wolfgang, this was just recently in the news (CTV news Canadian news channel in which they talked about close to a 30% decline in Arctic sea ice- specifically the perennial sea cover)

but this is a direct quote from a nasa website

nasa arctic ice decline

'On Sept. 21, 2005, sea ice extent dropped to 2.05 million sq. miles, the lowest extent yet recorded in the satellite record.

The ice that survives the summer is called the perennial sea ice cover which consists mainly of thick multiyear ice floes. "Since 1979 we've seen that Arctic perennial sea ice cover has been declining at 9.6 percent per decade," said Joey Comiso, a senior scientist of GSFC.'

9.6% per decade - in 27 years