You should read this

Regular readers know that I keep referring to the worrying theory that the great mass extinctions in earth's pre-history are connected with high atmospheric CO2, and its effect on ocean chemistry. The Scientific American article which first brought this to my attention said that the great extinctions seemed to be associated with CO2 levels of just under 1000 ppm. That's a fair way from where we are now, but still within sight of a century or so of current CO2 increases.

However, there's now a paper here, found via the Ocean Acidification blog, which suggests that maybe the "unifying theory" for mass extinctions is a CO2 level that specifically interferes with the operation of a particular enzyme. The biology is a little complicated, but the big concern with this theory is that severe trouble for the ocean environment may actually start at only 560ppm CO2. That could easily be reached by the middle of this century.

It's not too hard to read the full paper (in .pdf format), and it is well worth the effort. The implications for the future are set out towards the end. I'll put it here in full, and break it up a bit to make it easier to read:

Over the next century, if anthropogenic CO2 emissions proceed at the rates predicted by the IPCC scenarios (IPCC, 2001), then the identified pCO2 threshold concentration of 560 ppmv may be exceeded as early as 2050 (Fig. 3). Whilst the direct climatic impacts of this overshoot remain difficult to quantify with certainty, simple extrapolation of the central tenets of the urease hypothesis suggests that there is little doubt regarding the disruption and mass mortality that it will initiate within organisms that are heavily reliant upon the urease enzyme.

Previous mass extinction events appear to have guided the evolutionary process away from urease-dependence in higher vertebrate animals, but the threat remains for the lower invertebrates and plant communities. Importantly, these at-risk ecosystem elements are fundamental to: (i) the productive food chains, (ii) the essential habitat, and (iii) the stable climate cycles, upon which the higher vertebrate animals (including humans) rely for their survival.

Of particular note is the potential for a collapse in ocean productivity to initiate rapid greenhouse warming (Rampino and Caldeira, 2005). In this case, the cessation of marine biological export of 25 organic carbon from the surface would cause an increase in surface-ocean dissolved inorganic carbon, some of which would leak into the atmosphere to increase atmospheric pCO2. Modelling results suggest that a cessation of productivity today would result in a rapid doubling of pCO2 (Rampino and Caldiera, 2005). It is therefore a plausible scenario that a collapse of ocean productivity occurring at 560 ppm could trigger a rapid “post-apocalyptic” rise in pCO2 levels beyond 1000 ppm – leading to rapid global warming of 3–6C.

Recovery of atmospheric pCO2 from such a perturbation would be governed by the time scale of equilibrium of the ocean chemistry 5 with the carbonate system (c 104 years) (Archer et al., 1997). Post-apocalyptic greenhouse spikes of similar duration have been associated with previous mass extinctions (Retallack, 2005), and may be responsible for triggering additional climate change dependent kill responses (Elewa, 2008).

Clearly, the urease hypothesis forewarns of the global imperative that atmospheric pCO2 levels are stabilised well below 560 ppmv. This will require the development of technologies and solutions that are presently unavailable – thus demanding our immediate attention and resources.

Now, this is clearly presented by the author as a hypothesis, and it is called a "discussion paper". The author is a researcher from the Australian Institute of Marine Science in Townsville, but his exact qualifications are not clear. However, it certainly sounds like an idea worthy of some pretty intense investigation.

It is consistent with my position that arguing about whether the earth is currently warming or cooling is irrelevant to the issue of whether keeping CO2 levels down is a good idea.

What's more, the climate change sceptics are still largely silent on the issue of ocean changes as a result of high CO2. (Pointing out that corals have re-established in nuclear test lagoons is hardly relevant.) I don't think they can find any credible scientist who has compelling arguments as to why such concerns are not worthy of being taken seriously. Correct me if I am wrong, Jennifer Marohasy. (I note that she ran with suggestions earlier this year that the cooling temperature would see the alarmists start to run with ocean acidification instead. This is not an answer to the actual concerns, however.)