"This is a major milestone in the U.S.-China relationship," Obama told a news conference in Beijing, with Xi at his side. "It shows what's possible when we work together on an urgent global challenge."
The Associated Press reported that the deal was the result of months of secret discussions between U.S. and Chinese officials.
It was unclear how feasible it would be for either country to meet their goals, and Obama's pledge was sure to confront tough opposition from ascendant Republicans in Congress.
"This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs," said incoming Senate Majority Leader Mitch McConnell, R-Ky, in a statement.