(Corrects paragraph 11 to "trillion tonnes" from "billion tonnes")
* Figueres says three quarters of reserves may have to stay in ground
* UN wants "urgent transformation" towards greener future
By Alister Doyle, Environment Correspondent
OSLO, April 3 (Reuters) - The U.N.'s climate chief called on the oil and gas industry on Thursday to make a drastic shift to a clean, low-carbon future or risk having to leave three-quarters of fossil fuel reserves in the ground.
"The time for experimentation, for marginal changes and for conditional response is now over," Christiana Figueres told the International Petroleum Industry Environmental Conservation Association (IPIECA) in a speech in London.
She urged an "urgent transformation" to greener production after top scientists warned on Monday that climate change would damage food supplies, slow economic growth and aggravate the underlying causes of armed conflicts.
Limiting global warming to an agreed U.N. ceiling "means that three quarters of the fossil fuel reserves need to stay in the ground, and the fossil fuels we do use must be utilized sparingly and responsibly," she said.
Oil and gas firms say they are addressing global warming, for instance by focusing on energy efficiency and low-carbon technologies.
In a report on climate change risks on March 31, Exxon Mobil said that all energy sources, including fossil fuels, had to be exploited to meet growing world demand.
"All of Exxon Mobil's current hydrocarbon reserves will be needed, along with substantial industry investments, to address global energy needs," William Colton, vice president of corporate strategic planning, said in a statement.
Figueres has become more outspoken in criticising the fossil fuel industry in recent months as part of efforts to promote renewable sources such as solar or wind power. In November, she called on the coal industry to clean up.
She said oil and gas firms should start by reporting risks to their business after governments agreed in 2010 to limit warming to less than 2 degrees Celsius (3.6 Fahrenheit) above pre-industrial times.
Temperatures are heading to breach the ceiling.
Figueres, the head of the U.N.'s Climate Change Secretariat in Bonn, noted the U.N.'s Intergovernmental Panel on Climate Change says the world has already burnt more than half a budget of a trillion tonnes of carbon if it wants to stay below 2C.
Companies should also take measures such as cutting methane leaks, lobby for an effective price on carbon emissions and invest in carbon capture and storage (CCS), she said.
CCS, which includes technologies to strip carbon dioxide from the flue gases of power plants, would allow continued output by eliminating most carbon emissions.
So far, however, there are few projects.
Saskatchewan Power in Canada will start its $1.35 billion Boundary Dam coal-fired CCS project this year, capturing a million tonnes annually of carbon dioxide in what it says is the world's first post-combustion coal-fired CCS project.
(Reporting By Alister Doyle, editing by David Evans)