The Earth is likely to experience a temperature rise of at least 3C, the UK government's chief scientist says.
The UK's own carbon emissions have gone up recently
Professor Sir David King warned this would happen because world governments were failing to agree on cutting emissions of greenhouse gases.
He told the BBC that nations had to act now to tackle the warming expected to happen over the next 100 years.
And he said even if a global agreement could be reached on limiting emissions, climate change was inevitable.
The UK government and the EU want to try to stabilise 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.
A recent report called Avoiding Dangerous Climate Change, produced by the Hadley Centre, one of the top world centres for projecting future climate, modelled the likely effects of a 3C rise.
It warned the situation could wreck half the world's wildlife reserves, destroy major forest systems, and put 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.
The Hadley forecasts hinge on stabilising the greenhouse gas carbon dioxide (CO2) at a level of 550 parts per million in the atmosphere. Professor King said this was the figure Prime Minister Tony Blair wanted world leaders to agree on.
He admitted politicians were taking a big risk to push CO2 levels as high as 550ppm. This figure is almost double the pre-industrial level of two centuries ago.
But he said the UK government believed 550ppm was the lowest figure achievable worldwide as developing countries continued to increase their emissions, and the US refused to cut its CO2.
The Institute for Public Policy Research (IPPR) has criticised Professor King for accepting global temperatures could rise above 2C.
And 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 three 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.