Charles Darwin was born on February 12, 1809. The events commemorating Darwin’s birthday anniversary last Sunday, together with the recent conclusion of an important court case concerning the teaching of Intelligent Design (ID) in public schools prompts me to some musing concerning the relation of the Evolution/ID dialog to similar issues arising in connection with anthropogenic global warming. The age of the two theories is similar as well: Darwin introduced his theory in 1859, whereas Fourier initiated the study of the effect of atmospheres on climate with his 1821 treatise, stimulating the chain of developments leading to Arrhenius’ enunciation in 1896 of the theory that human influences on the atmosphere’s CO2 content could change the climate.
I don’t propose to wade into questions of religion, or the question of whether or in what form ID could be taught in public schools. However, the discussion surrounding ID is significant because it has focused a lot of public attention on the question of : "What is science?" A Nov. 5, 2005 letter to the Chicago Tribune by one Mr. Ross Williams makes the connection explicit: In his letter, Mr Williams implies that the Theory of Global Warming is more like ID than it is like Evolution. Referring to global warming, he states: "It is no more than an idea, a notion." and goes on to say:
- " The scientists pursuing this hypothesis are struggling to test it and make predictions using their ideas. Thus far, they have had extremely limited success in testing, and virtually no luck in predicting–resulting in continually modified (and, consequently, less severe) forecasts. Despite this, they are spawning a whole cadre of non-scientific worry warts who are declaring that, well, really, the science doesn’t matter."
In Mr. Williams lexicon, a hypothesis is just "a notion," presumably not much better than ID. In this article, I will attempt to explain why the bleak picture painted by Mr. Williams and people of like mind is unwarranted.
Another relation between the two issues is that Evolution skeptics are motivated by ideology to deny a well-established scientific theory. In the case of Evolution, the ideological motivation is a perceived conflict between the picture of the operation of the natural world presented by the Theory of Evolution, and the tenets of certain faiths (a perceived conflict that, I am happy to see, is not shared by all people of faith, as witness the extensive "Evolution Sunday " activities ). Similarly, most Global Warming denialists are for the most part motivated not by abstract curiosity about the behavior of climate systems, but by a perceived conflict between the actions that would need to be taken to avert unacceptable climate change, and their beliefs about the extent to which economic growth and material prosperity based on fossil-fuel energy use should be unfettered. (Again, not all economists or members of the business community perceive a conflict here). In both cases, the skeptics prosecute not just an attack on the policy implications of science, but on the scientific method itself, often using similar rhetorical devices. In fact, sometimes skepticism about global warming and about evolution are combined in one and the same person, as is the case for Roy Spencer, for example (see his article on evolution here.)
Just what is the theory here?
First, we need to get straight on just what we might be talking about when referring to "The Theory of Global Warming." There’s a natural tendency to identify such a theory with the statement that "The Earth is Warming." That’s wrong because it confuses a theory with observations that might be used to test a theory. It’s also wrong because it would imply that the only reason we think that the Earth will continue warming in response to increased CO2 is that we already see it warming today; it loses the chain of physical causation. Somewhat better would be the statement, "The Earth is warming, and the warming is largely due to increases in atmospheric CO2 and other long lived greenhouse gases." This is defensible as a hypothesis, but I think it would be far better to consider this statement, too, as more properly in the domain of one of the tests we might apply to the Theory of Global Warming.
My own preferred statement of The Theory of Global Warming is this:
- An increase in the atmospheric concentration of CO2 and other long lived greenhouse gases requires the surface temperature to ultimately increase so as to maintain a balance with the absorbed solar radiation. The increase is amplified by water vapor (also a greenhouse gas), which increases with temperature in such a way as to keep relative humidity approximately constant. Melting of ice will further amplify the warming, particularly in high latitudes. The resulting widespread warming corresponding to a doubling of CO2 will be large enough and rapid enough to be well outside the range of past experience of the human species, by an amount comparable to the difference between a glacial and interglacial climate. Changes in atmospheric cloud properties may somewhat reduce or increase the sensitivity, but do not substantially alter the conclusion.
The last part of the statement of the theory is, of course, the hard part, and the most uncertain.
I have deliberately left the matter of the severity of the impacts of such a climate change out of the hypothesis. Theories regarding the impact are nascent and in many regards still rather ill-formed, in comparison to the theory dealing with the physical dimensions of climate change. Also, insofar as there are uncertainties about the severity of the impacts of climate change, it is a matter for the political apparatus to decide how to deal with the uncertainties, and the extent to which one should pay attention to the worst case vs. the most likely case. The question of how to factor in the uneven distribution of harms (and possibly benefits) across the peoples of the Earth, and between human societies and natural ecosystems, is also at heart a matter of ethics and values. These are questions that can be informed by science, but they are not themselves scientific questions.
Finally, one must be careful not to be confused by the usage of the word "theory" in common everyday English. Statements like, "Oh, that’s just a theory, not a fact" have little to do with the scientific understanding of the word "theory." Linguistic confusion goes the other direction as well: Scientists often talk about "believing" in a theory, but this expresses a judgement of whether the balance of tests of a theory against observations lends sufficient support to the theory to rely on it in drawing further inferences. It does not declare that subscribing to the theory or not is an article of faith, to be left to one’s conscience. If I say that I "believe in" quantum theory, that is expressing a different kind of judgement than if I say I "believe in" the tenets of Buddhism.
Judge Jones on "What is Science"
Judge Jones (a George W. Bush appointee, by the way) of the Middle District Court of Pennsylvania, presided over the case Kitzmiller v. Dover Area School District, which dealt with the constitutionality of an attempt to introduce some limited teaching of Intelligent Design into science classes. His decision that teaching ID in public school science classes would be an unconstitutional establishment of religion, is a masterpiece of wit, scholarship and clear thinking. Most of the decision deals with application of tests (such as the "Lemon Test") of whether a government action constitutes an establishment of religion. These make fascinating reading, and show Judge Jones’ wide ranging intellect, but they are not of concern to me here. What’s relevant to the point at hand is the rather extensive part of the decision devoted to the question "How do we know whether something is science?" This question wasn’t entirely central to the basis of the Judge’s decision, but he devoted a lot of attention to it because, in his words,
- "Having so concluded, we find it incumbent upon the Court to further address an additional issue raised by Plaintiffs, which is whether ID is science. To be sure, our answer to this question can likely be predicted based upon the foregoing analysis. While answering this question compels us to revisit evidence that is entirely complex, if not obtuse, after a six week trial that spanned twenty-one days and included countless hours of detailed expert witness presentations, the Court is confident that no other tribunal in the United States is in a better position than are we to traipse into this controversial area. Finally, we will offer our conclusion on whether ID is science not just because it is essential to our holding that an Establishment Clause violation has occurred in this case, but also in the hope that it may prevent the obvious waste of judicial and other resources which would be occasioned by a subsequent trial involving the precise question which is before us."
In other words, Judge Jones had already seen enough irreducible complexity, bacterial flagella, fossil record interpretations and panda’s thumbs to last a lifetime (maybe two), and didn’t want any of his colleagues to have to go through the same business all over again.
For the most part, the good judge takes a positivist approach to the definition of science, following Karl Popper. This approach emphasizes that a scientific theory should be falsifiable. The centrality of this notion has been challenged by Thomas Kuhn and a few other philosophers of science , but as a description of the way most of us in the trenches actually see our enterprise, Popper does pretty well, as long as we allow a little flexibility in the matter of what counts as falsifiability. The important thing is that a scientific theory should be productive. It should make predictions that can be tested against observation and experiment, the more the better. Thus, Ptolemy’s epicycle theory of planetary motion is not bad as a scientific theory: it does make predictions about where planets will be, that can be tested against data. Newton’s theory is far better, though, because it makes far more predictions over a vastly wider range of circumstances, while requiring far fewer assumptions. It’s not just that it’s more economical than epicycles. It’s far more productive of testable predictions — all of which prove true, so long as one steers clear of speeds close to that of light and very strong gravitational fields. Einstein’s General Theory of Relativity is even more productive, covering the extreme cases while reducing to Newton’s theory in the low speed and weak gravity limit.
Further, the notion of prediction has to be broadly construed. The fact that we can’t predict the exact weather a year out is no refutation of the basic theory of climate, any more than the fact that we can’t predict the position of Pluto in its chaotic orbit is a refutation of Newtonian mechanics. In the context of testing scientific theories, a prediction need not refer to something that happens in the future; this is important in observational sciences such as Earth science or cosmology, where one’s "predictions" often deal with things that happen in the past. A prediction in this context is any inference drawn on the basis of a theory, that can be objectively tested against observations. For that matter, a prediction need not even come in advance of an observation. Obviously, it is a more convincing test of a theory if the inference is made before the observation, since this provides some protection against the accusation of tuning unknown parameters; however, there are other ways to check whether a match succeeds only because of unwarranted tuning.
Judge Jones’ considers three basic arguments in his consideration of whether ID is science. The detailed application of each argument to ID is buttressed by numerous citations to theological, scientific and ID-advocacy writings, which are not reproduced in detail below.
The first argument is against ID as science is that science does not rely on untestable supernatural causes. Supernatural explanations are "science stoppers" which preclude further inquiry. This is, in essence, a restatement of the falsifiability (positivist) criterion. Among the many documents Judge Jones cites is a National Academy of Sciences statement that notes that the publications arguing for ID "do not offer hypotheses subject to change in light of new data, new interpretations, or demonstration of error. This contrasts with science, where any hypothesis or theory always remains subject to the possibility of rejection or modification in the light of new knowledge." The Judge declares, on the basis of the evidence, that "ID fails to meet the essential ground rules that limit science to testable, natural explanations."
The Judge notes that the preceding alone is sufficient to disqualify ID as science, but given a surfeit of evidence, he does not want to stop halfway. The next argument he produces is quite different from the positivism criterion, namely that the arguments for ID rest on a contrived dualism. "ID is at bottom premised upon a false dichotomy, namely, that to the extent evolutionary theory is discredited, ID is confirmed, " he writes. He then points out that arguments for ID based on this contrived dualism are, from a scientific standpoints, not arguments for ID at all, but merely tests of the Theory of Evolution — and hence only serve to further establish that Evolution is science. Judge Jones, in this connection, disassembles some of the arguments against Evolution made by ID proponents, but this is a matter of evaluating tests of Evolution as a scientific theory, not a matter of deciding whether ID is science. The notion of "irreducible complexity," for example, is a refutable and testable negative argument against evolution, but that does not make it a testable argument for ID. The discussion of the merit of ID proponents’ arguments against Evolution does, however turn up a point that has some relevence also to the argument brought to bear against the Theory of Global Warming. Discussing testimony on the ID case against Evolution, the judge writes: "We find that such evidence demonstrates that the ID argument is dependent upon setting a scientifically unreasonable burden of proof for the theory of evolution." (My emphasis added). The judge finds that Evolution skeptics argue by "pointing to real gaps in scientific knowledge, which indisputably exist in all scientific theories, but also by misrepresenting well-established scientific propositions." This description applies word-for-word to many skeptics’ arguments against global warming, for example to most of Richard Lindzen’s testimony to the House of Lords (discussed here)
Judge Jones’ third argument is a pragmatic one: it assumes that there is such a thing as a recognized scientific community, and that it knows science when it sees it even if it may be hard to rigorously and unambiguously define the criteria. He specifically looks to the peer-reviewed publication process as an indicator: "A final indicator of how ID has failed to demonstrate scientific warrant is the complete absence of peer-reviewed publications supporting the theory." After some further discussion of the publication record of ID, he concludes "ID is not science and cannot be adjudged a valid, accepted scientific theory as it has failed to publish in peer-reviewed journals, engage in research and testing, and gain acceptance in the scientific community."
Does "Global Warming Theory" pass Judge Jones’ science test?
In one sense, the Theory of Global Warming is clearly a falsifiable scientific theory: all we need to do is wait around a while until industrial activities have doubled CO2, and observe what has happened to atmospheric temperature, water vapor and clouds. This indeed seems to be the experiment that most of the world seems intent on carrying out.
However, when we talk about "verifying" the Theory of Global Warming, what most of us have in mind is doing something to test the theory right now, so that (to the extent that it is correct) necessary policy decisions can be informed by the predictions of the theory.
Earth science shares the full range of difficulties generic to observational sciences, in that we can carry out laboratory experiments testing individual basic physical principles making up our theories, but have only limited opportunities to conduct experiments on the collective behavior of the whole system. For the latter, we must do the best we can with those ready-made examples that Nature provides. In this regard, the situation of the Theory of Global Warming is rather similar to that of the Theory of Evolution.
There are indeed a great many aspects of the Theory of Global Warming that are falsifiable without waiting for the next century’s climate to come upon us. There are, to start, all the laboratory tests of basic physics, such as the infrared absorption properties of CO2 and water vapor. There are also field tests of the predictions of these basic physical theories, as is done when one measure water vapor and temperature in the atmosphere, and compares the predictions of radiative transfer theory with observed infrared radiation measured at the top of the atmosphere by satellite, or at the surface by radiation sensors. One can check the evaporation formulae used in climate models against the measured evaporation at buoys in the ocean, or the predictions of cloud models against observed cloud reflectivity. Going up the scale in complexity, one can compare the predictions of the theory against observations of recent climates, and of climates of the more distant past. General circulation models encapsulate the assumptions of the theory, and provide the tool necessary for testing hypotheses in such a complex system.
A further point regarding the positivist criterion is the the Theory of Global Warming is productive. The implied influence of CO2 (or methane) on climate can be, and has been, applied to the understanding of the Last Glacial Maximum, to Snowball Earth, to the Faint Young Sun, and to Cretaceous warmth. Variants apply also to Venus, Mars (present and past) and Titan. It is fair to say that this theory plays as central a role in the theory of planetary climate as the Theory of Evolution plays in biology. A relatied point is that the theory can be and has been challenged by data, and forced to adapt accordingly. This was the case in the precursor to the theory, when Tyndall discovered that minor constituents (CO2 and water vapor) dominated the greenhouse effect; the resulting adaptation of Fourier’s theory opened the way for Arrhenius to conclude that human influences on the atmosphere could change the climate. A more recent adaptation was the incorporation of aerosol effects in the late 1980′s which was forced upon the theory by the inability to explain the pattern of 20th century climate change with greenhouse gas increases alone. Contrary to the assertion in Mr. Williams’ letter to the Chicago Tribune, revisions to the theory have not led to any systematic downward revision of the appraisal of the magnitude of the thread caused by doubling CO2. Indeed, some discoveries, notably the prevalance of abrupt climate change in the past record, have raised concerns that the current understanding may underpredict the magnitude of the response.
What of Judge Jones’ other two criteria applied in the Kitzmiller case? The false duality issue does not arise in the judgement of Global Warming Theory itself, since the theory has never been argued for on the basis of such a stipulated duality ("The world is warming, and if it’s not the Sun, then it must be CO2!"). On the other hand, a false duality has often been invoked in arguing against the Global Warming Theory. This typically takes the form of pointing out some aspect of the observations that Global Warming Theory doesn’t explain, and then jumping to the conclusion that the observed warming must be due to the local skeptic’s favorite cause: maybe solar variability, maybe some unspecified sort of "natural variability." Often such arguments involve holding Global Warming up to unreasonable standards of proof ("If we don’t understand everything about climate, then we understand nothing about climate."), and often, like ID proponents arguing against Evolution, the arguments offered against Global Warming are at best distortions of scientific truth. In this regard, Global Warming plays the role of the Theory of Evolution, with the Global Warming Skeptics playing the role of ID advocates.
On Judge Jones’ final criterion (presence in the peer-reviewed literature) the Theory of Global Warming gets an easy and obvious pass. Here, the Global Warming skeptics are in a somewhat better position than the ID advocates, in that a very few of the skeptics arguments have appeared in the peer reviewed literature. This doesn’t make them right, but it does mean that to some extent, some of them are playing by the rules of science. Still, the relative paucity of skeptics arguments being played out in the peer reviewed literature suggests that they may not be as wrong as the ID advocates, but that they are not as right as the vastly greater number of researchers who have published in support of the Global Warming Theory.
To what extent is "Global Warming Theory" verified?
The basic physical principles upon which the Theory of Global Warming is based include the notion of interconvertibility amongst forms of energy (introduced by Fourier in his formulation of planetary energy balance), thermodynamics (air cools when it rises), thermodynamics of phase change (cold air holds less water), quantum theory (absorption and emission of infrared by CO2 and other greenhouse gases), blackbody radiation, and Newton’s laws of motion. Each of these components has passed literally thousands of tests in the laboratory. There is essentially zero uncertainty in the validity of such things, which form the basic physical underpinning of the Theory of Global Warming. If any of these parts of the theory didn’t work, neither would microwave ovens, computers, steam engines, infrared remote controls, and any number of other everyday devices.
Tests of the collective behavior of the Earth’s climate system are somewhat harder to come by, but there has been substantial progress here as well. I would highlight the following, which is far from an exhaustive list:
- Reproduction of the temporal and spatial pattern of 20th and 21st century warming. To be sure, models with varying assumptions about clouds and aerosols can fit the observed warming equally well, indicating that the job is not complete. However, no quantitative model based on physical principles can match the 20th century warming without incorporation of a substantial warming component from greenhouse gas increases.
- The rapid increase in atmospheric greenhouse gases should throw the Earth’s radiation budget out of balance, because the ocean has not yet had time to warm up to restore balance. The expected imbalance has been observed. (Hansen et al. 2005)
- The planet’s energy imbalance has implications for the pattern of subsurface ocean warming. The predicted pattern has been observed. (Discussed here.)
- Satellite observations indicate that mid-tropospheric water vapor is indeed increasing with temperature, as the theory requires and as models predict (Discussed here).. Note that the water vapor assumption I included as part of the statement of the Theory of Global Warming is not itself built into the general circulation models used to predict climate change. It is an emergent property that is deduced from more basic assumptions made in the models. In this regard, the statement regarding the presumed behavior of water vapor amounts to a statement that the models capture the same processes governing water vapor in the real atmosphere. There is now a wealth of evidence (in the "large scale control" literature) supporting this viewpoint.
- Melt-back of Northern Hemisphere sea ice
- Nearly worldwide melting of mountain glaciers, many of which survived previous naturally occurring warm periods
- The theory predicts that the stratosphere should be cooling at the same time the surface is warming. This pattern is observed.
- The degree of cooling of the Tropics and Southern Hemisphere during the Last Glacial Maximum, for which there would be no explanation if we were to assume that current models substantially overestimate sensitivity to CO2. An interesting bit of history concerning this point is that in the 1980′s the tropical behavior in glacial times was considered an indication that models were wrong: CLIMAP data indicated little surface cooling in the tropics, while mountain snowlined data did show cooling. This led to all sorts of theories spun about exotic thermostat mechanisms and strange lapse rate behavior. In the end, it turned out that the models were right and that the CLIMAP data was wrong. Thus, in this instance, the models (based on theory) made a true prediction, which was verified after the fact.
The scientific community is still searching for a really good way to evaluate the nature of cloud effects, though comparisons with past and recent climates provide some reassurance that we are not too far off base with cloud effects. More importantly, there is not yet a physically based hypothesis on the table which is compatible with data and which reduces climate insensitivity to inconsequential levels. Lindzen’s "Iris" hypothesis comes closest, but it has been evaluated in the scientific literature and most of the community remains unconvinced.
Besides the ongoing problem with clouds, the general theory of Earth’s climate, like any good scientific theory, continues to be confronted by phenomena it cannot yet fully explain, and to evolve in response. Some notable problems include the tendency of many coupled general circulation models to produce double Intertropical Convergence Zones in the Tropics, inconsistencies in the prediction of the regional distribution of climate change, inability to make firm inferences concerning the effect of global warming on El Nino, and the inability of general circulation models to reproduce recurrent abrupt climate change events like D-O events or even the full magnitude of response to the Younger-Dryas event. An especially notable unresolved challenge is the inability of models to reproduce the low North-South gradient in warm climates such as the Cretaceous. In this case as well as in others (such as the problem of vertical structure of tropical tropospheric warming) the problem may lie as much in the data sets being used to test the theories as in the theories themselves.
A theory can never be definitively proved; there is always the possibility that some new observation will overturn it, and most theories are imperfect and fail in one way or another to account for some of the data. The question thus emerges as to the extent to which global warming skeptics are holding the theory up to an "unreasonable standard of proof," much as ID proponents do in the case of Evolution. Given that the intensity of interest in the Theory of Global Warming stems largely from its policy implications, it is fair to ask how the standards of proof to which global warming has been held stack up against other theories that have been used to make policy decisions of enormous consequence. "Supply Side Economics" (the theory that tax cuts pay for themselves by stimulating economic growth) is a telling example that comes to mind (to say nothing of the "theory" that Iraq had WMD).
And speaking of intelligent design, I feel compelled to remark that the CO2 molecule seems rather admirably designed from the standpoint of regulating climate. It’s a good infrared absorber even in small quantities so you don’t need to much of it, yet the radiative effect is logarithmic in concentration, so you don’t have to tune its concentration too terribly precisely to get a habitable climate. There’s plenty of it in the form of carbonates in the Earth’s crust, so you can always get more if you need some to keep the climate warm enough. Most importantly, it plays well with liquid water, so that if the planet gets too warm or too cold the rate of removal tends to adjust to reset the atmospheric carbon dioxide at a point where the climate will stay relatively equable. It has thermodynamic properties that keep it from condensing out of the atmosphere (in contrast to water vapor), resulting in it having a long enough lifetime to even out the vicissitudes of climate forcing fluctuations. How strange it is, then, that the Earth should have an abundant supply of so attractive and convenient fuel as coal. A fuel which, unfortunately, messes up the system by releasing CO2 when it is burned.
Bad design? Or just forbidden fruit?
Hansen, J., et al. 2005. Earth’s energy imbalance: Confirmation and implications. Science 308, 1431-1435, doi:10.1126/science.1110252.