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. 2015 Jul 22;2(7):150181.
doi: 10.1098/rsos.150181. eCollection 2015 Jul.

Keep up or drown: adjustment of western Pacific coral reefs to sea-level rise in the 21st century

R van Woesik  1 , Y Golbuu  2 , G Roff  3
Affiliations

Keep up or drown: adjustment of western Pacific coral reefs to sea-level rise in the 21st century

R van Woesik et al. R Soc Open Sci. .

Abstract

Since the Mid-Holocene, some 5000 years ago, coral reefs in the Pacific Ocean have been vertically constrained by sea level. Contemporary sea-level rise is releasing these constraints, providing accommodation space for vertical reef expansion. Here, we show that Porites microatolls, from reef-flat environments in Palau (western Pacific Ocean), are 'keeping up' with contemporary sea-level rise. Measurements of 570 reef-flat Porites microatolls at 10 locations around Palau revealed recent vertical skeletal extension (78±13 mm) over the last 6-8 years, which is consistent with the timing of the recent increase in sea level. We modelled whether microatoll growth rates will potentially 'keep up' with predicted sea-level rise in the near future, based upon average growth, and assuming a decline in growth for every 1°C increase in temperature. We then compared these estimated extension rates with rates of sea-level rise under four Representative Concentration Pathways (RCPs). Our model suggests that under low-mid RCP scenarios, reef-coral growth will keep up with sea-level rise, but if greenhouse gas concentrations exceed 670 ppm atmospheric CO2 levels and with +2.2°C sea-surface temperature by 2100 (RCP 6.0 W m(-2)), our predictions indicate that Porites microatolls will be unable to keep up with projected rates of sea-level rise in the twenty-first century.

Keywords: Pacific; climate change; coral reefs; corals; sea-level rise.

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Figures

Figure 1.
Figure 1.
(a) Evidence of stable sea level along the ubiquitous limestone islands of Palau, (b) notches extend approximately 2–3 m horizontally at lowest spring tide (red line, highest tide mark), (c) study sites in Palau, western Pacific Ocean (dark blue, open ocean; pale blue, lagoon; yellow outline, outer reef locations; green, land; tide data from Malakal tide gauge marked with a purple square).
Figure 2.
Figure 2.
(a) In intertidal reef-flat environments, massive Porites form characteristic ‘microatoll’ formations, with living tissues around the perimeter, and dead skeleton on the exposed upper surface. Microatoll growth is predominantly lateral, as vertical growth is limited by a lack of accommodation space. (b) In sub-tidal reef environments (approx. 1–2 m depth), massive Porites are unconstrained by sea level, and form dome shaped colonies through lateral and vertical growth.
Figure 3.
Figure 3.
(a) Local daily tide data (grey line) from Malakal A (1926–1939) and Malakal B (1969–2015) fitted with linear regression (orange line), showing a 2.87 mm increase per year since 1969 in the Malakal B time series; dotted black line is the mean for each time period. (b) Tide data from Malakal B showing monthly averages (grey line), annual averages (red points) and long-term increase above the 1969–2015 average indicated by the significant cubic fit (red curve ± 95% CIs); double-headed red line shows recent difference in tidal height from average, the red-dashed vertical line signifies the approximate timing of the onset of Porites vertical extension. (c) Tide data from Malakal B showing monthly averages (grey line), annual mean low water spring tide height (blue points) and long-term increase in mean low water spring tide height above the 1969–2015 average (thick dashed black line) indicated by the significant quadratic fit (blue line ± 95% CIs); double-headed blue line shows recent accommodation space, the blue dashed vertical line signifies the approximate timing of the onset of Porites vertical extension.
Figure 4.
Figure 4.
(a) High tide across a reef flat in Palau showing field measurement protocol, (b) average vertical growth of Porites microatolls at each of 12 sites (means ± s.d.), (c) relationship between microatoll growth rate and vertical extension of Porites microatoll rims (means ± s.d.) and the shaded polygon is the 95% CI, (d) hindcasted timing of the onset of microatoll-rim extension (means ± s.d.) and the shaded polygon is the 95% CI, and Porites microatoll rim sections from the (e) Ngerdis and (f) Ngatpang sites, showing a cross section of the skeleton, and a schematic interpretation of the annual growth bands derived from X-rays (X1, initial height constrained by sea level; X2, height of vertical extension unconstrained by sea level).
Figure 5.
Figure 5.
(a) Measured tide gauge data from Malakal, Palau and projected global mean sea level (m) (relative to 1984–2005) for four different IPCC (Intergovernmental Panel on Climate Change) RCP (Representative Concentration Pathway) scenarios (2000–2100); (b) Global sea-surface temperature (SST °C) past and projected increases for four different IPCC RCP scenarios (2000–2100); and for (c–f), projected rates of global sea-level rise (SLR) (mm yr−1) for four different IPCC RCP scenarios (2000–2100) and modelled estimates of massive Porites growth rates assuming an average of 51% reduction in growth rates for every 1°C rise in SST (41–56% upper and lower bounds), where (c) depicts RCP 2.6, (d) depicts RCP 4.5, (e) depicts RCP 6.0 and (f) depicts RCP 8.5.
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