Anthony commented yesterday on the question of atolls and sea level rise here, and I had previously written on the subject in my post “Floating Islands“. However, Anthony referenced a paper which was incorrectly linked by New Scientist. So I thought I’d provide some more information on the actual study, entitled “The dynamic response of reef islands to sea level rise: evidence from multi-decadal analysis of island change in the central pacific”, by Arthur Webb and Paul Kench.
One of the ironies of the new paper involves the atoll of Amatuku in the island nation of Tuvalu. Amatuku became the first poster child of “drowning atolls” due to an article in the July/August 2003 issue of Sierra Magazine, the magazine of the Sierra Club. The article was entitled “High Tide in Tuvalu”, with the sub-title “In the tropical Pacific, climate change threatens to create a real-life Atlantis.” Here’s a recent photo of “Atlantis”:
Figure 1. Photo taken in the South Pacific nation of Tuvalu (8°S, 179°E), showing Amatuku Atoll and the abandoned causeway. PHOTO SOURCE
In the Sierra Magazine article the author described the terrifying effects of “global warming” on Amatuku Atoll, site of the Tuvalu Maritime Training Institute:
To explain global warming in stark detail, all Tito Tapungao has to do is show a visitor around the grounds of his school. Dressed in his sailor’s pressed whites, the chief executive officer of the Tuvalu Maritime Training Institute points out a small brick cabin built by missionaries in 1903. Now, a century later, annual high tides rise halfway up the bedposts.
YIKES! Be very afraid. So what is the irony in the new study?
Well, I’ll get to that. But first, a bit of history. The Sierra Magazine article was what impelled me to write my 2004 paper (Word Doc) on Tuvalu. I read that article, and my urban legend detector started ringing like crazy. Consider: the missionaries’ cabin was likely built a metre or so above high tide. Add another half metre for the floor, and a half metre to get “halfway up the bedposts” … no way, I thought, that the sea level has risen two metres in Tuvalu.
Upon further investigation, I found out that the answer was already known, because geologists had studied (pdf) the area. They found the changes in the shape of Amatuku Atoll were a result of changing currents from major alterations made in the reef during World War Two. A channel was cut from the lagoon to Amatuku, and a causeway was constructed between Amatuku and nearby Malitefale Atoll. Fill to make the causeway came from “borrow pits”, holes dug in the reef flats to provide coral rubble for the construction. And some decades after the war, further borrow pits were dug to provide building materials for the Maritime Institute. The swimmers in the Fig. 1 are swimming in one of the old borrow pits. Here’s an aerial view of the changes:
Figure 2. Amatuku and Malitefale Atolls, Tuvalu, South Pacific. Amatuku is less than a kilometre long.
As you can see, the changes in the reef structure were quite extensive. All of these alterations in the reef changed the currents around the two atolls. And of course, as a result, the shape of the atolls changed. This change in shape is to be expected – after all, atolls are just piles of sand and rubble in the middle of a wild ocean. One of the results was the erosion (not from CO2, not from warming, not from sea level rise, but erosion from man-made changes in the reef) of the corner of the atoll where the missionaries’ cabin was located.
Over the years since I published my paper, I’ve taken a lot of heat for my claims. I’ve gotten plenty of irate emails from folks in Tuvalu and around the world, emails castigating me for suggesting that the rising sea levels won’t drown the atolls, emails impugning my ancestry, emails saying we’d soon see thousands of “climate refugees” from Tuvalu, emails proposing that I perform anatomically implausible acts of sexual auto-congress, and mostly emails saying that I was clearly wrong, that it was patently obvious that rising sea levels would inevitably drown the atolls, duh, so there.
OK, enough history. I got a pre-publication copy of the current paper under discussion from one of my secret underground (underwater?) sources, my thanks to WS. The abstract of the paper says (emphasis mine):
Low-lying atoll islands are widely perceived to erode in response to measured and future sea level rise. Using historical aerial photography and satellite images this study presents the first quantitative analysis of physical changes in 27 atoll islands in the central Pacific over a 19 to 61 year period. This period of analysis corresponds with instrumental records that show a rate of sea level rise of 2.0 mm.y-1 in the Pacific.
Results show that 86% of islands remained stable (43%) or increased in area (43%) over the timeframe of analysis. Largest decadal rates of increase in island area range between 0.1 to 5.6 hectares. Only 14% of study islands exhibited a net reduction in island area. Despite small net changes in area, islands exhibited larger gross changes. This was expressed as changes in the planform configuration and position of islands on reef platforms. Modes of island change included: ocean shoreline displacement toward the lagoon; lagoon shoreline progradation; and, extension of the ends of elongate islands. Collectively these adjustments represent net lagoonward migration of islands in 65% of cases.
Results contradict existing paradigms of island response and have significant implications for the consideration of island stability under ongoing sea level rise in the central Pacific. First, islands are geomorphologically persistent features on atoll reef platforms and can increase in island area despite sea level change. Second; islands are dynamic landforms that undergo a range of physical adjustments in responses to changing boundary conditions, of which sea level is just one factor. Third, erosion of island shorelines must be reconsidered in the context of physical adjustments of the entire island shoreline as erosion may be balanced by progradation on other sectors of shorelines. Results indicate that the style and magnitude of geomorphic change will vary between islands. Therefore, Island nations must place a high priority on resolving the precise styles and rates of change that will occur over the next century and reconsider the implications for adaption.
Ahhh, vindication is sweet. The authors agreed totally with what I had written in 2004. Rising sea levels don’t destroy atolls, and their shape is always changing. Exactly what I had taken so much heat for saying.
In addition to the Abstract, the Conclusions of the paper are quite interesting. Here are some extracts (emphasis mine):
The future persistence of low-lying reef islands has been the subject of considerable international concern and scientific debate. Current rates of sea level rise are widely believed to have destabilised islands promoting widespread erosion and threatening the existence of atoll nations. This study presents analysis of the physical change in 27 atoll islands located in the central Pacific Ocean over the past 20 to 60 years, a period over which instrumental records indicate an increase in sea level of the order of 2.0 mm y-1.
The results show that island area has remained largely stable or increased over the timeframe of analysis. Forty-three percent of islands increased in area by more than 3% with the largest increases of 30% on Betio (Tarawa atoll) and 28.3% on Funamanu (Funafuti atoll [the main atoll in Tuvalu – w.]). There is no evidence of large scale reduction in island area despite the upward trend in sea level. Consequently, islands have predominantly been persistent or expanded in area on atoll rims for the past 20 to 60 years.
… Results of this study contradict widespread perceptions that all reef islands are eroding in response to recent sea level rise. Importantly, the results suggest that reef islands are geomorphically resilient landforms that thus far have predominantly remained static or grown in area over the last 20 – 60 years. Given this positive trend, reef islands may not disappear from atoll rims and other coral reefs in the near-future as speculated. However, islands will undergo continued geomorphic change. Based on the evidence presented in this study it can be expected that the pace of geomorphic change may increase with future accelerated sea level rise. Results do not suggest that erosion will not occur. Indeed, as found in 15% of the islands in this study, erosion may occur on some islands. Rather, island erosion should be considered as one of a spectrum of geomorphic changes that have been highlighted in this study and which also include: lagoon shoreline progradation; island migration on reef platforms; island expansion and island extension. The specific mode and magnitude of geomorphic change is likely to vary between islands. Therefore, island nations must better understand the pace and diversity of island morphological change and consider the implications of island persistence and morphodynamics for future adaptation.
Couldn’t say it better myself … and oh, yeah, what about the irony?
Well, Amatuku, the poster child of disappearing atolls, the threatened “real-life Atlantis”, home of the disappearing missionaries’ cabin, happened to be one of the atolls considered in the study. The authors found that despite the loss of the missionaries’ cabin, Amatuku increased in area by about 5% over the nineteen year period during which it was studied … ah, the irony, it burns.