By Alex Kirby BBC News Online environment correspondent

The UK government's leading scientist says levels of carbon dioxide in the atmosphere already represent a danger.

Professor Sir David King told a London audience the world had to adapt to prepare for significant changes ahead, and also to reduce greenhouse gases.

He said climate change was "the most serious issue facing us this century and beyond", needing global solutions.

On present trends, Sir David said, the world was just 60 years from triggering an irreversible climate disaster.

Sir David, the government's chief scientific adviser, was delivering the annual Greenpeace business lecture, entitled Global Warming: The Science Of Climate Change - The Imperatives For Action.

'Tipping point'

He said measurements of atmospheric CO2 taken at the Mauna Loa observatory in Hawaii, and published earlier this year, were significant.

They showed that while carbon levels had increased in recent years by an average of 1.5 parts per million (ppm) annually, in 2002 and 2003 the increase had been more than 2ppm.

It's a damned good thing we put the London Thames Barrier up. A flood would knock out the City of London and cost about £30bn Sir David King

Levels had risen by 2.08ppm in 2002 and 2.54 the following year. Sir David said: "This is taking us up into relatively dangerous levels of CO2 for our planet."

If warming temperatures one day melted the Greenland ice cap, he said, that would mean global sea levels would ultimately rise by 6-7m (19-22ft).

"Is there a point where the melting becomes irreversible?" he asked. "Yes, there is. When the temperature around Greenland is 2.7C above the pre-industrial level - that is the tipping point.

"We're already 0.6C above it, and to avoid raising temperatures too far we should prevent atmospheric CO2 going beyond 500ppm."

Atmospheric concentrations have risen from about 280ppm before the Industrial Revolution to 315 in 1957, when the Mauna Loa data collection began, to a high recently of 379ppm.

Thames concern

Asked how long it would take to reach 500ppm, Sir David told BBC News Online: "We're now close to an annual rise of 2ppm, so on present trends it will take us about 60 years - assuming we're not on an exponential growth curve."

Sir David is the UK government's chief scientific adviser

He said much of the UK could face an increased risk of flooding as the climate changed, and said the Thames Barrier, built to protect London from catastrophic floods, was being used six times a year, not once every three to five years as planned.

"It's a damned good thing we put it up," he said. "A flood would knock out the City of London and cost about £30bn."

Sir David also challenged the argument that rail is more environmentally friendly than road.

As trains were now running at up to 125mph (200km/h), he said, they were responsible for much more carbon than cars.