UN climate chief Christiana Figueres expressed "regret" at the move and others at the worldwide negotiations in Warsaw attacked it as a fresh blow to an already-troubled process.
But Japan stood its ground and said the about-turn had been forced by the Fukushima nuclear accident in 2010.The country announced it would aim for a 3.8-percent cut in emissions by 2020 over 2005 levels, replacing an earlier goal to cut them by 25 percent from 1990 levels.
"I want to assure you my country is still ambitious on climate change. My prime minister is committed to the climate change challenge," embattled envoy Hiroshi Minami told journalists at the annual 11-day talks on limiting carbon emissions.
The previous, more ambitious goal, had been based on the assumption that ever more of Japan's energy would come from nuclear power stations, which turned out not to be the case with no plants currently operational.
"Japan is a highly advanced economy and they have made impressive advances both in increasing their efficiency as well as solar energy investments and our sense is that Japan can and will continue those and will soon see that the current target is actually conservative," Figueres said at a separate briefing.
The European Union urged Japan to consider the consequences of its actions, while the Alliance of Small Island States (AOSIS) said the move was "a huge step backwards" for the global effort.
In a press statement, the European Commission said EU members "recognise" Japan's difficulties after the 2011 earthquake and tsunami, which shuttered its nuclear energy sector, but said it should nevertheless keep its promise.
"The European Union and its 28 Member States call on Japan to consider the implications of the new target for Japan's contribution to international mitigation action." AOSIS, gathering poor island nations vulnerable to rising seas, accused Japan of retreating on promises it made at the 2009 UN climate summit in Copenhagen.
"AOSIS is extremely concerned that the announcement represents a huge step backwards and puts our populations at great risk," it said.
The grouping pointed to Super Typhoon Haiyan, which ravaged the Philippines, as "just the latest in a series of climate-related extreme weather catastrophes" that would increase under global warming.


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