climate change, language change

As someone who teaches students about the precision of language, I find these two sentences from a CNN article on the latest global warming research amusing:

"The phrase 'very likely' translates to a more than 90 percent certainty that global warming is caused by man.

What that means in layman's language is 'we have this nailed,' said top U.S. climate scientist Jerry Mahlman, who originated the percentage system."

Is Jerry Mahlman really the one who decided that "very likely" = 90%? Does that mean that whenever I use "very likely" in the future, it's going to mean I'm 90% certain about something? And why does "very likely" need to be translated into layman's terms? It it really such technical language that no ordinary mortal could grasp it?