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HANGZHOU, China — President Barack Obama has announced the U.S. will formally join a sweeping global emissions-cutting accord reached last year, boosting efforts to bring the plan into effect by the end of 2016.

The Paris Agreement could well be remembered as "the moment we finally decided to save our planet," Obama said on Saturday in advance of the Group of 20 summit that starts Sunday in the eastern Chinese city of Hangzhou.

Chinese President Xi Jinping (right) and President Barack Obama. AFP - Getty Images

"For all the challenges that we face, the growing threat of climate change could define the contours of this century more dramatically than any other challenge," he said.

The announcement marked a major victory for Obama. He has pushed hard for the world to recognize the need to cut greenhouse gas emissions and protect the planet. Some of his most intense critics don’t even agree with the basic premise that the world climate is changing in a way that threatens future generations.

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"This signals a new era in global efforts to address climate change"

Significantly, China’s President Xi Jinping also formally ratified the accord earlier on Saturday. That means China and the U.S., which produce nearly 40 percent the world’s greenhouse gases, are now officially part of the agreement negotiated by 195 nations in Paris last year.

"The world’s two largest economies and two largest emitters, our entrance into this agreement continues the momentum in Paris and to give the rest of the world confidence, either developed or developing countries," Obama said. "That a low carbon future is where the world is heading."

#China ratified Paris #Climate Agreement ahead of @G20_China. New hopes to bring accord into effect by end of 2016. pic.twitter.com/0PJIF0A9pe — Andy Eckardt (@ameckardt) September 3, 2016

Republicans in Congress have said the agreement is a treaty that requires Senate ratification, something that would not happen. And the Obama administration is locked in a case that’s gone all to the Supreme Court, with at least 29 states fighting an Environmental Protection Agency requiring them to make big cuts in emissions from power plants.

Global environmental campaigner Greenpeace hailed Obama's announcement, but said it was just a start.

"This signals a new era in global efforts to address climate change. [China and the U.S.] now need to scale and speed up their efforts in charting a future that avoids the worst impacts of climate change," Executive Director of Greenpeace International Jennifer Morgan said. "Today’s commitment brings the possibility of the Paris Agreement entering into force early much closer to reality."

The Paris accords, which are not legally binding, set goals for nations around the world to cap and reduce emissions, with a long-term goal to keep global warming below 3.6 degrees Fahrenheit.

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More than 20 nations have signed on, or are expected to before the end of the year. The pact goes into effect when joined by at least 55 nations responsible for 55 percent of the world’s carbon emissions.

For the U.S. and China, climate change is a rare issue where the two powers have worked as partners. The two countries' relationship has grown increasingly contentious in recent years, with Obama pushing a regional trade deal that pointedly excludes China, U.S. charges of Chinese cyber attacks, and China making territorial claims in the South China Sea.

Obama was also planning to hold one-on-one talks with the leaders of China, Turkey and Great Britain’s new premier, Theresa May, concerns about the U.K.'s decision to leave the European Union. The President is also expected to meet informally with Russian President Vladimir Putin on the sidelines of the gathering.