By by Coral Atolls

There is much to-do in the press about the immanent drowning of coral atoll islands due to rising sea level.  While climate change could well destroy coral atolls, it  won't be primarily due to the rise in sea level.  Some background:

The present ice age, which we are in the middle of,  started roughly 2.5million years ago.  It has had numerous glaciated periods (glacials) and warm periods (interglacials).   The  interglacial before the one we are in now, we call the Eemian.    It was centered about 125,000 years ago.  That is 62 times as long as from now back to the Roman empire.  The present interglacial we are living in we call the Holocene.  As the previous (Eemian) interglacial, faded away sea level fell as more and more water was deposited as snow on the continental glaciers*.  At its greatest extent, sea level was 120m below its present level.  Of course the corals that were growing within 120 meters of the surface of the ocean during the Eemian interglacial, were killed as sea level dropped.  Without live corals to resist the effect of waves, these islands would have eroded.  They may well have eroded down to the level of the  low tide mark, 120 meters below present low tide.   A lot of erosion can occur in more than 100,000 years.

 

*In the western hemisphere, glaciers start to accumulate on the high lands of Baffin Island and spread out from there.  

 
As the ice started to melt, some 20,000 years ago and really got under way 15000 years ago, sea level rose quickly as the continental glaciers flowed into the oceans.  The main melt ended 7000 years ago with a slow rise since then of about one mm per year.  Today, coral reefs all over the world are at about the current low tide level and Atoll islands are a few meters above high tide.  Clearly, corals have grown as the ice melted and sea level rose.  The corals have filled in the 120 or so meters between the low tide level at the maximum extent of the recent 'glacial' to the present low tide level*. The lesson is, as sea level rises, the restraint on coral growth is removed and they grow up to the current low tide mark.  The average sea level rise was about 14mm per year during the rapid melt with isolated periods of as much as 56mm per year. (today sea level is rising at about 3.5mm per year)

* Incidentally, the Calcium carbonate of which coral skeletons are made are a tad over 60% Carbon dioxide!!which ultimately comes from the atmosphere. This, under natural (without humans) conditions would be contributing to sliding us into the next glaciation as Carbon dioxide was built into coral.

OK,,,,,, so if corals are limited by low tide, why are the coral atoll islands meters above the level of the growing corals.  This question pertains to the present fear that coral islands will be swamped as the sea rises.

The answer, to a large extent,  is Parrot fish.  Parrot fish eat corals and digest the polyps.  They poop out coral sand.  A parrot fish typically produces 90kg of sand per year.  A thousand parrot fish in a lagoon and you have a production of 90 tons of sand per year.  The sand is moved by wind, currents and waves, especially during hurricanes,  and collects where the total energy is low*.  As long as the seas are healthy for corals, new ones grow providing more food for Parrot fish and hence more sand.

 Now we come to the function of hurricanes.  In a hurricane waves are longer and higher*. so they pick up, not only coral sand but coral rubble and deposit it where the energy of wind, currents and waves is low.  It is a sort of  one way valve.  This keeps the atolls supplied with sand and rubble which keeps them growing upward.  As long as nothing kills the corals (or the Parrot fish) coral islands will keep up with sea level rise.

*For those of you with a mathematic bent, for every ninth of a wave length  you go down in the ocean, the circle of rotation of a wave halves.  For instance, if you have a wave length of 18m and a height of 1m, if you go down 4m, the circle of rotation will be 25cm.  A long wave has an effect much deeper than a short wave.
 
Once the sand forms a bit of land above sea level, bird transported seeds can germinate and the resulting plants will dampen the force of the wind crossing the island.  This results in an increased catch of wind-blown sand on the island and a root system to retain what sand there is.  Once there is a bit of an island above the high tide, rain will accumulate in the soil of the island, floating as a lens above the sea water.  Varieties of plants, which need fresh water can then grow. Bird poop (Guano) will add to the fertility of the soil created.


So under natural conditions, it is unlikely that sea level rise will destroy the Atolls.  In fact some satellite pictures show them growing.  The real problem that climate change will cause is primarily due the increase in Carbon dioxide.  Two effects are at play here.  As the sea becomes more acidic due to the absorption of CO2, it becomes harder and harder for Calcium carbonate depositing animals to extract the calcium from sea water.  A bit more acidic, and  shells and corals will start to dissolve.  

The second problem which could come from climate change is temperature rise*.  The lethal temperature for corals is only just above the temperature of maximum growth which is, in turn, only a few degrees above the present water temperature.  There are a number of reasons that tropical seas could warm.  A major one is the shut down of the ocean circulation which is powered by two phenomenon.  The first is  freezing of Arctic (and Antarctic) water.  Fresh ice crystallizes out of the sea water leaving the salt behind.  This forms brine which sinks down to the bottom of the Arctic ocean and flows out of the Arctic.  This out-flowing deep water causes surface water to be sucked toward the poles, bringing with it calories.  Heat is being pulled from warmer regions of the earth, toward the poles.  Stop the formation of deep salty ocean water and no longer will this flow of heat exist and tropical areas will warm.

The second effect that powers the Gulf stream is the cooling of the somewhat saltier water that flows up the East coast of North America.  It only stays on top because it is warmer than the underlying water.  As it flows north into cooler climes, it cools as it gives up it's heat to the atmosphere until it is dense enough to sink down through the colder water below.  If there is an outflow of fresh water from melting continental glaciers, the mix of the surface, salty water with the fresh water from the ice will no longer sink as it cools.  Once again you get a reduction of the formation of deep oceanic water, and hence the reduction of surface water being pulled northward and the heat remains in tropical waters.

If the arctic overturn is stopped by increased melting of Greenland ice sheets, and by a reduction in the formation of oceanic ice,  we will have very cold winters in Europe despite the general warming of the planet.  The corollary is that heat will not be removed from southern waters.  If either acidification or temperature rise occurs, there is nothing that the people of the coral atolls can do**.  Without live corals and parrot fish to provide a constant source of coral sand, the islands will erode.   

** (Added Jan 2017)Note that in 2016, toward the end of a very severe El Nino there was wide spread coral bleaching.  Overall, each bleaching event seems to be more severe than the previous one.


Jason Buchheim reports 
As reef building corals live near their upper thermal tolerance limits, small increases in sea temperature (.5 –1.5 degrees C) over several weeks or large increases (3-4 degrees C) over a few days will lead to coral dysfunction and death. Anomalously high sea temperatures have often been reported in the Caribbean-wide series of bleaching events that occurred during 1986-88, leading to hypothesis that global warming was having an effect on the coral reefs in this region.

*If climate change results in an ice free Arctic ocean, it becomes a massive solar panel and could rapidly melt the Greenland Ice Sheet.  If fresh water pours into the sea sufficiently fast, this could shut down the ocean circulation system.  This system, as it warms northern Europe, cools the tropics.  Stop this cooling and tropical waters could reach a lethal level for corals.


However, Short of global acidification or a rise in the temperature  of the tropical oceans, the health of the coral atolls is in the hands of the local people. 

The three basic principles to avoid your atoll being swamped by the sea are: 

 
A) do nothing that damages corals, 
B) never kill a parrot fish and 
C) make sure the islands are vegetated so that any wind born sand across the island will land on the island for the root system to hold there.  

More specifically:

*  Don't use fishing methods that damage coral reefs.  This includes dynamite.
*  Don't use chemical fertilizers on land.  They can damage corals when they seep into the sea.  If land sourced nutrients are sufficient they can lead to phytoplankton blooms that shade the zoozanthellae that are necessary for coral health.They can also fertilize sea weed growth which can smother reefs.
*  Don't allow sewage to flow into the sea or into the water table unless it is fully treated.  Primary or even secondary treatment is not sufficient.  The nutrients must be removed.
*  Don't use pesticides or herbicides as they can harm sea organisms.
*  Don't over utilize the fresh ground water.  The vegetative cover of the island 
depends on this fresh water
*  Never ever ever harm a parrot fish
*  Leave the rabbit fish (Siganid sp.) alone too.   They eat algae that can smother corals. (third fish down in the link)
*  Reintroduce the system of Tapu (taboo) in which large sections of the reef are off limits to utilization of any kind for a number of years.  Every decade or so the area is changed.  Fishing in  areas not under Tapu will be greatly improved as a bonus because of the recruitment of fish from the tapu area.

Short of a global situation that kills the corals, the fate of the atolls is in the hands of the local people.   The elephant in the room, of course is population control.  All the strains on coral atolls mentioned above are exacerbated by over-population.  Atolls are microcosms of the situation the whole world is in at present.  With stable or decreasing numbers of people on coral islands, all the bad effects decrease to manageable proportions.

By the by, an interesting experiment to try would be to plant some mangroves in shallow water by the land. If they grow, they will catch sand from the water currents which will further increase the available real estate and will protect the land during hurricanes.  Mangrove  areas are also apparently great breeding grounds for fish. They are also areas of low energy where sand will accumulate during hurricanes.  Just a thought.

Final conclusion 
The future existence of coral atolls depends on the health of the corals and the presence of fish which turn corals into sand.  The corals respond by growing upward and providing fresh material to be turned into sand.  Kill the reef and the supply of sand and coral rubble will dwindle and storms will no longer have material to keep pace with sea level rise.