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The Arctic sea ice is shrinking to almost a record level

Arctic sea ice

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MOSAIK / AWI / Steffen Graupner

This summer’s Arctic sea ice shrank to the second lowest level ever in the age of satellite observation.

The clods retreated to just under 3.74 million square kilometers last week, preliminary data show.

The only time this minimum was exceeded in the 42-year spacecraft record was in 201

2, when the pack ice was reduced to 3.41 million km².

Shorter autumn days and the cold mean that the clods now grow back again.

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It is normal for the Arctic sea ice to expand each year over winter and melt again in summer, but the September minima, which are responsible for some variability, keep getting deeper as the polar north gets warmer.

The downward trend since routine satellite monitoring of the floes began has been around 13% per decade averaged over the month.

Computer models assume that the summer sea ice will regularly be less than one million square kilometers later this century.

This is bad news for the climate. Extensive sea ice cools the Arctic and the rest of the planet. In its absence, more sunlight is absorbed by the ocean’s darker surface waters, promoting further warming and ice loss.

“I see it now that we will always have low sea ice; it will never be the same as it was in the 1980s or 1990s,” said Prof. Julienne Stroeve of the Center for Polar Observation and Modeling (CPOM) at University College London ( UCL), UK.

“But whether or not we hit a new record low from one year to the next – that really depends a lot on what happens in the summer weather patterns,” she told BBC News.

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Prof. Stroeve was in the Arctic when it was coldest and darkest

Twenty was notable for some late-night storms that helped break up diffuse ice that went into its September low. Twenty-twenty didn’t have that, but there were some very warm conditions, especially on the Siberian side of the ocean, which melted much of the early season.

Prof. Stroeve worked on the ice for four and a half months last winter and examined the conditions with an international team based on the German research vessel Polarstern.

Last October, the ship set itself the task of drifting with the floes for a whole year, although supply and crew exchange difficulties as a result of the Covid 19 crisis interrupted this plan somewhat.

The CPOM-UCL scientist used the Polarstern mosaic expedition to study how precisely spacecraft sensors see the ice.

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Graphics: The Cristal satellite system is planned to fly towards the end of this decade

Of particular interest to them are the radar altimeters, which measure the thickness of the floes by measuring the height difference between the top of the sea ice and the surface of the ocean – the ice freeboard.

Satellites like the European Space Agency’s Cryosat-2 platform can use this observation to infer the depth of the submerged part of a floe – the ice draft – and thus obtain a 3D view of the pack ice, not just its 2D extent.

The complication with this approach is the consideration of snow that may be on the ice. This changes the horizon from which radar measurement signals bounce back to the satellite.

Prof. Stroeve’s winter experiments show that Esa’s Cryosat mission tends to judge the sea ice to be thicker than it actually is.

The space agency is currently working with the European Union to develop a new spacecraft called the Cristal, which will operate at two different radar frequencies.

“This would then give you the ability to get both ice thickness and snow depth on the same satellite system. Snow depth on the ice has always been one of those great unknowns that has contributed to the fact that we couldn’t really map sea ice thickness as well as we could want, “said Prof. Stroeve.

Esa announced on Monday that it had placed an order worth EUR 300 million with aerospace manufacturer Airbus to start developing Cristal.

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Investigations on the ice should improve the measurements from space

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