World leaders converge on New York and Pittsburgh this week for pivotal talks in the two-year effort to remake global climate rules, with success far from assured.
In New York, UN Secretary-General Ban Ki-Moon opens a top-level "Climate Summit" on Monday, kicking off a week peppered with policy debates, meetings and the informal chatter of diplomats attempting to zero in on a deal.
Climate negotiators have spent the last two years working toward a make-or-break summit in Copenhagen this December, which is expected to ink new targets for global emissions beyond 2012, when the Kyoto Protocol expires.
Ban has called on leaders attending this week's meetings to "publicly commit to sealing a deal in Copenhagen," as concerns mount that time is running out.
Despite months of extensive talks, sharp differences still exist between rich and poor countries over a future climate change treaty, with funding for reform emerging as one the key blocks to progress.
Climate negotiators from the world's 17 largest developing and developed economies met in Washington on Thursday and Friday, for talks described by the top US climate envoy as a "pretty full ventilation of views."
"I think there was some narrowing of differences," Todd Stern, the US special envoy for climate change, said after the two-day talks. But he acknowledged, "there are plenty of differences that remain."
A series of meetings this autumn, beginning with those at the United Nations and the G20 next week, aims "fundamentally to narrow differences in an effort to get us toward a successful outcome in Copenhagen," he said.
Environment expert Michael Levi of the Council on Foreign Relations warned against a "very dangerous" temptation to use the UN summit to crank up pressure for a final deal in Copenhagen.
"It will be a much better outcome if the heads of state set a realistic agenda for their negotiators," he told AFP, noting the UN negotiations "are still fairly primitive."
At the G20 meeting in Pittsburgh of the world's biggest industrialized and developing nations, which account for 80 percent of the world's carbon dioxide emissions, leaders are expected to discuss the vexed question of who pays for reform.
"There are actually a lot of important questions that have to do with kind of the structure, architecture of a financial package," said Stern.
Clarification, he said, is needed on where the money comes from, through which institutions it is channeled and how financing decisions are taken.
On Friday, European Union leaders called on rich countries to provide at least 7.3 billion dollars (five billion euros) next year to help poor nations tackle climate change.
They estimated as much as 147 billion dollars (100 billion euros) a year might be need for poor countries each year by 2020.
"The G20 should recognize the need to fast-start international public support for addressing urgent climate financing needs in developing countries," the EU leaders said in a statement.
EU ambassador to the United States John Bruton earlier called on Washington to act, warning a deal at Copenhagen might be made more difficult if the US Congress does not pass a climate change bill this year.
"It would open the United States to the charge that it does not take its international commitments seriously, and that these commitments will always take second place to domestic politics," Bruton said.
Levi cautioned that even "if you have a bill in the Congress, that doesn't mean you have a deal... A bill is necessary, but it's not enough."
President Barack Obama is expected to respond to some of those criticisms when he addresses the UN meeting on Tuesday.
Stern said Obama will likely highlight what his fledgling administration has done to tackle climate change and what steps should be taken in the future.
"Eighty billion dollars of our stimulus is devoted to green investments," noted Stern, pointing to a recent 787-billion-dollar plan to kick-start the US economy.
Both the European Union and the United States are facing pressure from poorer nations to act.
On Monday, a group of 42 low-lying and largely developing nations will renew their demand that any Copenhagen deal limit temperature increases to less than 34.7 degrees Fahrenheit (1.5 degrees Celsius).
"Current pledges for emissions reductions put forward by these (industrialized) countries risk temperature increases in excess of three degrees centigrade" (37.4 degrees Fahrenheit), the Alliance of Small Island States (AOSIS) said.