L'AQUILA, Italy — The chasm between rich and poor on how to address climate change burst into the open at the G-8 summit Thursday, showing how difficult it will be to persuade the world to make lifestyle and economic sacrifices needed to save the planet from global warming.

President Barack Obama urged emerging economies to do more to curb global warming, while the U.N. chief demanded developed countries set an example and take more concrete steps to reduce pollution.

Especially reluctant to commit to change were two budding powers that are just now getting comfortable economically: India and China.

Obama said industrialized countries, the United States included, had a "historic responsibility" to take the lead in emissions reduction efforts because they have a larger carbon footprint than developing nations.

"And I know that in the past, the United States has sometimes fallen short of meeting our responsibilities. So, let me be clear: Those days are over," he said.

But he said developing nations have to do their part, as well.

"With most of the growth in projected emissions coming from these countries, their active participation is a prerequisite for a solution," Obama said.

Two days of negotiations between the world's major industrial polluters and developing nations failed to make any major breakthrough on firm commitments to reduce carbon emissions. While both sides said for the first time that global average temperatures shouldn't rise over 2 degrees Celsius, they didn't set any joint targets to reach that goal.

And significantly, the Group of Eight industrialized nations made no firm commitment to help developing countries financially cope with the effects of rising seas, increased droughts and floods, or provide the technology to make their carbon-heavy economies more climate friendly.

The results indicate how difficult it will be to craft a new climate change treaty by December, when nations from around the world will gather in Copenhagen, Denmark, to negotiate a successor to the 1997 Kyoto Protocol, which expires in 2012.

"That leaves us with quite a lot of work to do," said the chief U.N. climate change negotiator, Yvo de Boer.

The comments came at the conclusion of a meeting of the 17-nation Major Economies Forum, which includes the G-8 — Canada, Britain, France, Germany, Italy, Russia, Japan and the United States — and other emerging countries: China, which is overtaking the U.S. as the world's biggest polluter, and India, which is close behind. Mexico, Brazil, South Africa, Indonesia, Australia, South Korea and the European Union also are in that club of the world's major polluters.