Fabricius et al., 2007:

"Data compiled in the previous sections on the exposure, sensitivity, impacts, adaptive capacity and resilience of coral reef habitats confirm the findings from many previous studies. That the presently observed extent and rate of climate change, and the associated higher frequencies of extreme weather events, constitute a severe threat to the presence and future health of coral reefs. The diagram emphasises that some of the direct effects on reefs are common across the five main climate change variables: they all reduce coral cover, structural complexity and available habitat, and the number of sensitive species. The effects of acidification and temperature are of most concern, whereas changing irradiance is probably of least concern. These direct effects lead to flow-on effects on major ecosystem properties, including:

• Shift in balance from net calcification to net erosion (see Figure 1)
• More bare substratum available for algae to settle, resulting in a shift from coral to algal dominance and presence of algae retarding coral recovery
• Lower structural complexity leading to reduced habitat and shelter for fish and other coral-associated organisms
• Local extinctions of sensitive, rare and highly specialised species; possibly some global extinctions of endemic species that are unable to migrate or compete with other species for resources
• Reduced population sizes leading to reduced reproduction and recruitment, and longer recovery times
• Simpler, ecologically less complex ecosystems, overall reduction in biodiversity.

A dramatic loss in reef biodiversity appears inevitable at atmospheric CO₂ concentrations approaching 500 parts per million. Given that impacts on many other ecosystems also become extreme at 450 to 500 parts per million, limiting emissions to below this point is critical for coral reefs."

Figure 1: The rate of reef formation is determined by the balance between processes of accretion and erosion. In growing reefs today (1), accretion exceeds erosion, leading to net growth. Ocean acidification, brought about by increased absorption of atmospheric carbon dioxide, reduces the rate of reef accretion. If erosion exceeds accretion, net loss results (2). If erosive processes increase (for example, through increased storm damage), the difference between erosion and accretion will be even greater (3). (Figure courtesy of Ken Anthony, UQ)

Figure 17.1: Predicted direct and indirect impacts of the five main climate change variables on coral reefs and how this will influence coral reefs in the future