Ice is melting so fast in the Arctic that the North Pole will be in the open sea in 30 years, according to leading climatologists.

Ships will be able to sail over the top of the world and tourists will be able visit what was, until climate change, one of the planet's most inaccessible landscapes.

American researchers, assessing the impact of carbon emissions on world climate have calculated that late summer in the Arctic will be ice-free by 2040 or earlier, well within a lifetime.

Some ice would still be found on coastlines, notably Greenland and Ellesmere Island, but the rest of the Arctic Ocean, including the pole, would be open water.

The researchers, who were funded by NASA, said that the ice retreat is likely to remain fairly constant until 2024 when there will be a sudden speeding up of the process.

In 30 to 50 years, they concluded, summer sea ice will have vanished from almost the entire Arctic region.

Their forecast may, however, already be out of date and over-optimistic, said Professor Chris Rapley, head of the British Antarctic Survey.

He said a recent study by the Global Carbon Project suggested that emissions were rising more than twice as fast as in 2000, which was likely to speed up ice-loss even further.

"The study may be an underestimate of when the Arctic summer ice might be all gone," he said. "It could well be their assumptions are more optimistic than they might be."

He described the report as "worrying" but said it fitted into recent findings based on satellite observations of the speed at which ice was retreating.

Over the past 25 years Arctic ice has been reduced by 25 percent. Scientists have long realised that ice reflects heat and as the quantity reduces so, too, does the amount of heat that can be bounced away from the Earth. However, the study team from the National Centre for Atmospheric Research in Colorado, and two U.S. universities, identified warmer ocean currents as an additional factor to be considered.

Disappearing ice is already causing problems for the polar bear. Other wildlife, including seals, are also likely to suffer. The U.S. study team modified one of the climate models used by the United Nations' Intergovernmental Panel on Climate Change to establish what will happen to the ice as carbon emissions rise.

"We have already witnessed major losses in sea ice, but our research suggests that the decrease over the next few decades could be far more dramatic," said Marika Holland who led the study.

"As the ice retreats, the ocean transports more heat to the Arctic and the open water absorbs more sunlight, further accelerating the rate of warming and leading to the loss of more ice."
Melting the sea ice will not lead to an increase in sea levels because it is solid water sitting in liquid water. However, if the ice and snow on Greenland melt sea levels are expected to rise by 23ft.

September ice levels are measured in the models because that is the time of year when it has had longest to melt. The scientists gave warning: "The rate and manner in which sea ice retreat affects the ability of ecosystems and societies to adapt to these changes."

Other climate models were tested to see if the disappearance was replicated and the scientists found rapid retreats in the majority of the simulations.

However, in a statement likely to be seized on by environmentalists, they said that a reduction in carbon dioxide emissions, widely accepted as the prime cause of global warming, would slow down the ice loss significantly: "Reductions in greenhouse gas emissions moderate the likelihood of these events."

Dr Jeff Ridley, a climate scientist at the Met Office's Hadley Centre, was cautious about the results of the study and pointed to previous research which has suggested it will be 65-75 years before the summer ice vanishes. "All our models and the other global models in the IPCC report indicate that the ice won't disappear until 2070-80," he said.

"I'm surprised by the findings but it's not way out of bounds."