Floating Islands

Guest Post by Willis Eschenbach

Much has been written of late regarding the impending projected demise of the world’s coral atoll islands due to CO2-caused sea level rise. Micronesia is suing the Czech Government over CO2 emissions that they claim are damaging their coral atolls via sea level rise. Tuvalu and the Maldives are also repeating their claims of damage from CO2. If the sea level rises much, they say they will simply be swept away.

Recently, here in the Solomon Islands, the sea level rise has been blamed for saltwater intrusion into the subsurface “lens” of freshwater that forms under atolls. Beneath the surface of most atolls, there is a lens-shaped body of fresh water. The claim is that the rising sea levels are contaminating the freshwater lens with seawater. On other atolls, increased sea levels are claimed to be washing away parts of the atoll.

In this paper, I will discuss the two interrelated claims that people are making as illustrated above. The claims are:

1. Sea level rise causes salt water to intrude into the freshwater lens

2. Sea level rise gravely endangers low-lying coral atolls like Tuvalu, Kiribati, and the Maldives. A mere 1-meter rise would see them mostly washed away.

I will look at the real causes of the very real problems faced by atoll dwellers. Finally, I will list some practical measures to ameliorate those problems.

And before you ask, how do I know this atoll stuff? For three years I lived on and worked on and had wells dug on and watched the moon rise over and dived in the lagoon and on the reef wall of a coral atoll in the South Pacific … hey, somebody has to … that plus a lot of study and research.

Claim 1. Can a sea level rise cause salt water to intrude into the freshwater lens?

Short answer, no. To understand what is really happening with the freshwater lens, we’ll start with the geology. Here is a cross-section of a typical atoll that I drew up.

Figure 1. Typical cross section through a coral atoll. The living coral is in the ring between the dotted green line and the beach. The atoll used for the photo in this example is Tepoto Atoll, French Polynesia. Click image to embiggen.

Note that the seawater penetrates throughout the porous coral rubble base. Because fresh water is lighter than salt water, the freshwater lens is floating on this subsurface part of the ocean. The weight of the freshwater pushes down the surface of the seawater underneath it, forming the bottom of the “lens” shape. The lens is wider in areas where the atoll is wider. The amount of fresh water in the lens is a balance between what is added and what is withdrawn or lost. The lens is only replenished by rain.

The important thing here is that the freshwater lens is floating on the sea surface. It’s not like a well on land, with an underground freshwater source with a water-tight layer below it. There is no underground freshwater source on an atoll. It is just a bubble of water, a rain-filled lens floating on a seawater table in a porous coral rubble and sand substructure. If there is no rain, the freshwater will eventually slowly mix with the salt water and dissipate. When there is rain, you get a floating lens of fresh water, which goes up and down with the underlying seawater.

So the second claim, that a sea level rise can cause the sea water to intrude into the freshwater lens, is not true either. The freshwater lens floats on the seawater below. A rise in the sea level merely moves the lens upwards. It does not cause salt water to intrude into the lens.

Claim 2. Would a sea level rise gravely endanger low-lying coral atolls?

Regarding atolls and sea level rise, the most important fact was discovered by none other than Charles Darwin. He realized that coral atolls essentially “float” on the surface of the sea. When the sea rises, the atoll rises with it. They are not solid, like a rock island. They are a pile of sand and rubble. There is always material added and material being lost. Atolls exist in a delicate balance between new sand and coral rubble being added from the reef, and atoll sand and rubble being eroded by wind and wave back into the sea or into the lagoon. As sea level rises, the balance tips in favor of sand and rubble being added to the atoll. The result is that the atoll rises with the sea level.

Darwin’s discovery also explained why coral atolls occur in rings as in Fig. 2 above. They started as a circular inshore coral reef around a volcanic rock island. As the sea level rose (or equally, if the island sank), flooding more and more of the island, the coral grew upwards. Eventually, the island was drowned by the rising sea levels, and all that is left is the ring of reef and coral atolls.

Why don’t we see atolls getting fifty feet high? Wind erosion keeps atolls from getting too tall. Wind increases rapidly with distance above the ocean. The atolls simply cannot get taller. The sand at that elevation is blown away as fast as it is added. That’s why all atolls are so low-lying.

When the sea level rises, wind erosion decreases. The coral itself continues to grow upwards to match the sea level rise. Because the coral continues to flourish, the flow of sand and rubble onto the atoll continues, and with reduced wind erosion the atoll height increases by the amount of the sea level rise.

Since (as Darwin showed) atolls float up with the sea level, the idea that they will be buried by sea level rises is totally unfounded. Despite never being more than a few meters tall, they have survived a sea level rise of up to three hundred plus feet (call it a hundred meters) within the last twenty thousand years. Historically they have floated up higher than the peaks of drowned mountains.

So the third claim is not true either. Atolls are created by sea level rise, not destroyed by sea level rise.

What is the real cause of salt water in the lens?

Given that the saltwater intrusion can’t be a result of sea level rise (because the lens is floating), why is there saltwater in the islanders’ wells? Several factors affect this. First and foremost, the freshwater lens is a limited supply. As island populations increase, more and more water is drawn from the lens. The inevitable end of this is that the water in the wells gets saltier and saltier. This affects both wells and plants, which draw from the same lens. It also leads to unfounded claims that sea level rise is to blame.

The second reason for saltwater 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. When the atoll shrinks, the lens shrinks.

The third reason is that roads and airstrips and changes in land use and land cover has reduced the amount of rain making it to the lens. Less freshwater in, more saltwater in.

What is the real cause of loss of beach and atoll land?

An atoll is not solid ground. It is not a constant “thing” in the way a rock island is a thing. An atoll is a not-so-solid eddy in a river of sand and rubble. It is an ever-changing body constantly replenished by a (hopefully) unending stream of building materials. It is a process, not a solid object. On one side, healthy reefs plus beaked coral-grazing fish plus storms provide a continuous supply 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 by waves and blown away by the wind. 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.

So if the atoll is shrinking, there are only a few possibilities. Erosion may have increased. The supply of sand and rubble, the raw atoll construction materials, may have decreased. Currents may have changed from reef damage, dredging, or construction.

Water erosion and current changes are increased by anything that damages or changes the reef. That thin strip of living coral armor is all that stands between a pile of sand and the endless waves. When the reef changes, the atoll changes.

Erosion is also caused by a variety of human activities. Road and path building, house construction, ground cover change, clearing of channels through the reef, the list is long.

The reduction in the supply of coral sand and coral rubble, however, is harder to see. This reduction has two main causes – using of coral for building, and killing the wrong fish. The use of coral as a building material in many atolls is quite common. At times this is done in a way that damages the reef. Anything that affects the health of the reef affects how much atoll building material it produces each year. This is the somewhat visible part of the loss of building materials, 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 major 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 around a hundred kilos of coral sand per year. Parrotfish and other beaked reef fish create the sand by grinding up the coral with their massive jaws and bony throats, digesting the food, and excreting the ground coral.

Beaked grazing fish are vital for overall coral health, growth, and production. This happens in the same way that pruning makes a tree send up lots of new shoots. The constant grazing by the beaked fish keeps the corals in full production mode. This greatly increases the annual production of coral for sand and rubble.

Unfortunately, these fish sleep at night, and thus are easily wiped out by night divers. The invention of the diving flashlight has meant that their populations have plummeted in many areas in recent years. Result? Less sand means less beaches, and means more claims of “CO2 is to blame, you can see the damage!”.

Some Practical Suggestions

What can be done to turn the situation around for the atolls? From the outside, not a whole lot. Stopping the Czechs from burning coal won’t do a damned thing. From the outside, we can offer only assistance. The work needs to occur on the atolls themselves.

There are, however, a number of low-cost, practical steps that atoll residents can take to preserve and build up their atolls and protect the freshwater lens. In no particular order, these are:

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 into 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. On some atolls, 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 underfoot. The parrotfish and the other beaked reef-grazing fish are constantly building up the atoll. Every year they are providing tonnes and tonnes of fine white sand to keep the atoll afloat in turbulent times. They should be honored and protected, not killed. Caring for the reef is the single most important thing you can do.

5. Be cautious regarding the use of coral as a building material. The atoll will be affected if anything upsets that balance of sand added and sand lost. It will erode if the supply of coral sand and rubble per year starts dropping (say from reef damage or extensive coral mining or killing parrotfish) or if the total sand and rubble loss goes up (say by heavy rains or strong winds or human erosion or a change in currents).

So when 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 that is absolutely necessary to keep the atoll afloat. 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 down 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 the precious sand into the ocean. Invisibly, the wind blows the ground out from underfoot. Protect the island from erosion. 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 and recording and photographing the reef. Remind 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 often quite possible to “grow” an atoll. The key is to slow down the water as it passes by. The slower the water moves, the more sand drops out to the bottom. Slowing the water is accomplished by building low underwater walls perpendicular to the coastline. These start abovewater, and run out until the ends are a few metres underwater. Commercially this is done with a geotextile fabric tubes which are pumped full of concrete. See the references for more information. 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 pattern, 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.  This triangular shape does not attempt to stop the water currents. Think of it as a speed bunp. It just slows the currents 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 and rubble … like for example a healthy reef with lots of parrotfish. That’s why I said above that the most important thing is to protect the fish and the reef. If you have a healthy reef, you’ll have plenty of sand and rubble to keep the atoll afloat 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 with the sea level. 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 overfishing, or overpumping the water lens, or unrestrained coral mining. Those are what is killing the atolls, not the same rate of sea level rise that we’ve had for the last hundred years.

FURTHER REFERENCES:

On global sea level rise levelling off: University of Colorado at Boulder Sea Level Change,  http://sealevel.colorado.edu

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), http://conf.sopac.org/virlib/TR/TR0234.pdf This atoll was cited by the Sierra Club as an example of the dangers of sea level rise. The truth is more prosaic.

On the same topic: Xue, C., Malologa, F. (1995) Coastal sedimentation and coastal management of Fongafale, Funafuti, Tuvalu, SOPAC Technical Report 221

More information on how parrotfish increase reef production: http://www.esajournals.org/doi/abs/10.1890/1051-0761(2006)016%5B0747:TIOEGS%5D2.0.CO%3B2

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, available here (PDF doc).

On expanding island beaches: Holmberg Technologies, http://www.erosion.com/

On the atolls getting larger: Global-scale changes in the area of atoll islands during the 21st century

On the dangers of overpopulation: Just look around you …

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[UPDATE June 3, 2010] Other scientists are catching up with me (emphasis mine).

Global and Planetary Change, Article in Press, Accepted Manuscript, doi:10.1016/j.gloplacha.2010.05.003

The dynamic response of reef islands to sea level rise: evidence from multi-decadal analysis of island change in the central pacific

Arthur P. Webb a, and Paul S. Kench b; a South Pacific Applied Geoscience Commission, SOPAC. Fiji; b School of Environment, The University of Auckland, Private Bag 92019, Auckland, New Zealand

Received 22 February 2010; accepted 13 May 2010. Available online 21 May 2010.

Abstract

Low-lying atoll islands are widely perceived to erode in response to measured and future sea level rise. Using historical aerial photography and satellite images this study presents the first quantitative analysis of physical changes in 27 atoll islands in the central Pacific over a 19 to 61 year period. This period of analysis corresponds with instrumental records that show a rate of sea level rise of 2.0 mm.y-1 in the Pacific.

Results show that 86% of islands remained stable (43%) or increased in area (43%) over the timeframe of analysis. Largest decadal rates of increase in island area range between 0.1 to 5.6 hectares. Only 14% of study islands exhibited a net reduction in island area.

Despite small net changes in area, islands exhibited larger gross changes. This was expressed as changes in the planform configuration and position of islands on reef platforms. Modes of island change included: ocean shoreline displacement toward the lagoon; lagoon shoreline progradation; and, extension of the ends of elongate islands. Collectively these adjustments represent net lagoonward migration of islands in 65% of cases.

Results contradict existing paradigms of island response and have significant implications for the consideration of island stability under ongoing sea level rise in the central Pacific.

First, islands are geomorphologically persistent features on atoll reef platforms and can increase in island area despite sea level change.

Second; islands are dynamic landforms that undergo a range of physical adjustments in responses to changing boundary conditions, of which sea level is just one factor.

Third, erosion of island shorelines must be reconsidered in the context of physical adjustments of the entire island shoreline as erosion may be balanced by progradation on other sectors of shorelines. Results indicate that the style and magnitude of geomorphic change will vary between islands. Therefore, Island nations must place a high priority on resolving the precise styles and rates of change that will occur over the next century and reconsider the implications for adaption.

In other words, the islands are floating upwards with the sea level rise, just as I had said. So for those in the comments section who think I’m just making this up … think again. In particular, the final comment by lkrndu22 says that I am “hoist by my own petard” because ocean acidification has already caused “evident and severe” damage … ‘fraid not. The islands continue to rise. The main cause of damage to the corals is … coral mining and killing the fish. And islands where that is happening are in danger, as I indicated above.

But sea level rise? The atolls have lived through that for thousands of years without damage.