SAN FRANCISCO - THERE is already enough carbon in Earth's atmosphere to ensure that sea levels will rise several metres in coming decades and summertime ice will vanish from the North Pole, scientists warned.

To mitigate global warming's worst effects, including severe drought and flooding, people must not only cut current carbon emissions but also remove some carbon that has collected in the atmosphere since the Industrial Revolution, they said on Thursday.

'We're a lot closer to climate tipping points than we thought we were,' said Dr James Hansen, director of the National Aeronautics and Space Administration (Nasa)'s Goddard Institute for Space Studies. 'If we are to have any chance in avoiding the points of no return, we're going to have to make some changes.'

'If you see that even a small amount of warming has had a notable effect on the ice, it's a good question what effect future warming will have,' said Pennsylvania State University Professor Richard Alley.

Record melting occurred at the North Pole this summer, when for the first time in recorded history, ships sailed across the Arctic Ocean on water once covered by the polar ice cap.

In the summer of 1980, the North Pole was covered by an ice sheet about the size of the continental United States, but this year the ice would not have covered the states west of the Mississippi River.

Stabilising the climate
'The only hope for a recovery is considerable and persistent atmospheric cooling, but with the warming trend so high, everything is working in concert toward a bluer or totally blue Arctic Ocean,' said Mr Josefino Comiso, a senior researcher at Nasa's Goddard Space Flight Center.

Scientists say global warming is caused by carbon dioxide and other gases that trap heat in the atmosphere.

The gases are produced by burning fossil fuels and are long-lasting: About 20 per cent of the carbon produced by burning coal remains in the atmosphere for at least 1,000 years, Dr Hansen said.

The concentration of carbon in the atmosphere is now about 380 parts per million and increasing by 2 parts per million each year. To stabilise Earth's climate, the concentration needs to fall to at least 350 parts per million, Dr Hansen said.

This goal might be achieved by using alternative fuels and new technology.

Researchers are working on a project that scrubs carbon dioxide from the stacks of coal-burning power plants. The gas is then permanently stored in rock formations and oil wells thousands of feet below Earth's surface, said Ms Julianna Fessenden of the Los Alamos National Laboratory.

But that technology is at least a decade away, and while carbon emissions escalate, some scientists are investigating emergency solutions such as dispersing fine particles to reflect sunlight in the stratosphere.

'There is a sort of desperation and an opportunism that's emerging, because it's obvious that it's going to be difficult to reduce carbon emissions in the near term,' said Professor Richard Turcoat of the University of California, Los Angeles. -- REUTERS