By Richard Black
Environment correspondent, BBC News website
A Nasa satellite has documented startling changes in Arctic sea ice cover between 2004 and 2005.
Satellite instruments can distinguish "old" Arctic ice from "new"
The extent of "perennial" ice - thick ice which remains all year round - declined by 14%, losing an area the size of Pakistan or Turkey.
The last few decades have seen summer ice shrink by about 0.7% per year.
The drastic shrinkage may relate partly to unusual wind patterns found in 2005, though rising temperatures in the Arctic could also be a factor.
The research is reported in the journal Geophysical Research Letters.
The Arctic is warming about twice as fast as the global average; and recent studies have shown that the area of the Arctic covered by ice each summer, and the ice thickness, have been shrinking.
September 2005 saw the lowest recorded area of ice cover since 1978, when satellite records became available.
This latest study, from scientists led by Son Nghiem of Nasa's Jet Propulsion Laboratory in California, measures something slightly different from the extent of summer ice cover - the extent of "perennial" ice cover.
Perennial or "multi-year" ice is up to 3m thick and survives through at least one summer. It is different from "seasonal" ice, which is thinner and melts more easily, surviving for just one winter before succumbing to the summer sun.
"Perennial ice contains less salinity," explained Dr Nghiem. "It's freshwater ice - there are more bubbles in it and typically its surface is much rougher - and a scatterometer [a radar-based instrument] can distinguish between the two types."
Using the scatterometer on Nasa's Quikscat satellite, researchers scanned the Arctic for perennial and seasonal ice. From October 2004 to March 2006 they plotted a steady decline.
When they compared figures for the 2004 and 2005 northern hemisphere winter solstices - 21 December - a huge change showed up.
"In previous years there is some variability, but it is much smaller and regional," Dr Nghiem told the BBC News website.
"However the change we see between 2004 and 2005 is enormous."
The area of perennial sea ice lost was about 730,000 sq km, with a huge loss in the East Arctic (defined as north of Russia and Europe) and a small gain in the West Arctic, north of the Americas and the Atlantic Ocean.
Continuous scatterometer data has been available only since 1999, so for comparison researchers must use the records of summer ice extent - which is almost, but not exactly, the same thing as perennial ice extent.
"If we average that over the long term we find a reduction of between 6.4% and 7.8% per decade," said Dr Nghiem. "What we have here is 14% in one year - 18 times the previous rate."
Annual Arctic air temperatures relative to the 1961-1990 average
The key questions are what caused it, and whether it is an anomaly or the first sign of a major change of pace for Arctic melting.
Quikscat also monitors winds, and noted unusual patterns of wind in the East Arctic in 2005 which could be related, perhaps propelling old ice from east to west, though how that could explain such a drastic loss of perennial ice is not clear.
If the pace of Arctic melting is quickening, the implications for the future are not reassuring.
Ice reflects the Sun's energy back into space; open water absorbs it. So a planet with less ice warms faster, potentially turning the projected impacts of global warming into reality sooner than anticipated.