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.
It is misleading, though perhaps not technically false, to say that CO2 "absorbs" infrared radiation. CO2 is not like an absorbent sponge for heat. It is better to say that CO2 is a radiative gas. The mechanism is this: In-coming short-wave radiation (SWR) from the sun passes right through CO2, because the resonant frequencies of the CO2 molecule are different than the SWR from the sun. When the SWR hits the earth, it is transformed into heat and then (immediately or eventually, depending on the properties of the earth's surface) reflected out as long-wave radiation (LWR). This out-going LWR hits CO2 molecules, and because of the resonant frequency of CO2 molecules, the LWR "excites" the molecules temporarily. But it is only a temporary excitation; ere long, the CO2 molecule sheds LWR in another direction and returns to it's unexcited state. Some (but only a small fraction) of the LWR that is shed by atmospheric CO2 is reflected back to the earth: it is this back-reflection that is conjectured to be the driver of catastrophic climate change. The improbability of this conjecture is not hard to see when you consider: (a) how tiny the changes in CO2 in the atmosphere have been (from 280 ppm to 410 ppm); (b) how low a percentage of LWR is actually reflected back to earth by CO2 molecules, rather than being dispersed into space; (c) that the radiative frequencies of CO2 are narrow bands that completely overlap with the radiative frequencies of water vapour; and (d) that water vapour is 95% of radiative gasses in the atmosphere. No wonder the original discoverer of this process, the Nobel Prize winner Arrhenius, thought that we should pump more CO2 into the atmosphere to make our climate more pleasant.
All those CO2 causation believers should plot a graph of CO2 versus Time starting 500 million years ago. Unlike all the phoney (ie incredibly complex and excluding critical known variables) climate models, this too is a "model", incredibly simple and sufficient. It's a straight line on a logarithmic scale which demonstrates 2 important facts.
1. Life on earth thrived at 5 times the current levels.
2. CO2 levels are dangerously low. Projecting 25,000 years forward, there will be insufficient CO2 to sustain life on earth.
The only assumption made in this model is that the 500 million year trend will change.