Rapidly declining levels of sea ice are a well-recorded phenomenon in the Arctic, but until recently, the Antarctic was seeing the opposite effect, with slight increases in average sea ice extent year on year.
However, this year sea ice around the Earth’s southern pole has hit a new record low, with less than 2 million square kilometres of ice surrounding the continent in February – which marks the end of the Antarctic summer.
The extent is the lowest seen since the launch of satellite observations of the poles in 1978.
During this timeframe sea ice in the Antarctic has generally been increasing, with a modest growth of around one per cent a decade.
But this year’s low level of ice is not the first time this has occurred – though it is the most extreme reduction recorded so far. It is the second such event in five years – in 2017, a record minimum was also recorded.
The low level of ice this Antarctic summer was also a factor which helped researchers locate the wreck of Ernest Shackleton’s famous lost ship Endurance.
Sea ice is a critical part of the Antarctic ecosystem and wider environment, with the ice not only being essential to species such as emperor penguins, but also playing a role in holding back the ice shelves hemming in glaciers, which would otherwise drain more rapidly into the sea, raising global sea levels even faster.
The new data taken reveals that there was significantly lower-than-normal ice cover in the Bellingshausen/Amundsen Seas, the Weddell Sea and the western Indian Ocean sector around Antarctica.
The scientists, from China’s Sun Yat-sen University and Southern Marine Science and Engineering Guangdong Laboratory, said that sea ice extent was some 30 per cent lower than the average across the 1981-2010 three-decade baseline period.
The team used existing data to work out the various factors contributing to the formation of ice and its melting, and also how currents, winds and weather systems move the ice, which also directly affects sea ice extents.
They found that in the summer, the dominating factor controlling sea ice extent is the melt rate which depend on "anomalies in the transport of heat toward the pole in the Bellingshausen/Amundsen Seas, the western Pacific Ocean, and the eastern Weddell Sea in particular".
Furthermore, there is also an increase in overall energy from the sun, which creates an ice-loss feedback loop.
The less ice there is, the more the sunlight will directly hit dark water, where instead of being reflected back into space, as it does when it hits ice, it is instead absorbed by the sea, raising water temperatures.
“Sea ice is whiter than the dark unfrozen sea, thus there is less reflection of heat and more absorption,” said climatologist Qinghua Yang, a co-author of the study.
“[This] in turn melts more sea ice, producing more absorption of heat, in a vicious cycle.”
In the spring, as well as these two processes occurring, ice loss in the Amundsen Sea drives a northward ice motion that pushes more ice towards the tropics, thus increasing melting.
In addition, a thinner sea ice freeboard – the thickness of the sea ice that sticks out above the waterline – along the coast of the Amundsen Sea plays a critical role in relation to the spring and summer melting.
Finally, another set of factors controlling ice levels include major weather systems.
When this year’s record low sea ice extent occurred, there was a combination of both La Niña and a positive southern annular mode (SAM)
The SAM describes a belt of strong westerly winds or low pressure that surrounds the continent, moving north or south, while La Niña describes a weather pattern of powerful winds that blow warm ocean surface water from South America to Indonesia in the tropics.
Both of these phenomena deepen the Amundsen Sea low (ASL) – a centre of low atmospheric pressure over the far south of the Pacific Ocean and off the coast of west Antarctica. The variability of atmospheric conditions in the region is greater than anywhere else in the southern hemisphere.
The researchers found that all the atmospheric impacts on the sea-ice extent anomalies originate with the intensity and position of the Amundsen Sea low.
The researchers were able to explain a lot, but their findings only produced further questions.
“If tropical variability is having such an impact, it’s that location that needs to be studied next,” said Jinfei Wang, one of the other authors of the paper.
The research is published in the journal Advances in Atmospheric Sciences.