(AP) -- Prospects for a limited deal at the latest climate talks appeared to brighten with the U.S. and China narrowing differences on a key element: how to monitor greenhouse gas emissions.
But other issues that go to the heart of a new global warming treaty - long-term commitments for cutting emissions - proved stubbornly unmoving, and out of reach for any resolution during the annual two-week conference.
Nonetheless, analysts said an understanding on measuring emissions would be an important step that could help break the long-standing deadlock on reducing pollutants that scientists say have caused global temperatures to steadily rise over recent decades.
The World Meteorological Organization is due to present data Thursday on worldwide temperatures this year, and scientists say they expect 2010 to turn out to match the hottest years on record.
The Cancun meeting is the first since the Copenhagen summit last December, which defied early hopes for a broad treaty prescribing emissions reductions and instead ended with a brief statement of principles that fell short of the unanimous approval required.
After a series of acrimonious meetings since then, the tone at the 193-nation Cancun conference appeared markedly improved, especially between the U.S. and China. Over the past year they had repeatedly exchanged accusations of reneging on commitments and undermining the talks.
The ultimate objective of the talks is a treaty to replace the 1997 Kyoto Protocol, which required 37 countries and the European Union to cut emissions by 5 percent by 2012. The United States rejected the accord, partly because it made no demands on rapidly developing countries like China and India.
This week Japan said it was not interested in negotiating an extension of the Kyoto targets, arguing it was pointless unless the world's largest polluters also accepted binding targets.
The fate of the Kyoto Protocol, or the shape of any agreement that succeeds it, is one of the most divisive issues in the negotiations, and no one expected it to be settled at the Mexican conclave.
However, an agreement on measurement, reporting and verification of emissions - MRV in negotiating parlance - would be a morale booster for negotiators, although the details remain to be worked out. The much disputed issue involves how nations account for their actions to limit emissions and to what extent they will allow other countries to review their books.
It also is only one of several elements that negotiators want to adopt as a package in Cancun that has something for everyone.
China's chief negotiator, Su Wei, said the differences with the U.S. over MRV "are not that huge. In general, both countries would like to promote the process" and emerge from Cancun with a deal.
The veteran diplomat said China had put in place a rigorous system for measuring and assessing its carbon emissions and had no objection if other countries examined its reports. "We have no problem with MRV," he said.
Previously, China said only some of its actions would be open to international scrutiny.
Earlier this week, U.S. negotiator Jonathan Pershing said the U.S. and China had "spent a lot of energy in the past month working on those issues where we disagree and trying to resolve them. My sense is we have made progress." He did not specify those issues.
Kathrin Gutman, who follows the talks for the World Wildlife Fund, said an agreement on verification would be an important piece of a deal that could "unlock the larger discussion" on emissions reductions.
She said the two sides had refused to formally discuss the subject as recently as the last preparatory meeting a month ago, which was held in the Chinese city of Tianjin.
The shift apparently derived from compromise proposals by India and Singapore.
Barbara Finamore, the China expert for Natural Resources Defense Council, said the Chinese attitude at Cancun reflected "a sea change" in approach.
"China made a strategic decision to be as positive, open and forthcoming as they can," she said in an interview.
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