Carbon conundrum prompts some greens to think the unthinkable: nuclear

* Many experts say all technologies needed in climate fight

* U.N. talks in Warsaw working on global climate deal

By Nina Chestney and Alister Doyle

WARSAW, Nov 19 The lack of progress at United Nations talks to agree a deal to tackle global warming is calling into question the insistence of many environmental groups that low-carbon nuclear power can never be the answer.

Japan dismayed many at the 195-nation conference last week by saying it would fail to reach its greenhouse gas emissions targets for 2020 after the 2011 Fukushima disaster because it is replacing nuclear power with the fossil fuels gas and oil.

In Europe, more coal is being burnt due to cheap U.S. exports displaced by the U.S. shale gas boom. The continent's biggest economy, Germany, is also phasing out its nuclear plants by 2022.

Most environmental groups advocate scaling up renewable energy such as wind and solar power but do not support nuclear due to safety issues, cost, and the time it takes to build.

But some experts say their stance is too narrow, making it harder to agree an effective deal in 2015 to cut greenhouse gases and keep temperature rises within safe limits of 2 degrees Celsius (3.6 Fahrenheit).

"If you are thinking seriously about holding climate change to no more than 2 degrees you can't afford to rule out any of the low-carbon technologies," said Nicholas Stern, chair of the Grantham Institute on Climate Change and the Environment at the London School of Economics.

"You need to be investing in all of them - including nuclear," he told Reuters.

Robert Stavins, director of the Harvard Environmental Economics Program, said economists wanted all technologies to be considered, whether it was carbon capture and storage, nuclear power or geoengineering.

"It may turn out to be necessary, so you need to know the possible benefits, costs and risks," he said.

Despite the setback in Japan, many environmental groups are adamant that both high-emissions coal, the main target of their ire in Warsaw, and nuclear can be phased out.

"Nuclear power = dirty power" and "Don't nuke the climate" were among signs held by protesters outside the U.N. meeting on Tuesday.

After shuttering its nuclear industry, Japan said it was easing its emissions goal for 2020 to a maximum 3 percent rise above 1990 levels from a previously planned cut of 25 percent. That is because fossil fuels will take over.

"We need to have everything on the table. We shouldn't be ruling things out," said Nathaniel Keohane, vice-president for international climate at the Environmental Defense Fund, which is among the few green groups not totally opposed to nuclear.

Alden Meyer, representing the Union of Concerned Scientists think tank, said his group was neither pro- nor anti-nuclear.

"I am not getting the sense that many (green) organisations are relaxing their stance on nuclear. If anything, Fukushima has hardened the opposition to nuclear," he said.

Earlier this month, four leading climate experts, including former NASA scientist James Hansen, sent an open letter to green groups urging them to rethink.

"We appreciate your organizations' concern about global warming, and your advocacy of renewable energy. But continued opposition to nuclear power threatens humanity's ability to avoid dangerous climate change," they wrote.

Worldwide, 435 nuclear reactors generate about 13.5 percent of the world's electricity, according to the World Nuclear Association's website. It said more than 70 reactors were under construction around the world.

Last month, Britain signed a deal with French utility EDF to build a 16 billion pound ($26 billion) nuclear plant, the first new one agreed in Europe since Fukushima.

A report by a U.N. panel of climate scientists in 2011 said renewables could make up between 15 and 77 percent of world energy by 2050, depending on the scenario.

So renewables are still the main focus for greens.

"Our concern is that we need to get away from the obsession with nuclear and focus on renewables," WWF-UK chief executive David Nussbaum said. "All the ingenuity, technology and private-sector focus needs to be put into renewables." ($1 = 0.6206 British pounds) (Editing by Dale Hudson)