Energized by new targets set by China and the United States, the world's top climate polluters, U.N. global warming talks resume Monday with unusual optimism despite evidence that human-generated climate change is already happening and bound to get worse.

Negotiators from more than 190 countries will meet in the Peruvian capital for two weeks to work on drafts for a global climate deal that is supposed to be adopted next year in Paris. Getting all countries aboard will be a crucial test for the U.N. talks, which over two decades have failed to reduce the greenhouse gas emissions blamed for global warming.

Pledges by Chinese President Xi Jinping and U.S. President Barack Obama earlier this month to limit their emissions after 2020 sent a powerful signal that a global deal could be possible next year. The two countries, which produce about 40 percent of all global emissions, long have been adversaries in the U.N. climate talks.

"Climate change will not be solved only by the United States and China. But it certainly will not be solved without them," U.N. climate chief Christiana Figueres told The Associated Press.

Earlier this year, the European Union announced an emissions target for 2030, meaning the world's three biggest emitters have made pledges. The hope now is that other big polluters including India, Japan, Russia and Australia will set their goals.

In Lima, delegates hope to specify what information should be included when countries submit their formal emissions targets early next year, so that the targets can be compared against each other. There is little expectation, however, that negotiators will agree on enforceable legal terms. The U.S. and other countries oppose a legally binding emissions treaty, which would face stiff opposition in a Republican-controlled Congress.

The talks in Lima come just a month after a scientific assessment by the U.N.'s expert panel on climate change warned that rising global temperatures could have an irreversible impact on people and ecosystems as glaciers melt, sea levels rise, heat waves intensify and oceans become warmer and more acidic.

The Intergovernmental Panel on Climate Change said it's now 95 percent certain that humans are to blame for most of the warming, mainly by burning oil, coal and gas. Those emissions keep rising as fossil fuels power the expansion of China and other emerging economies.

The delegates will be meeting in a country particularly threatened by the impact of climate change.

"Peru is hit from all angles," said Walter Vergara, a top Latin America climate change expert formerly with the World Bank. "Many ecosystems are going to be affected."

The Andean nation has 70 percent of the world's tropical glaciers, which are melting at an accelerated pace, putting 300,000 highlanders under severe stress as pastures and croplands slowly dry up.

Those glaciers have lost more than one-fifth of their mass in just three decades, and the 70 percent of Peru's 30 million people who inhabit the country's Pacific coastal desert, depend on glacial runoff for hydropower and to irrigate crops, meaning their electricity and long-term food security could also be in peril. Higher alpine temperatures are killing off plant and animal species in cloud forests and scientists predict Pacific fisheries will suffer.

The overall goal of the U.N. talks is to keep the average global temperature rise below 2 degrees Celsius (3.6 degrees F) compared with pre-industrial times. That would require a massive energy shift to renewable sources and, perhaps, capturing airborne CO2 and storing it underground.

The U.N. Environment Program earlier this month warned that there's a growing gap between what countries promise to do about carbon pollution and what scientists say needs to be done to meet that goal.

And global temperature data show 2014 is shaping up to become one of the hottest — if not the hottest — years on record.

"We're still heading in the wrong direction," said Andrew Steer, the head of the World Resources Institute, a Washington-based environmental group.

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Associated Press writer Karl Ritter reported this story from Stockholm and Frank Bajak reported in Lima.