“We have combined information about the climate found in ice cores from an ice cap on Svalbard and from the annual growth rings of trees in Finland, and this gave us a curve of the past climate,” said Aslak Grinsted, a geophysicist with the Centre for Ice and Climate at the University of Copenhagen’s Niels Bohr Institute.
By combining the curve of the climate with actual historical records of the distribution of the ice, researchers have been able to reconstruct the extent of the sea ice all the way back to the 13th century. Although the 13th century was a warm period, scientists said, the calculations show that there never has been so little sea ice as in the 20th century.
“There was a sharp change in the ice cover at the start of the 20th century,” according to Grinsted, when the ice shrank by 300,000 square kilometres in the years from 1910 to 1920. Sudden changes have occurred throughout time, he noted, and in the last few years very little ice extent has been recorded. “We see that the sea ice is shrinking to a level which has not been seen in more than 800 years,” he said.
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