Environmentalists have hailed the decision by China and the United States to ratify the Paris Agreement on climate change, with the move being praised as a pivotal step toward controlling global warming.

WWF-China CEO Lo Sze Ping described it as an encouraging signal to the world.

"This clearly shows China's determination to implement the Paris Agreement. This also shows that China is taking more of a leadership role in the global effort to prevent climate change," he said.

China and the US announced that they had ratified the agreement on Saturday on the eve of a meeting of G20 leaders in Hangzhou.

The joint formal approval marks a crucial milestone, according to the United Nations Environment Programme.

"This announcement is hugely important. The leadership of China and the US is crucial to taking the Paris Agreement forward," said Erik Solheim, UN environment head.

"The fight against climate change remains difficult and urgent, but having heavy-hitters like China and the US on your side is extremely heartening."

The Paris Agreement has so far been signed by 180 counties, but it will only take effect after 55 nations responsible for 55 percent of greenhouse gas emission have ratified it.

Before this weekend's joint announcement, only 24 countries representing just more than 1 percent of global greenhouse gas emissions had ratified the deal.

China the US are responsible for 38 percent of global greenhouse gas emissions, making their ratifications crucial.

"This is a pivotal moment for our planet. And there's no turning back. The world's largest emitters of carbon pollution are now on an unstoppable path to protect us from climate change," said Rhea Suh, president of the Natural Resources Defense Council, an international nonprofit environmental organization based in New York.

Now that China and the US have ratified the deal, aiming to cap global warming at 2 C above pre-Industrial Revolution levels, more countries are being urged to follow suit.

"President Obama and President Xi Jinping are signaling to other countries that the Paris Agreement can come into force this year," said Lou Leonard, WWF-US senior vice president for climate and energy.

He added that an urgent and accelerated implementation of the deal is necessary in order to prevent the worst affects of climate change.

Ping, from WWF-China, said: "Now other countries must act swiftly to ratify the deal, and to reduce their emissions in line with the Paris Agreement's long-term goals, according to science and equity, and therefore increase their current pledges."

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