Warmer waters in North Atlantic melted Antarctic ice sheets

"The last interglacial is an interesting time to study the behaviour of the climate system under conditions slightly warmer than today," Jean-Claude Duplessy told environmentalresearchweb. "We accumulated enough precise data to estimate the temperature of the North Atlantic Deep Water during the last interglacial."

To achieve this estimate, Duplessy and colleagues measured the oxygen isotope content of bottom-dwelling foraminifera to reveal both the temperature and seawater δ¹⁸O content present during the last interglacial period.

According to the researchers, air temperatures in the northern hemisphere were a few degrees warmer than today (most likely as a result of insolation changes), while surface waters in the North Atlantic were warmer and more saline. As a result, during winter these waters sank into the abyss in the Norwegian Sea at a temperature slightly warmer than at present and gave birth to a North Atlantic Deep Water mass about 0.5°C warmer.

"Our model experiments show that this warming was transferred to the Antarctic Circumpolar water," said Duplessy. "As this warmer water bathes the bottom of the Antarctic ice shelves at around 600 m depth, it provides heat which favoured the retreat of the grounding lines of the West Antarctic Ice Sheet and its partial melting."

This melting, together with the partial melting of Greenland, led to sea levels about 6 m higher than at present. "Glaciologists estimate that the partial melting of each ice sheet contributed to a sea level rise of 3 m," said Duplessy.

Ocean temperatures next to Antarctica's continental shelf have already increased by around 0.2°C, a warming comparable to that in the last interglacial period.

"If the current global warming continues and follows the present trends, we could reach conditions similar to those of the last interglacial period during the present century and initiate an irreversible partial melting of the West Antarctic Ice Sheet," said Duplessy. "This will obviously take some time, which our study does not allow us to estimate."

Now the team plans to analyse deep sea cores to find out about the variability of interglacial periods during the Quaternary and the link between ocean temperature and sea level. The researchers will then use coupled models of the ocean-atmosphere system forced by insolation to determine whether they can simulate the main trends of these interglacials.

The researchers reported their work in Science.