* U.N. talks must focus on piecemeal deals, not treaty
* Mexico meeting faces 'formidable' obstacles
LONDON, Sept 23 (Reuters) - U.N. climate talks must set aside hopes of a treaty to curb global warming and, to avoid failure, must accept rich country emissions targets which many scientists say are inadequate, a U.N. official said on Thursday.
The U.N.-led process failed at a conference in Copenhagen last December to agree a successor to the Kyoto Protocol which expires in 2012. Countries and experts have widely dismissed hopes for a full treaty at the next major meeting in Cancun, Mexico at the end of this year.
A serious risk still lurked that the Cancun conference would hit the same rich-poor rift as Copenhagen over sharing the burden of fighting climate change, said Halldor Thorgeirsson, a director at the U.N. climate agency.
"The obstacles towards a significant outcome in Cancun remain formidable. The likelihood of continued deadlock remains significant, we really need to face that fact," he told a climate conference in London.
"But concerted efforts ... could potentially push Cancun into a range of incremental if not significant progress."
The world must focus on piecemeal deals, he said, for example to curb deforestation, and accept developed countries' existing post-2012 emissions pledges, in order to extend Kyoto and avoid a gap where there was no global deal at all.
"Although the mitigation pledges fall short of expectations ... they are not likely to change in the short-run," said the senior U.N. executive who has long helped steer the talks.
A U.N. panel suggested in 2007 that industrialised countries would have to cut greenhouse gases by 25-40 percent below 1990 levels by 2020 to put the world on track to avoid dangerous warming, but their actual targets would achieve about half that. [ID:nLDE6561GX] [ID:nLDE6371AV]
A formal, new legal treaty to combat climate change would have to wait, added Thorgeirsson.
"A new treaty is by no means the only, single measure of success. Parties ... seem to have accepted that the Cancun conference will not deliver a legally binding instrument. It does not mean that a legally binding instrument will not emerge in the future."
Simply extending the present Kyoto Protocol with new targets may not require ratification by capitals around the world, at least immediately, experts told the conference, an important factor as time runs out before the 2012 cut-off.
Thorgeirsson listed seven areas where he thought agreement was possible in Cancun.
These included: turning national carbon pledges into formal U.N. commitments; designing a system for tracking performance on these pledges; designing a fund for long-term finance; agreeing a long-term, global goal to combat climate change; and launching pilot schemes to protect tropical forests.
China hosts in Tianjin next month the last meeting before the Cancun conference, which runs from Nov. 29 to Dec. 10.
Our Standards: The Thomson Reuters Trust Principles.