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.

The G-8 did set a long-term commitment to reduce their carbon emissions by 80 percent by 2050. But they made no shorter-term target, despite warnings from a U.N. panel that they must cut emissions between 25 percent and 40 percent by 2020 to keep average global temperatures from rising more than 2 degrees Celsius above levels 150 years ago. Most scientists agree that even a slight increase in average temperatures would wreak havoc on farmers around the globe, as seasons shift, crops fail and storms and droughts ravage fields.

Countries like China and India — the next generation of big polluters — want the industrial countries to commit to reducing carbon emissions by 40 percent over the next decade before they commit to any reductions of their own. Without that commitment from the G-8, they refused to make any targets of their own.

“The ground for a breakthrough can only be prepared if the G-8 leaders reach consensus on the midterm binding goals of cutting greenhouse emission and stop asking the developing nations to act first as an excuse for their not committing to the binding goals,” China’s official Xinhua News Agency said in a commentary earlier this week. The failures earned the G-8 a sharp rebuke from U.N. Secretary-General Ban Ki-moon.

Obama did announce Thursday that the Group of 20 major economies would take up the climate financing issue at their meeting in September in Pittsburgh — a move environmentalists said could help break the logjam while sending developing countries a signal that the G-8 is serious about financing.