Antarctic ice-sheet destabilized within a decade

After the natural warming that followed the last Ice Age, there were repeated periods when masses of icebergs broke off from Antarctica into the Southern Ocean. A new data-model study led by the University of Bonn (Germany) now shows that it took only a decade to initiate this tipping point in the climate system, and that ice mass loss then continued for many centuries. Accompanying modeling studies suggest that today's accelerating Antarctic ice mass loss also represents such a tipping point, which could lead to irreversible and long-lasting ice retreat and global sea level rise. The study has now been published in the journal Nature Communications.

To understand what the consequences of current and future human-induced climate warming may be, it helps to take a look at the past: how did sea-level changes look like during times of natural climate warming? In a recent study, an international research team led by Dr. Michael Weber from the Institute of Geosciences at the University of Bonn investigated this question. In doing so, they focused on the Antarctic Ice Sheet as the largest remaining ice sheet on Earth.

There, they searched for evidence of icebergs that broke off the Antarctic continent, floated in the surrounding ocean and melted down in the major gateway to lower latitudes called "Iceberg Alley." In the process, the icebergs released encapsulated debris that accumulated on the ocean floor. The team took sediment cores from the deep ocean in 3.5 km water depth from the area, dated the natural climate archive and counted the ice-rafted debris.

The scientists identified eight phased with high amounts of debris which they interpret as retreat phases of the Antarctic Ice Sheet after the Last Glacial Maximum about 19,000 to 9,000 years ago, when climate warmed and Antarctica shed masses of icebergs repeatedly into the ocean. The result of the new data-model study: each such phase destabilized the ice sheet within a decade and contributed to global sea-level rise for centuries to a millennium. The subsequent re-stabilization was equally rapidly within a decade.

The research team found three other independent pieces of evidence for such post-glacial tipping points: Model experiments showing the melting of the entire Antarctic ice sheet, a West Antarctic ice core documenting ice-sheet elevation draw-down and drill cores revealing a step-wise ice-sheet retreat across the Ross Sea shelf.

Today's ice mass loss could be start of long-lasting period

The results are also relevant for ice retreat observed today: "Our findings are consistent with a growing body of evidence suggesting the acceleration of Antarctic ice-mass loss in recent decades may mark the begin of a self-sustaining and irreversible period of ice sheet retreat and substantial global sea level rise," says study leader Dr. Michael Weber from the University of Bonn.

Combining the sediment record with computer models of ice sheet behaviour the team showed that each episode of increased iceberg calving reflected increased loss of ice from the interior of the ice sheet, not just changes in the already-floating ice shelves. "We found that iceberg calving events on multi-year time scales were synchronous with discharge of grounded ice from the Antarctic Ice Sheet," said Prof. Nick Golledge from the University of Wellington (New Zealand), who led the ice-sheet modelling.

Dr. Zoë Thomas, a co-author of the study from the University of New South Wales in Sydney, Australia, then applied statistical methods to the model outputs to see if early warning signs could be detected for tipping points in the ice sheet system. Her analyses confirmed that tipping points did indeed exist. "If it just takes one decade to tip a system like this, that's actually quite scary because if the Antarctic Ice Sheet behaves in future like it did in the past, we must be experiencing the tipping right now," Thomas said.

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Climate changed abruptly at tipping points in past

Abrupt changes in ice core samples and other records indicate dramatic changes in climate occurred at certain points in the past.

In Chaos, by AIP Publishing, climate scientists identify abrupt transitions in climate records that may have been caused by the climate system crossing a tipping point. This happens when self-reinforcing feedbacks in a system push it away from a stable state, leading to dramatic change.

Identifying these events in the Earth's past is critical to understanding the tipping points likely to be encountered this century as a warming climate destabilizes the Earth's physical systems and ecosystems.

The researchers from CNRS (France), UCLA, and Columbia University devised a statistical method to determine whether transitions seen in climate records such as ice cores are simply noise or evidence of a more significant change. This has typically been done by visual inspection, a process that is time-consuming and subjective.

Their method is less error-prone, since it doesn't rely on human determination of whether a jump is a significant transition. It allows comparing different records consistently and can identify important events that may have been overlooked in older studies.

An augmented Kolmogorov-Smirnov (KS) test, a statistical technique named after its original authors, provided an alternative approach to recurrence analysis. The KS test has been successfully applied to other inherently noisy systems, such as finance and signal processing.

The method compares two samples taken before and after the potential transition point to test whether they come from the same continuous distribution. If they don't, the transition point is identified as a significant abrupt change indicative of a true climactic shift.

"We applied our method to two paleoclimate records of the last climate cycle, a Greenland ice core and a speleothem composite record from China," said author Witold Bagniewski.

Analysis of ice cores reveals that the ratio of two oxygen isotopes varies over time. This ratio depends on the local temperature at the time the ice formed, providing a measurement of the climate at that particular time.

Speleothems are mineral deposits in caves showing a similar pattern of isotope ratios varying as the climate changes.

"Many of the abrupt transitions in the Greenland ice core record correspond to shifts between a warmer climate, known as Greenland Interstadials (GIs), and a colder climate, the Greenland Stadials (GSs)," said Bagniewski.

The existence of these two climate states, GI and GS, is an example of a bistable climate system, in which two distinct states are both stable. The climate may jump abruptly from one to the other when crossing a tipping point.

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Why did glacial cycles intensify a million years ago?

Something big happened to the planet about a million years ago. There was a major shift in the response of Earth's climate system to variations in our orbit around the Sun. The shift is called the Mid-Pleistocene Transition. Before the MPT, cycles between glacial (colder) and interglacial (warmer) periods happened every 41,000 years. After the MPT, glacial periods became more intense -- intense enough to form ice sheets in the Northern Hemisphere that lasted 100,000 years. This gave Earth the regular ice-age cycles that have persisted into human time.

Scientists have long puzzled over what triggered this. A likely reason would be a phenomenon called Milankovitch cycles -- cyclic changes in Earth's orbit and orientation toward the Sun that affect the amount of energy that Earth absorbs. This, scientists agree, has been the main natural driver of alternating warm and cold periods for millions of years. However, research has shown that the Milankovitch cycles did not undergo any kind of big change a million years ago, so something else likely was at work.

Coinciding with the MPT, a large system of ocean currents that helps move heat around the globe experienced a severe weakening. That system, which sends heat north through the Atlantic Ocean, is the Atlantic Meridional Overturning Circulation (AMOC). Was this slowdown related to the shift in glacial periods? If so, how and why? These have been open questions. A new paper published today in the journal Proceedings of the National Academy of Sciences proposes an answer.

The researchers analyzed cores of deep-sea sediments taken in the south and north Atlantic, where ancient deep waters passed by and left chemical clues. "What we found is the North Atlantic, right before this crash, was acting very differently than the rest of the basin," said lead author Maayan Yehudai, who did the work as a PhD. student at Columbia University's Lamont-Doherty Earth Observatory.

Prior to that oceanic circulation crash, ice sheets in the Northern Hemisphere began to stick to their bedrock more effectively. This caused glaciers to grow thicker than they had before. This in turn led to a greater global cooling than before, and disrupted the Atlantic heat conveyor belt. This led to both stronger ice ages and the ice-age cycle shift, says Yehudai.

The research supports a long-debated hypothesis that the gradual removal of accumulated slippery continental soils during previous ice ages allowed ice sheets to cling more tightly to the older, harder crystalline bedrock underneath, and grew thicker and more stable. The findings indicate that this growth and stabilization just before the weakening of the AMOC shaped the global climate.

"Our research addresses one of the biggest questions about the largest climate change we had since the onset of the ice ages," said Yehudai. "It was one of the most substantial climate transitions and we don't fully understand it. Our discovery pins the origin of this change to the Northern Hemisphere and the ice sheets that evolved there as driving this shift towards the climate patterns we observe today. This is a very important step toward understanding what caused it and where it came from. It highlights the importance of the North Atlantic region and ocean circulation for present and future climate change."

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