A study in Nature Climate Change, ostensibly celebrating the 40th anniversary of weather satellites, claimed that the influence of CO2 on the climate is now known to a "5-sigma" level of certainty, which according the authors say puts it into the "gold standard of proof" category alongside events like the discovery of the Higgs Boson. Naturally the alarmist press was quick to assume the Gold Standard of Proof slogan applied to all climate claims. But University of Guelph economist Ross McKitrick says not so fast. Their statistical tests only detect warming without isolating the cause or amount. And even at that they exaggerate the certainty.
McKitrick points out that, even if the authors are entirely correct in their analysis, they are measuring the existence of an effect, not its magnitude. Their conclusions are fully consistent, for instance, with recent satellite-based evidence that warming from greenhouse gases is minimal--below the low end of the range projected in climate models.
As for their statistical model, he says it makes a comparison that didn't support the headline claims of the paper. They used climate models to construct an artificial alternative climate history in which there were no greenhouse gases or climate trends of any kind. When they compared that simulation to observations of the last 40 years, unsurprisingly, it didn't fit very well. Then they used a simulation tuned to match observed changes, including a role for greenhouse gases, which of course fit better and said “Gotcha”. But McKitrick likens this to a rigged police lineup: "It is as if a bank robber were known to be a 6 foot tall male, and the police put their preferred suspect in a lineup with a bunch of short women. You might get a confident witness identification, but you wouldn’t know if it’s valid."
It's more or less a given that carbon dioxide absorbs infrared radiation, which (all else being equal) means with more CO2 in the atmosphere there should be slightly higher temperatures. That's been known since the 1800s. All the questions that matter for society and public policy depend on the difficult and much-disputed details that follow. Is it a large effect in the troposphere? Does it even matter at the Earth's surface? Will it be harmful? How does it affect weather extremes? None of these questions are settled. Breathless claims of 5-sigma certainty create a pleasant illusion that everything is now sorted out and the alarmists have won every debate. But beware the glittering certainty: fool's gold has taken in many a prospector before.