CNN - If we humans warm the world more than 2 degrees Celsius (3.6 Fahrenheit), we greatly up the odds of climate catastrophes. Think super droughts, rising seas, mass extinctions and acidifying oceans. We don't want to cross that mark.
For climate change, the point is we don't know the world will fall into crisis at exactly 2 degrees and not 2.1 or 1.8. But that range is certainly dangerous.
"The way I think of it is like smoking," said Katharine Hayhoe, an atmospheric scientist and director of the Climate Science Center at Texas Tech. "How many cigarettes are too many? I don't know. I have no idea. Nobody has any idea. But we do know the more cigarettes you smoke, the greater the risk."
Two degrees, then, is our most educated estimate. A way to focus attention.
It's incredibly valuable in that respect.