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Dynamic atolls give hope that Pacific Islands can defy sea rise

It is widely predicted that low-lying coral reef islands will drown as a result of sea-level rise, leaving their populations as environmental refugees. But new evidence now suggests that these small islands…

The tiny Pacific nation of the Marshall Islands could avoid being swamped entirely, although it will still suffer profoundly from sea-level rise. Christopher Johnson/Wikimedia Commons

It is widely predicted that low-lying coral reef islands will drown as a result of sea-level rise, leaving their populations as environmental refugees. But new evidence now suggests that these small islands will be more resilient to sea-level rise than we thought.

That is not to say that these tiny nations won’t face significant environmental challenges. Built of sand and shingle and lying just 1-3m above the current sea level, coral reef islands in the central Pacific and Indian Oceans are considered among the most vulnerable places on Earth.

The new findings suggest that, rather than being passive lumps of rock that will be swamped by rising seas and eroded by storms, the islands are dynamic structures that can move and even grow in response to changing seas.

But although the islands may survive into the future, the changes could still affect issues like fresh water and agriculture, potentially making life on these islands much more difficult than it is today.

Long-term island formation

We have closely examined how reef islands formed over the past 5,000 years in response to past changes in sea level, in a bid to find out how islands might behave in the future. In our most recent study, we show that Jabat Island in the Marshall Islands, central Pacific, was created 5,000 years ago as sea level rose to 1.5m above its present level.

Since that time, sea levels have fallen once more, leaving the island much higher relative to the current sea level. Over the coming century, future sea-level rise will simply reoccupy the levels under which the island formed. This finding is consistent with our case studies in the Great Barrier Reef and the Maldives, which show that islands can form under a range of sea-level conditions including rising, falling, and stable.

Together, these studies show that sea level alone is not the main factor that controls the formation and subsequent change of reef islands. These processes also depend on the surrounding coral reef generating sufficient sand and shingle to build islands.

Changes over the past century

We also looked at how islands have physically changed over the past 60 to 100 years. Our study sites are in regions of the Pacific Ocean where sea level has been rising at more than 2mm per year for the past five decades.

Using comparisons of historical maps, aerial photographs and satellite images, we have been able to test the hypothesis that central Pacific Islands have begun to erode away in response to this sea-level rise.

One example is the reef islands in Funafuti Atoll, Tuvalu, in the central Pacific. Our study found that most of these islands either remained stable in size or grew larger over the past few decades, in spite of rising sea levels.

Changes in vegetated shoreline on Tepuka Island, Funafuti Atoll, Tuvalu 1896-2005.

Another of our studies found that islands in Nadikdik Atoll, Marshall Islands, have been rebuilt over the past century despite being destroyed by a typhoon in 1905. All of this shows that reef islands are able to grow under current climate conditions.

Dynamic islands

This suggests that coral islands are very dynamic landforms that adjust their shape and position on reef surfaces over decades. Low-lying islands are built by the action of waves and currents, which deposit sand and gravel at the shoreline. Just like any beach, as wave and current processes change, island sand and shingle is mobilised and deposited elsewhere on the shoreline. Through this ongoing process islands can change their shape and migrate across reef surfaces.

We are now aiming to work out the scale and speed of these changes – which will be crucial for helping island communities to adapt to the rising seas. One question is whether islands can build vertically to keep pace with rising sea levels.

Our results suggest that islands can grow upwards when waves wash over them during storms or tsunami, depositing sand in the process. This suggests that islands may be able to withstand rising sea levels and increased storminess – although life on those islands may be very different to today.

What does this mean for small island nations?

On the face of it, this is potentially good news for Pacific communities. The islands they call home may be less vulnerable than is commonly thought.

But our findings also suggest that although the islands may not be swamped by rising seas, they are likely to change in size and shift their position on the surface of reefs. The rate of these changes may also increase as sea level rises.

This raises questions for their ongoing habitation. How will physical changes to the islands affect drinking water supplies, and how will communities need to adapt their farming practices? Questions about island change must be addressed urgently in order to inform decision making and secure the future of Pacific nations.

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24 Comments sorted by

  1. Barry Woods

    logged in via Twitter

    New evidence....?

    http://www.darwinproject.ac.uk/darwin-coral-reefs

    http://darwin-online.org.uk/EditorialIntroductions/Chancellor_CoralReefs.html

    BBC - Low-lying Pacific islands 'growing not sinking' - 2010
    http://www.bbc.co.uk/news/10222679

    A new geological study has shown that many low-lying Pacific islands are growing, not sinking.

    The islands of Tuvalu, Kiribati and the Federated States of Micronesia are among those which have grown, because of coral debris and sediment.

    One…

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    1. Mike Hansen

      Mr.

      In reply to Barry Woods

      Difficult to know what your point is.

      Paul Kench, the author of this article is also co-author of the 2010 New Scientist article cited in that BBC article.

      And from the current article
      "But although the islands may survive into the future, the changes could still affect issues like fresh water and agriculture, potentially making life on these islands much more difficult than it is today."

      From the 2010 article
      "But although these islands might not be submerged under the waves in the short-term, it does not mean they will be inhabitable in the long-term, and the scientists believe further rises in sea levels pose a significant danger to the livelihoods of people living in Tuvalu, Kiribati and the Federated States of Micronesia. One scientist in Kiribati said that people should not be lulled into thinking that inundation and coastal erosion were not a major threat."

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    2. David Arthur

      resistance gnome

      In reply to Barry Woods

      I wonder if changes to sea temperatures and acidity will have an effect on coral growth rates?

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  2. Peter Boyd Lane

    geologist

    Sea levels were 1.5m higher 5000 years ago than they are today? While not challenging this, this is interesting and, while far from expert in this field, news to me. What is the evidence for this and what was the cause?

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    1. Steve Hindle

      logged in via email @bigpond.com

      In reply to Peter Boyd Lane

      That surprised me too although by this graph it looks to be in the ball park. It is also interesting that you could have walked from PNG to Tassie around 18000 yrs ago. The average sea level is very much a moving target and I guess all those thousands of atolls sitting level just above the high water mark could never have been a coincidence. The article does a good job explaining how they keep up with changing sea levels. The question is, how quickly can they keep up if climate change moves the sea levels more rapidly?
      http://upload.wikimedia.org/wikipedia/commons/c/c0/Sea_level_temp_140ky.gif

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    2. Peter Boyd Lane

      geologist

      In reply to Steve Hindle

      it's not that sea levels have changed, they have, and quite dramatically, but all the data I have seen show sea levels some 120m lower than to today 18,000 years ago (peak of northern hemisphere glaciation), then rising at around 1m/century to about 6000 years ago, from which time they have risen very slowly. I have not seen anything to suggest that 5000 yrs ago they were higher than today.

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    3. Mike Hansen

      Mr.

      In reply to Peter Boyd Lane

      Peter

      An explanation can be found here.
      http://www.skepticalscience.com/Sea-Level-Isnt-Level-Ocean-Siphoning-Levered-Continents-and-the-Holocene-Sea-Level-Highstand.html

      Here is my partial summary but a full read of the article is probably required
      1. the influx (mass) of melt water from the ice age deglaciation caused the ocean floors to sink at the same time that the melt water was raising sea levels.
      2. the melt-water influx was the dominant factor causing sea levels to rise
      3. ~5000…

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    4. Ian Rudd
      Ian Rudd is a Friend of The Conversation.

      Retired accountant & unapologetic dissident

      In reply to Steve Hindle

      "The question is, how quickly can they keep up if climate change moves the sea levels more rapidly?"

      And also how some will respond to increased sea water acidity that is likely to inhibit the formation of coral reefs if not lead to their destruction.

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    5. Steve Hindle

      logged in via email @bigpond.com

      In reply to Ian Rudd

      That's a good question Ian. I was very surprised at how quickly the shape of Tepuka Island in the article is changing, it is a very dynamic process. How quickly the vertical change can be is another question.
      Water acidity gets less news time than global warming, but some of the articles on the subject make it sound like it is a bigger problem than the actual warming.

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    6. David Arthur

      resistance gnome

      In reply to Peter Boyd Lane

      Sea levels were 1.5 m higher on one particular part of the globe? For all we know, they were lower elsewhere.

      Factors that might account for this could include
      1) ocean height is affected by how the water sloshes around as the planet moves in space, as well as
      2) how the crust deforms in response to varying ice distribution.

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    7. takver takvera

      Journalist and Editor at Indymedia

      In reply to Ian Rudd

      Coral bleaching due to warming ocean temperatures is probably a greater impact in tropical waters than ocean acidification. Colder waters absorb CO2 more readily causing greater ocean acidification in polar waters. But both processes reduce coral reef resilience. Most coral reefs won't survive 2C of global warming (Frieler et al 2012) http://www.nature.com/nclimate/journal/v3/n2/full/nclimate1674.html

      Interesting to ponder what will be the physical impact on coral atolls ability to change shape and grow once coral reef ecosystems are thoroughly degraded. I suspect the capacity for atolls to change with varying sea levels is dependant on both mechanical and biological processes.

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  3. Paul Prociv

    ex medical academic; botanical engineer at University of Queensland

    So, what’r really new here? Charles Darwin had it all worked out about 180 years ago (see links in Barry Woods' comment, at top). The Pacific (and to a lesser extent, Indian) Ocean is spattered with atolls at all levels of development and decay. Some have risen well out of the water to become high, dry limestone islands(makateas), while others are still sinking, and so consist of just broken rings of reef. A major factor not even mentioned in this article is changing ocean floor levels – sitting…

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    1. Mike Hansen

      Mr.

      In reply to Paul Prociv

      That is a rather unfair comment Paul. Darwin did not have it **all** worked out 180 years ago. His theory of evolution does not mean that scientists should not study or research biology or botany.

      And your claim that "I'd say it reflects political agitation more than science" could equally apply to your comment.

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    2. Paul Prociv

      ex medical academic; botanical engineer at University of Queensland

      In reply to Mike Hansen

      Sorry, Mike, you misunderstood my meaning. It was Darwin's explanation of coral reef formation, not his ideas on speciation, that I was referring to. The article above says no more than did Darwin, and fails completely to mention ocean floor movements, considering only changes in sea level, which are something else. In Tuvalu, as well as the Maldives, political leaders are aggressively demanding compensation from the rest of the world for occasional flooding of their atolls, on this assumption…

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    3. Paul Kench

      Professor, School of Environment at University of Auckland

      In reply to Paul Prociv

      There is a distinct difference between the growth of atoll structure and the formation and ongoing change of islands situated on top of the coral reef platforms. Darwin was primarily concerned with the growth and distribution of reef structure. As you point out this theory was proven to be correct. However, Darwin had remarkably little to say about reef islands - the accumulations of sand and gravel depositied by waves and currents on top of reef structures. Indeed he misinterpreted island shoreline evidence in the Cocos (Keeling) Islands to infer subsidence. In fact all he obeserved was an ever changing shoreline. What is new here is the first evidence from the Pacific Ocean of reef islands forming under rising sea level conditions. We also add to our recent dataset depicting the decadal scale dynmaics of reef islands that show they are highly changeable features on reef surfaces.

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    4. Paul Prociv

      ex medical academic; botanical engineer at University of Queensland

      In reply to Paul Kench

      Thanks for that explanation, Paul -- much appreciated. But are you finally able to put my mind at ease about that rising/falling tectonic plate business? Is Tuvalu typical of all the world's atolls, in that it seems to be flooding more often or more seriously, or are they all being similarly affected? Are any others actually rising, i.e. experiencing FALLING see levels? BTW, I bear no grudges against Tuvaluans. About 40 years ago I worked with a great bunch of them (Ellice Islanders then) on Ocean island (Banaba), and was adopted into one of their island groups -- they are wonderful people.

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    5. Rob Painting

      logged in via Facebook

      In reply to Paul Kench

      Paul Kench - does anyone live on Tepuka Island? There doesn't appear to be much sign of habitation.

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    6. Rob Painting

      logged in via Facebook

      In reply to Paul Prociv

      Paul Prociv - the Tuvalu flooding episodes you may have seen on TV are generally related to astronomical factors ('king tides') and/or low pressure associated with the passage of tropical storms (lower atmospheric pressure means less atmospheric mass' pushing down' on the ocean, and thus the water level rises temporarily).

      It would have preferable if these reports did not neglect such important details, however rising sea level combined with the ongoing global die-off of coral reefs does not bode well for atoll islanders, such as Tuvalu.

      On the subsidence issue, <a href="http://www.skepticalscience.com/Tuvalu-sea-level-rise.htm">sea level at Tuvalu has been rising at around three times the global average</a> - between 1950-2009 sea level at Tuvalu rose at the rate of 5.1 (±0.7) mm per year. 10% of that is due to subsidence.

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  4. Doug Hutcheson

    Poet

    Atolls may be dynamic, in that they may redistribute their existing payloads of sand and gravel, but how will they cope with the need to grow vertically beyond the limits of redistribution? Under global warming conditions of increased acidity, surely reef-building organisms will be disadvantaged?

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    1. Paul Kench

      Professor, School of Environment at University of Auckland

      In reply to Doug Hutcheson

      Good question. We have documented island surfaces building vertically by 10-20 cm and in some cases more. How this occurs is through storm waves and tsunami waves, which can wash across the surface of low-lying islands. As they do so, they carry sand and gravel from the neighbouring reef and beach and deposit it on the island surface.

      The increased ocean acidity is also an interesting question and one which little work has been done. Some species may be impacted more than others. What we might see is a change in the dominant carbonate producing organisms that supply sand and gravel to islands.

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  5. Timothy Sorenson

    Math Professor

    @PaulK
    Upon reading "The Structure and Distribution of Coral Reefs" by Mr. Darwin, I don't see how you can conclude that 'what is new here...." It seems the only difference is Darwin, perhaps thought the islands were sinking and the coral rising to match this or as we see it now the rising sea. But he was well aware of the changing dynamic and described a natural progression from volcanoe to ring atoll. Don't want to say the study is bad or wrong, but it was significantly predated and almost every aspect including the changing dynamic and response to relative water height to the island whether to sinking land or rising sea.

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    1. Paul Kench

      Professor, School of Environment at University of Auckland

      In reply to Timothy Sorenson

      Reef islands and the structure of atolls are different geological entities. Darwin was primarily concerened with how coral reef structures evolved and as you note he concluded this was caused by the interplay between subsidence and the ability of coral to stay within the photic zone. We say very little about the long-term structural development of reefs.

      The work we have been progressing over the past decade is to resolve the evolution of islands deposited on top of the coral reefs. In particular…

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