L’AQUILA, Italy — The world’s major industrial nations and newly emerging powers failed to agree Wednesday on specific cuts in heat-trapping gases by 2050, undercutting an effort to build a global consensus to fight climate change, according to people following the talks.

Group of 8As President Obama arrived for three days of meetings, negotiators for the world’s 17 leading polluters dropped a proposal to cut global greenhouse gas emissions by 50 percent by mid-century, and emissions from the most advanced economies by 80 percent. But both the G-8 and the developing countries agreed to set a goal of stopping world temperatures from rising by more than 2 degrees Celsius from pre-industrial levels.

The discussion of climate change was among the top priorities of world leaders as they gathered here for the annual summit meeting of the Group of 8 powers. Mr. Obama invited counterparts from China, India, Brazil, South Africa, Mexico and others to join the G-8 here on Thursday for a parallel “Major Economies Forum” representing the producers of 80 percent of the world’s greenhouse gases. But since President Hu Jintao of China abruptly left Italy to deal with unrest at home, the chances of making further progress seemed to evaporate.

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