The United States has joined China to formally ratify the Paris agreement to curb climate-warming emissions, a major diplomatic achievement for U.S. President Barack Obama, who ends his term in January.

Obama and Chinese President Xi Jinping submitted their plan to join the agreement to U.N. Secretary-General Ban Ki-moon during a ceremony on the sidelines of the Group of 20 major economies summit in Hangzhou, China.

Obama said the U.S. is committed to being a global leader in the fight against climate change. He said the Paris Agreement may be remembered as “the moment we finally decided to save our planet.”

Xi called the agreement a milestone that marks the “emergence of a global government system” for climate change.

Senior Obama adviser Brian Deese said the joint declaration should push other countries, mainly a critical mass of polluting states, to formally join the agreement.

The U.S. is hopeful that will happen this year.

Ban also expressed a similar viewpoint during the ceremony. He said he was “optimistic” that the Paris accord would be in force by year-end.

He told reporters that he will hold a high-level event in New York to which he will invite country leaders to formally ratify the agreement.

While 180 countries have now signed the agreement, 55 nations – covering at least 55 percent of global emissions – need to formally ratify the treaty to put it into legal effect.

Before China and the United States, 23 nations had ratified – including North Korea – but they collectively accounted for just 1.08 percent of global emissions, according to the U.N. Framework Convention on Climate Change.

China represents just over 20 percent of global emissions while the United States accounting for 17.9 percent, Russia 7.5 percent and India 4.1 percent.