Also last year, then Japanese Prime Minister Shinzo Abe proposed a global target to halve greenhouse gases by 2050. The target was shrugged off as too vague and lacking teeth without binding targets.

But the 2050 date is still being discussed by some of the world's largest greenhouse gas emitters as a target for imposing reductions. Such a target would be too far off for businesses to start taking meaningful action to fight climate change, Yvo de Boer, the head of the UN Climate Change Secretariat said in a telephone interview.

He said 2050 targets would be an easy way for politicians to push the hard work of cutting emissions into the future because most of them would be dead by then. "2050 is in a way committing the unborn, and I think that the signal that businesses are looking for is where rich nations intend to be in 2020," he said.

Japan's current Prime Minister Yasuo Fukuda is set to host the Group of Eight summit in July. He is under pressure on climate change and is likely to urge major emitters to each set targets for reducing carbon dioxide to be achieved before 2050, Japanese media has said.

De Boer said a 2050 goal is among the things being discussed by a group of major emitters led by the United States. But he said this could complicate the setting of a nearer goal that would spur businesses to start taking real steps on fighting climate change.

"If it's hard to fix the nature of something for 2050 when most politicians will be under the ground, how much more difficult is it going to be to be clear on 2020?" said de Boer.

"I really hope that the Japanese G8 presidency can provide a breakthrough on that sense of direction." he said.