President Barack Obama and Chinese President Hu Jintao each vowed urgent action Tuesday to cool an overheating planet, even as prospects dimmed for a full treaty by the end of the year.

The world’s two biggest greenhouse-gas polluting nations were the focus at the U.N.’s unprecedented daylong climate change summit, which drew more than 50 presidents and 35 prime ministers, along with many environment ministers and at least one prince.

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