By Richard Black
Environment correspondent, BBC News website, Poznan
John Kerry on plans to combat climate change
The US is set to lead the world towards a new climate deal, according to John Kerry - but only if other countries pledge emission curbs too.
The former US presidential candidate said here at the UN climate conference that the aim of agreeing a new global deal next year must remain on track.
But a deal could not work unless it covered all countries, he added.
Mexico indicated intentions to halve emissions by 2050, one of the strongest plans from a developing nation.
The country's environment minister Juan Elvira invited other developing countries to join the Mexican initiative, which could see the establishment of a cap and trade system, enlarging the global carbon market.
But the idea will have to go before the president for final approval, possibly in February.
Environmental groups applauded the Mexican move, which would make it one of the first developing countries to set itself targets for reducing emissions, although they are "aspirational goals" rather than binding commitments.
The conference here marks the halfway point in a two-year process, agreed at the UN meeting last December in Bali, that aims to establish a new global deal on climate change by next year's gathering in Copenhagen.
Some observers doubt that the timescale can be met, bearing in mind that the incoming administration of Barack Obama has yet to become involved in the UN process.
But Senator Kerry, who will shortly take over as chairman of the influential Senate committee on foreign relations, said there could be no slippage.
"We believe it's vital to keep the Copenhagen date on target, and talk of not meeting it is premature," Mr Kerry told reporters.
"We believe it is a date that the science mandates us to meet."
Without a new global deal, he said, temperatures could be between 3C and 5C higher by mid-century than they are now - figures that fit within the projections of the Intergovernmental Panel on Climate Change (IPCC).
But the Copenhagen deal would have to include other parties, including the major developing economies, accepting that they would have to restrict emissions in some way - though perhaps not as deep cuts as Mexico is promising.
"The US will not pass a treaty unless it's a global treaty," he said.
UN Secretary-General Ban Ki-moon was among the speakers who said they looked forward to working with Mr Obama, emphasising that the world was looking to the US "for leadership".
During his election campaign, Mr Obama said he would like the US to adopt targets of reducing emissions down to 1990 levels by 2020 - which would mean a cut of about 15% from current levels - and by 80% by 2050.
Europe has played a very important role in recent years, but the US is emerging now with a new administration, and we look to big countries like China and India, Brazil and South Africa as increasingly playing a constructive role
Achim Steiner, Head of UN Environment Programme
As part of that plan, Mr Kerry said, there would be mandatory caps on US emissions, although the details are still to be decided.
Mr Obama will be pressing for heads of state to attend the Copenhagen summit.
If his vision matches Mr Kerry's, he will also be pressing for tougher targets than any bloc has so far endorsed.
The senator said that none of the numbers on the table - the EU's 20% by 2020, the US return to 1990 levels, the Chinese pledge of a 40% reduction in "carbon intensity" (the amount of carbon produced per unit of GDP) - was enough to stave off dangerous climate change.
Meanwhile in Washington, Mr Obama's picks for the senior environment posts in his administration have emerged.
Carol Browner, a lawyer who headed the Environmental Protection Agency under Bill Clinton, will take on a key new White House role of co-ordinating and developing climate policy.
The appointments have been welcomed by environment groups.
As Mr Kerry was speaking here, EU heads of state were meeting in Brussels with the aim of finalising a package of measures on energy and climate.
Among other things, they are set to endorse the target of cutting 20% of emissions by 2020, or 30% if there is an international deal.
A number of delegations here believe it is vital that the EU shows strong leadership by turning down bids from Poland and Italy, among other countries, to dilute some aspects of the package.
But the head of the UN Environment Programme, Achim Steiner, said there were other options.
"Europe has played a very important role in recent years, but the US is emerging now with a new administration, and we look to big countries like China and India, Brazil and South Africa as increasingly playing a constructive role," he told BBC News.
"I think the first one who would actually be hurt from such a step back in Europe would be Europe itself, because it would actually hurt its own competitiveness internationally."
With countries such as the US, India and China stepping into the renewable energy arena, he said, European countries could find themselves falling behind without incentives to expand these industries.
The Poznan talks are scheduled to end on Friday evening, but delegates believe they may spill over into Saturday, which has become something of a habit in UN climate negotiations.
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