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Monday, September 12, 2011

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 due to the rise in sea level.  Some background:

The present ice age, which we are in the middle of,  started 2.5million years ago.  It has had numerous glaciated periods (glacials) and warm periods (interglacials).   The  interglacial before the one we are in now was  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 is called the Holocene.  At the end of Eemian interglacial, sea level started to fall 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 100,000+ years.

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.  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 this rapid melt with isolated periods of as much as 56mm per year. (today sea level is rising at about 3mm per year)

* Incidentally, the Calcium carbonate of which coral skeletons are made are a tad over 60% Carbon dioxide!!which ultimately came from the atmosphere.

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 is Parrot fish.  Parrot fish eat corals to get at 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*.

 *  In hurricanes, coarser material as well as sand can be racked up, adding to the size and even altering the location somewhat of coral islands.

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.

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 still, 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; 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 crystalizes 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.

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 until it is dense enough to sink down through the colder water below.
If the arctic overturn is stopped by increased melting of Greenland ice sheets, 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.   

** 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 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 and the root system will stabilize it.  

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 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 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.


balanceact said...

This is a really informative post on how coral atolls form and recede.

So as Anthropogenic Climate Change melts all the glaciers and ice sheets on the planet and sea level rises around 240 ft., the Marshall Islands will disappear during this SLR, but the coral atolls should keep up and at the end of this SLR, in oh, 5K-10K years, or however long it takes to fully melt Antarctica and Greenland, the islands would reappear, assuming the fish and corals haven't been completely wiped out by high ocean temperatures and ocean acidity.

Not really a pretty picture.. It would be so much better if we could get our act together and stop emitting CO2 and find a way to remove and store the excess atmospheric CO2 we have already emitted. No small task.

William Hughes-Games said...

Sadly, the coral islands will only be able to keep up with Sea Level Rise if there are no people on them. It would be possible for people to continue on the islands with them keeping up with the rise in the sea but they would have to understand how it works and then act accordingly. For instance, they would have to limit their population, never kill a parrot fish or a rabbit fish, put their houses on stilts so that during extreme storms, sand and broken coral could be washed up on to their islands. They should also treat their sewage to level three, plant mangroves, never use herbicides or pesticide and so forth.
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