Broadly Misleading

Let’s take a closer look at the question of CO2 variations over deep time. In contrast to the situation for the late Pleistocene, there is no one method for reconstructing CO2 at earlier times which is fully satisfactory. Methods range from looking at carbon isotopes in microfossils to looking at the density of pores on fossil leaves, with many other exotic geochemical tracers (e.g Boron) coming in in recent times. There is also some data for the very early Earth associated with the CO2 conditions under which certain exotic minerals (uraninites and siderites) form. None of the methods is unambiguous, and none provide information about other greenhouse gases that might be playing a role (though there may be some hope to do something abou methane). As an example of the difficulty faced by the field, take a look at the compilation of various estimates of CO2 since the Permian presented in the following figure (From Donnadieu et al, G3, in press; the red squares come from an attempted geochemical model fit to the data. The data set comes from Royer et al. (2004) and is available here).

By the time one gets back to the Permian, the error bars are huge. At earlier times, the estimates are even more problematic. Broad’s article does make reference to a very interesting paper by MIT’s Dan Rothman, writing in PNAS. This paper attempts a peek at the CO2 over the past 500 million years, using a clever and novel reconstruction technique. It is innovative, but far from the last word on the subject. Broad inappropriately cherry-picks Rothman’s statement that there appears to be no clear connection between warm climates and CO2 (except in "recent" times, about which more anon). However, Broad’s article neglects all the caveats in the paper, which clearly point to the real problem being that the reconstructions of CO2 and climate over such time scales are so uncertain that it’s not clear that the data is up to the task of teasing out ssuch a connection.

Even in Rothman’s reconstruction, during the past 50 milllion years — when the data is best and continents are most like the present — the long term cooling trend leading into the Pleistocene is clearly associated with a long term CO2 decline. This is not our main reason to infer that increasing CO2 will warm the climate in the future, but insofar as the data supports CO2 decline as a main culprit in the long slide from the Cretaceous hothouse climates of 60 million years ago to the cold Pleistocene climate, it also lends weight to the notion that as industrial activity busily restores CO2 to levels approaching those of the Cretaceous, climate is likely to turn the climate clock back 60 million years as well. From Broad’s flip dismissal of the CO2-climate connection in the "recent" part of the record, the reader would never guess at the length and particular significance of this period.

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