Japan should shun a politically inexplicable option of a rise in greenhouse gas emissions when it sets a 2020 target in coming days, the U.N.'s top climate change official said on Wednesday.

Yvo de Boer told Reuters he hoped that both Japan and Russia, the two largest industrialized nations which have not yet outlined 2020 goals, would issue goals for cuts during June 1-12 talks in Bonn working on a new U.N. climate treaty.

Japanese Prime Minister Taro Aso is to announce by mid-June a choice from six 2020 options outlined by a government advisory panel that range from a rise in emissions of 4 percent compared to 1990 levels to a cut of 25 percent.

I think it would be politically inexplicable for Japan to take on a target that would amount to an increase in its emissions above its current Kyoto commitment, de Boer said of the weakest goal, which is backed by many industries.

Under the U.N.'s Kyoto Protocol for curbing global warming -- named after the Japanese city where it was agreed in 1997 -- Japan has committed to cut it emissions to at least 6 percent below 1990 levels by 2008-12, he noted.

Japan is the world's fifth largest emitter of greenhouse gases, mainly from burning fossil fuels and its emissions were 8 percent above the 1990 benchmark in 2007. By that yardstick, a four percent target implies a slight cut.

De Boer said Aso had indicated that the minus 25 would be prohibitively expensive amid a recession. That meant Japan's goal would be somewhere in between the extremes, said de Boer, who is head of the U.N. Climate Change Secretariat in Bonn.

EFFICIENT

Japan argues that its economy is efficient compared to other developed nations partly because it cleaned up its industry after a 1970s shock of high oil import prices.

I hope that they (Japan) submit their number at this session, de Boer said during the 181-nation talks on a new climate treaty due to be agreed in December in Copenhagen.

Developing nations such as China and India want rich nations on average to sign up to cut emissions by at least 40 percent below 1990 levels by 2020 to avert the worst effects of climate change such as heatwaves, extinctions and rising sea levels.

They say developed nations have to outline promised cuts in emissions -- both national and collective goals -- by June 17, or six months before a Copenhagen deal.

De Boer said it was too early to tell if the deadline would be met. It's not a legal requirement but it was a decision that those ranges would be agreed here. So if they're not, that deadline will be missed, de Boer said.

There are still numbers missing from Japan and Russia. Once the list is complete the question is: 'can we reach an agreement on an aggregate figure, and on individual figures?'.

Developed nations say they have no intention of unveiling a collective goal in Bonn. De Boer reiterated his view that cuts promised so far are on average not deep enough.

A study issued by the Potsdam Institute for Climate Impact Research on Monday estimated that 2020 cuts outlined so far by developed nations added up to a reduction of between 8.2 and 14.9 percent below 1990 levels.

-- For Reuters latest environment blogs click on: blogs.reuters.com/environment/

(Editing by Janet Lawrence)

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