Coral Reefs Under Rapid Climate Change and Ocean Acidification

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Science  14 Dec 2007:
Vol. 318, Issue 5857, pp. 1737-1742
DOI: 10.1126/science.1152509

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  1. Fig. 1.

    (A) Linkages between the buildup of atmospheric CO2 and the slowing of coral calcification due to ocean acidification. Approximately 25% of the CO2 emitted by humans in the period 2000 to 2006 (9) was taken up by the ocean where it combined with water to produce carbonic acid, which releases a proton that combines with a carbonate ion. This decreases the concentration of carbonate, making it unavailable to marine calcifiers such as corals. (B) Temperature, [CO2]atm, and carbonate-ion concentrations reconstructed for the past 420,000 years. Carbonate concentrations were calculated (54) from CO2atm and temperature deviations from today's conditions with the Vostok Ice Core data set (5), assuming constant salinity (34 parts per trillion), mean sea temperature (25°C), and total alkalinity (2300 mmol kg–1). Further details of these calculations are in the SOM. Acidity of the ocean varies by ± 0.1 pH units over the past 420,000 years (individual values not shown). The thresholds for major changes to coral communities are indicated for thermal stress (+2°C) and carbonate-ion concentrations ([carbonate] = 200 μmol kg–1, approximate aragonite saturation ∼Ωaragonite = 3.3; [CO2]atm = 480 ppm). Coral Reef Scenarios CRS-A, CRS-B, and CRS-C are indicated as A, B, and C, respectively, with analogs from extant reefs depicted in Fig. 5. Red arrows pointing progressively toward the right-hand top square indicate the pathway that is being followed toward [CO2]atm of more than 500 ppm.

  2. Fig. 2.

    Reduction in the resilience of Caribbean forereefs as coral growth rate declines by 20%. Reef recovery is only feasible above or to the right of the unstable equilibria (open squares). The “zone of reef recovery” (pink) is therefore more restricted under reduced coral growth rate and reefs require higher levels of grazing to exhibit recovery trajectories.

  3. Fig. 3.

    Ecological feedback processes on a coral reef showing pathways of disturbance caused by climate change. Impact points associated with ocean acidification (e.g., reduced reef rugosity, coralline algae) are indicated by the blue arrows, and impact points from global warming (e.g., bleached and dead corals) by the red arrows. Boxes joined by red arrows denote that the first factor has a negative (decreasing) influence on the box indicated. Green arrows denote positive (increasing) relationships. Over time, the levels of factors in hexagonal boxes will increase, whereas those in rectangular boxes will decline. Boxes with dashed lines are amenable to local management intervention.

  4. Fig. 4.

    Changes in aragonite saturation {Ωaragonite = ([Ca2+]. [CO32–])/Ksp aragonite)} predicted to occur as atmospheric CO2 concentrations (ppm) increase (number at top left of each panel) plotted over shallow-water coral reef locations shown as pink dots (for details of calculations, see the SOM). Before the Industrial Revolution (280 ppm), nearly all shallow-water coral reefs had Ωaragonite > 3.25 (blue regions in the figure), which is the minimum Ωaragonite that coral reefs are associated with today; the number of existing coral reefs with this minimum aragonite saturation decreases rapidly as [CO2]atm increases. Noticeably, some regions (such as the Great Barrier Reef) attain low and risky levels of Ωaragonite much more rapidly than others (e.g., Central Pacific).

  5. Fig. 5.

    Extant examples of reefs from the Great Barrier Reef that are used as analogs for the ecological structures we anticipate for Coral Reef Scenarios CRS-A, CRS-B, and CRS-C (see text). The [CO2]atm and temperature increases shown are those for the scenarios and do not refer to the locations photographed. (A) Reef slope communities at Heron Island. (B) Mixed algal and coral communities associated with inshore reefs around St. Bees Island near Mackay. (C) Inshore reef slope around the Low Isles near Port Douglas. [Photos by O. Hoegh-Guldberg]