Coral atolls and sea level rise
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 as Darwin discovered, 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 of 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, or 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.