90% is Good Enough

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Professor David Budescu and his colleagues asked 223 volunteers to read sentences from reports by the Intergovernmental Panel on Climate Change (IPCC). The responses revealed some fundamental misunderstandings about how science works.

Science is a process. Scientists gather and compare evidence, then construct hypotheses that “make sense” of the data and suggest further tests of the hypothesis. Other scientists try to find flaws in the hypothesis with their own data or experiments.

Eventually, a body of knowledge builds, and scientists become more and more certain of their theories. But there’s always a chance that a theory will be challenged. And so the scientists speak about degrees of certainty. This has led to some confusion among the public about the scientific consensus on climate change.

[volunteers] interpreted statements such as “It is very likely that hot extremes, heat waves and heavy precipitation events will continue to become more frequent” to mean that scientists were far from certain.

In fact, the term very likely means more than 90 percent certain, but almost half the subjects thought it meant less than 66 percent certain, and three quarters thought it meant less than 90 percent.

The IPCC considers “virtually certain” to mean more than 99 percent likely; “very likely” to mean more than 90 percent certain; “likely” to be more than 66 percent; “more likely than not” more than 50 percent; and so on. The IPCC has concluded it is “very likely” that human emissions of greenhouse gases rather than natural variations are warming the planet’s surface. Remember, that means they are more than 90 percent certain. That’s about as close to unequivocal as science gets.

This is science that has been rigorously peer-reviewed. …We may never reach 100 percent certainty on climate change and its causes—that’s not what science is about.