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Sea-level rise could overwhelm coral reefs

An assessment of the capacity of coral reefs to grow fast enough to keep up with projected rises in sea level finds that most reefs will fall behind if nothing is done to restore them.
Ilsa B. Kuffner is at the US Geological Survey, St. Petersburg Coastal and Marine Science Center, St. Petersburg, Florida 33701, USA.
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Coral reefs are famous for housing biodiversity and attracting tourists, and the economic benefits that reefs provide for tropical, coastal communities around the globe measure in the billions of dollars1. One of the main services provided by reefs is that they act as natural breakwaters (Fig. 1), protecting shorelines and human-built infrastructure from storms. In a paper in Nature, Perry et al.2 report a detailed analysis of the ability of coral reefs in two ocean basins to keep growing upwards in the face of the ecological degradation they have already experienced, and taking into account future sea-level rise. The findings show that, as living coral populations wane, their capacity to build reefs might be diminished to the point at which the reef community fails to keep up with the rising ocean surface.

Wave crashing over coral reef

Figure 1 | Natural breakwaters. Coral reefs, such as this one in the Red Sea off the coast of Saudi Arabia, protect shorelines from stormy seas. Perry et al.2 report that most coral reefs might not be able to grow fast enough to keep up with future sea-level rises, putting crucial coastal defences at risk.Credit: Thomas P. Peschak/Getty

Corals are simple invertebrates that are related to sea anemones and jellyfish, but a trait that sets them apart is their ability to create rock from sea water. Each year, corals add a new layer of calcium carbonate on top of their existing exoskeletons, growing larger and intertwining over thousands of years to form a coastal barrier capable of quelling enormous amounts of wave energy. Corals are under threat from warming oceans and from an onslaught of localized environmental pressures, which collectively cause coral bleaching, slower growth, disease and death3. If there are not enough corals alive to keep a reef growing, then erosion takes over and the reef loses elevation.

There are some uncertainties associated with understanding the fate of coral reefs as geological structures. Coral growth does not translate millimetre for millimetre into vertical reef growth. Many constructional and erosional processes are at work simultaneously, adding to and subtracting from the net amount of calcium carbonate produced or lost (the carbonate budget), and determining whether a reef builds or winnows away. In previously published work, Perry and colleagues4 were among the first to use field data that account for the organisms responsible for reef building (corals and calcifying algae) and reef breakdown (excavating parrotfish, sea urchins and reef-infesting sponges) in budgeting projections of reef growth and destruction.

In the current work, Perry et al. take those budgeting efforts a step further by combining them with projections of sea-level rise under two scenarios published in the Fifth Assessment Report5 from the Intergovernmental Panel on Climate Change (IPCC). What they find is not encouraging: 16 reef areas in the tropical western Atlantic Ocean and 6 in the Indian Ocean are barely keeping up with the present sea level. Even worse, only 9% of the 202 reefs they assessed have the capacity to keep up with the rates of sea-level rise associated with even the more optimistic of the two scenarios (IPCC Representative Concentration Pathway 4.5), which predicts that atmospheric greenhouse-gas emissions will peak around 2040.

The authors acknowledge that their accounting does not adequately address some reef processes that are important in the budget. If you used a rotary drill to take a peek inside a reef, you would find that only about half of the structure is composed of intact coral skeletons; the rest is either void space or reef detritus, including rubble and sediment6. The processes that control the breakdown of reefs and determine whether the resulting material fills the cracks and crevices or gets swept away are not well studied. Although Perry and colleagues’ budget did account for the biologically mediated erosion responsible for producing reef rubble and sediments, they did not factor in the chemical dissolution of carbonates, nor evidence suggesting7,8 that both of these processes will be accelerated by increased levels of carbon dioxide absorption by the ocean (ocean acidification).

Additionally, the transport of loose sediment away from reefs and into deeper water is largely driven by sporadic storms. This makes it difficult to estimate an average rate at which sediment contributes to reef building. Moreover, sediment transport away from reefs might increase as cyclones become more intense as a result of ocean warming9 — a factor that was also not considered by the authors. Taken together, the processes not accounted for by Perry and colleagues could mean that the projections of reef-building rates are, if anything, too optimistic.

The implications of the study are dire: many island nations and territories are set to quickly lose crucial natural resources responsible for coastal defence. Prompt action is warranted to slow and reverse this loss. Fortunately, reef restoration has come a long way since the twentieth century, when piles of discarded car tyres and engine blocks were used as artificial reefs. Restoration using live corals farmed in offshore nurseries is fast becoming more common and feasible as coral-gardening techniques have been streamlined. So far, these efforts have been driven largely by conservation organizations and hoteliers, but reef-restoration programmes are poised to benefit from initiatives that coordinate restoration practitioners, scientists, governments, resource managers and local communities (see, for example, go.nature.com/2rljaqh).

The feasibility and efficacy of enhancing coastal and community resilience through live-coral planting have not been quantified, but Perry et al. provide convincing evidence that the time has come to make reef restoration a priority. A recent analysis10 indicates that ecological restoration projects aimed at protecting shorelines might be more cost-effective than conventional projects that use engineered concrete structures. Although it is uncertain how much time we can buy for coral reefs through restoration, such projects might extend the existence of the reefs long enough to bridge the gap until global efforts start to decrease the concentration of atmospheric greenhouse gases, thereby slowing the rates of global warming and sea-level rise.

Nature 558, 378-379 (2018)

doi: 10.1038/d41586-018-04879-7
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