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.
Do we have any idea why calcification was also that low in the 16th-17th centuries?
Koen, I don’t know. The little ice age? But given the size of the confidence intervals (spanning practically the whole range of calcification plotted) there is a good chance that is wasn’t.
John and Ove’s interpretation of De’ath et al.’s paper focuses mainly on the possibility of a causative link with increasing ocean acidification. The paper itself is much more circumspect, since the authors were unable to ascribe the declining calcification to any specific environmental signal (either temperature or seawater acidity). A subsequent study (Tanzil et al. 2009 Coral Reefs – DOI 10.1007/s00338-008-0457-5) from South Thailand on reefs in warmer waters closer to the equator (7 deg N) also demonstrated a similar decline in calcification, which like the GBR study was due to decreased linear extension and not to any changes in bulk density. Whilst the data available to Tanzil et al. for ocean acidification was sparse and incomplete, nonetheless no obvious link existed, but the authors were able to demonstrate that the decrease in linear extension was associated with a significant long term increase in sea temperature (R squared between 10 and 30%). The common finding in both studies, that there was no change in bulk density, may also be important, for it implies that whilst these corals may be growing less slowly, they are still producing robust skeletons, although an earlier study by Cooper et al. (2008 Global Change Biology 14:529-538) had found a small decrease (6%). As Atkinson & Cuet (2008 MEPS 373:249–256) point out, we are still a long way from understanding how changes in ocean acidity may affect coral calcification, particularly in natural conditions. At the moment whilst rising sea temperatures clearly seem to be reducing the growth of corals, there is still much work to be done to demonstrate whether the same is true for changes in ocean acidity.
Abstract of Tanzil et al. paper: Of the few studies that have examined in situ coral growth responses to recent climate change, none have done so in equatorial waters subject to relatively high sea temperatures (annual mean >27degC). This study compared the growth rate of Porites lutea from eight sites at Phuket, South Thailand between two time periods (December 1984–November 1986 and December 2003–November 2005). There was a significant decrease in coral calcification (23.5%) and linear extension rates (19.4–23.4%) between the two sampling periods at a number of sites, while skeletal bulk density remained unchanged. Over the last 46 years, sea temperatures (SST) in the area have risen at a rate of 0.161degC per decade (current seasonal temperature range 28–30degC) and regression analysis of coral growth data is consistent with a link between rising temperature and reduced linear extension in the order of 46– 56% for every 1 degC rise in SST. The apparent sensitivity of linear extension in P. lutea to increased SST suggests that corals in this part of the Andaman Sea may already be subjected to temperatures beyond their thermal optimum for skeletal growth.
Thanks for that thoughtful and informative post Richard.