Researchers at DTU Space, at The Technical University of Denmark, have used unique NASA satellite data to create a picture of Antarctic sea ice extent dating back to the early 1970s. This new mapping may contribute to a better understanding of developments in recent years, where sea ice extent has reached record lows.

In 2016, the extent of sea ice in Antarctica dropped abruptly. Prior to this, the ice had been stable and slightly increasing. Since 2016, Antarctic sea ice has remained at a lower level, both in winter, when the extent is at its peak, and in summer, when it is at its smallest. In 2023, Antarctic sea ice reached its lowest recorded levels for both winter and summer.

A somewhat similar scenario, however, may have occurred in the early 1970s, according to new research from sea ice scientist and DTU Space associate professor, Rasmus Tonboe, and his team. He explains that back then, the sea ice extent also suddenly declined after being stable or slightly increasing for several years, before starting to grow again towards 2016.

”Our findings indicates that something might have happened to the sea ice extent around the early to mid-1970s, resembling what we’ve observed since 2016 - a period of growth followed by a dramatic decline over a short time,” says Rasmus Tonboe.

”However, we also see that the ice extent began to grow slightly again from the late 1970s. Observing such significant historical variations in sea ice extent allows us to investigate the dynamics of the Antarctic sea ice system during a period before the pressures of global warming became as intense as they are today. This is particularly intriguing as Antarctic sea ice, or its absence, influences the melting of land ice and, consequently, sea level rise”.

A very accurate picture of sea ice extent
Rasmus Tonboe and his team has assembled a very accurate picture of sea ice extent in the 1970s by using unique old data from experimental satellites.

”We have extended the time series backwards by analysing old satellite data made available by NASA and cleaning it of noise. This has allowed us to fill gaps in the monitoring of the Antarctic sea ice extent during this crucial period,” explains Rasmus Tonboe.

So far, the efforts has resulted in a scientific article published in the journal Earth System Science Data (ESSD) and one in Remote Sensing Applications, with more articles underway.

New knowledge found by using AI on NASA's data
The research draws on data from NASA’s Nimbus satellite series operational in the 1970s, specifically Nimbus 5 and Nimbus 6. The first satellite capable of observing Antarctic sea ice, with available data, dates back to 1972. It was the Nimbus 5 ESMR that had a microwave radiometer onboard.

”We developed a model that convert data from satellite images on old TIF files from scanned films into datasets for sea ice extent in a given area. We’ve also developed an AI algorithm that estimates uncertainties in the calculations, enabling us to account for them,” explains Rasmus Tonboe.

The data reveals that in 1972-74, sea ice extent was relatively high - comparable to 2016, when it covered 19 million square kilometers in winter. Subsequently, a decline was observed from 1975-77, though precise data is missing for 1978. Thereafter, satellite data shows a gradual increase in sea ice extent up until 2016.

”If we can improve our understanding of the processes behind these relatively sudden changes in sea ice extent, we’ll also be better equipped to predict whether the shift we saw in 2016 might reverse, allowing for more sea ice formation, or if it was the beginning of a longer-term decline. Such knowledge is essential for understanding climate change and its future impacts on sea level rise and global temperature,” says Rasmus Tonboe.

Sea ice helps to retain ice on land in Antarctica
In recent years, vast amounts of ice have disappeared from mainland Antarctica.

Between 2002-2023, the continent lost approximately 150 gigatons of ice annually. By comparison, Greenland loses approximately 270 gigatons of land ice each year.

Recent studies show that the melting of Antarctic ice shelves - ice tongues stretching from land into the sea - is linked to the retreating sea ice.

”When sea ice is affected by climate change, it’s a sign that all other parts of the system are also undergoing changes,” says Rasmus Tonboe.