Arctic researchers have discovered a clue as to why sea ice in the North is melting so much faster than anyone thought it would.

Scientists have long puzzled over why Arctic sea ice is retreating at up to three times the rate that climate models say it should.

In an effort to answer that question, a group of U.K-based explorers walked more than 500 kilometres of sea ice in the High Arctic, taking temperature readings of the ocean below them.

They found a layer of cold, salty water about 200 metres down that they suspect has come from the melting of first-year ice.

That meltwater has forced the relatively warmer water to the surface, where it's speeding up the decay of more ice.

'Complicated processes'

"We're trying to understand why the ice is melting so fast," said Simon Boxall of the Catlin Arctic Survey. "It's not just down to simple warming. There are more complicated processes."

The speed at which sea ice is disappearing in the Arctic has far exceeded almost all predictions and alarmed climate scientists.

A 2007 paper from the National Snow and Ice Data Center in Boulder, Colo., found that the projections of the UN-sponsored Intergovernmental Panel on Climate Change were already obsolete three years after they were published.

When projections from the panel were compared with actual observations, the authors found that between 1953 and 2006 the sea ice was retreating three times faster than it should have. Between 1979 and 2006, when satellite data was available, the actual retreat was twice as fast as climate models predicted.

The report concluded that sea ice retreat is 30 years ahead of where scientists thought it would be.

"Decay of the ice cover is proceeding more rapidly than expected based on the model simulations," said the report published in Geophysical Research Letters.

Polar trek

The team at the Catlin Arctic Survey, sponsored by the Catlin Group insurance company, thought the answer might lie in different temperatures at different levels of Arctic seas.

Such data is usually obtained from ships. But during the spring, when melting is greatest, there's still too much sea ice for ships to make it through.

So the scientists walked from Borden Island to Ellef Ringnes Island and also from near the North Pole all the way down to the northern tip of Ellesmere Island, slogging about 10 kilometres a day in below-deep-freeze temperatures over rugged, uneven ice.

What they found was a surprise — a layer of seawater about 200 metres below the surface that was actually colder than when it had been measured by previous expeditions.

"That's counterintuitive," said Boxall. "We would expect to see, with global warming, warming conditions generally."

But when they realized that the colder water was also saltier than they expected, an explanation began to suggest itself.

Boxall points out that the older sea ice is, the less salt it contains. Ice that's two or three years old already contains very little salt.

Saltier, denser water

Year-old ice, however, remains fairly salty. And when it melts, it produces meltwater that's denser than the relatively fresh water from older ice.

As multi-year ice declines throughout the Arctic, more of the saltier meltwater from younger ice is mixing into the ocean. That colder, denser water sinks more quickly and forces less dense water from deeper in the ocean up to the surface.

Because fresh meltwater is colder than seawater, that means relatively warm water is being forced upwards. And that, said Boxall, may be part of the reason that sea ice is melting so much faster than anyone thought it would.

"What we're seeing is that [fresh meltwater] being taken away from the surface and replaced by slightly warmer water," said Boxall. "The evidence is that the surface waters are [now] slightly warmer."

More research needed

Boxall cautions that his conclusions are based on a preliminary review of data that the team brought back from the ice.

"We need to compare our results with previous data and with groups from other areas."

A paper is being prepared for publication.

The results do show that the effects of climate change and global warming are not always obvious, suggested Boxall.

"The evidence is that there's something interesting going on. The fact that [the climate] is getting warmer is one reason for the ice melting, but it's more complex than that."