Saturday, June 03, 2006

Comforts of credulity

I was driving about this afternoon minding my own business when Skeptics Society founder Michael Shermer snuck on to the radio (“again,” believe I heard him on AirTalk just the other day). He had a limited amount of time to say a limited number of things I agree with. The topic of the discussion was the Skeptics Society conference (The Environmental Wars) going on at Caltech this weekend. Shermer was asked what the dominant topic of debate would be, and responded: “global warming.”

This was spoken not in the sense of there being controversy as to whether the climate is warming or not but rather as to the cause of climate change. Shermer assures us of controversy on that point, and I’m not anyone to deny that, generally. However, he rambles on about how the climate naturally warms and cools, the host obligingly responds “yes, we heard this week how the poles were like Miami,” and Shermer approves, noting that indeed the temperature was significantly higher than present without the assistance of SUVs, ~55 million years ago.

They refer to recent headlines regarding Moran et al. (2006), a new study published in Thursday’s issue of Nature concerning the Paleocene-Eocene Thermal Maximum (PETM), ~55 million years ago. The still-controversial PETM has been previously attributed to catastrophic methane release from marine clathrate deposits, leading to greenhouse gas buildup and global warming. There are some inconsistencies to resolve concerning this theory. A recent study evaluated shoaling of the carbonate compensation depth (enhanced carbonate dissolution resulting from acidification of the oceans), arguing that the amount of carbon released to the oceans is larger than would be consistent with a methane source (Zachos et al., 2005). What is clear is that substantial warming took place contemporaneously with substantial carbon input to the oceans and atmosphere.

With the fact in hand that the climate was substantially warmer at the Paleocene-Eocene boundary without anthropogenic input, the implication of the radio discussion that caught my attention was that such events in the past imply doubt concerning causes of modern warming. However, this conclusion does not logically follow from the interpretation of the past record. There is no mechanistic connection between the PETM and the present trend of global warming. Scientific evidence is clear on the question of the derivation of modern perturbations to the carbon budget. Modern buildup of carbon dioxide is due to human activity, not to the destabilization of methane hydrates or any other feasible mechanism for the PETM event. The PETM example is not at all encouraging for our purposes… a greenhouse earth in which the polar caps are wholly removed is a catastrophic scenario beyond the direst of present forecasts. This example does not provide any rational basis for disputing anthropogenic global warming… nor does any example of past climate change that lacks relevance to modern forcing of climate. Simply asserting that modern warming may be natural given variability in the past is not the province of skepticism, but rather a declaration of credulity.

As an aside, Chris Mooney is participating in a debate at the aforementioned conference (discussed here and here), and I expect he will have an interesting report on the proceedings worth reading, by and by.

Moran, K., Backman, J., Brinkhuis, H., et al., 2006, The Cenozoic Palaeoenvironment of the Arctic Ocean, Nature 441, 601-605.

Zachos, J.C., Rohl, U., Schellenberg, S.A., et al., 2005, Rapid Acidification of the Ocean During the Paleocene-Eocene Thermal Maximum, Science 308, 1611-1615.

Update: In fairness to Shermer, it appears he is a recent global warming convert, so one might generally assume that he does not consider the debunked argument above to be compelling, but it's odd contrasting his newsletter statement with the NPR Weekend America appearance. Shermer singles out a presentation from Al Gore in convincing him of global warming, yet on the radio criticizes An Inconvenient Truth for implying a lack of controversy.

Chris Mooney has also posted brief comments on the conference.