Michael Asten: cautious on CO2 causation

Climate sense

Writing in The Australian, Michael Asten again questions the climate science orthodoxy in an excellent article (his earlier article was posted here). Here he addresses the Palaeocene-Eocene thermal maximum, a time when CO2 levels rose sharply and temperatures also rose, often cited by the warmists as “proof” that CO2 causes rapid and dangerous warming:

I argue there are at least two possible hypotheses to explain the data in this study: either the link between atmospheric CO2 content and global temperature increase is significantly greater (that is, more dangerous) than the existing models show or some mechanism other than atmospheric CO2 is a significant or the main factor influencing global temperature.

The first hypothesis is consistent with climate change orthodoxy. Recent writings on climate sensitivity by James Hansen are consistent with it, as was the Intergovernmental Panel on Climate Change in its pre-Copenhagen update, The Copenhagen Diagnosis.

Indeed, the 26 authors of the IPCC update went a step further, and encouraged the 46,000 Copenhagen participants with the warning: “A rapid carbon release, not unlike what humans are causing today, has also occurred at least once in climate history, as sediment data from 55 million years ago show. This ‘Palaeocene-Eocene thermal maximum’ brought a major global warming of 5C, a detrimental ocean acidification and a mass extinction event. It serves as a stark warning to us today.”

We have to treat such a warning cautiously because, as Pearson and his colleagues pointed out in their letter two weeks ago, “We caution against any attempt to derive a simple narrative linking CO2 and climate on these large time scales. This is because many other factors come into play, including other greenhouse gases, moving continents, shifting ocean currents, dramatic changes in ocean chemistry, vegetation, ice cover, sea level and variations in the Earth’s orbit around the sun.”

Sound science also requires us to consider the second of the above two hypotheses. Otherwise, if we attempt to reconcile Zeebe’s observation by inferring climate sensitivity to CO2 is greater than that used for current models, how do we explain Pearson’s observation of huge swings in atmospheric CO2, both up and down, which appear poorly correlated with temperatures cooling from greenhouse Earth to moderate Earth.

The two geological results discussed both show some discrepancies between observation and model predictions. Such discrepancies do not in any sense reduce the merit of the respective authors’ work; rather they illustrate a healthy and continuing process of scientific discovery.

Read it here.

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