A group of scientists and students, including Ron Kwok of NASA's Jet Propulsion Laboratory visited the Northwest Passage over this summer to run a check up on the health of the sea ice.
In particular, the scientists were looking at the thickness of the ice and trying to determine the longer term changes that have been going on.
Satellites have only monitored sea ice extent since 1973. NASA's Ice, Cloud, and land Elevation Satellite (ICESat) has been on the task since 2003, allowing researchers to estimate ice thickness as well, according to the NASA article.
In order to extend that record, the research team recently combined the high spatial coverage from satellites with a longer record from Cold War submarines to piece together a history of ice thickness that spans close to 50 years.
In total, declassified submarine data span nearly five decades--from 1958 to 200--and cover a study area of more than 1 million square miles, or close to 40 percent of the Arctic Ocean.
Their analysis shows that the Arctic Sea Ice thickness has declined 53% since a peak in 1980. Also, the current thinning of Arctic sea ice has actually been going on for quite some time.
"We need to understand the long-term trends, rather than the short-term trends that could be easily biased by short-term changes," Kwok said. "Long-term trends are more reliable indicators of how sea ice is changing with the global and regional climate."
That's why a long-term series of data was necessary. "Even decadal changes can be cyclical, but this decline for more than three decades does not appear to be cyclical," according to Drew Rothrock of the University of Washington.
"A fantastic change is happening on Earth -- it's truly one of the biggest changes in environmental conditions on Earth since the end of the ice age," said Tom Wagner, cryosphere program manager at NASA Headquarters.