As an addendum to my 2020 series of posts on the CO₂ global warming hypothesis (here, here and here), this post presents a further challenge to the hypothesis central to the belief that humans make a substantial contribution to climate change. The hypothesis is that observed global warming – currently about 1 degree Celsius (1.8 degrees Fahrenheit) since the preindustrial era – has been caused primarily by human emissions of CO₂ and other greenhouse gases into the atmosphere.

The new challenge to the CO₂ hypothesis is set out in a recent research paper by French geologist Pascal Richet. Richet claims, by reexamining previous analyses of an Antarctic ice core, that greenhouse gases such as CO₂ and methane had only a minor effect on the earth’s climate over the past 423,000 years, and that the assumed forcing of climate by CO₂ is incompatible with ice-core data. The paper is controversial, however, and the publisher has subjected it to a post-publication review, as a result of which the paper has since been removed.

The much-analyzed ice core in question was drilled at the Russian Vostok station in East Antarctica. Past atmospheric CO₂ levels and surface temperatures are calculated from ice cores by measuring the air composition and the oxygen ¹⁸O to ¹⁶O isotopic ratio, respectively, in air bubbles trapped by the ice. The Vostok record, which covers the four most recent ice ages or glaciations as well as the current interglacial (Holocene), is depicted in the figure below. The CO₂ level is represented by the upper set of graphs (below the insolation data), and shows the substantial drop in CO₂ during an ice age; the associated drop in temperature ΔT is represented by the lower set of graphs.

It is seen that transitions from glacial to interglacial conditions are relatively sharp, while the ice ages themselves are punctuated by smaller warming and cooling episodes. And, though it’s hardly visible in the figure, the ice-age CO₂ level closely mimics changes in temperature, but the CO₂ concentration lags behind – with CO₂ going up or down after the corresponding temperature shift occurs. The lag is more pronounced for temperature declines than increases.

The oceans, which are where the bulk of the CO₂ on our planet is stored, can hold much more CO₂ (and heat) than the atmosphere. Warm water holds less CO₂ than cooler water, so the oceans release CO₂ when the temperature rises, but take it in when the earth cools.

Richet noticed that the temperature peaks in the Vostok record are much narrower than the corresponding CO₂ peaks. The full widths at half maximum, marked by thick horizontal bars in the figure above, range from about 7,000 to 16,000 years for the initial temperature peak in cycles II, III and IV, but from 14,000 to 23,000 years for the initial CO₂ peak; cycle V can’t be analyzed because its start is missing from the data. All other peaks are also narrower for temperature than for CO₂.

The author argues that CO₂ can’t drive temperature since an effect can’t last for a shorter period of time than its cause. The fact that the peaks are systematically wider for CO₂ than for temperature implies that the CO₂ level responds to temperature changes, not the other way round. And for most of cycles II, III and IV, CO₂ increases correspond to temperature decreases and vice versa.

Richet’s conclusion, if correct, would deal a deathblow to the CO₂ global warming hypothesis. The reason has to do with the behavior of the temperature and CO₂ level at the commencement and termination of ice ages.

Ice ages are believed to have ended (and begun) because of changes in the Earth’s orbit around the sun. After tens of thousands of years of bitter cold, the temperature suddenly took an upward turn. But according to the CO₂ hypothesis, the melting of ice sheets and glaciers caused by the slight initial warming could not have continued, unless this temperature rise was amplified by positive feedbacks. These include CO₂ feedback, triggered by a surge in atmospheric CO₂ as it escaped from the oceans.

The problem with this explanation is that it requires a similar chain of events, based on CO₂ and other feedbacks, to have enhanced global cooling as the temperature fell at the beginning of an ice age. But, says Richet, “From the dual way in which feedback would work, temperature decreases and increases should be similar for the same concentrations of greenhouse gases, regardless of the residence times of these gases in the atmosphere.” The fact that temperature decreases don’t depend in any straightforward way on CO₂ concentration in the figure above demonstrates that the synchronicity required by the feedback mechanism is absent.

Next: Fishy Business: Alleged Fraud over Ocean Acidification Research, Reversal on Coral Extinction