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.
Highlights
Sea level stabilized in the Pacific Ocean around 5500 years ago [2]. Evidence of this static sea level is apparent along many limestone islands in the Pacific Ocean, including Palau (Micronesia), where notches extend approximately 3 m horizontally at sea level. These notches have been eroded through the millennia by freshwater and reduced alkalinity caused by terrestrial run-off, and suggest that tectonic subsidence has been minimal through the Late-Holocene [4]
We modelled whether microatoll growth rates will potentially ‘keep up’ with predicted sea-level rise in the near future under different climate-change scenarios
A simple linear analysis of the 45-year daily tidal data since 1969 showed an increase in sea-level of 2.87 mm yr−1. This increase is consistent with the regional Pacific average of 2.7 ± 0.6 mm yr−1 (1970–2008) and is higher than the global average of 1.8 ± 0.5 mm yr−1 (1970–2008) [24,25]
Summary
Evidence of this static sea level is apparent along many limestone islands in the Pacific Ocean, including Palau (Micronesia), where notches extend approximately 3 m horizontally at sea level (figure 1a,b) These notches have been eroded through the millennia by freshwater and reduced alkalinity caused by terrestrial run-off, and suggest that tectonic subsidence has been minimal through the Late-Holocene [4]. Palau and in the western Pacific Ocean, massive forms of Porites species have been the primary reefframework builders throughout the Late-Holocene, and a high and consistent density of massive Porites colonies are most typical of near-shore habitats in the tropical Pacific Ocean [4,10,11] These coral colonies grow rapidly, and accrete framework at rates of approximately 1 m in 100 years [4]. We modelled whether microatoll growth rates will potentially ‘keep up’ with predicted sea-level rise in the near future under different climate-change scenarios
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