Another round of U.N. climate talks closed Friday without resolving how to share the burden of curbing man-made global warming, mainly because countries don't agree on who is rich and who is poor.

China wants to maintain a decades-old division between developed and developing countries, bearing in mind that, historically, the West has released most of the heat-trapping gases that scientists say could cause catastrophic changes in climate.

But the U.S. and Europe insisted during the two-week talks in Bonn that the system doesn't reflect current economic realities and must change as work begins on a new global climate pact set to be completed in 2015.

"The notion that a simple binary system is going to be applicable going forward is no longer one that has much relevance to the way the world currently works," U.S. chief negotiator Jonathan Pershing said.

Countries like Qatar and Singapore are wealthier than the U.S. per capita but are still defined as developing countries under the classification used in the U.N. talks. So is China, the world's second largest economy.

Finding a new system that better reflects the world today, while also acknowledging the historical blame for greenhouse gas emissions, is the biggest challenge facing the U.N. process as it seeks a global response to climate change.

"That is a fundamental issue," said Henrik Harboe, Norway's chief climate negotiator. "Some want to keep the old division while we want to look at it in a more dynamic way."

The U.N. climate talks are based on the premise that industrialized countries must take the lead on climate change by committing to reducing emissions of carbon dioxide and other greenhouse gases. They are also expected to provide money to help poor countries grow in a sustainable way and to protect the most vulnerable nations from rising sea levels, droughts and other consequences of a warming world

Disputes on how to categorize countries going forward was behind much of the procedural wrangling that slowed down the talks in Bonn. Delegates failed to agree on an agenda until the last day, leaving most of the work for a bigger summit in Qatar in November.

A separate dispute between developing countries delayed the appointment of officials to steer the talks. That stalemate was also unlocked on the last day.

The slow pace frustrated climate activists who fear that there won't be enough political will to rein in emissions to avoid dangerous levels of warming in coming decades.

"The talk here doesn't match the action that science says is required," said Mohamed Adow, senior climate change adviser at Christian Aid.

China's lead negotiator Su Wei told The Associated Press that the proposed new deal, which would have binding commitments for all countries after 2020, must be based on the principle of "common but differentiated responsibility" enshrined in previous climate agreements.

"That means we still would continue the current division between developed and developing countries," Su said.

He said China is still a developing country because if you look at wealth per capita, it barely makes the world's top 100. More than 100 million Chinese live below the poverty line, which Beijing has defined as about $1 a day.

Still, Western officials say China's fast-growing energy needs and rising emissions mean it can no longer be off the hook in climate negotiations.

"We need to move into a system which is reflecting modern economic realities," EU negotiator Christian Pilgaard Zinglersen said.

In the early 1950s, China accounted for just 2 percent of global emissions while the U.S. accounted for more than 40 percent, according to Climate Analytics, a climate research group based in Potsdam, Germany.

Today China's share of global emissions exceeds 25 percent, while the U.S. share has fallen toward 20 percent.

China and its supporters reject blame for stalling the climate talks, saying it is the U.S. and other developed nations that are unwilling to live up to their obligations to cut carbon emissions.

The U.S. refused to join the only binding accord to limit emissions — the 1997 Kyoto Protocol — partly because it didn't include China.

Seyni Nafo, spokesman for a group of African countries in the climate talks, noted that the U.S. also said that joining Kyoto would harm the U.S. economy. Years later, the U.N. climate effort still has little support in the U.S. Congress, which includes outspoken climate skeptics.

"We are hoping that they will get on board this time, which is not a given," Nafo said.