How certain are they about global warming?
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 certain that cigarettes kill.
They are as sure about climate change as they are about the age of the universe. And 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 the number isn't 100 percent -- it's 95 percent.
And for some non-scientists, that's just not good enough.
This week, scientists from around the world have gathered in Stockholm for a meeting of a U.N. panel on climate change. They are expected to issue a report saying it's "extremely likely" -- which they define as 95 percent certain -- that humans are mostly to blame for temperatures that have climbed since 1951.
Some climate change deniers say the 95 percent figure shows there's doubt.
But scientists say it's about as close as you can get to certainty. One says it's not even 100 percent certain that the sun will come up in the morning.