Broadly Misleading

Just when we were beginning to think the media had finally learned to tell a hawk from a handsaw when covering global warming (at least when the wind blows southerly), along comes this article ‘In Ancient Fossils, Seeds of a New Debate on Warming’ by the New York Times’ William Broad. This article is far from the standard of excellence in reporting we have come to expect from the Times. We sincerely hope it’s an aberration, and not indicative of the best Mr. Broad has to offer.

Broad’s article deals with the implications of research on climate change over the broad sweep of the Phanerozoic — the past half billion years of Earth history during which fossil animals and plants are found. The past two million years (the Pleistocene and Holocene) are a subdivision of the Phanerozoic, but the focus of the article is on the earlier part of the era. Evidently, what prompts this article is the amount of attention being given to paleoclimate data in the forthcoming AR4 report of the IPCC. The article manages to give the impression that the implications of deep-time paleoclimate haven’t previously been taken into account in thinking about the mechanisms of climate change, whereas in fact this has been a central preoccupation of the field for decades. It’s not even true that this is the first time the IPCC report has made use of paleoclimate data; references to past climates can be found many places in the Third Assessment Report. What is new is that paleoclimate finally gets a chapter of its own (but one that, understandably, concentrates more on the well-documented Pleistocene than on deep time). The worst fault of the article, though, is that it leaves the reader with the impression that there is something in the deep time Phanerozoic climate record that fundamentally challenges the physics linking planetary temperature to CO2. This is utterly false, and deeply misleading. The Phanerozoic does pose puzzles, and there’s something going on there we plainly don’t understand. However, the shortcomings of understanding are not of a nature as to seriously challenge the CO2.-climate connection as it plays out at present and in the next few centuries.

The use of the more recent Pleistocene and Holocene record to directly test climate sensitivity presents severe enough difficulties (discussed, for example, here), but the difficulties of using deep-time Phanerozoic reconstructions for this purpose make the Pleistocene look like childs’ play. The chief difficulty is that our knowledge of what the CO2. levels actually were in the distant past is exceedingly poor. This situation contrasts with the past million years or so, during which we have accurate CO2. reconstructions from ancient air trapped in the Antarctic ice. Obviously, if you don’t know much about how CO2. is changing, you are poorly placed to infer its influence on climate, even if you know the climate perfectly and nothing else is going on besides variation of CO2.. But, neither of these latter two conditions are true either. Our knowledge of climates of the distant past is sketchy at best. Even for the comparatively well-characterized climates of the past 60 million years, there have been substantial recent revisions to the estimates of both tropical (Pearson et al., Nature 2001) and polar (Sluijs et al, Nature 2006) climates. Most importantly, one must recognize that while CO2. and other greenhouse gases are a major determinant of climate, they are far from the only determinant, and the farther back in time one goes, the more one must contend with confounding influences which muddy the picture of causality. For example, over time scales of hundreds of millions of years, continental drift radically affects climate by altering the amount of polar land on which ice sheets can form, and by altering the configuration of ocean basins and the corresponding ocean circulation patterns. This affects the deep-time climate and can obscure the CO2-climate connection (see Donnadieu, Pierrehumbert, Jacob and Fluteau, EPSL 2006), but continental drift plays no role whatsoever in determining climate changes over the next few centuries.

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