The share of renewable energy will have to rise "dramatically" if the world is to have a chance of limiting global warming to a maximum 2 C temperature rise, a leading expert said.
Ottmar Edenhofer, co-chairman of a scientific group due to present a UN report on renewable energy in 2010, said clean technology such as wind and solar power needed a big role even if the world also turned increasingly to nuclear power.
"To achieve a 2C target the share of renewables has to be increased substantially and dramatically," he told the Reuters Global Climate and Alternative Energy Summit in a telephone interview.
"This is valid across all the scenarios I have seen," said Mr Edenhofer, who is also chief economist of the Potsdam Institute for Climate Impact Research. He gave no precise figures for the needed rise.
Renewable energies dominated by biomass - such as firewood - and including wind, hydro, solar and tidal power made up 13 per cent of world energy demand in 2006, according to the International Energy Agency. Fossil fuels make up about 81 per cent and nuclear power the other six.
The renewables report, by the UN's Nobel Prize-winning Intergovernmental Panel on Climate Change, is to be released in December 2010, a year after a new UN climate pact is due to be agreed in December in Copenhagen.
To cut reliance on fossil fuels, options include renewable energies, boosting nuclear power, seeking to improve energy efficiency or capturing and burying emissions of carbon dioxide, the main greenhouse gas released by burning fossil fuels.
"In most of these scenarios renewables play an important role even if you make nuclear and CCS (carbon capture and storage) a large part of your portfolio," Mr Edenhofer said.