Tahitian coral dating provides evidence of rising sea levels

These results are particularly important in view of the current rise in sea levels, which is one of the most worrying manifestations of global warming. Since the beginning of the ^20th century, tide gauge records suggest a sea level rise of 1.5 to 2 mm/year. More recently, satellite altimetry observations indicate a global average rise of 3.3 mm/year over the last two decades. This increase is attributed to global warming over the past century. While the estimation of their respective contributions is still the subject of active research, it has been established that the thermal expansion of the oceans and the melting of the polar ice caps, Greenland and Antarctica, and mountain glaciers are the major causes.

Compilations of the latest IPCC report (2007) indicate that sea level rise by 2100 is likely to be between 20 and 60 cm, depending on the different greenhouse gas emission scenarios. However, as this report points out, the climate models used up to 2007 do not realistically simulate the dynamics of the polar ice caps in response to global warming. The 2007 IPCC projections are therefore only considered as lower limits of sea-level rise. The most recent simulations, based on semi-empirical statistical models, suggest a sea-level rise of between 60 and 180 cm by the year 2100. As we can see, the stakes are high, since some 3.2 billion people - half the world's population - live on a coast or within 200 km of one, and a tenth of the population today lives less than 10 meters above sea level.

One way of understanding the dynamics of ice caps, and thus improving modelling and forecasting of future sea-level variations, is to draw on geological archives that have recorded past sea-level variations. Cores drilled in coral reefs such as Tahiti and Barbados provide particularly valuable information on these variations, and therefore on the behavior of ice caps in the past. Records obtained from these archives have revealed extremely rapid rises in sea level in the past, particularly during the last deglaciation. During this period, which saw sea level rise from the -120/130 metre mark since the last glacial maximum 21,000 years ago to its current level, the rise in sea level was not constant, but was punctuated by rapid accelerations in sea level associated with massive break-up of the ice caps. The most significant of these accelerations, referred to by paleoclimatologists as Melt-Water Pulse 1A (MWP-1A), remains enigmatic in many respects (Bard et al., Nature 1990). Based on corals collected as part of an international expedition off the coast of Tahiti, the results published in the journal Nature (Deschamps et al., 2012) lift the veil on this climatic event, undoubtedly one of the most significant of the last 20 ,000 years.

Tahitian corals, markers of sea-level variations

Hermatypic corals are organisms that live exclusively in tropical waters. Highly sensitive to light and temperature, they grow flush with the water, within a very narrow depth range, making them excellent markers of sea level. This is why the study of fossil corals formed over the last few hundred thousand years makes it possible to reconstruct sea-level variations and environmental changes over time. One of the characteristics of corals that makes them a material of choice for paleoclimatologists is that they can be dated with excellent precision using the Uranium-Thorium method, which relies on the radioactive decay of the natural uranium present in the coral skeleton. For example, the dating precision obtained at CEREGE as part of this project is of the order of 30 years for samples 15 000 years old.

The IODP 310 "Tahiti Sea Level" International Expedition

In 2005, as part of the international IODP(Integrated Ocean Drilling Program) co-financed by INSU-CNRS (Institut National des Sciences de l'Univers), ECORD(European Consortium for Ocean Research Drilling; http://www.ecord.org/) carried out a drilling campaign on the slopes of the current Tahitian reefs. This expedition was prompted by work initiated in the 90s based on coring carried out by the IRD, also in Tahiti, and which was the subject of landmark publications by the CEREGE team (Bard et al., Nature, 1996; Bard et al., Science, 2010). It was on the basis of a project submitted by CEREGE as early as 1999 that the expedition was able to get underway and, in 2005, core three sites off the island of Tahiti (see also Thomas et al., Science, 2009). CEREGE researchers' work on the corals of Tahiti has received financial support from the Comer Foundation (USA), the European Science Foundation (ESF-EuroMARC), the European Community (Past4Future Project), the Collège de France and the Institut de Recherche pour le Développement.

The history of MWP-1A revealed

The results confirm the existence of MWP-1A, which was still partly controversial, but also clarify its amplitude, chronology and duration. MWP-1A began 14,650 years ago, coinciding with the start of the first warm period, the Bølling, which marked the end of deglaciation in the Northern Hemisphere. During this lecture, sea level rose by almost 16 metres in less than 350 years. The rate of sea-level rise would have reached a minimum of 40 mm/year, which compares favourably with the average rate of 10 mm/year estimated for the last deglaciation, or the 3 mm/year observed today. Using geophysical model simulations, the researchers were also able to establish that the Antarctic ice cap had made a very significant contribution to MWP 1A, shedding new light on the complex relationships between global climate, ocean circulation and sea level, and the role that this catastrophic flow of freshwater to the ocean may have played in the end of the deglaciation. This work illustrates the complex response of ice caps, particularly the Antarctic ice cap, to a major climatic disturbance.

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Article published in La Lettre du Collège de France n° 33, May 2012.