Originally published on Grist.

China and the US really want you to know we’re in it together on climate change. On Saturday, Presidents Obama and Xi Jinping formally joined the Paris climate agreement in a joint event in China, giving the deal a big boost from the two top polluters.

The future of the climate agreement is something of a numbers game: 55 countries representing 55 percent of global greenhouse emissions must ratify it before the deal becomes official. China and the US together represent 38 percent of global emissions.

If all the countries that said they will try to ratify the deal this year do so, including Brazil, Japan, Argentina, and South Korea, then the agreement could be entered into force before year’s end.

The sooner Paris is official the better, the thinking goes: It gives nations a head start on how they’re going to meet their (non-legally binding) promises, and makes Donald Trump’s promises to “cancel” the agreement look foolish.

“This is momentum with purpose,” a White House adviser said in a press call Friday.

Just six years ago, Obama famously crashed a secret meeting held by China, India, and Brazil because the Copenhagen climate negotiations were deadlocked. Considering their complete transformation in years since, their joint ratification is a remarkable symbolic moment.

Further reading

The world has agreed to a major climate deal in Paris. Now comes the hard part.

Past climate treaties failed. So the Paris deal will try something radically different.

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