Coral Atolls and Sea Level Rise
- a guest blog by Willis Eschenbach
Much has been written of late regarding the impending demise of the world’s coral atolls due to sea level rise. Recently, here in the Solomon Islands, the sea level rise has been blamed for salt water intrusion into the subsurface “lens” of fresh water under some atolls. Beneath the surface of most atolls, there is a lens shaped body of fresh water which floats on the seawater underneath. The claim is that the rising sea levels are contaminating the fresh-water lens with seawater.
These claims of blame ignore several facts. The first and most important fact, discovered by none other than Charles Darwin, is that coral atolls essentially “float” on the surface of the sea. When the sea rises, the atoll rises with it, and when the sea falls, they fall as well. Atolls exist in a delicate balance between new sand and coral rubble being added from the reef, and sand and rubble being eroded by wind and wave back into the sea.
When the sea falls, more sand tumbles from the high part, and more of the atoll is exposed to wind erosion. The atoll falls along with the sea level. When the sea level rises, wind erosion decreases. The coral grows up along with the sea level rise. The flow of sand and rubble onto the atoll continues, and the atoll rises. Since atolls go up and down with the sea level, the idea that they will be buried by sea level rises is totally unfounded. They have gone through sea level rises much larger and much faster than the current one.
Given that established scientific fact, why is there water incursion into the fresh water lenses? Several factors affect this. First and foremost, the fresh water lens is a limited supply. As island populations increase, more and more water is drawn from the lens. The inevitable end of this is the intrusion of sea water into the lens. This affects both wells and plants, which both draw from the same lens. It also leads to unfounded claims that sea level rise is to blame. It is not. Seawater is coming in because fresh water is going out.
The second reason for salt water intrusion into the lens is a reduction in the amount of sand and rubble coming onto the atoll from the reef. When the balance between sand added and sand lost is disturbed, the atoll shrinks. This has two main causes — coral mining and killing the wrong fish. The use of coral for construction in many atolls is quite common. At times this is done in a way that damages the reef as well as taking the coral. This is the visible part of the loss of reef, the part we can see.
What goes unremarked is the loss of the reef sand, which is essential for the continued existence of the atoll. The cause for the loss of sand is the indiscriminate, wholesale killing of parrotfish and other beaked reef-grazing fish. A single parrotfish, for example, creates about half a tonne of coral sand per year. Parrotfish and other beaked reef fish create the sand by grinding up the reef with their massive jaws, digesting the food, and excreting the ground coral.
In addition to making all that fine white sand that makes up the lovely island beaches, beaked grazing fish also increase overall coral health, growth, and production. This happens in the same way that pruning makes a tree send up lots of new shoots, and in the same way that lions keep a herd of zebras healthy and productive. The constant grazing by the beaked fish keeps the corals in full production mode.
Unfortunately, these fish sleep at night, and are easily wiped out by night divers. Their populations have plummeted in many areas in recent years. Result? Much less sand.
The third reason for salt water intrusion into the lens is the tidal cycle. We are currently in the high part of the 18 year tidal cycle. The maximum high tide in Honiara in 2008 was about 10 cm higher than the maximum tide in 1996, and the highs will now decrease until about 2014. People often mistake an unusually high tide for a rise in sea level, which it is not. There has been no increase in the recorded rate of sea level rise. In fact, the global sea level rise has flattened out in the last couple years.
What can be done to turn the situation around for the atolls? There are a number of essential practical steps that atoll residents can take to preserve and build up your atoll, and protect the fresh water lens:
1. Stop having so many kids. An atoll has a limited supply of water. It cannot support an unlimited population. Enough said.
2. Catch every drop that falls. On the ground, build small dams in any watercourses to encourage the water to soak in to the lens rather than run off to the ocean. Put water tanks under every roof. Dig “recharge wells”, which return filtered surface water to the lens in times of heavy rain. Catch the water off of the runways. In Majuro, they have put gutters on both sides of the airplane runway to catch all of the rainwater falling on the runway. It is collected and pumped into tanks. On other atolls, they let the rainwater just run off of the airstrip back into the ocean …
3. Conserve, conserve, conserve. Use seawater in place of fresh whenever possible. Use as little water as you can.
4. Make the killing of parrotfish and other beaked reef grazing fish tabu. Stop fishing them entirely. Make them protected species. The parrotfish should be the national bird of every atoll nation. I’m serious. If you call it the national bird, tourists will ask why a fish is the national bird, and you can explain to them how the parrotfish is the source of the beautiful beaches they are walking on, so they shouldn’t spear beaked reef fish or eat them. Stop killing the fish that make the very ground under your feet. The parrotfish and the other beaked reef-grazing fish are constantly building up your atoll. Every year they are providing tonnes and tonnes of fine white sand to keep your atoll afloat in turbulent times. You should be honoring and protecting them, not killing them. This is the single most important thing you can do.
5. Be very cautious regarding the use of coral as a building material. An atoll is not solid ground. It is is not a constant “thing” in the way a rock island is a thing. An atoll is an eddy, an ever-changing body constantly replenished by a (hopefully) unending river of coral sand and rubble. It is a process, wherein on one side healthy reef plus beaked coral-grazing fish plus storms provide a continuous stream of coral sand and rubble. This sand and rubble are constantly being added to the atoll, making it larger. At the same time, coral sand and rubble are constantly being eaten away, and blown away, and eroded away from the atoll. The shape of the atoll changes from season to season and from year to year. It builds up on this corner, and the sea washes away that corner.
And of course, if anything upsets that balance of sand added and sand lost, if the supply of coral sand and rubble per year starts dropping (say from reef damage or coral mining or killing parrotfish) or if the total sand and rubble loss goes up (say by heavy rains or strong winds or a change in currents) the atoll will be affected.
So if coral is necessary for building, take it sparingly, in spots. Take dead or dying coral in preference to live coral. Mine the deeps and not the shallows. Use hand tools. Leave enough healthy reef around to reseed the area with new coral. A healthy reef is the factory that annually produces the tonnes and tonnes of building material. You mess with it at your peril.
6. Reduce sand loss from the atoll in as many ways as possible. This can be done with plants to stop wind erosion. Don’t introduce plants for the purpose. Encourage and transplant the plants that already grow locally. Reducing water erosion also occurs with the small dams mentioned above, which will trap sand eroded by rainfall. Don’t overlook human erosion. Every step a person takes on an atoll pushes sand downhill, closer to returning to the sea. Lay leaf mats where this is evident, wherever the path is wearing away. People wear a path, and soon it is lower than the surrounding ground. When it rains, it becomes a small watercourse. Invisibly, the water washes your precious sand into the ocean. Invisibly, the wind blows the ground out from under your feet. Protect your island. Stop it from being washed and blown away.
7. Monitor and build up the health of the reef. You and you alone are responsible for the well-being of the amazing underwater fish-tended coral factory that year after year keeps your atoll from disappearing. Coral reseeding programs done by schools have been very successful. Get the kids involved in watching the reef. Educate the people that they are the guardians of the reef. Talk to the fishermen.
8. Expand the atoll. Modern coastal engineering has shown that it is quite possible to “grow” an atoll. The key is to slow down the water as it passes by. The slower the water, the more sand builds up. Slowing the water is accomplished by building low underwater walls perpendicular to the beach. These run out until the ends are a few metres underwater. Normally this is done with a geotextile fabric tubes which are pumped full of concrete. In the atolls the similar effect can be obtained with “gabbions”, wire cages filled with blocks of dead coral. Wire all of the wire cages securely together in a triangular shape, stake them down with rebar, wait for the sand to fill in. It might be possible to do it with old tires, fastened together, with chunks of coral piled on top of them. It will likely take a few years to fill in. Here’s a before and after picture of the system in use on a beach (not an atoll), taken three years apart. Note the low height and triangular shape of the wall extending out from the beach and continuing underwater (made of 3 concrete-filled geotextile fabric tubes). This triangular shape does not attempt to stop the water currents. It just slows them down and directs them toward the beach to deposit their load of sand. Eventually, the entire area fills in with sand.
Of course to do that, you absolutely have to have a constant source of sand … like for example a healthy reef … with lots of parrotfish. That’s why I said above that the single most important thing is to protect the fish and the reef. If you have beaked fish and a healthy reef, you’ll have plenty of sand and rubble forever. If you don’t, you’re in trouble.
Coral atolls have proven over thousands of years that, if left alone, they can go up and down with any sea level rise. And if we follow some simple conservation practices, they can continue to do so and to support atoll residents. But they cannot survive an unlimited population increase, or unrestricted fishing, or overpumping the water lens, or unrestrained coral mining.
On sea level rise in Honiara: Pacific Country Report Sea Level & Climate: Their Present State Solomon Islands June 2006
On global sea level rise levelling off: University of Colorado at Boulder’s Sea Level Change
On Darwin’s discovery: Darwin, C., The Autobiography of Charles Darwin 1809-1882, 1887
No other work of mine was begun in so deductive a spirit as this; for the whole theory was thought out on the west coast of S. America before I had seen a true coral reef. I had therefore only to verify and extend my views by a careful examination of living reefs. But it should be observed that I had during the two previous years been incessantly attending to the effects on the shores of S. America of the intermittent elevation of the land, together with the denudation and deposition of sediment. This necessarily led me to reflect much on the effects of subsidence, and it was easy to replace in imagination the continued deposition of sediment by the upward growth of coral. To do this was to form my theory of the formation of barrier-reefs and atolls. (Darwin, 1887, p. 98, 99)
On the results of coral mining and changing the reef: Xue, C. (1996) Coastal Erosion And Management Of Amatuku Island, Funafuti Atoll, Tuvalu, 1996, South Pacific Applied Geoscience Commission (SOPAC)
On the same topic: Xue, C., Malologa, F. (1995) Coastal sedimentation and coastal management of Fongafale, Funafuti, Tuvalu, SOPAC Technical Report 221
On parrotfish creating sand: this link
On the cause of erosion in Tuvalu: Tuvalu Not Experiencing Increased Sea Level Rise, Willis Eschenbach, Energy & Environment, Volume 15, Number 3, 1 July 2004 , pp. 527-543
On expanding island beaches: Holmberg Technologies
On the dangers of overpopulation: Just look around you …