 The UK's own carbon emissions have gone up recently |
That would cause drought and famine and threaten millions of lives, said Professor David King in a report based on computer predictions.
Tony Blair wants a global consensus on stabilising greenhouse gas emissions, blamed for climate change. The government shares the EU's 2C limit.
The US refuses to cut emissions and those of India and China are rising.
The government report says a 3C rise would cause:
A drop worldwide of between 20 and 400 million tonnes in cereal crops About 400 million more people at risk of hunger Between 1.2bn and 3bn more people at risk of water stress
At such a temperature, it said, few ecosystems (like natural forests) could adapt; half of nature reserves would cease to be worthwhile and a fifth of coastal wetlands would be lost.
Agreement 'unlikely'
Prime Minister Tony Blair said recently in New Zealand he wanted the world to agree a level for stabilising greenhouse gas emissions.
He wants to stabilise the climate at an increase of no more than 2C, according to the Department for Environment, Food and Rural Affairs and Downing Street.
Professor King told BBC Radio 4's Today Programme it was unlikely Mr Blair would gain international agreement on this target.
But he said: "We don't have to succumb to a state of despondency where we say that there is nothing we can do so let's just carry on living as per usual.
"It is very important to understand that we can manage the risks to our population.
"What we are talking about here is something that will play through over decades - we are talking 100 years or so. We need to begin that process of investment."
He said it would be a major challenge for developing countries, in particular.
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His comments define the scale of the mis-match between climate science and climate politics and highlight a discrepancy between the government's stated goals and the figures on which its policies are based.
The government's domestic ambition to cut CO2 by 60% from 1990 levels by 2050 was founded on a report from the Royal Commission on Environmental Pollution that assumed, given the level of knowledge at the time, that 550ppm was a safe level.
Threat
The forecast effects of a 3C temperature rise are made in Avoiding Dangerous Climate Change, a report based on computer modelling by the UK's Hadley Centre, a world leader in climate projection.
To hold temperature rise below 2C with a high degree of certainty, it says, global levels of CO2 should be kept below 400ppm.
The 550ppm level envisaged by Professor King is almost double the pre-industrial level of two centuries ago.
He said it might be unavoidable, given the refusal of the US to cut emissions and the need for China and India to increase their economies to tackle poverty.
 Professor King says the government is taking a big risk |
This uncertainty has led critics to accuse them of either exaggerating the threats to the planet, or under-playing them.
The government's favourite think tank, the Institute for Public Policy Research (IPPR), has criticised Professor King for being prepared to countenance global temperatures rising above 2C.
Environment specialist Tony Grayling said the government should be doing much more at home to tackle emissions - which have risen under the Labour government - and pushing for stricter targets internationally.
Environmentalists argue the government has no right without public debate to settle on a figure so important and so controversial.
So far, the US, the world's largest emitter of greenhouse gases, has been unwilling to enter a debate on a threshold for CO2.
President Bush's chief climate adviser James Connaughton said recently that he did not believe anyone could forecast a safe CO2 level.
He said cutting greenhouse gas emissions could harm the world economy, which would have damaging effects of its own.
