too.

They have authoritatively stated that “interfering with plants without a valid reason as ‘morally inadmissible.’” This means the next time you carve you and your sweetheart’s name into a tree can lead to a nice, long jail sentence. (If the famed Swiss police ever catch you, that is.)

The ethics committee did grudgingly admit—for now—that “all action involving plants for the preservation of the human race was morally justified.” Meaning, I suppose, that it’s still OK to eat them. I probably don’t need to explain to you the fix we’d be in if we could not. But there is only direction for the Enlightened to go, so stay tuned for an announcement banning the use of “higher” plants, such as maybe corn and tomatoes, for use in the “preservation of the human race.”

The august Swiss body has also found that “genetic modification of a plant did not contradict the idea of its ‘dignity’.” Yes, I can see how a kumquat would not find it an affront to be genetically probed. Until, that is, the kumquat learns how easily this sort of thing can sully one’s reputation. It’s only matter of time before a lawyer figures this out and brings a case to Brussels.

Just keep all this in mind, think about what you are doing—raise your awareness!—next time you are at the salad bar.

The Devil’s Delusion: Atheism and its Scientific Pretensions by David Berlinski

There are, as everybody knows, a recent number of books seeking to either demonstrate, scientifically, that God does not exist, or to show that the love of religion is the root of all evil. Some familiar names: Daniel Dennet, Richard Dawkins, Stephen Weinberg, Victor Stenger, Christopher Hitchens, and even John Allen Paulos. All proclaim that the weight of scientific evidence is either completely or heavily on the side of the non existence of God.

The question is, of course: Has the authority of eminent scientists enabled them to prove their case? Berlinski says, “Not even close.” Not only have they not come close, Berlinski goes further and shows how easily they are persuaded by weak or demonstrably false arguments, and the extraordinary lengths that some scientists will go, in the sense of believing bizarre theories, to avoid ceding any ground to the “religionists.” Their distaste of religion has also lead them to say some rather stupid things. For example, Berlinski quotes the eminent biologist Emile Zuckerkandl as saying that if God exists, He would represent “something like a pathology of the state of being.” An enjoyable, sputtering rant by that author published in the peer-reviewed journal Gene is summarized later in the book.

Incidentally, before we get too far, it is worth mentioning that like most (all?) books in this genre, Berlinski does not attempt a definition of who or what God is—and neither do those on the other side. I haven’t one to offer, either. This curiosity can very well mean that everybody is talking at cross purposes. But since nobody delineates or bounds God, I can’t say much more than this, except that it should be borne in mind when reading any of these books.

A non-Enlightened disease

Berlinski puts the claim that religion is bad for you in perspective. Some anti-religion authors won’t settle for anything less than damning religion in all its stripes, disallowing, even, the crumb of comfort given to people when their loved ones die. Even Carl Sagan, in his Demon-Haunted World allowed this kind of solace, without recognizing that since, I must point out, everybody dies, this is an enormous amount of comfort to go around that would be denied mankind if religion were absent. But you never hear of our authors breaking open Mill to assist in calculating the utility of comforts versus torments of religion.

Many scientists feel that religion, while still a cancerous growth, is benign and only mostly harmful, and not immediately deadly. Sort of like smoking, which the more Enlightened among us would like to ban. Presumably, those who would prohibit smoking are same people who would support legalizing assisted suicide. Which happened in Holland in 1984 (and where a partial smoking ban does exist). Since then, about three percent of all deaths in that country are assisted, of which the government admits that about one-fourth are “involuntary.” We call that involuntary method of exiting “murder” here in the States, but Europeans are often considered more Enlightened, so they might be one step ahead of us in legal definitions.

Arguments for assisted suicide are usually intentionally religion-free. Thus, the point of the Holland example, of course, is that the world would not necessarily become a more moral, or safer place, if religion were to disappear. More proof is given by Berlinski in the form of a table, ordered by number of “excess”, or untimely, twentieth-century deaths due to non- or even anti-religious behavior. Leading the pack are of course the two World Wars, but not far behind in the body count are mankind’s experiments with various communist utopias. Since one of the top arguments used by those who would wish to bar religion is that the religious can be cruel and have killed, the evidence that the non-religious can be cruel and have killed in equal or larger number only proves that there will always be a class of people who adore pain, misery, and bloodshed, irrespective of creed.

The disease religion is also seen as congenital, in the sense that people have religion on the brain, literally. Somehow, we are assured, the brain has genetically encoded religion into itself, and that if we’d just grow up and recognize this, we would become Enlightened (or brightened, these days). This is one of the sillier arguments put forth by scientists. If religion is genetically encoded, then it cannot be overcome, unless some of us, the superior ones naturally, have somehow managed to escape expressing those particular genes that activate, say, the praying response. Look for one of those fMRI studies that “proves” this, soon.

Berlinski shows that because some scientists cannot countenance religious arguments of any kind, they refuse to accept any evidence that is any way tainted by religion. This leads to the fallacy that one should not listen to arguments against, say, stem cell research or abortion because they are religious. You will surely certainly recognize this ploy when you meet it.

Scientific ontology

Everybody already knows that physics, and its offshoots, has done brilliantly at explaining more and more of the universe. But it cannot keep doing so forever. At some point, meta-physics must enter into the discussion. This is because, no matter what physical laws we have identified, we will never have explained through observation why these particular laws and not some other are in force, nor can we answer what the laws mean. It is obvious that it is here that God can slip in and offer the needed explanations. Some scientists are therefore anxious to fill in these gap with…something, anything but God. Or, if that cannot be accomplished, then to prove that God does not exist.

Dawkins, in his The God Delusion offers a particularly weak argument. His first premise is that the universe is improbable. And we can stop right there, because that is a nonsensical statement, so his argument fails. Any thing or statement cannot be improbable. A thing can only be improbable with respect to something else. Further, a thing can be improbable with respect to one set of evidence and entirely probable with respect to other evidence. So, in Dawkin’s case, the universe is improbable with respect to what?

Weak Anthropic evidence is sometimes offered, in the guise of certain physical constants having particular values, in the sense that if these constants did not have these values, then human life would be impossible (which is not the same as saying the universe is impossible, but let that pass). Now the burden is on those who tout this evidence to show that this is the best evidence with which to measure the improbability of the universe. And there are many hints that it is not the best evidence. It is, after all, by its very name, suspiciously self indulgent and human centered evidence. Why would the universe care if humans, or other sentient beings, evolved enough to notice that they might not have evolved had the universe been arranged differently anyway? Besides, to say that things might have been different and humans might not have evolved is just a tautology, and therefore of no interest.

Still, accept it if you like, so that we can move to Dawkins’s second premise, which is that God Himself is improbable. Again, the statement is nonsensical: improbable with respect to what? Dawkins suggests that God must be more improbable than the universe, which again makes no sense. Anyway, improbable is not impossible, as Dawkins often argues with respect to evolution by natural selection, arguments he has apparently forgotten. Still, Dawkins moves to his conclusion that God is so improbable that He doesn’t exist, and advises people to accept some recent conjectures in cosmology that seem to do away with the need to explain why the universe, or universes, are the way they are.

These are the Landscape and multiverse hypotheses, put forward by various authors to help them cope with the insolubilities of quantum mechanics and cosmology. These are attempts to shift the questions of “Why?” one step back. That they do not answer them, I would have thought obvious. Even pushing the grand questions a little deeper down is enough to please some people. Berlinski, a mathematical physicist, covers these speculations well, without any math, and gives pointers to books where we might learn more. See especially his very clever “Catechism of Quantum Cosmology.” Briefly, however, the solutions offered posit an uncountable number of alternate universes that are coming into and out of creation always. There are no mechanisms to observe these other universes directly or indirectly. Even if we could, these theories might answer some questions of quantum mechanics and gravity, but they never answer why it is infinities of universes instead of just one. The theories are also mind-boggling complex, and by no means are they consistent with one another. Nobody even knows what the full scope of these ideas are.

Berlinski quotes Dawkins, who is nevertheless satisfied, as saying, “The key difference between the radically extravagant God hypothesis and the apparently extravagant multiverse hypothesis, is one of statistical improbability.” Presumably, he means that God is more improbable. He never says how much more. Infinities, of universes or anything else, are a dangerous thing. More foolishness has been generated by jumping to infinity than by any other reason (see chapter 15 of Jaynes’s remarkable Probability Theory for appropriate words of admonition).

Argument from design

It has long been convincing to many that the wonderful biological complexity that is everywhere in evidence must have had a designer. How else, Darwin himself wondered, can one explain the human eye? This argument is less convincing than it once was, because of the success of modern biology and genetics, and the seeming success of evolution by natural selection.

(It is just as well to point out here that I accept that evolution accounts for some or most of the observed biological variation on Earth, and that the mechanism driving it is natural selection, or something like it.)

Wait a minute. Did he just say seeming success? He did. Which brings us back to Dawkins, the best-known anti-religion author. Was there ever a man who published so much nonsense that was taken so seriously by the scientific community? Nobody else even comes close. Just mentioning the word memes proves my point. Is not believing in God a meme? Berlinski doesn’t discuss memes, but does offer some well known criticisms of “selfish” genes—incidentally, the best are due to the philosopher’s Mary Midgley (Evolution as a Religion) and David Stove (Darwinian Fairytales; if you haven’t read either of these books, please do so, especially Stove’s, before you comment).

Not all biologists are satisfied with present-day theory. Berlinski writes

[Darwinian] theory is what is always was: It is unpersuasive. Among evolutionary biologists, these matters are well known. In the privacy of the Susan B. Anthony faculty lounge, they often tell one another with relief that it is a very good thing the public has no idea what the research literature really suggest.

“Darwin?” a Nobel laureate in biology once remarked to me over his bifocals. “That’s just the party line.”

There are still gaps in the evolutionary record. Nobody knows how life original arose, and nobody knows how species originate. Some fill these gaps with God. Scientists argue that the gaps will be filled in eventually. Berlinski says that this assumption is “both intellectually primitive and morally abhorrent—primitive because it reflects a phlegmatic absence of curiosity, and abhorrent because it assigns to intellectual future a degree of authority alien to human experience” because filling gaps “has created [new] gaps all over again.”

The answer

The best summation on the side of (non-apoplectic) scientists is probably from Richard Feynman, who said, “Today we cannot see whether Schrödinger’s equation [which describes the time evolution of physical systems] contains frogs, musical composers, or morality. We cannot say whether something beyond it like God is needed , or not. And so we can all hold strong opinions either way.”

To say whether or not God exists is the hardest question in the world; yet it is the one people find easiest to answer, and everybody seems delighted to meet an argument, however weak, that agrees with their desires. This leads very smart people to say exceptionally stupid things.

My own surmise is that any proof—for or against—is impossible. And so any belief you have is based entirely on faith.

Why multiple climate model agreement is not that exciting

There are several global climate models (GCMs) produced by many different groups. There are a half dozen from the USA, some from the UK Met Office, a well known one from Australia, and so on. GCMs are a truly global effort. These GCMs are of course referenced by the IPCC, and each version is known to the creators of the other versions.

Much is made of the fact that these various GCMs show rough agreement with each other. People have the sense that, since so many “different” GCMs agree, we should have more confidence that what they say is true. Today I will discuss why this view is false. This is not an easy subject, so we will take it slowly.

Suppose first that you and I want to predict tomorrow’s high temperature in Central Park in New York City (this example naturally works for any thing we want to predict, from stock prices to number of people who will vote for a certain USA presidential candidate). I have a weather model called MMatt. I run this model on my computer and it predicts 66 degrees F. I then give you this model so that you can run it on your computer, but you are vain and rename the model to MMe. You make the change, run the model, and announce that MMe predicts 66 degrees F.

Are we now more confident that tomorrow’s high temperature will be 66 because two different models predicted that number?

Obviously not.

The reason is that changing the name does not change the model. Simply running the model twice, or a dozen, or a hundred times, does not give us any additional evidence than if we only ran it just once. We reach the same conclusion if instead of predicting tomorrow’s high temperature, we use GCMs to predict next year’s global mean temperature: no matter how many times we run the model, or how many different places in the world we run it, we are no more confident of the final prediction than if we only ran the model once.

So Point One of why multiple GCMs agreeing is not that exciting is that if all the different GCMs are really the same model but each just has a different name, then we have not gained new information by running the models many times. And we might suspect that if somebody keeps telling us that “all the models agree” to imply there is greater certainty, he either might not understand this simple point or he has ulterior motives.

Are all the many GMCs touted by the IPCC the same except for name? No. Since they are not, then we might hope to gain much new information from examining all of them. Unfortunately, they are not, and can not be, that different either. We cannot here go into detail of each component of each model (books are written on these subjects), but we can make some broad conclusions.

The atmosphere, like the ocean, is a fluid and it flows like one. The fundamental equations of motion that govern this flow are known. They cannot differ from model to model; or to state this positively, they will be the same in each model. On paper, anyway, because those equations have to be approximated in a computer, and there is not universal agreement, nor is there a proof, of the best way to do this. So the manner each GCM implements this approximation might be different, and these differences might cause the outputs to differ (though this is not guaranteed).

The equations describing the physics of a photon of sunlight interacting with our atmosphere are also known, but these interactions happen on a scale too small to model, so the effects of sunlight must be parameterized, which is a semi-statistical semi-physical guess of how the small scale effects accumulate to the large scale used in GCMs. Parameterization schemes can differ from model to model and these differences almost certainly will cause the outputs to differ.

And so on for the other components of the models. Already, then, it begins to look like there might be a lot of different information available from the many GCMs, so we would be right to make something of the cases where these models agree. Not quite.

The groups that build the GCMs do not work independently of one another (nor should they). They read and write for the same journals, attend the same conferences, and are familiar with each other’s work. In fact, many of the components used in the different GCMs are the same, even exactly the same, in more than one model. The same person or persons may be responsible, through some line of research, for a particular parameterization used in all the models. Computer code is shared. Thus, while there are some reasons for differing output (and we haven’t covered all of them yet), there are many more reasons that the output should agree.

Results from different GCMs are thus not independent, so our enthusiasm generated because they all roughly agree should at least be tempered, until we understand how dependent the models are.

This next part is tricky, so stay with me. The models differ in more ways than just the physical representations previously noted. They also differ in strictly computational ways and through different hypotheses of how, for example, CO2 should be treated. Some models use a coarse grid point representation of the earth and others use a finer grid: the first method generally attempts to do better with the physics but sacrifices resolution, the second method attempts to provide a finer look at the world, while typically sacrificing accuracy in other parts of the model. While the positive feedback in temperature caused by increasing CO2 is the same in spirit for all models, the exact way it is implemented in each can differ.

Now, each climate model, as a result of the many approximations that must be made, has, if you like, hundreds (even thousands) of knobs that can be dialed to and fro. Each twist of the dial produces a difference in the output. Tweaking these dials, then, is a necessary part of the model building process. The models are tuned so that they, as closely as possible, first are able to produce climate that looks like the past, already observed, climate. Much time is spent tuning and tweaking the models so that they can, at least roughly, reproduce past climate. Thus, the fact that all the GCMs can roughly represent the past climate is again not as interesting as it first seemed. They better had, or nobody would seriously consider the model as a contender.

Reproducing past data is a necessary but not sufficient condition that the models can predict future data. Thus, it is also not at all clear how these tweakings affect the accuracy in predicting new data, which is data that was not used in any way to build the models, that is, future data. Predicting future data has several components.

It might be that one of the models, say GCM1 is the best of the bunch in the sense that it matches most closely future data. If this is always the case, if GCM1 is always closest (using some proper measure of skill), then it means that the other models are not as good, they are wrong in some way, and thus they should be ignored when making predictions. The fact that they come close to GCM1 should not give us more reason to believe the predictions made by GCM1. The other models are not providing new information in this case. This argument, which is admittedly subtle, also holds if a certain group of GCMs are always better than the remainder of models. Only the close group can be considered independent evidence.

Even if you don’t follow—or believe—that argument, there is also the problem of how to quantify the certainty of the GCM predictions. I often see pictures like this:
GCM predictions
Each horizontal line represents the output of a GCM, say predicting next year’s average global temperature. It is often thought that the spread of the outputs can be used to describe a probability distribution over the possible future temperatures. The probability distribution is the black curve drawn over the predictions, and neatly captures the range of possibilities. This particular picture looks to say that there is about a 90% chance that the temperature will be between 10 and 14 degrees. It is at this point that people fool themselves, probably because the uncertainty in the forecast has become prettily quantified by some sophisticated statistical routines. But the probability estimate is just plain wrong.

How do I know this? Suppose that each of the eight GCMs predicted that the temperature will be 12 degrees. Would we then say, would anybody say, that we are now 100% certain in the prediction?

Again, obviously not. Nobody would believe that if all GCMs agreed exactly (or nearly so) that we would be 100% certain of the outcome. Why? Because everybody knows that these models are not perfect.

The exact same situation was met by meteorologists when they tried this trick with weather forecasts (this is called ensemble forecasting). They found two things. The probability forecasts made by this averaging process were far too sure—the probabilities, like our black curve, were too tight and had to made much wider. Second, the averages were usually biased—meaning that the individual forecasts should all be shifted upwards or downwards by some amount.

This should also be true for GCMs, but the fact has not yet been widely recognized. The amount of certainty we have in future predictions should be less, but we also have to consider the bias. Right now, all GCMs are predicting warmer temperatures than are actually occurring. That means the GCMs are wrong, or biased, or both. The GCM forecasts should be shifted lower, and our certainty in their predictions should be decreased.

All of this implies that we should take the agreement of GCMs far less seriously than is often supposed. And if anything, the fact that the GCMs routinely over-predict is positive evidence of something: that some of the suppositions of the models are wrong.

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