As we’ve covered here before at Climate Shifts, the build up of carbon dioxide in the atmosphere has the potential to slow the growth of reef-building corals by increasing the acidity of the world’s oceans.   By burning immense amounts of fossil fuels, humans have increased the concentration of carbon dioxide (CO2) in the atmosphere by nearly 40%.  Roughly a quarter of this CO2 is being absorbed by oceans, where it reacts with seawater to form carbonic acid, acidifying the upper layers of the ocean.

X-ray photographs of core sections from a massive coral (Porites lutea) showing annual density banding patterns similar to tree ring growth (Image courtesy of Bob Dunbar, Stanford University)

Several laboratory experiments suggest this could make it more difficult for corals and other organisms such as crabs and clams to secrete the calcium carbonate skeletons they depend on for survival.  The increased acidity essentially makes it more energetically costly to secrete skeletons and could eventually literally dissolve them.

A new research article published today in Science magazine (De’ath et al. 2009) takes the case for ocean acidification a step further.  The article suggests that the recent man-made increase in ocean acidity has lead to a reduced growth rate of massive reef-building corals.  A team of scientists from the Australian Institute of Marine Science measured the growth of hundreds of corals from 69 reefs on the Great Barrier Reef (GBR), Australia.  The team took core samples of the coral skeletons (up to 5m in length) and measured annual growth rings going back over 400 years. Their results strongly indicate that the vertical extension of massive, long-lived corals has slowed by roughly 13% since 1990.  This decrease in growth coincides with an increase in the acidity of tropical oceans. A previous paper from the group published earlier this year (see here) was widely criticized because it was based on corals from reefs close to shore and was limited to two geographic locations. However, the teams new paper found evidence of declines in calcification across over 300 corals growing across 2000 km of the GBR, which is indeed a cause for concern.

Whilst slower growth rates might not seem like a big problem in isolation, reef scientists are concerned that this will exacerbate the impacts of other threats to coral reefs.  Slower vertical growth of corals will make it harder for reefs to keep up with rising sea levels, whilst decreases in calcification makes corals more vulnerable to physical damage such as destructive storms and bioerosion.

Changes in (A) calcification, (B) linear extension, (C) density between 1905-2005 (328 corals from 69 different reefs across the length of the GBR. (D) shows trends in calcification between 1572-2001 (10 corals). Light blue bands indicate 95% confidence intervals for comparison between years, and gray bands indicate 95% confidence intervals for the predicted value for any given year.

The fact that coral growth since the 1990’s has fallen to its lowest rate for 400 years is particularly worrying, and in stark contrast to the usual inductive reasoning portrayed in the media that “corals grow faster in warm water, therefore warm water is good for coral reefs” (see here for more discussion).

Given that global CO2 emissions are now exceeding the worse-case IPCC scenario, and the likely lag time of several decades between the addition of CO2 and the resulting increase in ocean acidity, we will almost certainly see such problems with coral calcification escalate over the coming decades. We can add this to the list of ‘surprises’ (such as deep water anoxia and the runaway collapses of iceshelves), and strongly suggests that we have seriously underestimated not only the rate of climate change, but the scale of the impacts on ecosystems across the globe. These results should compel the Rudd government to make immediate and drastic steps to decarbonise Australian economic systems as a urgent priority.