There is no clearer example of how biological systems are affected by climate change than the impacts already apparent on coral reefs. The combined effects of warming and acidification have already affected coral reefs, and the effects will become devastating if atmospheric CO2 concentrations do not stabilise below 450 ppm. Stabilisation at or below these levels of CO2 in the atmosphere will require drastic action within political, economic and social spheres (at least a 90% reduction in greenhouse emissions by 2050). The details of the required actions are beyond the scope of this chapter. The fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC 2007), particularly the analysis of working group 3, is a good place to start for those people interested in this area.
Given the scale of the projected impacts of climate change on coral reefs, there might be a temptation to conclude that preventing and managing the other impacts on coral reefs (see Chapter 12) is now irrelevant. This is a valid point of view if we do not deal with the current unrestrained growth in greenhouse gas emissions. However, if society does pursue a policy of drastically reducing emissions such that atmospheric CO2 concentrations stabilise at 450-500 ppm, it must go hand in hand with good management of coral reefs themselves. Coral reefs will still experience increases in sea temperature and very low carbonate concentrations that will be outside any they experienced in millions of years, and bleaching events will become more commonplace. There is now substantial evidence that the ability of corals to recover from bleaching events is strongly affected by how much they are being influenced by other stresses. For example, elimination of grazing fish (from overfishing) appears to increase the recovery of corals after an ecological disturbance, like the mortality following coral bleaching, by 2-3 fold. Deteriorating water quality might act synergistically with under-grazing to further lower recovery after bleaching events. If, through improved reef management, the recovery of reefs from bleaching can be improved, then reefs will persist longer under the stresses expected over the next 50-100 years. Actively managing coral reefs in this current period of rapid global change will consequently translate as a greater abundance and diversity of coral reef organisms from which reefs will expand when the climate has finally stabilised once again.
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