Also being discussed are proposals to give industrialized countries credit for building low-carbon nuclear power stations in developing countries, to give nations credit for helping to avoid deforestation, and various schemes to raise the trillions of dollars needed in the coming decades to help poor countries adapt to the effects of their changing climates.
"I get the feeling that people just want to make a fresh start and get on with putting something in place that is going to measure up to the scientific challenge," Yvo de Boer, the top U.N. climate official, said Thursday.
"Any notion of changing the base year was still out of the question for many countries a year or two ago. There is now a willingness to consider that," De Boer told reporters.
The two-week conference in Bonn ends Friday. Talks will reconvene for another round in August in Ghana and again in December in Poland.
A new agreement must be concluded by December 2009 to allow countries time to ratify it before the current accord, the Kyoto Protocol, expires in 2012.
Advancing the starting point to 2005 could accommodate the United States, which has shunned the 1997 Kyoto Protocol but is participating in the negotiations to draft a successor agreement.
But it would be troublesome for countries which have been trying for years to slow their pollution.
The European Union is roughly on track for an 8 percent reduction from 1990 levels within four years, and is committed to a 20 percent target by 2020.
Any such goal would be out of reach for the United States, where greenhouse gas emissions have risen by 16 percent since 1990.
"Because we have done so little over the last 15 years means that we are starting from a different place," said Alden Meyer, of the Washington-based Union of Concerned Scientists.
De Boer said most countries now concede that increase in U.S. emissions must be taken into account.
A bill drafted in the U.S. Senate that failed to reach the floor for debate last week used the 2005 base line, calling for a 4 percent emissions cut by 2012 and a 71 percent reduction by 2050.
Though the bill failed, it likely will be the starting point for U.S. policy discussions next year after a new president is in the White House, Meyer said.
Both presidential candidates Democrat Barack Obama and Republican John McCain have said they would send their own negotiating team to the critical December negotiating round in Poland.
Japanese Prime Minister Yasuo Fukuda also took 2005 as his starting point when he announced details of his "Cool Earth Initiative," including a 60-80 percent reduction in greenhouse gas emissions by 2050.
Fukuda said Japan likely would reach a 14 percent reduction over the next dozen years, which he said would match Europe.
Meyer said, however, that taking different baseline years allowed countries to play a "shell game" with the numbers, and that Japan's commitment works out to be much less than Europe's.