Image caption The minimum level of cover is far below the average of 1979-2000

Sea ice cover in the Arctic in 2011 has passed its annual minimum, reaching the second-lowest level since satellite records began, US scientists say.

The National Snow and Ice Data Center (NSIDC) says the minimum, reached on 9 September, was 4.33 million sq km.

That value is 36% lower than the average minimum for 1979-2000.

NSIDC said the figure was preliminary, and that "changing winds could still push the ice extent lower" before final numbers are published in early October.

The preliminary value is 160,000 sq km - or 4% - above the record minimum seen in 2007.

"While the record low year of 2007 was marked by a combination of weather conditions that favoured ice loss - including clearer skies, favourable wind patterns and warm temperatures - this year has shown more typical weather patterns but continued warmth over the Arctic," they wrote.

"This supports the idea that the Arctic sea ice cover is continuing to thin."

NSIDC director Mark Serreze said: "Every summer that we see a very low ice extent in September sets us up for a similar situation the following year.

"The Arctic sea ice cover is so thin now compared to 30 years ago that it just can't take a hit any more. This overall pattern of thinning ice in the Arctic in recent decades is really starting to catch up with us."

In fact, an analysis released last week by researchers at the University of Bremen in Germany, who use a different satellite to assess ice cover, indicated that 2011's minimum was the lowest on record.

However, there is some controversy surrounding the result; the Bremen team's higher-resolution data can detect small patches of water where the NSIDC team would not, but the Bremen record goes back only to 2003.

These analyses are for the extent, or area, of Arctic ice, but recent estimates released by the University of Washington's Polar Science Center give an indication of the total amount of sea ice.

Their data indicate that the ice volume is at an all-time low for the second year in a row.

Analyses of Arctic ice in recent years consistently indicate a change in the nature of the ice itself - from one solid mass that melts and freezes at its edges towards more dispersed, piecemeal ice cover, and from robust "multi-year" ice toward seasonal floes that melt more easily.