When Earth warmed after the last Ice Age, Antarctica’s ice reacted surprisingly fast: It took just ten years, on average, to trigger massive outbursts of melting and birthing icebergs. These melting phases then continued for centuries, scientists discovered. This suggests that Antarctic ice could reach its tipping point just as quickly in the wake of current climate change.
Global warming is now affecting measurably even the supposed “eternal” ice in Antarctica. Large glaciers are melting faster and faster, especially along the West Antarctic Peninsula. As seawater flowing from the Southern Ocean to the shelf base of the continent gets warmer and warmer, it is melting the ice tongues floating on coastal glaciers and ice shelves from below.
How sensitive is the Antarctic reaction?
Scientists already fear that the melting of the Antarctic ice sheet has already passed a tipping point in at least part of West Antarctica – the threshold above which melt becomes independent and continues irreversibly for centuries. “Despite mounting evidence of current and future instability of the Antarctic ice sheet and concern about rising sea levels as a result, there has not yet been any data series showing the speed, timing and extent of past glacial losses combined,” explain Michael Weber of the University of Bonn and colleagues. This makes it difficult to estimate how quickly and forcefully the Antarctic ice may interact with climate change and overturning.
Help is now being provided by data from the so-called iceberg off the east coast of West Antarctica. This marine area represents the main path of icebergs that cut the Antarctic ice sheet and then drift northward with the current along the coast. As the icebergs gradually melt in this drift, their lower side gradually releases sediment and debris, which sinks to the sea floor along the way. The amount of accumulated iceberg debris reveals how many icebergs were born at a particular time and drifted away on the “Iceberg Highway”.
Eight melt waves with a sharp rise in sea level
This is where Weber and his team come into play: To see how global warming after the last ice age affected the Antarctic ice sheet, they examined sediment cores removed from two places on Iceberg Street. It contains layers that were deposited about 19,000 to 9,000 years ago. By using the amount of iceberg grit in these layers, researchers were for the first time able to more accurately understand when, how fast and for how long the phases of strong post-glacial defrosting took place. In addition, the team also used sediment core samples from the Ross Sea and ice cores from West Antarctica, which document the volume loss of ice masses during this time period.
Assessments have shown that there are eight phases in the post-glacial warm phase with severe melting of the Antarctic ice. They appear both in the iceberg debris that was deposited in abundance during these times and in the supplementary data. In these melting spurs, a large number of icebergs were generated and drifted northward. In one such eruption, known as MWP-1A, about 14,500 years ago, 1,300 gigatons of ice would have broken off annually. Its melting alone could have caused sea level rise of seven to eleven meters during the 400-year melt phase, Weber and his team report.
The tipping point was reached within a decade
More importantly, these stages of powerful defrosting began with surprising speed. It took less than a decade to destabilize the Antarctic ice sheet in a manner that was followed by a melting phase that lasted from centuries to thousands of years. “The speed of just one decade of regime reversal is very frightening, because if the Antarctic ice sheet behaves in the future as it did in the past, we will see the reversal now,” Weber says. So far, models have suggested that ice in West Antarctica needs about 60 years of the currently observed melt rate to break out of equilibrium and “inversion”. “Our data from Eisberg-Allee suggests that this could happen much more quickly, within a decade or two,” the researchers say.
If confirmed, the ice, especially in West Antarctica, could be much closer to the tipping point than is assumed – or even exceeded. “Our results are consistent with a growing body of evidence suggesting that the acceleration of Antarctic ice loss over the past few decades could mark the beginning of a long, irreversible period of ice sheet retreat,” Weber says.
Source: University of Bonn; Technical Article: Nature Communications, doi: 10.1038/s41467-021-27053-6
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