“History may look back and say this was the turning point on climate,” said U.S. Rep. Henry A. Waxman (D-Beverly Hills), ranking member of the House Energy Committee. The deal was immediately challenged by Obama’s Republican critics on Capitol Hill, who have pledged to make it a priority to roll back the president’s measures on the environment when they assume the majority in Congress next year.

The White House did not immediately say whether Obama will propose legislation or use his executive powers to enact changes without lawmakers, though his environmental team expects a fight on the kinds of regulations necessary to carry out his newly set goals. “This unrealistic plan, that the president would dump on his successor, would ensure higher utility rates and far fewer jobs,” Senate Republican leader Mitch McConnell of Kentucky said in a statement. The agreement, if it is adhered to and if it prompts similarly far-reaching cuts from other countries, would help put the world on “a straight path to 80% reductions in emissions over 2005 levels by 2050,” said Ken Lieberthal, a senior fellow at the Brookings Institution.

Those are the kinds of reductions scientists have recommended to avert the greatest temperature rises associated with climate change.The deal portends significant change for the climate, observers said, as it sets a bolder timeline for two countries that, together, account for more than a third of global greenhouse gas emissions. “History may look back and say this was the turning point on climate,” said U.S. Rep. Henry A. Waxman (D-Beverly Hills), ranking member of the House Energy Committee.

The deal was immediately challenged by Obama’s Republican critics on Capitol Hill, who have pledged to make it a priority to roll back the president’s measures on the environment when they assume the majority in Congress next year. The White House did not immediately say whether Obama will propose legislation or use his executive powers to enact changes without lawmakers, though his environmental team expects a fight on the kinds of