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In recent days, you’ve probably heard ad nauseum that the UN’s new IPCC report claims that it is “95% certain” that humans are causing climate change.

95% is a very specific number. So where does it come from?

The IPCC uses a “likelihood scale” that assigns percentages to various phrases, ranging from “exceptionally unlikely” (0-1% probability) to “virtually certain” (99-100% probability). This sounds like it is based on a precise scientific measurement or well-defined statistical process, but when it comes to deciding how likely it is that climate change is manmade, this is in fact a subjective decision that is made by the report’s authors.

According to the IPCC: “The approaches used in detection and attribution research […] cannot fully account for all uncertainties, and thus ultimately expert judgment is required to give a calibrated assessment of whether a specific cause is responsible for a given climate change.”

In other words, the “95% probability” that is making all of the headlines is nothing more than an arbitrary number decided on in closed door meetings between the report authors. Still, it serves an important propaganda purpose in giving a veneer of scientific credibility to the decision, one that a media that never bothers to explain these decisions to you thinks you will be too stupid to figure out for yourself:

So how reliable is the IPCC process in general? That will be the subject of next week’s report.