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Menopausal Mother Nature

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Ice flow in Antarctica changes with season, can impact sea-level rise estimates

Meltwater reaching base of glaciers or thinning out of ice shelves likely causes 

Antarctica’s ice moves faster in the summer, according to a new study. This means that the continent’s contribution to global sea-level rise is likely being miscalculated.

In summers, the ice flow from glaciers feeding into George VI, the second largest ice shelf in Antarctica, speeds up by 15 per cent, the study published in the journal Cryosphere found

Ice shelves are floating slabs of ice extending from glaciers.

“Current estimates of ice mass loss assume that Antarctic land ice flows at the same rate throughout the year,” Karla Boxall from Cambridge’s Scott Polar Research Institute and the study’s first author told Down To Earth.

Boxall and her colleagues used data collected from Sentinel-1 radar satellites to monitor the continent throughout the year, the researchers said.

They zoomed in on the George VI ice shelf, which covers an area of roughly 23,500 square kilometres. Its thickness ranges from 100-600 metres, the paper read.

Map: Frazer Christie

Their analysis revealed that during the Antarctic summer from December-January, ice flow velocity increased by 0.06 ± 0.005 metre per day, representing a roughly 15 per cent increase in speed.

Boxall and the team are unsure of the reason behind the seasonal variation. But they speculate that meltwater from the upper surface of the glaciers could be reaching their base through crevices and acting as lubricant.

This is already occurring in the Arctic and the Alps, the researchers said.

Alternatively, they theorise that warm ocean water could be melting the ice shelf from below, causing it to thin out. This, the paper said, allows upstream glaciers to move faster.

The seasonal cycles can be due to either mechanism or a mixture of the two, Frazer Christie from Scott Polar Research Institute highlighted.

Conducting detailed ocean and surface measurements can help them understand the reasons behind seasonal changes in ice flow, the researchers said.

These results suggest that other vulnerable glaciers in Antarctica such as Pine Island and Thwaites glaciers are likely experiencing seasonal changes too, the authors of the study speculated.

Wrong estimates

Annual volume of ice loss is the basis for estimating Antarctica’s contribution to sea-level rise. But seasonal changes in ice flow mean scientists could be underestimating or overestimating the continent’s contribution to sea-level rise, the paper noted.

“Some estimates of mass loss to the ocean are calculated from the change in [ice] flow between two points in time,” Boxall explained.

If the two points are during winter, she added, the mass loss over the year will be underestimated. 

In this case, experts ignore the summertime ice speed-up and the resulting mass loss, Boxall said.

But if estimates are conducted during summer, experts overestimate annual mass loss as they ignore the relatively slow flow during the remaining months.

The Antarctic Ice Sheet covers an area of almost 14 million square kilometres and contains 30 million cubic kilometres of ice, according to the British Antarctic Survey.

The ice sheet holds around 90 per cent of the world’s surface freshwater. This is equivalent to a 70-metre rise in global sea level, the polar research institute stated.

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