James C. Zachos
Earth and Planetary Sciences Dept., University of California, Santa Cruz, CA

Wed 8 Feb, 15:00 – 15:50
IMAS Sandy Bay seminar room

Abstract:
Over the last century roughly 380 Pg C has been emitted to the atmosphere contributing to nearly 1°C of global warming. Of the total emissions, the ocean has absorbed more than 30%. Because the ocean is thermally stratified and the vertical mixing time of the ocean is slow (~ 500 y), most of the absorbed CO₂ has accumulated in the thin surface layer. As a consequence, both the pH and carbonate saturation state of seawater are measurably decreasing.  With unabated carbon emissions over the next several centuries, the pH of the surface ocean is projected to decrease by as much as 0.7 pH units.  In addition, as the atmosphere and upper ocean warm, the % of emitted carbon absorbed by the ocean is likely to decrease partly as a consequence of the pH change, but also because of increased stratification (Friedlingstein et al., 2006; Le Quere et al., 2009).

In terms of rate, the anthropogenic carbon cycle perturbation appears to be unprecedented in Earth history. The closest analog is the release of carbon that triggered the Paleocene-Eocene Thermal Maximum (PETM; ~56 Mya), a transient global warming of 5 to 6°C. The primary evidence for the carbon cycle perturbation is a large negative carbon isotope excursion (CIE), and widespread dissolution of seafloor carbonate.  New estimates on the rate and magnitude of the CIE and extent of carbonate dissolution (Zachos et al., 2005) have proved critical for quantifying several features of the event including the magnitude of ocean pH change and hence the mass of carbon released during the PETM. Simulations using box and earth system models suggest that the total mass of carbon released was between 4500 to 6000 PgC (Panchuk et al., 2008; Zeebe et al., 2009).  Although this is similar to the projected anthropogenic mass (~4500 PgC), the flux was spread over 5 to 10 ky, a period significantly longer than the turnover time of the ocean, thus enabling some degree of buffering by mixing with the deep sea and dissolution of carbonate sediment.  As a consequence, the magnitude of peak warming, and impacts on surface ocean saturation state were nominal.  The changes in deep-sea carbonate chemistry, however, were clearly severe and long lasting (>100 ky).

Given the faster rate of release (a few 10² vs. 10³y), most impacts of the current and projected anthropogenic carbon emissions should be more severe than observed for the PETM, particularly the magnitude of global warming and surface ocean acidification.  However, other impacts and responses will be similar, for example the undersaturation of the deep sea, as well as the long time scale for C sequestration and climatic recovery (>100 k.y.). Finally, the large mass of carbon released during the PETM is difficult to reconcile with just a single source.  Aside from possible volcanic driven emissions (e.g.,Svensen et al., 2004) which would have been rate limited, the largest potential sources with sufficient capacity are the two large reservoirs of reduced carbon, soil peat and marine hydrates (CH₄), raising the specter of a predominantly feedback driven carbon release.