Representing the world's two biggest-emitting countries, President Barack Obama and China President Xi Jinping agreed to record-setting reductions of heat-trapping emissions that contribute to global warming.

The deal could give momentum to a new global climate agreement, slated to be finalized next year during United Nations-led talks in Paris. Yet actually meeting the reduction targets is another matter. They would require massive economic changes in both the U.S. and China that could be politically or socially unpopular.

The two leaders had been expected to make bold pledges early next year in advance of any Paris agreement, so the sweeping announcement surprised many climate change analysts.

"It's a new day to have the leaders of the U.S. and China stand shoulder-to-shoulder and make