The Arctic in 2005 saw little renewal of the thick, perennial sea ice that normally melts and is replenished every year, a NASA study has found.
Renewing the layer is crucial to maintaining the summer ice cover's stability, and the new findings suggest it may continue to decrease by as much as 10 per cent a year, researchers at the U.S. National Aeronautics and Space Administration said.
"The area of seasonal ice that survives the summer may no longer be large enough to sustain a stable perennial ice cover, especially in the face of accelerating climate warming and Arctic sea ice thinning," Ron Kwok of NASA's Jet Propulsion Laboratory in Pasadena, Calif., said in a written statement sent to CBC News Online.
The study published in the journal Geophysical Research Letters adds to the findings of a NASA study released late in 2006 that found the perennial ice had declined by about 14 per cent between 2004 and 2005.
"Recent studies indicate Arctic perennial ice is declining seven to 10 per cent each decade," Kwok said. "Our study gives the first reliable estimates of how perennial ice replenishment varies each year at the end of summer. The amount of first-year ice that survives the summer directly influences how thick the ice cover will be at the start of the next melt season."
Melting in the summer and migration of the three-metre thick sea ice alter the perennial ice coverage every year. Some of the seasonal layer of ice that builds up during the colder months is thick enough to survive the summer thaw.
But after studying six annual cycles of Arctic perennial ice coverage from 2000 to 2006 using data from NASA's QuikScat satellite, Kwok found that perennial ice coverage was 14 per cent lower in January 2006 than it was at the same time in 2005 — only about four per cent of the 2.5 million square kilometers of seasonal ice formed the previous winter survived the summer.
The data was collected from a device called a scatterometer on the QuickScat satellite. It sends radar pulses to the ice surface and measures the echo of the reflected radar pulses. The strength of the echo indicates to researchers the difference between seasonal ice and older perennial ice.
Abnormal wind conditions
The depletion of the sea ice was also affected by abnormal wind conditions that resulted in about seven per cent of the perennial ice coverage area migrating out of the Arctic — an unusually high amount.
The lack of refreezing during the summer heats up the ocean, contributing to even further thinning of the ice.
That pattern has been reflected in records that date back to 1958, according to Kwok, whose examination of historical data found a gradual warming trend in the first 30 years of that period that has rapidly increased after the mid-1980s.
"The record doesn't show any hint of recovery from these trends," he stated. "If the correlations between replenishment area and numbers of freezing and melting temperaturedays hold long-term, its expected the perennial ice coverage will continue to decline."