By Alex Kirby
Environment correspondent, BBC News website
The world can ward off a dangerous rise in temperature much more cheaply than many people think, a UK scientist says.
Friendlier farming is one way to tackle climate change
Professor John Schellnhuber, of the University of East Anglia, believes the cost of averting runaway climate change could be as low as 0.3% of global GDP.
He is telling a high-level conference in the German capital, Berlin, that the world can avoid a major catastrophe.
But he says there is no simple solution for reducing greenhouse gases, and the world will need a mix of strategies.
Professor Schellnhuber is research director of the Tyndall Centre for Climate Change Research, which is based at the university.
He is presenting his analysis at a conference on Wednesday at the British Embassy in Berlin, entitled Climate Change: Meeting The Challenge Together.
The Queen is expected to attend a reception for the participants, who will see a video address by the prime minister, Tony Blair.
Professor Schellnhuber says capacity-building, reducing vulnerability to the effects of a changing climate, and increasing resilience will all be necessary, but will not be enough to meet the threat.
His list of 15 possible ways of reducing emissions of carbon dioxide (CO2), the main greenhouse gas produced by human activities, is exhaustive.
It includes more efficient vehicles and buildings, reduced vehicle use, capturing CO2 at the point of emission, and where possible replacing coal, oil and gas with other fuels.
Professor Schellnhuber also argues for cutting deforestation, and for ploughing methods which release less CO2.
Controversially, in the eyes of some, he proposes the use of nuclear power to replace coal.
In a key part of his speech, he says the cost of preventing climate change reaching dangerous levels would be between 0.3 and 0.5% of the world's gross domestic product.
The way to achieve this, he believes, is by trying to mitigate the probable effects of a warming world, and at the same time adapting to them.
So the infrastructure of our cities must become "less brittle and more robust", to help them to withstand harsher conditions.
Professor Schellnhuber says the atmospheric concentration of CO2 has risen by over 30 % since the pre-industrial era to stand today at about 380 parts per million (ppm).
He says: "The magnitude and rate of the increase is unprecedented in the past two million years. The concentration is likely to reach 1,000ppm by 2100 if no action is taken to reduce emissions.
Professor John Schellnhuber: We can do it
"A CO2 doubling is likely to cause an increase in global mean surface temperature of 1.5-6C. Recent results suggest the 'most likely' increase is around 3C.
"The maximum global mean surface temperature since the emergence of H. sapiens was 1.5C above the present."
Professor Schellnhuber told BBC News: "My main message is that there's no magic bullet for climate change - but if we have a portfolio strategy, we can solve it.
Action needed now
"In the scientific community we think an atmospheric CO2 concentration of about 450ppm - the equivalent of a 2C rise in temperature - is about the most we can allow.
"Beyond that you start to reach the 'tipping points', the unpredictable areas where rapid changes can set in.
"We need to act over the next 50 years, though some of what has to be done will take a century or more to work through.
"Remember what the economist John Maynard Keynes said about Britain in the second world war: 'We threw good housekeeping to the winds. But we saved ourselves... and helped save the world'."