Scientists fear global warming has passed an ominous tipping point after new Nasa satellite data showed the already relentless melting of the Arctic increased greatly during the Northern Hemisphere's hot summer months.

One expert even speculated that summer sea ice would be gone in five years.

Greenland's ice sheet melted nearly 19 billion tons more than the previous high mark, and the volume of Arctic sea ice at summer's end was half what it was just four years earlier, according to Nasa data.

"The Arctic is screaming," said Mark Serreze, senior scientist at the government's snow and ice data centre in Colorado.

Just last year, two top scientists surprised colleagues by projecting the Arctic sea ice was melting so rapidly it could disappear by summer 2040.

This week, after reviewing his own new data, Nasa climate scientist Jay Zwally said: "At this rate, the Arctic Ocean could be nearly ice-free at the end of summer by 2012, much faster than previous predictions."

"The Arctic is often cited as the canary in the coal mine for climate warming," said Mr Zwally. "Now, as a sign of climate warming, the canary has died. It is time to start getting out of the coal mines."

It is the burning of coal, oil and other fossil fuels that produces carbon dioxide and other greenhouse gases, responsible for man-made global warming. For the past several days, diplomats have been debating in Bali, Indonesia, the outlines of a climate treaty calling for tougher limits on these gases.

What happens in the Arctic has implications for the rest of the world. Faster melting there means eventual sea level rise and more immediate changes in winter weather.

More than 18 scientists said they were surprised by the level of ice melt this year.

Waleed Abdalati, Nasa's chief of cyrospheric sciences, said: "This year the change is so big, particularly in the Arctic sea ice, that you've got to stop and say, What is going on here?'".

For the past 30 years, the data pattern of Arctic ice sheet melt has zig-zagged. A bad year, like 2005, would be followed by a couple of lesser years.

According to that pattern, 2007 should not have been a major melt year, said Konrad Steffen of the University of Colorado, which gathered the latest data. "I'm quite concerned," he said. "Now I look at 2008. Will it be even warmer than the past year?"

Nasa scientist James Hansen, the researcher called the godfather of global warming, will tell scientists at a meeting in California tomorrow that in some ways Earth has hit one of his so-called tipping points, based on Greenland melt data.

"We have passed that and some other tipping points in the way that I will define them," Hansen said. "We have not passed a point of no return. We can still roll things back in time - but it is going to require a quick turn in direction."-AP