Loss Of Coral Could Threaten Coastlines More Than Sea Level Rise

A researcher installing pressure transducers in the inner lagoon of Temae, Moorea, French Polynesia to measure the strength of waves passing over the reef. V. Parravicini

The death of coral reefs may be the most advanced consequence of climate change, already devastating endangered species, tourism, and fishing communities. Reefs also protect coastlines from wave erosion. New research shows that vulnerable islands may suffer more from the loss of reefs than sea level rise, and the two in combination could be catastrophic.

Many low islands can only survive because coral reefs remove power from waves that would erode them away. Dr Daniel Harris of the University of Queensland looked at the way waves impact four coastlines on Moorea and Tahiti, French Polynesia, and modeled the effects of higher seas and reduced coral.

The authors report in Science Advances that it's not just the height of coral reefs that determines protective capacity. The more complex a reef structure is, the more effective it is at shielding the islands behind it.

Using the most likely scenario for the end of the century, Harris reports that waves in the waters between reef and shoreline are likely to average 2.4 times higher than they do today, with devastating effects on the land behind. Most of this increase was driven by loss of reef structure, with sea level rise playing only a small part.

Harris told IFLScience that the roughness of healthy coral reefs “with lots of nooks and crannies” provides friction to waves going over the top, slowing them down and removing energy. “This complexity is a good indication of the health of a reef,” Harris added. Although he says reefs maintain their three-dimensional roughness for some time after coral dies in a bleaching event, dead reefs quickly become ground down by cyclones or other major storms, losing their effectiveness as wavebreaks.

Although engineers have turned their minds to creating artificial reefs with similar complexity, Harris said no one yet knows how to do this on a large scale.

Coral reefs are suffering from a variety of attacks. Some of these, such as over-fishing and local pollution, are within the power of the communities that depend on them to control. However, much of the damage done to reefs in recent times comes from global warming-induced amplification of high temperatures, leading to massive bleaching. Meanwhile, ocean acidification is also reducing the capacity of corals to recover after damage. Both are primarily driven by emissions of carbon dioxide, something the vulnerable island communities of the Pacific and Indian Oceans, can do little about.

The reefs Harris studied have shown themselves to be much more resilient to damage than apparently similar ones in the Caribbean and Seychelles. Harris told IFLScience it is not known why this is, but marine biologists are investigating in the hope of finding ways to replicated it and save other reef systems.

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