Global Warming and Coral

There are two reasons why scientists have long been concerned about the effects of climate change on coral reefs. Firstly, corals will only grow in a narrow range of water temperatures. Secondly, when there is more carbon dioxide in the air, more of the gas dissolves into the sea. When carbon dioxide dissolves in water, it makes the water slightly more acidic, and this change in chemistry is likely to make it more difficult for corals to build reefs.

While the growth rate of most marine organisms usually increases with warmer water temperature, it had always been thought that many corals are damaged by small increases in maximum water temperatures. On average, global sea surface temperatures (known as SSTs) have increased over the past century but, surprisingly, there have been different responses to this change from the corals on Australia’s west coast and in the east. A 2012 study , just released, has found that coral calcification rates of reefs in higher latitudes of Western Australia have significantly increased over the past century despite rapidly rising SSTs. On the other hand, a 2009 study found that coral calcification rates along the entire Great Barrier Reef (GBR) declined over the period 1990-2005.

The WA study sampled coral reefs growing over a range from Rowley Shoals to the Houtman Abrolhos Islands. The northern reefs--at Rowley Shoals and Clerke Reef--with the lowest rate of SST increase (0.02˚C/decade), showed little change in calcification rates over the 1900-2010 period. However the southern reefs-- at Coral Bay and the Houtman Abrolhos Islands--with the greatest rates of SST increase (0.08˚C/decade and 0.10˚C/decade) had significant increases in calcification rates (six per cent and 24 per cent respectively). (Note that one reef in Exmouth Gulf, just north of Coral Bay, experienced a significant decline in calcification rate of more than 11 per cent but another reef at the same latitude showed no significant change.)

The report concluded that rising SST is the primary driver of changes in calcification rates. One of the authors, Timothy Cooper, said that

Rapid warming of parts of the tropical oceans, observed to date, appears to be driving coral calcification responses. Some corals in some locations are able to keep up with these changes, whilst others are already showing that the temperature changes have exceeded optimal conditions for coral growth.
Corals rely on a series of chemical reactions to build their hard structures and these reactions are easily disrupted by changes in pH (the measure of acidity). The results of this study suggest that the process of ocean acidification caused by increasing carbon dioxide in the air is not currently limiting coral reef calcification rates. This was an unexpected finding. It would be expected that acidification, which reduces the saturation in seawater of aragonite (a calcium carbonate mineral used to calcify coral), would first occur in higher latitude waters. That’s because these waters are cooler and aragonite is more soluble at lower water temperatures. However, it was found that

... the significant recent above–long-term–average calcification anomalies recorded at the Houtman Abrolhos Islands lends support to the view that thermal changes are likely to be the principal immediate climate-change threat to the calcification potential of reef-building corals.
Timothy Cooper also said that while temperature change was presently the dominant factor affecting coral growth, ocean acidification could become more important in the near future.

The GBR study sampled corals over a wide range of latitudes and found that the calcification rates of 12 out of 13 reefs declined by 14 per cent over the 15 year period 1990-2005. This followed an increase in calcification rates that was taking place until about 1970.

The study excluded a number of factors as possible causes of the decline in calcification rates, such as competition for space, water quality, salinity, diseases, irradiance, currents and large-scale and long-term oceanographic oscillations. This left SST and the concentration of aragonite in seawater as potential causes. The authors noted from previous studies that, while calcification rates increase with increased SST, those corals that survive excessively high water temperature have lower reduced calcification rates for up to two years. Their study found that calcification rates declined in cooler than average years. However, during warmer than average years calcification increased in some years but decreased in others. This suggested that more frequent heat stress events are likely to have contributed to declining coral calcification during 1990–2005.

Little data on the acidification of the GBR waters was available. Nevertheless, the global decline in aragonite saturation in seawater resulting from this acidification was estimated to be 16 per cent since global industrialisation. The calcification rate decline in the 2009 study suggested that a tipping point was reached in the late 20th century which reflects synergistic effects of higher temperature and increasing acidification.

Timothy Cooper suggested that possible factors contributing to the difference in calcification rates between the reefs on either side of Australia could be the fact that there is not the same labyrinth of coral reefs in WA as there is in the GBR, that there are lower populations in the west and that there is also less runoff (which affects water quality).

It seems that, while some coral calcification rates are responding well to increase in temperatures at present, in the longer term these responses will be compounded by the progressive impacts of ocean acidification.

The question therefore becomes 'when?'. When will those corals in WA with increasing growth rates, living in areas where temperature rise is most rapid, start to experience SSTs which cause a decline in growth rates?


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