There is much to do in the press about the immanent demise of coral atoll islands due to rising sea level. While global warming could well destroy coral atolls, it won't be due to the rise in sea level. Consider this as a mind exercise.
The present ice age 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 125,000 years ago. That is 62 times as long as from now back to the Roman empire. At the end of Eemian interglacial, sea level started to fall as more and more water was deposited 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 or so years.
At the beginning of the present interglacial period, some 11,000 years ago, sea level rose quickly as the continental glaciers melted. 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.
* Incidentally, the Calcium carbonate of which coral skeletons are made are a tad over 60% Carbon dioxide!!
Now if corals are limited by low tide, why are the coral atoll islands meters above the level of the growing corals.
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 and collects where the total energy is low. 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 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 than that and the 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. 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.
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
* 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 of the corals.
* Don't allow sewage to flow into the sea or into the water table unless it is fully treated so that the nutrients are removed. Primary treatment is not sufficient.
* 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 few years the area is changed. Fishing in areas not under Tapu will be greatly improved as a bonus.
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. 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. Just a thought.
The present ice age 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 125,000 years ago. That is 62 times as long as from now back to the Roman empire. At the end of Eemian interglacial, sea level started to fall as more and more water was deposited 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 or so years.
At the beginning of the present interglacial period, some 11,000 years ago, sea level rose quickly as the continental glaciers melted. 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.
* Incidentally, the Calcium carbonate of which coral skeletons are made are a tad over 60% Carbon dioxide!!
Now if corals are limited by low tide, why are the coral atoll islands meters above the level of the growing corals.
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 and collects where the total energy is low. 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 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 than that and the 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. 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.
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
* 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 of the corals.
* Don't allow sewage to flow into the sea or into the water table unless it is fully treated so that the nutrients are removed. Primary treatment is not sufficient.
* 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 few years the area is changed. Fishing in areas not under Tapu will be greatly improved as a bonus.
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. 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. Just a thought.
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