May 1 (Bloomberg) -- The retreat of the sea ice in the Arctic is underestimated by computer models used by the United Nations in preparing its assessments on climate change, U.S. scientists said.

Observational records from satellites, ships and aircraft showed the extent of sea ice coverage in September, the annual low-point, declined by an average 7.8 percent per decade from 1953 to 2006, the scientists say today in the journal Geophysical Research Letters.

That's more than triple the average decline produced from simulations by 18 models used by the UN's Intergovernmental Panel on Climate Change. The UN panel this year is conducting its biggest study on climate change since 2001.

``This suggests that current model projections may in fact provide a conservative estimate of future Arctic change, and that the summer Arctic sea ice may disappear considerably earlier than IPCC projections,'' Julienne Stroeve, lead author of the paper and a scientist with the National Snow and Ice Data Center said late yesterday in an e-mailed statement.

The IPCC in February reported global warming is ``very likely'' caused my human emissions of greenhouse gases such as carbon dioxide and, last month, it warned of floods, droughts and extinctions as a result.

The group will on May 4 report on the technologies available to reduce the human impact on the climate and the costs associated with mitigation efforts.

Disappearing Ice

In its February report, the IPCC said ``in some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of the 21st century.''

The fastest rate of summer ice loss predicted by the IPCC's models puts melting at 5.4 percent per decade, lower than the observed record, according to the NSIDC.

``Because of this disparity, the shrinking of summertime ice is about thirty years ahead of the climate model projections,'' Ted Scambos, another scientist with the Boulder, Colorado-based NSIDC, said in the statement.

The study was conducted by the NSIDC and the National Center for Atmospheric Research, and will appear today in the online edition of the journal Geophysical Research Letters. The NCAR said in December the Arctic Ocean may become virtually ice- free in the summer by 2040.

The Arctic sea ice retreated in 2005 to a record low and five of the six biggest annual reductions dating back to 1979 occurred in the past five years, according to the NSIDC.

Failing to Capture

The UN panel's models may fail to capture the full extent of the effect of rising greenhouse gas levels on the ice, according to the study.

Whereas the models attribute about half of the melting to increases in greenhouse gases and half to naturally climate variability, emissions ```may be playing a significantly higher role,'' they said.

The melting of the sea ice is increased by so-called feedback loops, according to the scientists.

The ice reflects sunlight back to space and, when it melts, it leaves exposed dark patches of ocean, which absorb light and increase regional warming.

To contact the reporter on this story: Alex Morales in London at amorales2@bloomberg.net.

Last Updated: May 1, 2007 01:40 EDT