China has ratified the landmark Paris agreement on climate change just hours before the start of the G20 summit, where representatives of the world's leading economies will gather.

The standing committee of China's National People's Congress voted to adopt "the proposal to review and ratify the Paris Agreement" at the closing meeting of a week-long session, state news agency Xinhua said.

The ratification by China, the world's biggest polluter with about 25 percent of global carbon emissions, took place ahead of the G20 meeting in Hangzhou where the United States, the world's second biggest polluter, is also expected to announce it will ratify the accord.

G20 countries emit about 80 percent of global greenhouse gases, with China and the US responsible for about half of that.

"The timing of this announcement is important because it comes a few hours before the start of the G20 Summit," Al Jazeera's Adrian Brown, reporting from Hangzhou, said.

"We understand that President Barack Obama will announce the United States will also ratify the deal."

The Paris agreement was signed in December 2015 by nearly 200 countries. But before today's announcement, only 23 signatories, responsible for about one percent of global emissions, had ratified it.

"For the accord to go into legal effect, however, at least 55 countries need to ratify the agreement. What China and presumably the US will do is set an example for other countries to follow," Brown said.

READ MORE: COP21 - Any agreement is better than no agreement?

The main goal of the deal is to slash greenhouse gas emissions and keep global temperature increases to "well below" two degrees Celsius.

Experts have said that target is already in danger of being breached, with the UN weather agency saying that 2016 is on course to be the warmest year since records began.

Environmental groups have reacted to the Chinese announcement with cautious optimism, stressing other countries must also ratify the Paris deal.

Greenpeace East Asia's senior climate policy adviser, Li Shuo, said both China and the US should put pressure on other G20 countries to ratify the agreement: "Xi and Obama should seize the opportunity to lead the world's 20 wealthiest nations by joining and building on the Paris Agreement."