Will Earth’s coral islands become a second Atlantis?
Exploring the relationship between coral islands and climate change
(Note: This essay is designed to present a scientific article in a more accessible way to a particular audience. It is written for the community of adult geeks that I go on a geek-themed cruise with each year to the Caribbean. I thought it would be neat to explain to them what is going on with the islands we visit and what they might look like in the future.)
Imagine you are lying on a sandy beach on the shore of a beautiful, tropical island. You already are? Great. Now, whether you know it or not, there is a pretty good chance that this beautiful island is sitting on a coral reef. (If it’s not on a reef, it’s probably an old volcano, but that’s another story.) If you stand up and look around, you’ll notice that most of this island is pretty low-elevation. People come here for easy access to the ocean, not for mountain climbing. Unfortunately, not only do people have easy access to the ocean, but the ocean has easy access to the island, which can result in a lot of problems if the ocean level starts to rise.
Now, imagine standing in the same place on the same island, but this time there is ocean water lapping at your knees. The towel and half-empty sunscreen you left 50ft up the beach are gone, as are the 300 beach chairs that used to mark the line where the sand turned into shrubbery. This is the sort of scenario that most people imagine when they think of climate change, sea-level rise, and tropical islands. However, thankfully, this is not the only possible outcome for the fate of a coral island in the midst of a changing climate. It turns out that there are an immense number of factors that determine the stability of a sediment-based island when sea level starts to rise. What else can an island do but sit there and drown, you ask? Good question; but the answer has very little to do with the island and everything to do with the coral reefs around it.
Before I get any further, let’s start with some basics. Coral islands are born when enough sediments collect in the shallows created by a coral reef to breech the surface of the water and make a land mass. However, if you have ever built a sandcastle, you know that waves are really good at pushing sand around – and the ocean happens to have a lot of waves. Therefore, unless there is a constant influx of sediments from the surrounding water coming in with the waves and being added to the new island, the land will slowly disappear as the sediments hitch a ride with the retreating wave. So, where does the new, sustaining sediment come from? The surrounding reef, of course! Not only does the reef act as a breakwater for incoming waves, consequently mitigating their erosive force on the beach, but it also supplies the island with the sediment it needs to survive.
What do I mean by ‘sediments’? Well, if you look really closely at sand, you will find that it comes in LOTS of different shapes, sizes, and colors. (Seriously, go do a Google Image search for “magnified sand” if you’ve never seen what I’m talking about.) Unsurprisingly, these different types of particles come from different parent materials. In the case of coral islands, there are a few major players in the “particles and sediments” division. Corals are obviously a fairly important source of sediment, but they are by no means always the dominant factor. Crustose coralline algae (CCA), Halimeda, and foraminifera are all various types of organisms that live on or around a reef and leave calcium-carbonate versions of themselves behind when they die. Mollusks (snails, clams, and the like) are another such source of chalky remains that frequently end up on beaches. In addition to these skeletal sediments, there are also sediments that simply eroded away from their structure of origin through biophysical processes, such as from wave action or when a sponge loosens particles from inside the underlying frame of a coral structure. Usually, all these various types of sediments will be present in a coral island, but the dominant particle type will vary widely based on the particular location, history, and ecology of each island. This matters because a coral island’s sedimentary composition actually has significant influence over how the island is predicted to fare in the face of climate change.
So, how do you know if beautiful Coco Cay – or the imaginary island you’re standing on now – is destined for a watery death? Unfortunately, scientists still don’t understand many of the forces at work in these ecosystems, largely due to huge gaps in the research and data they need to draw informed conclusions. There are few simple cause and effect relationships when it comes to climate change; and, as in everything ecological, generalizations can only get you half-way. Everything depends on the specific context of the case you are looking at. That said, half-way is (arguably) better than nothing, so here is what we do know.
To determine the projected stability of a coral island, we first need to determine what sort of island-building sediments are being generated by the surrounding reefs, as well as the estimated influx rate of these sediments to the island. Then we need to look at the risk factors at play. Sea level rise is pretty much guaranteed to be of concern to all coral islands, but individual groups of islands will also have to mitigate the effects of more localized issues as well. Overfishing, ocean warming, storm intensity, and changes in nutrient availability due to pollution all play huge roles in the sediment production of a coral reef and should be taken into account where applicable. Different reef communities will respond to various environmental pressures in different ways, so it is the combination of these two conditions (sediment creation and environmental variation) that allow us to make a prediction about the fate of the island in question based on the fate of its reef.
While the array of possible outcomes to these combined conditions is too extensive to address here, I do wish to leave you with a sense of the range they encompass and some hope for the future of coral islands. In general, islands that are composed of finer grain sediments, such as eroded corals and smaller forms of foraminifera, are considered the most vulnerable to sea level rise. However, influxes of nutrients from human disturbance may result in an increase in the production of such finer sediment by the surrounding reefs, potentially countering some of the erosive effects of sea level rise. Conversely, islands that are comprised primarily of coarser-grained sediments, such a larger pieces of coral and CCA, are generally expected to weather changes in sea level with little consequence. However, adding overfishing to the mix would most likely cause coral and CCA particle creation by the reef to decline, which could seriously impact the stability of the island. Such a change could even cause a long-term shift in the composition of the primary island sediments from coral/CCA to Halimeda and certain kinds of foraminifera, which would become the dominant course-grained sediment being produced by the reef.
Ultimately, the complexity of the factors that influence the stability of a coral island is both daunting and poorly understood, but there is yet some hope for the longevity of coral islands as a whole in the face of changing climates and rising sea levels. Instead of slowly drowning or being swept away, some islands may be able to take advantage of a change in sediment availability to adapt to their changing environment, while others may simply remain relatively unaffected. We can do our part to help save our favorite vacation destinations (even if we only look at them appreciatively on the way to the Lido deck between rounds of our favorite board games) by acknowledging the vital role that coral reefs play in the life of islands, and to ensure that any cruise line or shore excursion we choose to endorse takes that role seriously.
Article Citation: Perry, Chris T., Paul S. Kench, Scott G. Smithers, Bernhard Riegl, Hiroya Yamano, and Michael J. O’Leary. “Implications of Reef Ecosystem Change for the Stability and Maintenance of Coral Reef Islands.” Global Change Biology 17.12 (2011): 3679-696. Web. 26 Sept. 2013.