If predicting climate trends was as easy as predicting the reaction of global warming pseudoskeptics there wouldn't be any deniers left. When I came across a new study in Nature Geoscience on the cause of the massive shift in the climate 55 million years ago, my first reaction was, "How long will it take before someone completely misrepresents this paper as evidence that undermines anthropogenic global warming?"
Not long. See here, here and here, if you have the time.
In the paper, Richard E. Zeebe of the School of Ocean and Earth Science and Technology at the University of Hawaii and his colleagues use some clever isotopic analysis techniques to determine how much carbon was released during the Paleocene-Eocene thermal maximum (PETM) ... I'll let David J. Beerling of the University of Sheffield explain it, as he does in an accompanying essay in Nature Geoscience:
Global warming 55 million years ago was accompanied by a massive injection of carbon into the ocean-atmosphere system, but the resulting climatic warming was much greater than expected from the modeled rise in atmospheric carbon dioxide alone.
Which means our models, the ones forecasting disastrous rises in global average temperatures as soon as the middle of this century if we don't stop pouring more carbon into the ecosystem, aren't up to the task yet of explaining what happened millions of years ago. Does this mean that the world isn't going to warm as much as we feared? Although pseudoskeptical blogospheric reaction would have you answer in the affirmative, the correct response is no. And here's why.
Anyone who has actually read the paper or the essay would understand that the research in question hasn't produced any evidence that would make us rethink the basic idea that more carbon in the air traps more heat. All it does challenge is the completeness of our models. Zeebe et al.'s analysis still finds that massive amounts of carbon-containing molecules found their way into the atmosphere and oceans, precipitating an extreme increase in temperature -- something like 7 or 9 °C -- in a relatively short timespan.
What we still don't know is where all that carbon came from. The resulting warming was twice what we would have expected from carbon dioxide levels alone. As Zeebe and company write:
The origin of this additional warming is unknown at present. Possible causes of the excess warming include increased production and levels of trace greenhouse gases as a consequence of the climatic warming (CH4) [methane].
Beerling, who is better positioned to extrapolate than I, offers "the release of large quantities of methane." What he's talking about are frozen hydrates -- or clathrates -- which are abundant on Earth, largely buried beneath the Arctic permafrost and seabed. If they thaw thanks to global warming, the result would a positive feedback loop in which the carbon from the carbon dioxide initiates the release of more carbon in the form of methane from the clathrates. And then you do see the kind of temperature increase inferred from the isotopic analysis. As Beerling ads:
The total mass of carbon involved in the PETM warming event is uncertain, but some estimates suggest it is roughly equivalent to that stored in fossil fuels (3,000-4,000 Pg C), heightening the relevance of the PETM to present-day climate concerns.
The Zeebe paper is forced to make many assumptions, and some of those assumptions may prove unwarranted. For example, it assumes that the climate response to a doubling of CO2 concentrations is the same regardless of the starting point for CO2 concentrations. Given the role of tipping points, that may be prove to just plain wrong.
Furthermore, this is just one paper, after all. And it hasn't had time to be digested by the entire climatology community. It could be that another few studies will come to opposite conclusions -- that our models do explain what happened 55 million years ago.
But the point is, Zeebe et al. haven't discovered some flaw the basic physics, only a shortcoming in our models' assumptions about the source of carbon-induced warming and the role of positive feedbacks. If anything, what they've found is quite worrisome. If the Beerling's conjecture about feedbacks is correct, then we could be looking at twice the warming hitherto expected this century. (Remember that a global rise of just 2 °C above pre-industrial levels is considered disastrous enough for the world to agree to try to keep things below that.)
Also, just because we don't understand what happened 55 million years ago perfectly, that's no reason to get complacent. The Earth was largely ice-free at the time. CO2 levels were three times higher than today's even before the subsequent spike. So although it's important to understand what happened then, because it's one of the few times the planet has experienced something sort of similar to what's going on now, there are limits to the analogy.
We are going where no one has gone before. The real take-home message from this paper is this: anyone who says the science is settled and we don't need to be spending any more money on such research is just plain wrong.
Zeebe, R., Zachos, J., & Dickens, G. (2009). Carbon dioxide forcing alone insufficient to explain Palaeocene-Eocene Thermal Maximum warming Nature Geoscience, DOI: 10.1038/ngeo578
Link to blog: http://scienceblogs.com/islandofdoubt/2009/07/gaps_in_climate_knowledge_oh_n.php
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