Exceptional rise in past sea levels revealed


Since the end of the last ice age 21,000 years ago, our planet has seen ocean levels rise by 120 meters to reach their current levels. This increase has not been constant, rather punctuated by rapid accelerations, linked to massive outburst floods from the ice caps. 

New research has shed new light on the complex relationship between climate, ocean circulation and sea levels [Credit: © IRD/J. Orempuller, Univ. Colorado/A. Jennings]

The largest increase, known by paleoclimatologists as ‘Melt-Water Pulse 1A’, proved to be enigmatic in many respects. A study recently published in Nature by a team from the CEREGE laboratory in collaboration with the universities of Tokyo and Oxford has revealed the mysteries of this event, without doubt one of the most important in the last deglaciation. 

A spectacular rise 

The research has primarily confirmed the existence of this exceptional event, which had been controversial in some regards. Its chronology, amplitude and duration have now been defined. It began precisely 14,650 years ago and coincides with the start of the warm period known as the ‘Bølling oscillation’, which marked the end of the ice age. The rise in sea levels at that time was an average of 14m worldwide, over less than 350 years. This corresponds to a rate of 40mm per year — compared to the 3mm per year we are currently experiencing. 

Coral: a climate archive 

To describe this remarkable event, researchers analysed cores taken from the coral reef surrounding Tahiti, Polynesia, during the international IODP 310 ‘Tahiti Sea Level’ expedition. The corals that built these reefs and atolls are excellent indicators of sea level variation and also provide a virtual archive of previous climates. 

Using reconstructions of sea levels from the fossilised corals as well as geophysical simulations, scientists have been able to identify the source of this accelerated rise in sea levels. They have demonstrated that the Antarctic ice cap was responsible for up to 50% of these increases. Experts had previously believed that only melting ice from the Northern hemisphere had contributed to Melt-Water Pulse 1A, particularly the Laurentide ice cap that covered a large part of North America. 

To demonstrate the mechanisms occurring, the research team compared the rise in water levels in Polynesia with those observed during a previous research expedition to Barbados, in the Caribbean. According to the previously accepted hypothesis, the amplitude of the increase in sea levels must have been twice as large in Tahiti than in Barbados, which was closer to the Laurentide ice cap. And yet, the study showed that the rise was equivalent in these two locations, implying a highly significant role played by the Antarctic ice cap. 

A modified global climate 

These studies have shed new light on the complex relationship between climate, ocean circulation and sea levels. By demonstrating the synchronicity between Melt Water Pulse 1A and the warming of the Bølling oscillation, they have specifically demonstrated the part played by the massive oceanic inflow of fresh water in planetary deglaciation. This almost certainly caused major disturbances to thermohaline circulation in the global ocean, which itself had a discernable impact on the global climate. This study also demonstrates the complex reaction of the ice caps to a major climactic disturbance, particularly for the potential instability of the Antarctic ice cap. 

From past to present… 

These results are highly important with regard to the current rise in sea levels, which is one of the most worrying effects of global warming dating from the start of the industrial era. In the past century, marigraphic data have suggested that ocean levels have risen by between 1.5 and 2mm per year. More recently, altimetry satellites have demonstrate an average global rise of 3.3mm per year over the past two decades. Although the estimates of their respective contributions are still being studied, it is clear that the thermal expansion of the oceans, in addition to melting polar ice caps and mountain glaciers, are the major causes. 

… to the future 

The most recent IPCC report, dating from 2007, indicates that the rise in sea levels by 2100 will be between 20 and 60cm, depending on the different greenhouse gas emission models being used. However, the models used until 2007 did not provide the realistic simulations of the dynamic response of the polar ice caps to global warming used by the present study. The 2007 predictions from the IPCC will no doubt be exceeded: more recent simulations suggest an increase in sea levels of between 60 and 180cm by 2100. 

The risks are high: more than three billion people, or half of the world’s population, live on the coast or less that 200km from a coastline, and one tenth of the population currently lives less than 10m above sea level. 

Authors: Pierre Deschamps and Gaëlle Courcoux | Source: Institut de Recherche pour le Développement (IRD) [March 2012]