Projected changes to growth and mortality of Hawaiian corals over the next 100 years

Abstract

Background: Recent reviews suggest that the warming and acidification of ocean surface waters predicated by most accepted climate projections will lead to mass mortality and declining calcification rates of reef-building corals. This study investigates the use of modeling techniques to quantitatively examine rates of coral cover change due to these effects.

Methodology/principal findings: Broad-scale probabilities of change in shallow-water scleractinian coral cover in the Hawaiian Archipelago for years 2000-2099 A.D. were calculated assuming a single middle-of-the-road greenhouse gas emissions scenario. These projections were based on ensemble calculations of a growth and mortality model that used sea surface temperature (SST), atmospheric carbon dioxide (CO(2)), observed coral growth (calcification) rates, and observed mortality linked to mass coral bleaching episodes as inputs. SST and CO(2) predictions were derived from the World Climate Research Programme (WCRP) multi-model dataset, statistically downscaled with historical data.

Conclusions/significance: The model calculations illustrate a practical approach to systematic evaluation of climate change effects on corals, and also show the effect of uncertainties in current climate predictions and in coral adaptation capabilities on estimated changes in coral cover. Despite these large uncertainties, this analysis quantitatively illustrates that a large decline in coral cover is highly likely in the 21(st) Century, but that there are significant spatial and temporal variances in outcomes, even under a single climate change scenario.

Figure 1. Greater Hawaiian Archipelago.

Colored boxes represent 1°×1° boxes around Johnston Atoll (JOH), the Island of Oahu (OAH), French Frigate Shoals (FFS), and Midway Atoll (MID); these coincide with historical SST data and the reference location for AOGCM data extraction for each location.

Figure 2. 3rd-order polynomial used to calculate relative coral growth curves at study locations.

Maximum growth occurs at maximum climatological mean monthly temperature – 2 standard deviations; minimum growth at minimum/maximum mean monthly temperatures ±5°C. Solid lines represent climatological values derived from ERSST v3; dotted lines from Pathfinder SST v5.

Figure 3. Degree Heating Months (DHM)/coral mortality relationships used to calculate mortality from episodic heat stress (coral bleaching) events.

Observations of colony mortality associated with the 2005 event in the eastern Caribbean (‘Carib05’ as compiled by Buddemeier et al. in review); the 2002 Northwestern Hawaiian Islands event (‘NWHI02, Kenyon et al. 2006); observations of coral mortality associated with heated effluent (‘MHI72’, Jokiel and Coles 1974); a laboratory study of Hawaiian corals (‘MHI77’, Jokiel and Coles 1977), and a the 1996 main Hawaiian Islands event (‘MHI1996’, Jokiel and Brown 2004), are plotted for comparison. The curves ‘all data’, ‘MHI+NWHI’, and ‘Carib’ are 2^nd order best fits of all of the data points, only data associated events in the Hawaiian islands, and only data from the 2005 Caribbean event.

Figure 4. Example of bias correction and seasonal scaling of AOGCM data.

The example in this case is FFS; temperature predictions for Scenarios 20C3M and A1B from the CSIRO-Mk3.5 model (blue points) are first bias corrected (yellow points), and then seasonally scaled (green points), with observed temperature data (ERSST v3, black points).

Figure 5. Example of temperature prediction using normal distribution of historic temperatures.

Figure 5a represents distributions of August temperatures at study locations from ERSST ver. 3 (solid lines) and Pathfinder SST ver. 5 (dotted lines). Figure 5b: an example of statistical inversion of historic SST (black lines) about the low-pass filtered multi-model mean from all scenario A1B AOGCMs (red line) to produce SST prediction (FFS) (yellow lines).

Figure 6. 20th century fractional change in coral cover.

Individual modal solutions (a–d) plotted for JOH, OAH, FFS, and MID respectively; and Monte Carlo solutions (e–h) for JOH, OAH, FFS, and MID, respectively. Gray lines represent individual solutions from each model (a–d) or PDF solutions (e–h); In this case, corals were assumed to have no temperature adaptation to episodic mortality; Ωa sensitivity at 30% (see methods). Colored lines in each subplot represent ensemble mean.

Figure 7. 21st century fractional change in coral cover, “less resilient” case.

Individual modal solutions (a–d) plotted for JOH, OAH, FFS, and MID respectively; and Monte Carlo solutions (e–h) for JOH, OAH, FFS, and MID, respectively. In this “less resilient” case, corals were assumed to have no temperature adaptation to episodic mortality; Ω_a sensitivity at 30%. Colored lines in each subplot represent ensemble mean.

Figure 8. 21st century fractional change in coral cover, “more resilient” case.

Individual modal solutions (a–d) plotted for JOH, OAH, FFS, and MID respectively; and Monte Carlo solutions (e–h) for JOH, OAH, FFS, and MID, respectively. In this “more resilient” case, the episodic heat stress mortality threshold was allowed to linearly increase 1°C over the century; effects of changing Ω_a were ignored (CO2 effects module turned off). Colored lines in each subplot represent ensemble mean.

Figure 9. 21st century fractional change in coral cover, no episodic mortality.

Individual modal solutions (a–d) plotted for JOH, OAH, FFS, and MID respectively; and Monte Carlo solutions (e–h) for JOH, OAH, FFS, and MID, respectively. In this case, the effects of coral bleaching were not accounted for (the episodic heat stress mortality module was turned off). Colored lines in each subplot represent ensemble mean.

Figure 10. Monte Carlo solutions for fractional change in coral cover using in situ temperature measurements.

In situ temperatures from 1 m and 20 m water depths (as indicated) at Pearl and Hermes Atoll (neighboring MID) were used to constrain and downscale predicted SST (rather than ERSST). In this case, the corals were assumed to have no temperature adaptation to episodic mortality; Ωa sensitivity at 30% (same as the “less resilient” case, Figure 7).