Doubt among climate change skeptics challenges definition of 'virtually certain'

WASHINGTON – 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.

That is an issue because this week, scientists have gathered in Stockholm for a meeting of a U.N. panel on climate change, and they will probably release a report saying it is "extremely likely" — which they define 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 skeptics 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.

"Will the sun come up in the morning?" said Johns Hopkins University epidemiologist Thomas Burke. Scientists know the answer is yes, but they can't really say so with 100 percent certainty because there are so many factors 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.