Coral Islands Adjust to Rising Sea Levels

This is a picture of the Maldives cabinet meeting that took place on October 17th, 2009.  (click for credit)

This is a picture of the Maldives cabinet meeting that took place on October 17th, 2009.
(click for credit)

On October 17th, 2009, the cabinet of the Republic of Maldives held a meeting underwater. Outfitted with scuba gear and using hand signals to conduct the meeting, they signed a document calling on all countries to reduce their carbon dioxide emissions. Why would they do such a thing? The average elevation of their country is a mere 1.5 meters above sea level, making it the lowest-lying country in the world. Thus, they consider themselves the most at-risk nation when it comes to rising sea levels. Since many think that human-produced carbon dioxide is warming the planet and contributing to sea level rise, they wanted to make it clear that if the world doesn’t do something to curb emissions, their island nation could soon be underwater.

It is well known that sea levels have been rising since the end of the 1700s1, but we don’t know how much of it is caused by human-induced global warming and how much is part of the earth’s natural climate variability. Some claim that sea level rise has accelerated due to human-induced global warming, while others claim that it has remained fairly constant for the past 100 years or more. Hopefully, more research will allow us to get a better handle on how much of the rise in sea level is natural and how much (if any) is caused by human activity.

Nevertheless, let’s grant the Republic of Maldives its assumptions. Let’s say that rising carbon dioxide levels are heating up the planet, melting its ice reserves. This is causing sea levels to rise, and as a result, nations like the Maldives are at risk. They could literally be underwater if something doesn’t change soon. There is a serious problem with this scenario. The Republic of Maldives is made of a collection of atolls, ring-shaped coral reefs that can form lovely islands. Coral reefs, of course, are made of living organisms (corals) and their remains. We know that living organisms respond to changes in their environment.

We now know that the corals which form atolls respond to rising sea levels by raising the level of the atoll.

To investigate how coral atolls are affected by rising sea levels, researchers from the University of Auckland and the University of New South Wales studied islands around the Funafuti Atoll, which holds the island nation of Tuvalu. The researchers chose to study these particular Pacific islands because they have experienced rapid sea level rise at a rate of about 5.1 mm per year over the past 60 years. In other words, they have seen the sea level rise nearly a third of a meter in the past three decades. If rising sea levels threaten any coral atolls, it should be these.

What did the researchers find? Here’s what they say in their paper:2

Despite the magnitude of this rise, no islands have been lost, the majority have enlarged, and there has been a 7.3% increase in net island area over the past century (A.D. 1897–2013). There is no evidence of heightened erosion over the past half-century as sea-level rise accelerated.

How is this possible? The corals that make up the atolls have responded by building the reef higher. This, of course, makes complete sense. Corals like to live in shallow water. That way, they can get a lot of sunlight to support the photosynthetic organisms with which they have a mutualistic symbiotic relationship. As sea levels rise, the coral reef rises in response. This, of course, makes the island bigger, not smaller.

Now this isn’t the first scientific paper to demonstrate that coral islands are not threatened by rising sea levels. In 2010, two researchers studied 27 atoll islands in the Pacific and found that 43% of them had remained stable in size, while another 43% had actually grown.3 Once again, the researchers attributed this to the response of the corals that form the atoll.

Will these scientific results keep island nations like the Republic of Maldives from pulling publicity stunts like the one pictured at the top of the post? I doubt it. Unfortunately, science plays little role in the public debate surrounding global warming.


1. S. Jevrejeva, J. C. Moore, A. Grinsted, and P. L. Woodworth, “Recent global sea level acceleration started over 200 years ago?,” Geophysical Research Letters 35, 2008 doi:10.1029/2008GL033611
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2. P.S. Kench, D. Thompson, M.R. Ford, H. Ogawa, and R.F. McLean, “Coral islands defy sea-level rise over the past century: Records from a central Pacific atoll,” Geology, 2015 doi:10.1130/G36555.1
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3. Arthur P. Webb and Paul S. Kench, “The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis of island change in the Central Pacific,” Global and Planetary Change 72(3):234–246, 2010
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  1. […] We now know that the corals which form atolls respond to rising sea levels by raising the level of the atoll.  […]

  2. Jake says:

    This is horrible, but here’s my favorite warming negative feedback mechanism:

    1. Melting glaciers/ice on the ground reduces pressure on the earth’s crust.
    2. The reduced pressure can lead to volcanoes.
    3. Volcanoes release into the atmosphere gases that reflect sunlight and otherwise contribute to lowering atmospheric temperature.

    I’m not sure how efficient this is – maybe not very – but it’s still amusing. I actually heard about this from an article in Time (though point 3 was my addition):

    Does that make any sense? It seems reasonable, but I tend to be skeptical of just about everything science-related I read in the news.

    1. jlwile says:

      There is some truth to it, Jake, but it’s important to separate the nonsense in the Time article from the science that was actually published. In the study (Kathleen Compton, Richard A. Bennett, and Sigrun Hreinsdottir, “Climate driven vertical acceleration of Icelandic crust measured by CGPS geodesy,” Geophysical Research Letters 42(3):743–750, 2015) the authors don’t try to blame “global warming” for increased volcanic activity. I suspect they knew such nonsense wouldn’t make it past peer review. That’s something the lead author wrote to Time in an email. The paper simply shows that Greenland is rising, and they correlate it with the ice loss that is occurring. Of course, they blame that ice loss on “global warming,” but somehow don’t mention the ice gain in the Southern Hemisphere. In their minds, ice loss in the Northern Hemisphere is caused by “global warming,” while ice gain in the Southern Hemisphere is the result of something else entirely.

      The ice loss in the Northern Hemisphere is most likely the result of the Atlantic Multi-decadal Oscillation, a completely natural phenomenon. This does reduce the weight on underlying rock, which will allow for rebound.

  3. Aaron says:

    Dr. Wile,
    Would you be able to explain how atolls are above sea level in the first place? That never made sense to me, since they are built by corals, which (I assume) are all subterranean. Also, I always assume that corals build up atolls by building on top of themselves, which would mean that the foundation that the human infrastructure is currently built upon wouldn’t be moving upward. Clearly I have much to learn about marine biology.

    1. jlwile says:

      Actually, Aaron, it was Charles Darwin who figured out how atolls form. Essentially, a mountain is formed underwater by volcanic activity. That mountain sticks out above the water, forming an island. Coral reefs grow around the island, just under the water. This is called a “fringing reef.” Sometimes, the island will sink due to its own weight and the nature of the oceanic crust upon which it is sitting. As the island sinks, the fringing reef continues to grow upwards so the living corals stay in shallow water. After a while, the island has sunk so far that it seems like there is nothing but a reef there. However, if the volcanic island had not been there originally, there would not be a coral reef there now.

      You are right that the foundation upon which a coral nation’s infrastructure is built doesn’t move upward. However, the edges of the reef do. Those edges form a ring that captures sediment, which expands the island. So it’s not that the base of the island is rising. The edges of the island are rising, and that allows the island to capture new sediment, which causes the island to expand.

      1. Aaron says:

        Fascinating. That makes me wonder, is there a critical rate of sea level rise above which the reefs can’t collect sediment quickly enough to keep up? Or is that process able to keep up with any theoretical acceleration in sea level? That would be good information to know, but I imagine that that would be tough to simulate in any realistic way. Of course, if the sea level continues to rise at such a steady rate, the whole question would be moot anyway.

        1. jlwile says:

          I would think there would be some rate of sea level rise that coral reefs couldn’t keep up with. However, I am not sure how that would be determined. Long ago, it was thought that coral reefs grow very slowly, such that the coral reefs we see must have taken hundreds of thousands of years to form. We now know that’s not true at all. I suspect that the rates at which coral can grow when they must would surprise most marine biologists.

  4. John S says:

    In all the publicized talk of sea level rise I rarely, if ever, hear of subsidence and erosion. It seems to me they are inconvenient facts that are being repressed because they don’t help the cause of AGW.