In the cult classic movie Harold and Maude, Maude recites a quote, but cannot remember who had said it. “Oh, well,” she said, “I did.”
The statement “Extraordinary claims require extraordinary evidence” is now famous. I’m not sure who said it first—I have heard it was Carl Sagan, but I’ll bet he was quoting one of the giants on whose shoulder he was standing. What I wish to consider now is, what constitutes extraordinary evidence? An important kind of extraordinary evidence is consilience.
Consilience means that a theory has been supported by numerous, independent lines of evidence—independent, that is, unless the theory itself is true. William Whewell (pronounced “hule”) invented the term, and used Isaac Newton as an important example. Newton’s theories of motion and gravitation were supported by the movement of objects on Earth and of the planets. Whewell rejected the evolutionary theory of Charles Darwin, even though evolution has proven to be one of the premier examples of consilience. Evolution is supported by fossils, biogeography, the process of natural selection, and now by molecular evidence. Without evolution, there is no reason to expect that older fossil assemblages should be less similar to modern organisms than younger fossil assemblages, for large placental mammals to be absent from Australia and Borneo, or for chimps to have the same pseudogenes that we do.
There are numerous examples of consilience in the history of science, though few as important as evolution. One example is DNA. Watson and Crick figured out the structure of DNA not only from chemistry and crystallography, but also because this structure made it obvious how could replicate and how genetic information could be stored.
The newest example of massive consilience is the theory of global warming. It is a theory because it is assembled from numerous hypotheses, all of which have been tested and confirmed. Among the lines of evidence are:
• Direct temperature readings by thermometers in the last century and a half
• Direct carbon dioxide measurements from the atmosphere in the last half century
• Indirect temperature estimates from ice cores
• Carbon dioxide measurements from ice cores
• Melting of polar sea ice in the Arctic
• Melting of land ice in Greenland and Antarctica
• Rising sea levels (despite the refusal of the North Carolina legislature to permit it)
• Phenological changes in many species of animals and plants
• Warmer winters allowing insect outbreaks in coniferous forests
• Methane bubbling out of Siberian lakes that no longer freeze even in winter
• Storms of increased intensity in coastal areas
• Droughts of increased intensity and duration in continental areas
And so on. I have contributed some of the phenological data to the above list. (Plants and animals have become active earlier in the spring by about two days per decade or, in some local cases such as my data set, two days earlier per year.) There was a time, about thirty years ago, when global warming was an extraordinary claim that required extraordinary evidence. The evidence is now in, and it constitutes consilience. There is no reason to expect all of those separate lines of evidence to converge on the same theory unless the theory is true. Consilience can be thought of as the opposite of pseudoreplication, in which you measure the same thing over and over.
Edward O. Wilson has written many books of great significance, and you all know about most of them. In case you might have overlooked it, I wish to suggest his book Consilience. He indicated that evolution provides a framework for the consilience of all branches of knowledge. This claim was sufficiently broad and sweeping to evoke some strong responses, perhaps most notably from essayist Wendell Berry, who wrote Life is a Miracle in a sincere but, I think, failed attempt to refute Wilson. I am an immense admirer of Berry, but admit that he was wrong about this one.
It is interesting that the anti-science movement has fought against both evolution and global warming. They have taken on two of the aspects of modern science that not only have the most evidence, but the most consilience.