The Wrong Conclusion

Climate change skeptics are using a recent study to criticize the science of global warming. They should get their facts straight first.

If predicting climate trends were as easy as predicting the reaction of global warming pseudo-skeptics, there wouldn't be any deniers left. When I came across a new study in Nature Geoscience about the causes of a shift in the climate 55 million years ago, my first reaction was, "How long will it take before someone completely misrepresents this as evidence that humans have nothing to do with current global warming?"

As it turns out, not long. See here, here, and here.

In the Nature Geoscience study, Richard E. Zeebe of the School of Ocean and Earth Science and Technology at the University of Hawaii and others use some clever isotopic analysis techniques to determine how much carbon was released during the Paleocene-Eocene thermal maximum (PETM), a sudden global warming event that occurred 55 million years ago. I'll let David J. Beerling of the University of Sheffield explain it, as he does in an accompanying essay:

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 that our models, the ones that forecast disastrous rises in global average temperatures as early as mid-century if we don't stop pouring more carbon into the ecosystem, aren't yet up to the task of explaining what happened millions of years ago. Does this mean that the world isn't going to warm as much as we fear? Although pseudo-skeptical blog reaction thinks so, the correct response is no. And here's why.

Anyone who actually read the paper or the accompanying essay would understand that the research in question hasn't produced any evidence to make us rethink the basic idea that more carbon in the air traps more heat. What it does challenge is the completeness of our current models. Zeebe et al's analysis still posits 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 amount of time.

What we still don't know is where all that carbon came from. The warming 55 million years ago was twice what we would have expected from the 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].

What they’re talking about are frozen hydrates – or clathrates – which are abundant on Earth, largely buried beneath the Arctic permafrost and seabed. If they thaw because of global warming, the result would be a positive feedback loop. The release of carbon dioxide warms the planet enough to cause the release of more carbon from methane, causing more warming. Then you would see the kind of temperature increase inferred from the isotopic analysis. As Beerling says:

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 to 4,000 petagrams of carbon), heightening the relevance of the PETM to present-day climate concerns.

The Zeebe paper made 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 level of CO2. Given the role of tipping points, that may prove to be just plain wrong.

Furthermore, this is just one paper. There hasn't been the time for it to be digested by the entire climatology community. It could be that other studies will come to different conclusions – that our models do explain what happened 55 million years ago.

The point is, Zeebe et al haven't discovered some flaw in the basic physics. They have just found a shortcoming in our models' assumptions about the source of carbon-induced warming, and the role of positive feedbacks. If anything, their conclusions are quite worrisome. If Beerling's conjecture about feedbacks is correct, we could be looking at twice the warming hitherto expected this century. (Remember that a 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 perfectly understand what happened 55 million years ago, that's no reason to get complacent. The Earth was largely ice-free at the time. CO2 levels were three times higher than today, even before the spike. So although it's important to understand what happened back then, because it's one of the few times that the planet experienced something similar to what's going on now, there are limits to the analogy.

The real 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 research is just plain wrong.