Coral reefs and rapid climate change: impacts, risks and implications for tropical societies

Over 500 million people worldwide rely on the important goods and services that corals provide. In addition to supporting billion-dollar fisheries and tourism industries, reefs provide a source of food for a great number of people. Despite this, the 0.1% of sea floor that is covered by coral reefs is constantly under threat from climate change and is experiencing a swift loss of coral cover. This paper reviews the effects of rising atmospheric CO₂ on coral reefs, bringing to our attention the predicted impacts, risks and consequences of ocean acidification and warming.

The industrial revolution has driven an increase in atmospheric CO₂, and since then the average tropical ocean temperature has risen by 0.5^oC, a sea level rise of 17cm and surface ocean acidity by 0.1pH unit (leading to a drop in surface seawater carbonate ion concentration by ~30mmol kg; according to IPCC 2007). These changes have moved coral reefs into thermal and chemical conditions which they have not experienced for at least 720, 000 years.

As a general rule , an important thermal threshold for corals occurs at approximately 1^oC above the long-term summer maximum for a region; as if this is exceeded for prolonged periods, corals undergo mass bleaching, furthermore if the temperature is greater than 2^oC above maximum then mass coral mortality is likely. The second threshold is the concentration of carbonate ions in seawater below which coral reefs can no longer maintain the carbonate framework. The increasing atmospheric CO₂ decreases the carbonate ion concentration, and the balance between calcification and loss (biological, physical and chemical erosion) determines if the coral reef is maintained. As atmospheric CO₂ increases, fewer coral reefs will be able to calcify at rates that can keep up with erosion.

Reef frameworks are critical for playing roles in the protection of inshore coastal ecosystems such as mangroves and sea grasses, and human infrastructure from the force of waves as well as providing a three dimensional habitat which is made home by millions of species. The paper states that according to a previous study by Cooper et al. in 2007 the rates of calcification have declined by almost 20% since 1980; and recent studies suggest that if coral reef structures are lost then so too will half of the fish species that inhabit them. If the loss of reefs were to occur then many countries would suffer as they rely on the tourism industry and fisheries.

There is little doubt among coral reef scientists that a failure to stabilise atmospheric CO₂ below 450ppm will eradicate carbonate coral reef systems. The authors believe that we must boost global efforts to reduce the impact of local stressors, while working consistently to dramatically decrease global CO₂ emissions; which will lead to the reduction of short-term impacts and buy time to create international agreements that will result in the stabilisation of atmospheric CO₂ below the dangerous level of 450ppm.

A review of:

Hoegh-Guldberg, O., Hughes, T., and Anthony, K. et al. (2009). Coral reefs and rapid climate change: impacts, risks and implications for tropical societies. IOP Conf. Series: Earth and Environmental Science 6. Found at: (http://iopscience.iop.org/1755-1315/6/30/302004/pdf/1755-1315_6_30_302004.pdf) Accessed on: 10/12/2011