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“We know sea ice in the Antarctic has increased slightly over the past 30 years, since satellite observations began. Scientists have grappled with this trend in the context of global warming, but these new findings suggest it may not be anything new.

“If ice levels were as low a century ago as estimated in this research, then a similar increase may have occurred between then and the middle of the century, when previous studies suggest ice levels were far higher.”

The study was based on the ice observations recorded in logs from 11 voyages made between 1897 and 1917, including three led by Scott and two by Shackleton. It is the first study to calculate sea ice in the period prior to the 30s and suggests levels in recent decades are just 14 per cent less than at the highest point of the 1900s and 12 per cent more than the lowest.

In the early 1900s, there was an estimated 5.3 to 6.4 million square km of sea ice at the South Pole, according to the explorers’ records. This then increased to the point where the extent of Antarctic sea ice was significantly greater in the 50s, before it began a steep decline and returned to around 5.9 million square km.

The findings show that the climate of Antarctica had fluctuated throughout the 20th century and that sea ice in the South Pole is much less sensitive to the effects of climate change than in the Arctic, which experienced a dramatic decline during the 20th century.

Separate research by the British Antarctic Survey, published in the journal Nature, also shows that the present-day loss of the Pine Island Glacier, which drains into the Amundsen Sea in West Antarctica, has been happening since the mid-20th century and was probably caused by El Nino activity in the Pacific Ocean in the 1940s.

Co-author Prof Bob Bindschadler, of Nasa, said: “A significant implication is that once an ice sheet retreat is set in motion it can continue for decades, even if what started it gets no worse.”