What happened when Antarctic ice melted in the past?

Antarctica, the Earth’s southernmost continent, holds a staggering 60 percent of the world’s fresh water in its ice sheets. This amounts to 30 million cubic kilometers of ice, a figure that is often difficult to comprehend. If all of the ice in Antarctica were to melt, global sea levels would rise by an average of 58 meters, wreaking havoc on coastal communities and dramatically altering our planet’s landscape.

Irina Rogozhina, an associate professor at the Norwegian University of Science and Technology (NTNU), explained the potential consequences of ice sheet melting in East Antarctica: “The ice sheet in East Antarctica stores enormous amounts of water. This means that this is the biggest possible source of future sea level rise – up to 53 meters if all of the East Antarctic ice melts – and is seen as the largest source of uncertainties in the future sea level adaptation planning.”

Antarctica’s ice loss occurs primarily through ocean-driven melting of ice shelves and ice calving. These processes accelerate the movement of ice streams on land, which discharge more ice into the ocean where it is eventually lost to melting and calving. Professor Rogozhina noted that this phenomenon was likely responsible for more significant ice loss during the Earth’s warmer periods. In Greenland, for example, these two processes account for approximately 65 percent of all ice loss.

However, it is important to understand that the entire ice sheet does not need to melt for significant consequences to occur. Researchers from NTNU, including Rogozhina’s group, recently examined the ice in Queen Maud Land in East Antarctica. They discovered that the ice sheet in this region has experienced significant variation over time. This research is crucial in improving our understanding of the Earth’s climate and how it is changing.

The team analyzed the East Antarctic ice sheet and a meltdown that occurred several thousand years ago. Their findings have been published in the Nature journal Communications Earth & Environment.

The distribution of ice in Antarctica is not uniform. In the west, large portions of the ice sheet lie below sea level, reaching depths of up to 2,500 meters, making them highly vulnerable to ocean warming. In contrast, much of the ice sheet in the east is located on land above sea level, rendering it less susceptible to the ocean’s influence.

The researchers found that the East Antarctic ice sheet was thinner in the past, particularly following the end of the last ice age, when massive ice sheets covered North America, northern Europe, and southern South America. As these ice sheets melted, sea levels rose by over 100 meters.

Rogozhina explained the team’s findings: “From the evidence we presented in our study, we concluded that the East Antarctic ice sheet in Queen Maud Land also melted rapidly along its margins between 9,000 to 5,000 years ago, in a period we call the mid-Holocene. At this time, many parts of the world experienced warmer-than-present summers.”

The rapid response of the East Antarctic ice sheet to warmer conditions during the Holocene epoch is alarming, as Rogozhina said, “it is still difficult and worrisome to believe that the sluggish East Antarctic ice sheet can change so rapidly.”

Determining the exact timing of the melting and finding a simple explanation for this behavior has proven challenging due to the inhospitable conditions in Antarctica. However, the researchers have begun to unravel this mystery, which will undoubtedly contribute to our understanding of the planet’s climate and inform future adaptation strategies to cope with rising sea levels.

The research group, led by Professor Ola Fredin, focused on analyzing rock samples from various nunataks in Queen Maud Land for evidence of exposure to cosmic radiation. Nunataks are mountains that protrude through ice sheets. 

The researchers then analyzed different isotopes or variants of elements such as chlorine, aluminum, beryllium, and neon in the rocks from the nunataks. Through the study of cosmogenic isotopes, they were able to determine how high the ice was over geological time in Queen Maud Land. Professor Fredin likens this process to “using a dipstick to measure the level of engine oil in your car.”

By determining how long the rocks have been exposed to cosmic radiation, the researchers could also estimate the duration since the rocks were last covered by a protective layer of ice, which shielded them from cosmic radiation exposure. This was achieved by utilizing data from different areas and running various computer simulations.

The study also shed light on a possible explanation for the rapid thinning of the ice sheet sector in East Antarctica following the last ice age. 

“We believe that the ice sheet became less stable due to higher, regional sea levels and warmer water rising from the ocean depths in the polar regions, penetrating under the ice margins and melting them from below,” explained Professor Rogozhina. 

This process led to the break-up of large icebergs and accelerated the movement of ice from land to ocean, consequently thinning the inland section of the ice sheet. Rogozhina likens this to “when a house on a hill slope loses its supporting foundation and starts sliding downhill.”

The researchers also used cosmic radiation to determine how frequently ice has covered certain areas. Their findings indicated that it is most common for the ice in Queen Maud Land to be thick along the coast, but not further inland, where mountain peaks protrude through the ice and land elevations can reach several thousand meters.

According to Professor Fredin, the land masses along the coast of Queen Maud Land have been covered by ice between 75 and 97 percent of the time during the last one million years. By contrast, mountain summits further inland have been ice-covered as little as 20 percent of the time. 

This suggests that the ice sheet thickness and movement speed vary significantly over longer periods, and the mountain range further inland serves as a critical division between the dynamic coast and the ice sheet closer to the South Pole, which experiences less variation in thickness.

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