WARSAW, Poland (AP) — Rich and poor nations are struggling with a yawning rift at the U.N. climate talks as developing countries look for new ways to make developed countries accept responsibility for global warming — and pay for it.

With two days left, there was commotion in the Warsaw talks Wednesday after negotiators for developing nations said they walked out of a late-night meeting on compensation for the impact of global warming.

"We do not see a clear commitment of developed parties to reach an agreement," said Rene Orellana, head of Bolivia's delegation.

U.S. climate envoy Todd Stern downplayed the dispute, saying American negotiators who had attended the meeting were surprised to hear of a walk-out.

"The meeting ended with everyone leaving," Stern told reporters.

Contrasting views on what's been said and done in closed discussions is not unusual in the slow-moving U.N. effort to curb global warming, which has often been held back by mistrust between rich and poor countries. The talks in Warsaw on a new global climate deal in 2015 have been going on since Nov. 11.

The question of who's to blame for global warming is central for developing countries, who say they should receive financial support from rich nations to make their economies greener, adapt to climate shifts and cover the costs of unavoidable damage caused by warming temperatures.

Also, they say the fact that rich nations, historically speaking, have released the biggest amounts of heat-trapping CO2 by burning fossil fuels for more than 200 years means they need to take the lead in reducing current emissions.

In Warsaw, developing nations are coming up with fresh ways to make their point. Brazil has proposed creating a formula to calculate historical blame.

"They must know how much they are actually responsible ... for the essential problem of climate change," Brazilian negotiator Raphael Azeredo said.

Developed nations blocked that proposal, however, saying the world should look at current and future emissions when dividing up the responsibility for global warming.

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China, considered a developing nation at these talks, overtook the U.S. to become the world's biggest carbon polluter in the last decade, and developing countries as a whole now have higher emissions than the developed world.

To focus only on past emissions "seems to us as very partial and not very accurate," Stern, the U.S. envoy, said.

The U.S. wants to get rid of the U.N.'s current division between developed and developing nations. Stern noted that a 2007 study showed that by 2020, the all-time emissions of developing countries will exceed those of the developed world, due to emissions growth in large emerging economies like China and India.

Those countries are trying to develop in a cleaner way but say it's unfair to expect them to abstain from the dirty fuels that built Western economies into powerhouses with high living standards.

Finding a way to share the burden of emissions cuts in an equitable manner is one of the top challenges for the climate negotiators, whose overall goal is to keep warming below 2 degrees Celsius (3.6 degrees F) since preindustrial times.

Scientists say the global average temperature has already risen by 0.8 C (1.4 F), resulting in melting glaciers, rising sea levels and other climate impacts.

The point of measuring the responsibility for these climate changes is to establish benchmarks to measure against when countries present their national emissions proposals for the new global agreement, explained Alden Meyer of the Union of Concerned Scientists, a nonprofit advocacy group.

"How do you evaluate the proposals against each other? If, as I assume, we fall short, who needs to do more and why?" Meyer said.

The Warsaw conference is supposed to lay the foundation for a 2015 climate agreement, but it was unclear Wednesday whether countries would be able to agree on basic stepping stones, including a timeline for when commitments should be presented.

Climate finance is also divisive issue, with rich countries being urged to step up their financial support to help developing countries transfer to clean energy sources and adapt to climate change.

A key dispute in Warsaw revolves around a proposed "loss and damage" mechanism that developing countries say is needed to help them cope with climate disasters that cannot be avoided.

They have pointed to the devastation caused by Typhoon Haiyan in the Philippines. Single storms cannot be conclusively linked to climate change but rising sea levels can make them more destructive.

The U.S. and other rich countries say "loss and damage" is related to climate adaptation and doesn't need to be dealt with as a separate issue. It was those talks that developing countries said they walked out of early Wednesday.

The government of Poland, which has been criticized at the talks for hosting a coal summit at the same time, stirred things up further Wednesday by replacing Environment Minister Marcin Korolec, who is presiding over the climate conference. Korolec told reporters the move would not affect his duties as conference president.

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Associated Press writer Monika Scislowska contributed to this report.

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Ritter can be reached at https://twitter.com/karl_ritter