WHAT DOES 95% REALLY MEAN?
Science in Context
When climate scien-sts say they are 95% certain that humans are mostly to blame for temperatures increases since 1951, it technically means that there is only a 5% chance that random effects produced these observed changes. But when this “science speak” is put into context, the statement denotes much more. It signifies overwhelming and undeniable proof that humans are altering the climate system. Scien-sts use theories to figure out how the world works, and then test them. Scien-sts build their theories in accordance with observable facts. They accept nothing as 100% proven, instead using sta-s-cal methods to quan-fy their specific degree of confidence that study results are not due to chance. This leaves the door open to adjust their theories when new informa-on is discovered.
Even concepts we all take as fact, like gravity, are scien-fically classified as theories. A recent Associated Press story highlighted, for example, that scien-sts aren’t even 100% certain the sun will come up tomorrow. This ar-cle also cited these examples to put 95% certainty into perspec-ve:
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95% certainty is equivalent to the current scien-fic certainty that the universe is 13.8 billion years old.
95% certainty is equivalent to the current scien-fic certainty that smoking cigareSes is linked to cancer.
This graphic featured in the journal Nature shows the progression of certainty levels about human contribu-on to climate change in IPCC reports (look at the boSom row: “Gaining Confidence”), as scien-sts have more and more evidence of human influences on climate change.
Relevant Quotes from Scien3sts
Dr. Heidi Cullen, Vice President and Chief Climatologist for Climate Central “We live in a world where hyperbole feels very commonplace. But within the scien-fic community that is very unacceptable. For scien-sts the word ‘likely’ means 66-­‐90% certain, ‘very likely’ means greater than 90% certain, ‘extremely likely’ means 95-­‐100% certain. We (scien-sts) tend to be far more precise with language, some-mes to our own disadvantage. A statement that there is 95% certainty that climate change is a result of our ac-ons, in and of itself, is quite profound. We’re about as precise and confident as you can get in that.”
Dr. James McCarthy, Professor of Biological Oceanography at Harvard University
“What if a body of comparable authority (to the IPCC), such as the world’s experts on lung disease or heart disease, like the American Cancer Society or the American Lung Associa-on, said that they are 95% certain that smoking will contribute to cancer or lung disease. There would be no doubt to the public what that means. We’re in a similar situa-on right now.”
“We need to make sure the non-­‐scien-fic community understands how confident the scien-sts are with these statements. The cau-on of scien-sts of never wan-ng to overstate on this has had us (the IPCC) seeing changes that now upon reflec-on need a stronger statement, further change, a stronger statement, we’re now at a point where you know we’re within a whisker of being virtually certain.”
Dr. George Gray, Director of the Center for Risk Science and Public Health at George Washington University, and Chief Scien-st for the U.S. Environmental Protec-on Agency during the George W. Bush administra-on “There's a group of people who seem to think that when scien-sts say they are uncertain, we shouldn't do anything. That's crazy. We're uncertain and we buy insurance.” -­‐ Associated Press
Dr. Michael Oppenheimer, Climate Scien-st at Princeton University
“(Climate change) is not as sure as if you drop a stone it will hit the Earth. It's not certain, but it's close.” -­‐ Associated Press