Ben McNeil and Richard Matear from the University of New South Wales have just published a very important article in the Proceedings of the National Academy of United States (Link to full text). This paper further emphasises the critical importance of keeping carbon dioxide levels lower than 450 ppm, as Chris McGrath highlighted earlier this week. While politicians fumble over the issue of gaining effective control of carbon dioxide, there is growing evidence that we must keep CO2 levels below 450 ppm or be prepared to suffer serious consequences to life on Earth.
This is a significant paper which highlights the importance of understanding the dynamics of carbonate equilibrium in seawater in our greenhouse world. Rapid acidification of our oceans, as we now know, is an important impact of rising anthropogenic carbon dioxide in the atmosphere. More importantly, this study confirms the worrying conclusion that calcification in the worlds oceans is in big trouble if atmospheric levels of carbon dioxide exceed 450 ppm. We came to a similar conclusion for coral reefs in a recent article in Science magazine (Hoegh-Guldberg et al 2007) – finding as well that net calcification on coral reef ecosystems dwindles to zero at about 450-500 ppm. The implications of failing ecosystems as important as those in the Southern Ocean are considerable. Rigorous observations such as these should spur our political leaders to make much more decisive steps to curb the rise of carbon dioxide in the atmosphere – anything less will be disastrous.
McNeil & Matear (2008) Southern Ocean acidification: A tipping point at 450-ppm atmospheric CO2. PNAS
Southern Ocean acidiﬁcation via anthropogenic CO2 uptake is expected to be detrimental to multiple calcifying plankton species by lowering the concentration of carbonate ion (CO32-) to levels where calcium carbonate (both aragonite and calcite) shells begin to dissolve. Natural seasonal variations in carbonate ion concentrations could either hasten or dampen the future onset of this undersaturation of calcium carbonate. We present a large-scale Southern Ocean observational analysis that examines the seasonal magnitude and variability of CO32- and pH. Our analysis shows an intense wintertime minimum in CO32- south of the Antarctic Polar Front and when combined with anthropogenic CO2 uptake is likely to induce aragonite undersaturation when atmospheric CO2 levels reach ~450 ppm. Under the IPCC IS92a scenario, Southern Ocean wintertime aragonite undersaturation is projected to occur by the year 2030 and no later than 2038. Some prominent calcifying plankton, in particular the Pteropod species Limacina helicina, have important veliger larval development during winter and will have to experience detrimental carbonate conditions much earlier than previously thought, with possible deleterious ﬂow-on impacts for the wider Southern Ocean marine ecosystem. Our results highlight the critical importance of understanding seasonal carbon dynamics within all calcifying marine ecosystems such as continental shelves and coral reefs, because natural variability may potentially hasten the onset of future ocean acidiﬁcation.