In 2016, the extent of sea ice in Antarctica dropped abruptly. Prior to this, the ice had been stable and slightly increasing. The loss of Antarctic sea ice within a year equalled the total loss of Arctic sea ice over 40 years.
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. During winter, it covered an area of 17 million square kilometres, while in summer, it was just 1.79 million square kilometres - approximately 20 percent below the pre-2016 levels.
By late March, when Antarctic summer sea ice typically reaches its minimum extent, the record low levels seemed to persist, with only 1.98 million square kilometres. This marked the fourth consecutive year with a minimum below 2.0 million square kilometres.
A similar scenario though, may have occurred in the early 1970s, according to sea ice expert and DTU Space associate professor, Rasmus Tonboe.
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 until 2016.
"This 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," he explains.
"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".
Together with a group of DTU students, Rasmus Tonboe has assembled a picture of sea ice extent in the 1970s using unique old data from experimental satellites that were poorly calibrated and not necessarily intended for such purposes.
New knowledge in old NASA data
The first satellite capable of observing Antarctic sea ice, with available data, dates back to 1972. It was NASA's Nimbus 5 ESMR that had a microwave radiometer onboard, though it was highly challenging to use for mapping sea ice.
"We have extended the time series backwards by analysing old 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, this effort has resulted in a scientific article published in the journal Earth System Science Data (ESSD), with two more articles underway.
The data reveals that in 1972-74, sea ice extent was relatively high - comparable to 2016, when it covered 19 million square kilometres 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.
Algorithm uncovers the ice in satellite images
Rasmus Tonboe and his students have examined data from multiple satellites to form a more accurate picture of sea ice extent during the 'dark' period of the 1970s.
"I am working with some incredibly skilled students who have developed models for calculating sea ice extend. For instance, a model that can 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.
"This has allowed us to piece together an image that is among the most accurate for this period. It has drawn attention from researchers around the world in this field, and they are very interested in our project - which, of course, we are delighted about".
The research draws on data from NASA's Nimbus satellite series, specifically Nimbus 5, equipped with ESMR and NEMS instruments, and Nimbus 6, which carried SCAMS and ESMR, both operational in the 1970s.
Sea ice helps to retain ice on land
In recent years, vast amounts of ice have disappeared from mainland Antarctica.
Between 2002-2023, the continent lost approximately 150 gigatons of ice annually. This melting has contributed to a global sea level rise of 0.4 millimetres per year during this period. Comparatively, Greenland loses about 270 gigatons of land ice annually.
The concern regarding Antarctica is that the stability of the West Antarctic ice sheet could be affected by climate change, potentially accelerating the melting of this enormous continent and leading to significant global sea level rise.
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. Sea ice helps retain vast amounts of land ice. When sea ice diminishes, it exacerbates land ice loss through a complex interplay of factors, including the albedo effect, warming the air and waters around Antarctica.
"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.
"That's why it's so crucial to understand what's happening with the sea ice and the processes driving the changes, both now and historically".
