The falsifiability rule asserts that for a theory to count as scientific, it must be logically capable of being disproven. The falsifiability of climate change theories has been contested by some of their critics.

Science covers such a broad range of subjects that it is very hard to come up with a simple definition of scientific proof. Many scientific ideas cannot be tested by simple experiments. Sometimes, even well-established scientific ideas require revision when new data are discovered, so scientists generally believe it is impossible to prove a theory to be absolutely true. Austrian-born British philosopher Karl Popper proposed that ideas could be regarded as legitimate science if they could be falsified--that is, if there were some test that might possibly prove the idea wrong. Falsifiability separates science from subjects such as philosophy, religion, and political ideology, for which either there is no way to disprove ideas or believers are unwilling to admit their ideas can be falsified.

The simplest form of falsification is to compare a theory with the results of an experiment or an observation in nature. If the theory does not agree with the experiment or observation, it has been falsified. Of course, the experiment or observation must be performed correctly. If there is any doubt, the test can be repeated. Therefore, replicability, the ability to repeat observations or experiments, is considered a vital part of the scientific method and is one aspect of falsification. Events outside the laboratory such as earthquakes, hurricanes, and epidemics are never exactly repeatable, but they all have features in common from one occurrence to the next.

Theories about events in the remote past or physically inaccessible events can be falsified in several ways. For example, one can compare ancient features in rocks with features forming now to test whether processes on Earth have changed over geologic time. One can develop theories of how stars develop and compare the results with actual stars. If the theories fail to match observations, they have been falsified.

Popper also noted that tests must be "risky." There must be a real possibility of the test failing. If one knows that a theory will always pass a given test, it is not a real test. A test in which someone dismisses or evades the falsification is also not a real test. Although many people claim their religion or ideology is scientific, very few are willing to submit to a risky test and abide by the results.

Sometimes falsifications are wrong. A famous example is that of astronomer Simon Newcomb, who "proved" that powered flight was impossible only a few months before Wilbur Wright and Orville Wright successfully flew, because he made overly conservative assumptions about the weight and power of engines. Erroneous falsifications are common enough that author Arthur C. Clarke described them in his First Law: "When a distinguished but elderly scientist says that something is possible, he is very likely right. When he says that something is impossible, he is very probably wrong." Generally, erroneous falsifications stem from overly strict assumptions or failure to anticipate new discoveries.

One of Popper's own errors, frequently cited by anti-evolutionists, was to say "Darwinism is not a testable scientific theory, but a metaphysical research program." In fact, Charles Darwin himself proposed one test of evolution. He stated that evolution would be falsified if it could be shown that some change could not have occurred in a series of gradual steps. In 1978, Popper retracted his statement, saying "I have changed my mind about the testability and logical status of the theory of natural selection; and I am glad to have an opportunity to make a recantation."

The more tests a theory has passed, the more confident one can be in its validity. For long-established theories, it is hard to imagine any additional test that might falsify them. However, many of the best-established ideas in science are the basis of modern technology and can be said to face the riskiest of all tests: producing practical results. If the theories ever fail to produce the expected results, they will have failed.

Many people equate falsifiability with experiment, but there are many other ways scientific ideas can be falsified. They can be shown to be contradicted by other scientific findings, or shown to be logically inconsistent. Some people believe ideas cannot be falsified simply because they are unaware of the possible tests. For example, study of light from distant galaxies shows clearly that the same laws of physics apply throughout the universe. The idea that the laws of nature are constant in space and time is falsifiable and so far has survived every test.

Many people, seeking to justify disbelief in some scientific idea, engage in naive falsification--the use of evidence that seems to contradict established science, even if the evidence is of poor quality or supported by bad reasoning. Whenever an observation is claimed to contradict a theory, there are two possibilities: Either the theory is wrong, or the observations are wrong. If a single observation is claimed to disprove a well-tested theory, almost certainly the observation is wrong. In addition, if the results are reported in popular media, the observation may be inaccurately reported or even a deliberate hoax. Finally, unsolved problems or unanswered questions do not disprove a theory.

There probably is no single, all-purpose definition of science, but falsifiability is one of the most widely useful. In the case of climate change, falsifiability must be understood and applied properly. Isolated cases of unusually warm or cold weather do not prove or disprove climate change. Long-term climate trends, unprecedented changes such as large-scale melting of Arctic sea ice, and the physics of heat absorption by greenhouse gases count much more than individual weather events.

Bibliography:

1) Clarke, Arthur C. Profiles of the Future: An Inquiry into the Limits of the Possible. New York: Harper & Row, 1962.

2) Godfrey-Smith, Peter. Theory and Reality: An Introduction to the Philosophy of Science. Chicago: University of Chicago Press, 2003.

3) Popper, Karl. The Logic of Scientific Discovery. 1959. Reprint. New York: Routledge, 2002.

4) Popper, Karl. "Natural Selection and the Emergence of Mind." Dialectica 32, nos. 3/4 (1978): 339-355.

5) Tesh, Sylvia Noble. Uncertain Hazards: Environmental Activists and Scientific Proof. Ithaca, N.Y.: Cornell University Press, 2000.

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