livescience.com Yahoo News 2 Jul 09;
A reconstruction of sea ice reveals the lowest levels in 800 years, according to new research published in the journal Climate Dynamics.
Researchers modeled sea ice levels between Greenland and Svalbard, an archipelago in the Arctic Ocean north of Europe, from the 13th century to present using data from a natural climate "archive" and from historic human records.
"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," Aslak Grinsted said in a press release. Grinsted is a geophysicist with the Centre for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen in Denmark. "We see that the sea ice is shrinking to a level which has not been seen in more than 800 years."
The scientists also combed through harbor records and logbooks of ships that traveled the area to record human observation of sea ice levels. Then they pieced together a picture of how much sea ice has existed through this time period.
Sea ice melting and re-freezing is a complicated process that is influenced by a number of factors such as wind patterns, ocean currents, and how much ice has frozen or melted in recent years. The authors did not point to any causes for the changes in sea ice levels in their study.
The scientists noted that even though the 13th century was a relatively warm period and ice levels were low then, 20th century sea ice levels are still the lowest. The "Little Ice Age," from 1700 to 1800 had the greatest cover of sea ice, according to their data.
Other studies have found that Arctic ice is getting thinner over time, so that when the normal summer melt occurs, the entire polar cap is retreating compared to decades past. Last year, this melting opened up the fabled Northwest Passage, as a substantial amount of older ice melted. Climate scientists say the North Pole could be ice-free during summer within a few decades.
Grinsted said there have been instances of sudden changes throughout time, such as when sea ice shrank by 115 square miles (300 square kilometers), about the size of the Maldives in the Indian Ocean, from 1910 to 1920.
