Today’s big report from the Intergovernmental Panel on Climate Change says it is “extremely likely” that people are responsible for global warming.  

That’s up from “very likely” in the panel’s 2007 report.

For most of us, the difference between saying something is “extremely likely” and “very likely” is pretty negligible. But for the IPCC scientists, the difference is five percentage points.

“Extremely likely” means that scientists think there’s a 95 percent chance global warming is caused by human activity.  “Very likely” indicates a 90 percent chance.

The IPCC has an entire likelihood scale, associating probabilities with words.

Likelihood Scale
Virtually certain	99-100% probability
Very likely	90-100% probability
Likely	66-100% probability
About as likely as not	33-66% probability
Unlikely	0-33% probability
Very unlikely	0-10% probability
Exceptionally unlikely	0-1% probability

“If you ask people what you think a term like” likely,” or “very likely,” or “extremely likely” means, without any kind of a definition, you get very different answers for what they think those mean in terms of probabilities,” says Michael Mastrandrea, an environment professor at Stanford University, who helped develop the scale.

The scale gives everyone a consistent language. The panel also has set tiers of confidence — from very low to very high. 

And, yes, says Mastrandrea, scientists make jokes with their certainty scales.

Hill: Do you guys just at some point start making jokes about: it’s extremely likely I’m going to order steak for dinner?

Mastrandea: I think we do that all too often, unfortunately.  But I have pretty high confidence, I should say I have high confidence, that they are not very good jokes. 

There are important reasons for scientists to have a structured language of certainty and uncertainty.  “In climate science we have to be very honest about what we know, and what we don’t know,” says UCLA climate researcher and IPCC contributor Alex Hall,  “and this uncertainty language is a way to talk about that.”

All science has uncertainty.

And, while human responsibility for global warming is now “extremely likely,” there’s less certainty about the speed and effects of climate change.  “The true state of knowledge is shades of gray, not black and white,” says Hall. 

This likelihood language helps scientists communicate those shades to the public. And makes clear that now is the time to do something about it.