Human interference with the global climate system can cause changes in coral growth rates by altering the coral’s physical or chemical environment. Currently, the atmospheric carbon dioxide levels are around 390 parts per million (an increase of 40% since preindustrial times). As atmospheric carbon dioxide levels rise, the oceans absorb most of it. In the process known as ocean acidification, carbon dioxide alters the seawater carbonate equilibrium and leads to a reduced pH and carbonate saturation state—both of which are currently thought to have detrimental effects on reef-building corals and other marine calcifers. Cooper et al. (2012) examined significant changes in coral calcification of southeast Indian Ocean corals over the last 110 years and related those changes to known changes in sea surface temperature (SST). Looking at a latitudinal range of 11 degrees, they determined that there was no widespread decline in calcification rates during the 20thcentury. Results of this study suggest that ocean acidification is not currently limiting the calcification of coral reefs on a global scale and that thermal changes, not ocean acidification, appear to be the largest climate change threat to reef-building corals at this time. —Kelsey Waite
Cooper, T.F., O’Leary, R.A., Lough, J.M., 2012. Growth of Western Australian corals in the Anthropocene. Science 335, 593–596.
Cooper and colleagues collected twenty seven long cores from massive Porites sp. colonies at six locations (covering about 1000 km) off the coast of Western Australia. All of the cores were collected from colonies larger than 2 m in height and were at a depth of at least 6 m below the lowest astronomical tide. Cores were then analyzed to determine spatial and temporal variations in annual extension, skeletal density, and calcification rate (the product of skeletal density and annual extension). Calcification anomalies were calculated as the percent difference between the annual calcification rate and the long-term period 1900–2010. Linear and non-linear regression models were used to investigate the relationship between time, SST, and location in comparison to both calcification averages and calcification anomalies.
Cooper et al. found no consistent pattern of declining calcification rates over time. In fact, the two most southern regions had the greatest recent SST warming and showed a significant increase in calcification. The two northern regions had a much lower rate of recent SST warming but showed no significant change in calcification rates over time either. Since the southern regions showed an increase in calcification despite an increasing temperature, the findings of Cooper and colleagues suggest that ocean acidification is not yet limiting the calcification of coral reefs uniformly on a global scale.