Stagnating and finger-pointing

But progress has stagnated amid foot-dragging and finger-pointing.

Broadly speaking, rich countries want China, India and Brazil ? already major polluters and set to be the big emitters of tomorrow ? to sign up to strong commitments for tackling their gases.

But the poorer countries say rich economies bear the historical responsibility for warming and should show goodwill by offering deep emissions cuts of their own.

Until now, the lead has been taken by the European Union, which has unitarily decided to cut its emissions by 20 percent by 2020, and offered to deepen this to 30 percent if others follow suit.

Japan, under the outgoing government, had proposed a reduction of eight percent over 1990. The United States would see a reduction of about four percent, under a bill going through Congress.

Huq said it was unclear whether Hatoyama's announcement would trigger a deeper EU cut, as the mechanism by which the Europeans would extend their offer is unclear.

Greenpeace International's Martin Kaiser hoped Hatoyama's "climate leadership" would be emulated by US President Barack Obama.

But he and others also sounded a note of caution, seeing apparent conditions attached to the announcement and fearing the blocking ability of Japan's powerful business lobby, which wants a cut of no more than six percent.

Hatoyama said a "prerequisite" for Japan was "a highly ambitious accord with participation by all major countries".

UNFCCC Executive Secretary Yvo de Boer praised Hatoyama's target for being "commensurate with what science says is needed".

Under a scenario described by the UN's Intergovernmental Panel on Climate Change Change (IPCC), rich countries would have to make cuts of 25-40 percent in these heat-trapping emissions by 2020 as compared with 1990 levels to peg global warming to two degrees Celsius above pre-industrial levels.

Major emerging economies that are already big carbon polluters would have to brake their emissions growth, although by how much is not spelt out.