Antarctica: Rapid ice loss 8,000 years ago

Using certain features of the ice in a drill core, the researchers were able to document the drastic loss in height of the ice sheet in the past. © University of Cambridge/British Antarctic Survey

How could current global warming affect Antarctica's ice sheets? In order to get new answers to this question, researchers have now taken a look at developments at the end of the last ice age. The analysis of an ice core shows that warming during this time caused the ice cover of the West Antarctic ice sheet to shrink surprisingly quickly and intensively: According to this, the ice layer at the sampling point became 450 meters thinner within just under 200 years. The possible sensitivity of the ice masses should now be taken into account when forecasting further developments in the wake of climate change, say the scientists.

It is a clear sign of global climate change and at the same time a threat: the earth's ice masses are retreating significantly and their meltwater is causing sea levels to rise. A particularly worrying focus is on the development of the enormous ice masses in Antarctica. The West Antarctic ice sheet is considered particularly at risk because it thaws quickly and is largely deposited on bedrock that lies below sea level. Scientists fear that the current rising temperatures could lead to destabilization, which could trigger a collapse of the ice masses once a critical threshold is exceeded. However, it is uncertain when exactly and how quickly the ice might be lost.

look in the past

One way to better assess future developments is to collect information about ice loss from warming periods in the past so that it can then be integrated into modeling. Researchers from the University of Cambridge and the British Antarctic Survey have now dedicated themselves to precisely this task. Her focus was on developments during the last ice age. At its peak 20,000 years ago, Antarctic ice covered a larger area than it does today. As the Earth slowly warmed, the West Antarctic ice sheet shrank more or less to its current size. “We wanted to know what happened to the West Antarctic ice sheet at the end of the last ice age, when temperatures on Earth were rising, albeit more slowly than the current anthropogenic warming,” says co-author Isobel Rowell from the British Antarctic Survey.

The researchers got the clues from an ice core. As they explain, these samples consist of easily dated layers of ice that once formed during snowfall and were then buried and compacted into ice crystals over thousands of years. Each layer of ice contains bubbles from the air of that time and certain substances that mixed with the snowfall every year. All of these aspects can provide clues about the changing climate and ice extent. The ice core on which the current study results are now based is 651 meters long and was obtained from the southern edge of the Ronne Ice Shelf. At this sampling point, the ice sheet lying on the ground merges into floating ice shelf.

After the ice core was transported cooled to Cambridge, the researchers first analyzed stable water isotopes in the layers. As they explain, their occurrence reflects the temperature at the time of snowfall. This can provide clues about the elevation of the ice surface onto which the snow fell. It is known that at higher altitudes the temperature decreases, so that higher temperatures can indicate a relatively deep, thinned ice layer. The team used the pressure of the air bubbles trapped in the ice as another clue. Because like temperature, air pressure also varies with altitude. Deeper, thinner ice therefore contains air bubbles with a slightly higher pressure.

Rapid loss at the end of the Ice Age

As the team reports, the analysis results showed that around 8,000 years ago the ice sheet lost around 450 meters in height in just under 200 years. Global warming at the end of the Ice Age led to a surprisingly rapid loss of ice. “As soon as the ice became thinner, it appeared to shrink very quickly. “This is a turning point and a process that is getting out of control,” says senior author Eric Wolff from the University of Cambridge.

Scientists suspect that the thinning was likely triggered by warming causing warm water to seep beneath the edge of the West Antarctic ice sheet, which normally rests on bedrock. This could have caused some of the ice to become detached from the bedrock, allowing it to suddenly float. The diminishing braking effect of the basic binding could then have led to accelerated ice drainage with a thinning effect.

The researchers also found that the sodium content of the ice, which came from blown-in sea spray, increased significantly about 300 years after the ice thinned. This finding suggests that the ice shelf had retreated significantly after the ice thinned, so that the sea was then hundreds of kilometers closer to the sampling site than before.

“We already knew from models that the ice thinned during the era, but exactly when that happened was uncertain,” says Rowell. Ice sheet models suggested the retreat occurred between 12,000 and 5,000 years ago and could not say how quickly it occurred. “Now we have a very precisely dated observation of this retreat that can be incorporated into improved models,” says the scientist. Although the West Antarctic ice sheet retreated rapidly 8,000 years ago, it stabilized at about its present extent. “Now it’s important to find out whether additional heat could destabilize the ice again and cause it to retreat further,” says Wolff.

Source: University of Cambridge, specialist article, Nature Geoscience, doi: 10.1038/s41561-024-01375-8

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