The United States and China have formally joined the global climate change agreement reached in Paris last year. The decisions mark a major step towards the pact's eventual implementation.
At a ceremony on the sidelines of the Group of 20 summit in China on Saturday, US President Barack Obama and Chinese President Xi Jinping delivered documents to UN Secretary-General Ban Ki-moon entering their countries into the pact.
China's National People's Congress adopted "the proposal to review and ratify the Paris Agreement," according to the state-run Xinhua news agency. Xi called the agreement a milestone that marked the "emergence of a global government system" for climate change.
The ceremony took place shortly after Obama arrived in Hangzhou for the annual G-20 summit. Speaking in the presence of the Chinese president and United Nations Secretary General Ban Ki-moon, Obama said: "Just as I believe the Paris agreement will ultimately prove to be a turning point for our planet, I believe that history will judge today’s efforts as pivotal."
He said the United States was committed to being a global leader in the fight against climate change: "This is not a fight that any one country, no matter how powerful, can take alone." He said of the Paris agreement: "Some day we may see this as the moment that we finally decided to save our planet."
Ban Ki-moon said he would invite country leaders to formally ratify the agreement at a high-level event in New York. "By formally joining the Paris agreement you have added powerful momentum" toward bringing the agreement into effect, Ban told Obama and Xi on Saturday. "I am optimistic we can do it before the end of this year."
Some 180 countries have signed on to the agreement made in Paris last December. But it doesn't take effect until at least 55 countries - responsible for at least 55 percent of global warming emissions - formally ratify the deal.
Previously 24 nations had ratified the agreement - including North Korea - but those nations collectively account for just 1.08 percent of global emissions, according to the UN Framework Convention on Climate Change.
Top two emitters
China, the world's most populous country, with 1.38 billion people, is the world's largest emitter of greenhouse gases. The United States, with just 324 million people, is number two. The two countries account for about 20 percent and 18 percent, respectively, of global warming emissions.
Russia accounts for 7.5 percent, while India pumps out 4.1 percent.
Since it is an executive agreement, rather than a treaty, Obama can sign it without needing a vote from Congress, which would, in all likelihood, reject it.
In Paris, the countries agreed to a binding global compact for each country to decide how best to slash their own greenhouse emissions with the aim of keeping global temperature increases to "well below" 2 degrees Celsius (3.6 degrees Fahrenheit) relative to the preindustrial revolution.
But experts have said the pace of global warming is already threatening to exceed the temperature target. The UN weather agency said 2016 is on pace to become the warmest since record-keeping began, breaking the previous record set last year.
The Paris agreement set ambitious goals for capping global warming and funneling trillions of dollars to poor countries facing an onslaught of climate damage.