14th April 2006
The world is likely to suffer a temperature rise of more than 3C, the government's chief scientist warned.
That would put up to 400 million people worldwide at risk of hunger, said Professor Sir David King in a report based on computer predictions.
He told the BBC the world had to act now to tackle global warming expected to happen over the next 100 years.
He said even if international agreement could be reached on limiting emissions, climate change was inevitable.
The UK Government and the EU want to stabilize the climate at an increase of no more than 2C, but the US refuses to cut emissions and those of India and China are rising quickly.
The government report says a 3C rise would cause a drop worldwide of between 20 and 400 million tonnes in cereal crops and put about 400 million more people at risk of hunger.
Professor King told BBC Radio 4's Today Programme: "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.
In a report for the UK's prestigious Hadley Centre he warned that even if international agreement could be reached on limiting emissions – for CO2 at 550 parts per million in the atmosphere – this could result in an increase of 3C or more.
The government's ambition to cut CO2 by 60% by 2050 was founded on a report from the Royal Commission on Environmental Pollution that assumed 550ppm was a safe level.
But to hold temperature rise below 2C with a high degree of certainty global levels of CO2 should be kept below 400ppm, the Hadley Centre says.
Scientists admit the Earth's mechanisms are so complicated their calculations are uncertain.
The Institute for Public Policy Research (IPPR) has criticised Professor King for accepting global temperatures could rise above 2C.
Friends of the Earth director Tony Juniper said: "It is technologically possible to significantly reduce our emissions and deliver 2C – Professor King should be pressing for government polices to deliver on this rather than accepting the current lack of political will and talking of 3 degrees as an inevitability."
So far, the US, the world's largest emitter of greenhouse gases, has been unwilling to debate a CO2 threshold.
President Bush's chief climate adviser, James Connaughton, said he did not believe anyone could forecast a safe level and cutting greenhouse gas emissions could harm the world economy.
