By Alex Kirby
BBC News Online environment correspondent
The cuts the world will have to make in emissions of carbon dioxide are so huge it will have to find other ways to deal with the gas, a British scientist says.
Carbon can be stored in old oilfields
He is Professor John Shepherd of the UK's Tyndall Centre for Climate Change Research, a leading scientific group.
Professor Shepherd says this will mean studying ways to store carbon and alter the Earth's albedo (its reflectivity).
He also believes nuclear power may be needed to fill the gap until cleaner sources can replace most fossil fuels.
Professor Shepherd was speaking at a seminar on emergency responses to climate change held by the Green College Centre for Environmental Policy and Understanding, Oxford.
While the historical record showed the climate had been fairly stable, he said, it had sometimes been highly and rapidly variable.
He said: "It's almost certain climate change won't be gradual, but will proceed by fits and starts and jerks and bumps."
But to stay near the bottom end of the range of warming forecast for the next century would mean cutting emissions of carbon dioxide (CO2), the main greenhouse gas caused by human activities, by more than half.
For industrialised countries, cuts of 90% or more would probably be needed, Professor Shepherd believes.
Renewable energy could produce only about a fifth of the cuts needed, and so the world should research other methods, including possibly nuclear power and macro-engineering solutions.
Professor Shepherd said these could range from storing ("sequestering") CO2 in deep aquifers or at depths of more than 3,000m (9,850ft) in the oceans to mixing it with serpentine to produce magnesite and burying the resultant solid waste.
He put the cost at $50 (42 euros) per tonne, and falling.
Storing CO2 in trees and soils, as envisaged by the Kyoto Protocol, the international climate treaty, he estimated, could probably cope with no more than about 100 billion tonnes of carbon.
But the amount likely to be burnt this century was at least 10 times as great.
Another possibility, Professor Shepherd said, was to increase the Earth's albedo so as to reflect more heat back out into space: a 1% albedo increase would roughly balance a doubling of CO2.
Methods could include releasing reflecting micro-balloons into the stratosphere, enhancing low-level clouds, or putting a very large mirror in space.
But Professor Shepherd said none of these would reduce the growing acidification of the oceans' surface by CO2, something he said was harming creatures like corals and increasingly worrying scientists.
The answer may lie in space
Economic incentives would be needed to encourage change, and the cost would be modest. A carbon tax of 50 euros ($59) a tonne would probably be enough to make sequestration attractive.
That would correspond to about 100 euros ($118) per person per year for the UK and the rest of Europe, or about three pence (five cents) per litre on the price of fuel in the UK. And it could be made revenue-neutral by reducing value-added tax from 17.5% to 15%.
Professor Shepherd said his proposals were an insurance policy in case there were no technological breakthroughs within the next 50 years, like the development of a hydrogen economy.
He said the investment needed over the next two or three decades would be on the scale of that required to send men to the Moon.