U.S. President Barack Obama and Chinese President Xi Jinping fortified commitments to reduce carbon emissions Saturday by formally joining the Paris agreement and pledging a continued bilateral climate cooperation.

The leaders of the worlds two largest emitters of greenhouse gases met Saturday at the G20 Summit in Hangzhou, China . By officially ratifying the climate agreement reached in April, the two countries move the Paris deal a major step toward taking effect this year, the White House said.

More than 170 nations signed the Paris agreement, committing to fight climate change by cutting carbon emissions. The deal requires at least 55 countries representing 55 percent of global emissions to take the additional step of ratification before it takes effect.

Before Saturdays action by the U.S. and China, 24 nations had formally joined , accounting for just 1.08 percent of total global greenhouse gas emissions, according to the United Nations. The addition of the U.S. and China boosts that to roughly 40 percent about three-fourths of the required total, according to the White House.

Brian Deese, senior adviser to Obama, said on a call with reporters ahead of Saturdays meeting that the U.S. and China have become very strong partners on climate change.

Over the last eight years we have effectively demonstrated that as the worlds two largest emitters, if we can come together, we can help move the world forward on combatting climate change, Deese said.