The Rhetoric of the Scientific "95%"

When scientists say that they are "almost certain" or "virtually certain," this does not mean that the scientists are not sure. Just like "theory" doesn't mean the same thing in scientific discourse as it does in public discourse, the idea of certainty is also not at all the same in science as it is in public life. Take the following sets of statements; the first is written in a more regular manner of speech, and the latter is written with scientific recognition of contingency, but they mean the same thing:

Regular speech mode: This is going to be the best weekend party, ever!
Scientific speech mode: This will likely be the best weekend party that we have yet gone to.

Regular speech mode: John is a fireman.
Scientific speech mode: All the available evidence indicates that John is a fireman.

Yes, it sounds robotic. Yes, it sounds kind of annoying. Yes, it is overly pedantic. But it is also more accurate and more precise. The party this weekend cannot literally be the best weekend party throughout all of time. Unless you have objective information about the quality of parties in the future and in the distant past, you are likely to be able to project the likelihood that this upcoming party is better than similar ones from your past experiences.

Well, what about "John is a fireman"? If he works at a fire station, he is certified to be a fireman, is in a union for firemen, and actually goes out and fights fires, what more evidence is needed to prove that John is a fireman? Well... it's not about proving that he is a fireman. It's about whether there is evidence to show that he isn't a fireman. So far, we have no credible evidence to show that he isn't, which means that we can only say that the available evidence is indicative, but not proof-positive, that John is a fireman.

Yeah, it's annoying to have to deal with positivistic statements in regular day-to-day life while simultaneously having to recognize that scientists - when speaking scientifically - often appear to prefer speaking in terms that indicate the possibility that the statement in question can be falsified. (See what I did there?)

Anyway, with the latest IPCC report on climate change coming out, remember that it's a science-based document written (primarily) by scientists and to be based on the justifications surrounding the practice of science itself. In other words, it's chock-a-block full of apparently uncertain statements. But what is publicly meant and what is scientifically meant by the same words are not the same thing. Check it out:

Top scientists from a variety of fields say they are about as certain that global warming is a real, man-made threat as they are that cigarettes kill.

They are as sure about climate change as they are about the age of the universe. They say they are more certain about climate change than they are that vitamins make you healthy or that dioxin in Superfund sites is dangerous.

They'll even put a number on how certain they are about climate change. But that number isn't 100 percent. It's 95 percent.

And for some non-scientists, that's just not good enough.

There's a mismatch between what scientists say about how certain they are and what the general public thinks the experts mean, experts say.

That is an issue because this week, scientists from around the world have gathered in Stockholm for a meeting of a U.N. panel on climate change, and they will probably issue a report saying it is "extremely likely"—which they define in footnotes as 95 percent certain—that humans are mostly to blame for temperatures that have climbed since 1951.

One climate scientist involved says the panel may even boost it in some places to "virtually certain" and 99 percent.
Some climate-change deniers have looked at 95 percent and scoffed. After all, most people wouldn't get on a plane that had only a 95 percent certainty of landing safely, risk experts say.

But in science, 95 percent certainty is often considered the gold standard for certainty.

"Uncertainty is inherent in every scientific judgment," said Johns Hopkins University epidemiologist Thomas Burke. "Will the sun come up in the morning?" Scientists know the answer is yes, but they can't really say so with 100 percent certainty because there are so many factors out there that are not quite understood or under control.

George Gray, director of the Center for Risk Science and Public Health at George Washington University, said that demanding absolute proof on things such as climate doesn't make sense.

"There's a group of people who seem to think that when scientists say they are uncertain, we shouldn't do anything," said Gray, who was chief scientist for the U.S. Environmental Protection Agency during the George W. Bush administration. "That's crazy. We're uncertain and we buy insurance."
With the U.N. panel about to weigh in on the effects of greenhouse gas emissions from the burning of oil, coal and gas, The Associated Press asked scientists who specialize in climate, physics, epidemiology, public health, statistics and risk just what in science is more certain than human-caused climate change, what is about the same, and what is less.

They said gravity is a good example of something more certain than climate change. Climate change "is not as sure as if you drop a stone it will hit the Earth," Princeton University climate scientist Michael Oppenheimer said. "It's not certain, but it's close."

Arizona State University physicist Lawrence Krauss said the 95 percent quoted for climate change is equivalent to the current certainty among physicists that the universe is 13.8 billion years old.

The president of the prestigious National Academy of Sciences, Ralph Cicerone, and more than a dozen other scientists contacted by the AP said the 95 percent certainty regarding climate change is most similar to the confidence scientists have in the decades' worth of evidence that cigarettes are deadly.

"What is understood does not violate any mechanism that we understand about cancer," while "statistics confirm what we know about cancer," said Cicerone, an atmospheric scientist. Add to that a "very high consensus" among scientists about the harm of tobacco, and it sounds similar to the case for climate change, he said.

But even the best study can be nitpicked because nothing is perfect, and that's the strategy of both tobacco defenders and climate deniers, said Stanton Glantz, a medicine professor at the University of California, San Francisco and director of its tobacco control research center.

George Washington's Gray said the 95 percent number the Intergovernmental Panel on Climate Change will probably adopt may not be realistic. In general, regardless of the field of research, experts tend to overestimate their confidence in their certainty, he said. Other experts said the 95 percent figure is too low.

Jeff Severinghaus, a geoscientist at the Scripps Institution of Oceanography, said that through the use of radioactive isotopes, scientists are more than 99 percent sure that much of the carbon in the air has human fingerprints on it. And because of basic physics, scientists are 99 percent certain that carbon traps heat in what is called the greenhouse effect.

But the role of nature and all sorts of other factors bring the number down to 95 percent when you want to say that the majority of the warming is human-caused, he said.