"These signals significantly increase the chance that the agreement will enter into force this year," said Jake Schmidt, director of the International Program at the Natural Resources Defense Council, which is based in New York City.

Eliza Northrop, a researcher at the World Resources Institute, also said the signing by China and the US will give a major boost to efforts to reach the threshold for entry into force.

"It is reasonable to think the entry into force would happen in 2017," she said. "But given the varying timelines for countries to complete their domestic approval processes, the timing of entry into force is uncertain."

Samantha Smith, leader of the World Wildlife Fund, said more efforts are urgently needed for the climate change meeting next month in Bonn to pick up on issues where the Paris meeting left off.

"We hope that leaders will not only send strong signals to their negotiators, but even instruct them, about the key elements needed to give life to the Paris Agreement."

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