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Arctic Sea Ice: This is a low, no?

Arctic-sea-ice extent is now at, or very near to, the summer minimum. As in recent years, the area of ocean with at least 15% ice cover — now around 4.3 million square kilometres — is massively (more than 1.6 million sq km) below the 30-year average. But new data challenge a previous conclusion that this year’s minimum is the lowest since satellite observations began in 1972.

Remote sensing experts with the University of Bremen in Germany had reported on 8 September that sea-ice extent has dropped below the record low observed in 2007.

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Now, scientists with the US National Snow and Ice Data Center (NSIDC) in Boulder, Colorado, have come up with a slightly different picture. Their maps and observations suggest that Arctic-sea-ice extent on 10 September was still 4.34 million sq km – 110,000 sq km above the 2007 NSIDC value on the same date. As the melting season is practically over, the 2007 low (4.17 million sq km according to NSIDC data) will not be undercut. Rather, 2011 will probably go on the record as the second-lowest year in the satellite era.

The small, but news-making, discrepancy is easily explained by different satellite observation technologies and algorithms used to translate satellite data into actual maps.

The NSIDC sea-ice data come from the Special Sensor Microwave Imager/Sounder sensor on the Defense Meteorological Satellite Program F17 satellite. The Bremen group’s maps are derived from microwave data from a Japanese sensor on board NASA’s Aqua satellite.

The high-resolution data used by the Bremen group allow small ice and open-water features to be detected that are not observed by other sensors. But NSIDC’s data-processing method is better-tested and undergoes somewhat more rigid quality control.

“While the University of Bremen and other data may show slightly different numbers, all of the data agree that Arctic sea ice is continuing its long-term decline,” the NSIDC group explains.

Wind could still push the ice together and reduce its extent even more. NSIDC scientists will make an announcement when ice extent has stopped declining. They will issue a more detailed analysis of this year’s Arctic melt season in October.

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