A European Union report says a United States proposal to cut greenhouse emissions slowly at first before making deeper cuts later will increase the risks of irreversible climate damage.
The report by Sweden, which holds the EU's rotating presidency, says it is not possible to fully compensate in future decades for higher emissions in the earlier years in a so-called "delayed action scenario".
The EU has pledged to cut emissions of carbon dioxide to 20% below 1990 levels by 2020 and will cut by 30% if other nations follow suit.
By contrast, the US is moving towards agreeing a target of cutting emissions 17% by 2020 and 83% by 2050 from the higher baseline of 2005.
The report says the EU should continue to press for an ambitious international outcome regarding 2020 commitments.