The poorest countries will be most vulnerable to these effects, it adds. 

The report, Avoiding Dangerous Climate Change, collates evidence presented by scientists at a conference hosted by the UK Meteorological Office in February 2005.

The conference set two principal objectives: to ask what level of greenhouse gases in the atmosphere is too much, and what the options are for avoiding such a level.

Environment Secretary Margaret Beckett said the report's conclusions would be a shock to many people.

"The thing that is perhaps not so familiar to members of the public... is this notion that we could come to a tipping point where change could be irreversible," she told BBC Radio 4's Today programme.

"We're not talking about it happening over five minutes, of course, maybe over a thousand years, but it's the irreversibility that I think brings it home to people."

Vulnerable ecosystems

The report sets out the effects of various levels of temperature increase.

Greenland heats up Enlarge Image

The European Union (EU) has adopted a target of preventing a rise in global average temperature of more than two degrees Celsius.

But that, according to the report, might be too high, with two degrees perhaps enough to trigger melting of the Greenland ice sheet.

This would have a major impact on sea levels globally, though it would take up to 1,000 years to see the full predicted rise of 7m.

Above two degrees, says the report, the risks increase "very substantially", with "potentially large numbers of extinctions" and "major increases in hunger and water shortage risks... particularly in developing countries".

'Without delight'

The report asked scientists to calculate which greenhouse gas concentrations in the atmosphere would be enough to cause these "dangerous" temperature increases.

Currently, the atmosphere contains about 380 parts per million (ppm) of carbon dioxide, the principal greenhouse gas of concern, compared to levels before the industrial revolution of about 275ppm.

To have a good chance of achieving the EU's two-degree target, levels should be stabilised at 450ppm or below, the report concludes.

But, speaking on Today, the UK government's chief scientific adviser, Sir David King, said that was unlikely to happen.

"We're going to be at 400 ppm in 10 years' time, I predict that without any delight in saying it," he said.

"But no country is going to turn off a power station which is providing much-desired energy for its population to tackle this problem - we have to accept that.

"To aim for 450 (ppm) would, I am afraid, seem unfeasible."

"There isn't one, but at the same time people do smoke and live until they're 90," he told Today.

On the other question asked at the 2005 conference - what are the options for avoiding dangerous concentrations of greenhouse gases in the atmosphere? - the report says that technological options to reduce emissions do exist.

It concludes that the biggest obstacles to the take up of technologies such as renewable sources of energy and "clean coal" lie in vested interests, cultural barriers to change and simple lack of awareness.