by Judith Curry
Small atoll islands may grow, not sink, as sea level rises.
Recent headlines highlight the plight of Kiribati:
- Kiribati’s dilemma: Before we drown we may die of thirst
- Kiribati crumbles
- Kiribati climate change refugee crisis
Can we blame climate change, or more specifically sea level rise, for the problems of the atoll islands? From a recent article in the New Scientist Small atoll islands may grow, not sink, as sea level rises, we find:
Rising seas are eating away at small islands and will eventually turn their inhabitants into climate refugees, right? Not so for some of the world’s most threatened islands, which have grown despite experiencing dramatic sea level rise.
After poring over more than a century’s worth of data, including old maps and aerial and satellite imagery, they conclude that 18 out of 29 islands have actually grown. As a whole, the group grew by more than 18 hectares, while many islands changed shape or shifted sideways.
“There is still considerable speculation that islands will disappear as sea level rises,” says Kench. “Our data indicates that the future of islands is significantly different.”
Storms and other disturbances that churn up the sea seem to be more important than sea level in influencing stability, says Kench. Storms break up coral, which then gets deposited on the atolls. He says other coral reef islands are likely to evolve in the same way, and that the Maldives seem to be showing a similar effect.
“There will be less emphasis on external migration of ‘environmental refugees’ from atoll nations that has gained such prominence in the last few years,” he says. But he notes that the atoll-building sediment comes from productive coral reefs, which face a range of threats such as warming oceans and pollution.
Section 10.2 of Alan Longhurst’s book Doubt and Certainty in Climate Science addresses the underlying science relevant to sea level rise and island living. The text below is a summary provided by Alan:
Atolls or coral reef islands are accretionary and dynamic landforms that respond to changes in ambient patterns of wind and wave height, standing on coral platforms: as Darwin postulated, many of these are based on volcanic cones, now deeply submerged. They are built of wind- and wave-transported coral sand and debris, derived from the growth of coral organisms as these maintain an optimal profile across the reef in relation to sea level as this rises and falls at all time scales.
When the continental shelves were progressively flooded by the sea at the end of the last glaciation, sea level rose much faster than it is predicted to do by the IPCC during this century. During the Bølling warming at 14.6 Kyrs BP, sea level exceeded 40 mm/yr, greatly in excess of IPCC predictions and of the same magnitude to be expected from massive events, such as the fracture of an Antarctic ice sheet.
The existence of drowned coral platforms indicate that some reefs did not survive this or other fresh-water pulses but all of the coral reefs that exist today survived such higher rates, you might well conclude that atolls and reefs will easily survive the rise predicted by the IPCC.
But, unfortunately, these atolls were in pristine form when such surges of sea level ccurred and many reefs are today in such a delapidated (it is the only word) state that their reef islands are regressing; unfortunately even such ordinary activities as fishing can destroy the ability of an atoll to respond to sea level rise by growth of coral colonies – the massive release of urban sewage, implicated in the bacterial destruction of reefs off Florida is not required, and neither is the mining of carbonate rock on reefs for the production of cement in the Indian Ocean.
The mechanism is very simple. Fishing is an essential activity of people living on reef islands and their consumption of fish is very high. But many of the prized large fish species are herbivores, and if they are removed the macroalgae on which they previously grazed may progressively smother the coral and prevent its continued growth. Other large reef fish are spongivores, and their removal by fishing may similarly result in overgrowth of corals by sponges. This problem is largely ignored, and a recent major multi-author review of coral reef problems noted that those visiting reefs today are unlikely to see the large fish that were a character of reefs in the past, but no mention was made of the probable consequence for coral organisms of their absence.
Despite the significant change in conditions in the open ocean recorded during the 20th century, including a rise in sea level close to predicted rates, a survey of reef islands over a large part of the SW Pacific shows no alarming changes: over periods of about 20-60 years, of 27 reef islands surveyed only 14% suffered any loss of area, while the other showed either some accretion or minor changes of outline – of which some could be attributed to aggregation of material caused by the building of piers or jetties.
General predictions on the future of coral reefs and descriptions of their present state are almost wholly negative and tend to emphasise global warming and to ignore other human activities: mining for coral rock to use in building runways and for cement production, fishing, pollution, the invasion of exotic species and all the other insults that this fragile habitat now receives.
Perhaps such problems are most serious in the densely-populated Caribbean than elsewhere, but they are also very important on some coral islands and atolls of the Indian Ocean; of these the tectonically-raised atoll forming the main island of the Maldives is perhaps the worst case of self-destruction by the development of mass tourism.
But some Pacific sea-level atolls are in no better shape; this image shows the southeast corner of the atoll-city of Majuro, not far from the ribbon-like International Airport of the Marshall Islands; special concern has been raised by the UNO for the future of the population of this island nation, which claims that it will be forced to abandon its homeland due to rising sea levels for which the developed world is responsible. But their argument ignores the fact that the living corals of their atoll home no longer retain the functionality required for them to compensate by growth for natural changes in sea level. One wonders where the carbonate rock for the cement for the runway was dredged from and where the sewage from such a large population goes to? Majuro, together with similarly urbanised coral islands, is a disaster waiting to happen – climate change or not.
But many of the atolls of the Marshalls are uninhabited and appear to be in a viable state, so far as their main features can be discerned in satellite images. Wake Island, also in the western Pacific, is an interesting case; intensely used as an air base by US forces in the Pacific war, it is now very sparsely occupied, although the military airport remains in place: this reef island is now a wildlife sanctuary for sea birds and its coral ecosystems must be gently returning to a pristine – or at least a viable – status. The Chagos archipelago in the Indian Ocean is a similar case; the population was removed to Mauritius in 1960 by the British government of the day so that a NATO surveillance and transit base could be established on Diego Garcia.
The rest of the archipelago has by now largely reverted to pristine state and is hopefully to be declared a Marine Protected Area – much against the wishes of the exiled islanders who still want to return home.
The reality is that the reef islands that are formed on living coral arcs or atolls are not a suitable long-term habitat for anything other than a very light human footprint: in the not-so-distant past, limitation was placed on population growth by the size of the freshwater lens that forms by the accumulation of rainwater in the compacted sand behind the beach, and above sea level. If this becomes exhausted by the withdrawal of too much water for drinking or for growing vegetables, the human population dies out, or must migrate.
The alarm surrounding the Kiribati refugee crisis is yet another example of blaming human-caused climate change for human-caused environmental problems that have nothing to do with climate change.
Addressing the real cause of such problems is needed, rather than hoping for global emissions reductions to solve these problems (at best near the end of the 21st century). Blaming this problem on human caused global warming trivializes both the plight of Kiribati and the global warming issue.