Scientific news Actualidad cientifica
N° 386
October 2011
The village of Lataw in the
Torres Islands, is subjected more and more to sea incursions. In 2004 this small community of Vanuatu in the middle of the South
Pacific had to move several hundred metres back from the shore. The United
Nations denoted its
70 inhabitants as possibly the first climate refugees in
History. Are they innocent victims of global warming?
Not entirely. IRD scientists and their research partners 1 recently described in the journal PNAS how this group of islands is sinking into the ocean at a rate of about one centimetre per year. The Vanuatu islands are sitting on the boundary of the Pacific tectonic plate, overlapping the
Indo-Australian plate which is plunging underneath it, dragging its base along with the islands placed above.
These motions lead an apparent rise in sea-level twice as rapid as expected.
Lataw has not been displaced to a suitable site.
The Torres islanders, but also communities of other islands of Vanuatu, will have to go back and live in the hills, as their ancestors had done. The research conducted will help the local authorities take better decisions for their people’s future.
Rising water over the Torres Islands has flooded coconut palm plantations.
On December 2005 the eyes of the world were fixed on the village of Lataw on the Torres Islands, in the north of the Vanuatu archipelago at large in the South Pacific. The United Nations had declared the village’s inhabitants to be the Earth’s first climate refugees. The coconut palm plantations were flooded and housing was threatened.
Between 2002 and 2004, with the support of the
Vanuatu government and Canadian aid, the village was displaced several hundred metres to escape the rising water. International bodies interpret this rise as an effect of global warming, which melts the ice caps and dilates the oceans’ surface waters. However, another phenomenon superimposes its influence on the global increase in sea levels. This is island subsidence.
Sciences, that this subsidence 2 almost doubled the apparent rise in sea level over the Torres
Islands. The research team took their first measurements on this small remote piece of the Earth in 1997, initially to assess the consequences of a strong earthquake which had struck that year.
They returned in 2009, not long before a new seismic event stronger than 7 magnitude. The two sets of data enabled them to evaluate the change between the two earthquakes of the effective sea level on the one hand –measured by satellite altimetry– and of the islands’ subsidence on the other –measured by precise GPS (Global Positioning System) readings. In 12 years, while the water level increased by about 15 cm, the Torres
Islands sank by 12 cm. The sea had risen by a about 27 cm on the coast of Torres between the two shocks.
Islands sinking 1 cm per year
IRD scientists and their research par tners 1 recently showed, in an article of the journal The
Proceedings of the National Academy of
Plate tectonics implicated
This small chain of islands reaches 200 m in

places and spreads over some 40 km, within the circum-Pacific seismic belt or “Ring of Fire” –the string of volcanic arcs stretching from New
Zealand all the way round to Tierra del Fuego, at the southern tip of the American continent.
Thethe Torres Islands are hence placed on the border of the Pacific tectonic plate, very close to the 8000 m deep New Hebrides ocean trench. At that point, the continental Indo-Australian Plate plunges beneath the oceanic plate at a convergence rate of about 7 cm per year. This subduction process 3 leads the continental plate to drag with it the base of the overlapping plate, triggering the subsidence of the islands situated above, which sink further into the ocean.
Earthquakes change the game
The seismic cycle alters the relative sea level over the islands. Earthquakes induce sudden vertical movements of the Earth’s crust. These co-seismic motions partly compensate for the accumulated deformation in the opposite direction between the two events. Sometimes the islands subside. This was particularly the case in
1997, when they dropped abruptly down one metre. However, most often, when the lithosphere 4 breaks under the shock of the tremor, they rise up again by some tens of centimetres in a rebound effect. After the 2009 event for example, they lifted up 20 cm. Although the islands subsided between the two earthquakes – as the geodesic measurements showed– on the geologic time scale the islands are being uplifted, as the coral reefs that have now emerged testify.
Forebears lived in the hills
Although global warming is contributing to the apparent rise in sea level on the Torres Islands, it does not take the dominant role with which it has been attributed. The sea level is rising much more quickly than government authorities’ predictions, based solely on the global rise in the level of the oceans. An error of interpretation which led the local authorities to remove Lataw to an inappropriate site. Even though the village was displaced several hundred metres, the new settlement has not gained enough in altitude to decrease the community’s vulnerability in the long term.
Most of the population of Torres, but also of the other isles of Vanuatu, lives on the narrow coastal plain. Communities will have to leave the littoral zones most exposed to sea-level variations to settle in the higher areas of these small steep-sided islands, where their ancestors lived still just a century ago. These investigations offer governments a way of understanding environmental changes better and should subsequently help them to take better decisions for their people’s future.
Copy editor – Gaëlle Courcoux - DIC, IRD
Translation – Nicholas FLAY
FOR FURTHER INFORMATION
Contacts
Valérie BALLU,
CNRS researcher hosted by the
IRD for these investigations
Tel.: +33 (0)1 57 27 84 78 ballu@ipgp.fr
Address
Institut de Physique du Globe de Paris
Case 89, Tour 26-16, 3ème étage
4 Place Jussieu
75252 Paris cedex 05
Bernard PELLETIER, director of research at the IRD
Tel.: (687) 26 07 72 bernard.pelletier@ird.fr
UMR 82 Géoazur (IRD / Université de
Nice Sophia Antipolis / CNRS /
Observatoire de la Côte d’Azur)
Address
IRD Nouméa
BP A5
98848 Nouméa cedex
Nouvelle-Calédonie
References
B allu
V alérie
, B ouin
M. N.
, S iméoni
P .,
C rawford
W ayne
, C almant
S téphane
,
B oré
J ean
-M ichel
, K anas
T .,
P elletier
B ernard
.
Comparing the role of absolute sea-level rise and vertical tectonic motions in coastal flooding, Torres Islands
(Vanuatu). PNAS, 2011, 108 (32), p.13019-
13022. doi:10.1073/pnas.1102842108
B ecker
M., M eyssignac
B., L etetreL
c., L
LoveL
W., c azenave
a., d elcroix
t hierry
. Sea
Level Variations at Tropical Pacific Islands since 1950, Global and Planetary Change,
2011. doi:10.1016/j.gloplacha.2011.09.004
Key words
Climate refugees, sea level, Vanuatu
1. This research work was conducted in partnership with scientists from the Institut de Physique du Globe in Paris, the Centre
National de Recherches Météorologiques, Brest, Géo-consulte Survey service and the Vanuatu Land Survey Department.
2. Slow sinking of the Earth’s crust.
3. Subduction is the process whereby one tectonic plate slides under another.
4. Literally “stone sphere”, the lithosphere is the Earth’s rigid outermost shell.
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Gaëlle Courcoux
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View the IRD photos concerning this article, copyright free for the press, on: www.indigo.ird.fr
Graphic design and layout
Laurent CORSINI
The village’s new site (left and centre) is still too close to the coast. Communities of these small islands will have to take steps to resist the natural hazards around
Vanuatu (right, volcanic measurements), such as living in the hills as their ancestors’ did.
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