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Sea Level

Cautionary Note:

The instrumental record of sea level in the Chagos (Diego Garcia tide gauge, and Satellite Altimetry) is both highly variable and very short in comparison to the factors which influence sea level on decadal or longer timescales. It is unrealistic to suppose that we can derive accurate rates of long-term sea-level rise from these records and thus be able to say that sea level is increasing by XX mm per year and will continue to do so. These studies therefore represent only a snapshot in time and are the best information that is available to us. A rate of sea-level rise this decade may well be quite different to that in the past or in the future. Nor is it simply the case that sea-level rise can be assumed to be increasing or accelerating over time.

"For fools rush in where angels fear to tread". Alexander Pope - An Essay on Criticism (1709). 

Expert study of sea-level rise in the Chagos Archipelago concludes human habitation remains feasible, 40 years after islanders were exiled. See 2015 - March update for the latest figures

Absurd claims by Charles Sheppard and the Chagos Conservation Trust

Those who attended the Chagos Regagné at the Royal Geographical Society in May 2011 would have heard Dr John Turner telling us during his presentation, that sea level in the Chagos was rising at a rate of 12 mm yr-1. You would also have seen an ‘expert’ review of Chagos science, circulated at the same time, led by Professor Charles Sheppard saying the same thing.** This is an astonishing rate which would see these low-lying islands inundated in less than 50 years time. It is also nearly four times the rate of global mean sea-level rise measured by the major expert research groups.

The inevitable consequence of this alarming news would be that there is very little point in contemplating resettlement anywhere in the Chagos, now or ever. It would thus be a clear vindication of a ‘prudent’ anti-resettlement policy pursued by the FCO ever since the findings of a Study in 2002 where consultants reported that rising sea levels of 5 mm yr-1 were one reason making resettlement precarious and costly. Moreover, members of the Chagos Conservation Trust will doubtless have seen the ‘CCT factsheet’ on “Sea-level change and shore erosion” Fact Sheet, with its equally disconcerting story of sea-level rise and its erosive effect on the islands.

The True Situation

Thankfully the truth is nowhere near as dire. Sea level in the Chagos is rising more slowly than in many other parts of the world’s oceans. In fact, the data from a tide gauge at Diego Garcia and from satellite measurements of the ocean height, which between them span the past 23 years show a ‘best estimate’ of 2.2 mm yr-1 but because of the variability we cannot confirm this as a ‘true’ rate. Further north at Peros Banhos and Salomon, sea-level rise is even smaller and virtually undetectable. The reason behind this is a complex interaction of oceanic and atmospheric processes across the Indian Ocean.

These are the new findings of an expert study published in the journal ‘Global and Planetary Change’ (Vol 82-3:25-37), entitled “Contemporary sea level in the Chagos Archipelago, central Indian Ocean”. The authors, Richard Dunne, Susana Barbosa, and Philip Woodworth present the first comprehensive analysis of sea level, storms and cyclones, waves and wind, and the past effects of tsunamis on the islands of the Chagos Archipelago. Their principle findings are that:

  • the dominant feature of sea-level variability in the Chagos Archipelago is one of large variability from year to year, driven it would seem by a process called the Indian Ocean Dipole, which is an oceanic and atmospheric interaction similar to El Nino.
  • there is no evidence of any established sea-level trend either from tide gauges on the island of Diego Garcia (1988-2000 and 2003-2011) or in the satellite altimetry record (1993-2011) for the sea area surrounding the Chagos between 70-74o E and 4-9o (an area of 250,000 km2).
  • ocean models suggest that any longer term sea-level trend since the middle of the 20th century will have been very small.
  • there is no evidence of subsidence in the islands (which would contribute to an apparent ‘rise’ in sea level), and on Diego Garcia there has even been a small crustal uplift of about 0.5 mm yr-1 between 1996-2009.
  • the Chagos Archipelago experiences relatively low wind speeds and low levels of storminess, lying outside the Indian Ocean cyclone belt. The last known cyclone to pass over the islands was in 1891. Similarly, there is no evidence of long term changes in the wave and wind fields in this area, which might lead to flooding or overtopping of the islands.
  • the ‘apparent’ erosion of the islands is most likely the effect of natural seasonal changes in wind/wave direction which has previously been documented in the Maldives to the north. The lack of any detailed studies in the Chagos throughout the year may have led to the observed erosion being misinterpreted as being due to sea-level rise.
  • the findings of earlier studies in 2003 (Chagos Conservation Management Plan) and in 2006 used incorrect statistical methods and gave an erroneous impression of the magnitude and significance of sea-level rise in this area.

The authors concluded that “Collectively, these results suggest that this has been a relatively stable physical environment, and that these low-lying coral islands should continue to be able to support human habitation, as they have done for much of the last 200 years.”

The Foreign Secretary, William Hague, and the FCO, have been told of these findings and that the 2002 Resettlement Feasibility Study, and the Government’s evidence to the House of Lords in 2008, that resettlement was unfeasible because of sea-level rise, was wrong.

Whilst the future is always uncertain and much has been written about climate change and sea levels rising throughout the world, with predictions of the collapse of the Greenland and Antarctic ice sheets, we should always carefully examine the facts. Alarm, misinformation and errors have all contributed to further exclusion of the Chagossians from their islands since the historic restoration of the right of return by the High Court in 2000. Indeed the judges in the House of Lords in 2008 might even have ruled differently had the present study been before them.

Full text of the paper:      Contemporary sea level in the Chagos Archipelago - Global Planet Change.pdf  

The published version is available on the Global & Planetary Change website at: Global & Planetary Change - Dunne et al 2012 - doi:10.1016/j.gloplacha.2011.11.009

** [see page 14 of draft of 'Review' of Chagos science:  Sheppard et al. This later increased to 15.6 mm yr-1 when it appeared in a published paper Sheppard et al 2012 Aquatic Conserv: Mar Freshw Ecosys 

Following publication, this article and letters were published in New Scientist: 
[these extracts reproduced under permissible syndication reuse].

Chagos islands in sea-level rise controversy

23 November 2011 - by Fred Pearce

SEA-LEVEL rise is bad news for many island nations, but it may not be severe enough to prevent Chagos islanders from returning home four decades after they were expelled by UK authorities.

In the late 1960s, the 1000 or so inhabitants of the Chagos islands - a British overseas territory - were forced out to make way for American military. The UK has resisted all demands made in its courts by the Chagossians to be allowed to return to the outer islands of the archipelago.

Central to the British refusal has been the claim that the coral islands would be uninhabitable within decades because of rising sea levels due to climate change.

A management plan for the islands, written in 2003 by biologist Charles Sheppard at the University of Warwick, UK, said annual sea-level rise since a tide gauge was installed in 1988 had averaged 5.4 millimetres a year - twice the global average. He added that the figure was accelerating, and earlier this year at an event at the Royal Geographical Society in London, he revised the annual figure to 12.0 millimetres. 

Definitely wrong

Philip Woodworth, a researcher at the UK's National Oceanography Centre in Liverpool, says these figures are "definitely wrong". He agrees that high year-on-year variability, created by ocean-current fluctuations, means there are legitimate arguments about the precise underlying rise. But his best estimate, based on an analysis of the same data and to be published in Global and Planetary Change, is that sea level has been rising by just 2.2 millimetres a year since 1988.

Sheppard dismisses the new findings, but the issue will no doubt come up again when the two researchers attend future meetings to discuss the islands. Whatever the true measure, it should "certainly not be used for extrapolation" of future sea levels, says Woodworth.

This is what the British government has being doing, though. Since publishing a study in 2002 on the feasibility of repatriating the Chagossians - who currently live in Mauritius, the Sychelles and the UK - ministers have repeatedly cited such extrapolations as a "clear and compelling" reason why resettlement is "unfeasible". 

Chagos islands

7 December 2011 - Charles Sheppard, University of Warwick 

May I clarify some points in your article about sea levels, island erosion and flooding in the Chagos islands (26 November, p 4).

It was reported that I dismiss new findings referring to the 2.2-millimetre rise per year in sea level indicated by satellite altimetry. I don't. We have known about this for years and I am sure those measurements are right.

But this central Indian Ocean sea-level value clearly isn't as important as other things causing the observed erosion of Chagos island shores and seawater flooding.

First, the area is mildly tectonic and land subsidence probably contributes. Secondly, the massive coral mortality from warming in 1998 severely damaged the breakwater effect of protective reefs in several Indian Ocean islands including, it seems clear, the Chagos islands. Along with other factors, this means flooding occurs increasingly at high tides.

A healthy coral reef at stable elevation would cope with the sea-level rise indicated by the satellite measurements. It would be nice if only oceanic sea-level rise counted. As noted, it is flooding that is important, and that is happening.

Coral chaos

4 January 2012 - Richard Dunne

Charles Sheppard's letter on erosion and flooding in the Chagos Islands in the Indian Ocean (10 December, p 33) highlights the need for good science in these low-lying, remote coral atolls. Hopefully the paper I co-authored in Global and Planetary Change (DOI: 10.1016/j.gloplacha.2011.11.009), which sparked the story is a contribution to that goal.

The work is not solely about satellite sea level. It examines sea level from tide gauges and satellites, vertical land movement, ocean modelling, storms, wind and waves, tsunamis, and the occurrence of extreme events in order to evaluate the dangers faced. We conclude that over the past few decades the Chagos Islands have been a relatively stable physical environment, with no evidence of any increases in those factors which might contribute to erosion and flooding.

Low-lying coral atolls are highly dynamic environments. They are largely the product of the very forces that erode and reform them - storms and extreme high tides. Indeed, zoologist Gilbert Bourne described these processes on Diego Garcia, part of the Chagos archipelago, over 130 years ago.

 There is an urgent need for an expanded science programme to investigate the issues across a wide range of disciplines, including factors such as these, rather than relying on anecdotal reports. Ironically, the modest amount of UK government funding that had been available in the past from the Overseas Territories Environment Programme has now evaporated, despite the fact that the Chagos is a Marine Protected Area of great importance to the Indian Ocean.