* Germany's opposition Greens climb to 20 percent in polls
* Merkel move to extend nuclear power will lift Greens
* Greens could win mayor's office in Berlin next year
By Erik Kirschbaum
BERLIN, Sept 7 (Reuters) - Germany's Greens, the world's most successful environmental party, have surged to record highs in opinion polls as voter frustration over Chancellor Angela Merkel's centre-right government climbs to new heights.
The Greens, who have doubled in strength to around 20 percent in national surveys in the last year, also are expected to be the chief beneficiary of Merkel's controversial decision this week to extend the lifespan of nuclear power in Germany.
The Greens have been siphoning off support from Merkel's conservatives and her coalition partners, the Free Democrats (FDP), in their climb to levels around 20 percent -- putting them close to the centre-left Social Democrats (SPD) nationally.
In fact, the Greens -- who shared power at federal level as junior partners with the SPD from 1998 to 2005 -- are ahead of the SPD in polls in the city-state of Berlin before a 2011 election that could see a first Greens-led state government.
"It would be a sensational development if the Greens ended up the strongest party in a German state," said Gerd Langguth, a political scientist at Bonn University, of polls showing the Greens at 27 percent in the capital, in front of the ruling SPD.
The Greens also are polling 24 percent in Baden-Wuerttemberg -- the same as the SPD -- ahead of a key state election in 2011. The Greens' firm opposition to a gigantic new rail station in Stuttgart has galvanised support in that southwestern state.
"The Greens are doing so well because they're not in government and a lot of people are just frustrated by Merkel's performance," Langguth added. "They're fed up with the disputes. The Greens' anti-nuclear stance will definitely help them."
An ARD TV poll last week found 81 percent of Germans dissatisfied with Merkel's government.
The Greens, who masterminded Germany's nuclear exit when in power with the SPD, should take advantage of public discontent over Merkel's decision to extend atomic energy beyond 2021.
"The government's move to extend nuclear power will benefit the Greens," said Uwe Andersen, a political scientist at Bochum University. "It should push them even higher in the polls."
CLIMATE CHANGE
The party, which traces its roots to the peace movement and anti-nuclear protests of the late 1970s, is also well placed to benefit from worries about climate change thanks to its uncompromising support for renewable energy.
Germany now gets about 16 percent of its electricity from renewables, making it the world's leader in wind and solar energy generation. By contrast, Merkel has pushed to extend nuclear power use despite overwhelming public opposition.
Her coalition ended months of wrangling on Sunday by agreeing the country's 17 nuclear power plants should get an extension of about 12 years each on average.
"The other parties have all been rather daft," said Manfred Guellner, head of the Forsa polling institute, when asked to explain the Greens' steady rise since winning 10.7 percent in the 2009 federal election.
"In an era when people are interested in green issues -- like climate change and the environment -- it's logical voters turn to the original rather than imitations of the original."
Guellner said the SPD, CDU and FDP all have tried to paint themselves green in recent years -- in vain.
The Greens also helped themselves by opening up to new tie-ups other than with the SPD. They rule in coalition with the CDU in Hamburg and with the CDU and FDP in Saarland -- that has made them more attractive to moderate conservatives.
Conservatives had reviled the Greens as dangerous militants when they first entered parliament in 1983 wearing dungarees and carrying potted plants. The Greens had viewed conservatives as handmaidens of an imperialistic U.S. nuclear arms buildup.
Times change.
"The Greens have become a middle-of-the-road movement," said Langguth in Bonn. "They're not a radical left-wing party now."
(Editing by Michael Roddy)