The disproportionate impact of icebergs in glacial-interglacial cycles

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The latest advances in paleoclimatology show the major role played by icebergs in the transitions between interglacial and glacial periods. Indeed, their impact on ocean circulation would act as an astronomical signal amplifier. The results were recently published in the scientific journal Nature.

The alternation of glacial and interglacial periods is a key feature of the climate of the last millions of years and an active field of research in paleoclimatology. Remember that these are the astronomical parameters – known as by Milanković – which bring glacial-interglacial alternations. However, the variations associated with these parameters are remarkably small with regard to the climatic changes caused on Earth. Also, amplifying processes internal to the climatic machine must necessarily intervene to explain the amplitude of the fluctuations.

Icebergs, players in glacial-interglacial cycles

The amplifying phenomena involved are multiple and heterogeneous. Indeed, they involve the ocean as well as the biosphere, the cryosphere or the geosphere. Moreover, not all are known and assessing the respective contribution of each of them remains a difficult subject. Consequently, if the general picture of glacial-interglacial cycles is relatively well understood, the modalities of the physical progression are still subject to controversy.

Only 10% of an iceberg emerges from the surface of the sea. Credits: iStock.

A team of researchers now explains that Antarctic icebergs probably play a major role in amplifying the astronomical signal. More precisely, when the parameters of Milankovic decrease in the amount of solar energy arriving near the pole, icebergs are melting at lower and lower latitudes. The fresh water produced by the melting gradually invades the other oceans, including the Atlantic.

However, the latter houses the heart of the thermohaline circulation. The arrival of an abnormally soft layer of water – coupled with a salinization of the southern ocean – then triggers profound changes in ocean currents. Result: the capture of carbon by the ocean is amplified, which significantly reduces the greenhouse effect and global temperature. This series of mechanisms is articulated in a few thousand years, a scale typical of the cycles studied.

Global warming makes its circus

“We are amazed to have discovered that this teleconnection is present in each of the different ice ages of the 1.6 million years”, notes Aidan Starr, lead author of the paper published in Nature on January 13th. “This indicates that the Antarctic Ocean plays a major role in the global climate, which scientists have long felt, but which we have now clearly demonstrated “.

icebergs
Historical trajectories of Antarctic icebergs between 1978 and 2019. Iceberg A-68 is in orange, the last image dating from July 2020. Credits: NASA.

These innovative results were obtained using multivariate analysis of marine sediments. The researchers notably measured the quantity of rock fragments deposited on the ocean floor. A Proxy which makes it possible to assess the size and the areas where icebergs are melting. These cores were taken off the coast of South Africa and provide a view of the past 1.6 million years. Associated with these measures, the authors resorted to climate simulations which confirm the advanced hypothesis. They also suggest that the current warming threatens to make the Southern Ocean too warm to allow for the next glaciation.

“Last year, during an expedition aboard the Hesperides, the Spanish navy’s research vessel, we observed the huge A-68 iceberg which had just broken into several pieces next to the islands. from South Georgia ‘ relates Jiménez Espejo, co-author of the study. “Warming could cause a change in the trajectories and melting patterns of these large icebergs in the future, affecting the currents and, therefore, our climate and the validity of the models that scientists use to predict it ”.

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