Happy Birthday, Charles Darwin!

  • 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."

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