Arctic ocean may lose all its ice by 2040, disrupting global weather

· Scientists say Europe may face intensified storms
· Retreating Antarctic glaciers threaten sea levels

Ian Sample, science correspondent
Friday March 16, 2007
The Guardian

Rapidly thinning Arctic sea ice may have reached a tipping point that threatens to disrupt global weather patterns, bringing intense winter storms and heavier rainfall to western Europe, scientists warn today.

Satellite images taken since 1979 show that ocean ice cover in the region has declined steadily, with an estimated 38,000 square miles now being lost every year. Computer models predict sea ice could vanish from the Arctic ocean completely as early as 2040.

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The loss of Arctic sea ice will have a dramatic impact on polar bears and other species that hunt among the ice floes, but it will also trigger erratic shifts in climate that will be felt around the world, scientists believe. The bitter Arctic fronts that blow across Colorado and the western United States may warm and bring less snow, while winter storm tracks across parts of Europe are expected to intensify and unleash torrential downpours.

Acceleration

Sea temperatures in the Arctic have risen 3C in recent decades due to a buildup of greenhouse gases in the atmosphere, according to Mark Serreze, who led the study at the University of Colorado, Boulder. He fears continued thinning of the ice will lead to a sudden acceleration of melting that will leave the ocean ice-free.

"When the ice thins to a vulnerable state, the bottom will drop out and we may quickly move into a new, seasonally ice-free state of the Arctic," he said. "There is some evidence that we may have reached that tipping point, and the impacts will not be confined to the Arctic region."

Shifting wind patterns in the 1980s and 1990s forced much of the thick sea ice out of the Arctic ocean into the North Atlantic where it drifted south and melted. It was replaced with a thinner coverage of sea ice that ebbs and flows, but is gradually vanishing, leaving more open water to absorb heat from the sun, driving a cycle of warmer waters and increased melting.

"While the Arctic is losing a great deal of ice in the summer months, it now seems that it also is regenerating less ice in the winter," Dr Serreze said. "With this increasing vulnerability, a kick to the system just from natural climate fluctuations could send it into a tailspin." His report is published in the journal Science today.

Writing in the same journal, a team of British scientists raise concerns over four of the largest glaciers in the Antarctic, warning they may pose a disproportionate threat to global sea level rises as they slide into the ocean.

Opposing effects

Thinning of the Antarctic ice sheets is countered, and in some places, compensated for, by heavier snowfall that acts to rebuild the ice packs to around 2km thick. The opposing effects have complicated attempts to assess the future impact that glaciers on the ice sheets are likely to have on sea level rises.

Duncan Wingham at University College London and Andrew Shepherd at Edinburgh University reviewed five years of glacier observations in the Antarctic and identified the four glaciers as retreating in unison, driving the thinning of the ice sheets and representing the greatest risk to sea levels.

The Pine Island and Thwaites glaciers on the western Antarctic ice sheet, and the Totten and Cook glaciers on the eastern Antarctic ice sheet are now sliding into the water between 20% and 100% faster than in recent decades, the researchers report. As the glaciers flow into the surrounding ocean, they begin to melt, together accounting for around 12% of global sea level rises, or 0.35mm per year.

"Although the amounts of water aren't yet that large, the concern is that we simply don't know what's causing this acceleration of these glaciers. It may be that warm ocean water is getting underneath them and making them flow more easily," said Dr Wingham.

"There are relatively few but very large glacial systems that are driving sea level rise and we should be concentrating our efforts on understanding them. We don't know if they're going to slow down or speed up, but there's a lot of ice backed up in them, so there's reason for concern."

Antarctica and Greenland hold enough ice to raise global sea levels by 70m and according to geological records, previous collapses of ice sheets have caused surges in sea levels of up to 20m in less than 500 years. Such a rise, were it to occur today, would have tremendous societal implications.

The four glaciers are similar, lying in deep-seated basins that flow directly into the ocean, said Dr Andrews. "These glaciers are vulnerable to small changes in ocean temperature, such as those that have occurred over the 20th century. A rise of less than 0.5C could have triggered the present imbalance," he said.