PARIS (AFP) -- Eight months after 195 nations concluded a hard-fought climate rescue pact, pressure is mounting to put its carbon-cutting promises into action as world leaders gather at G20 and UN meetings this month.

The historic deal reached in Paris in December has been signed by 180 countries, but will only take effect after 55 nations responsible for 55 percent of greenhouse gas emissions have ratified it -- making it binding.

China -- responsible for around 25 percent of global carbon emissions -- ratified the pact Saturday, ahead of a meeting of G20 leaders where the United States is also expected to follow suit, considerably boosting efforts.

Until Beijing joined the club, only 24 nations emitting just over one percent of the global total had officially acceded, according to the UN climate body overseeing the deal to cap global warming at two degrees Celsius over pre-Industrial Revolution levels.

"As 2016 heads into the record books as likely the hottest year ever recorded in history, it is a reminder that we have precious little time left to act to keep global temperature rise well below 2 C," Pascal Canfin of environmental group WWF said ahead of a two-day G20 summit opening in Hangzhou, eastern China, on Sunday.

"We have the Paris Agreement to guide our way. Now we need governments to implement it," he said in a statement.

China and the United States are jointly responsible for about 38 percent of emissions and had been widely expected to announce ratification at the Hangzhou gathering, which begins Sunday and brings together world leaders representing 85 percent of global GDP and two-thirds of its population.

This will be followed on September 21 by UN chief Ban Ki-moon hosting a gathering on the sidelines of the General Assembly to beat the drum for ratification.

The Climate Action Network, a global NGO grouping, urged G20 leaders in an open letter to ratify the pact "as soon as possible" as a restatement of political commitment.

Early ratification would "also give a strong signal globally to business, cities and ordinary citizens to act ambitiously on climate change," it said.

The pact sets out to curb warming by replacing atmosphere-polluting fossil fuels with renewable sources -- an ambitious goal towards which most UN nations have already pledged emissions curbs.

This is meant to stave off the worst-case-scenario effects of violent droughts, storms and sea-level rise threatened by excessive planet warming.

Only by ratification, however, does a country agree to be bound to an international agreement of this kind, explained the World Resources Institute, a climate think tank.

Depending on constitutional provisions, many countries need to pass domestic legislation to do so.

On Thursday, France's climate envoy Segolene Royal urged Paris-based ambassadors to agitate for speedy ratification by their respective nations.

France hosted the UN huddle dubbed COP 21 (21st Conference of Parties) which yielded the climate pact. It will preside over the process until Morocco takes over as host of the next round of talks from November 7 to 18.

Ratification before the Marrakech meet, said Royal, "will allow COP 22 to be a COP of action", focusing on practical solutions for reaching the set goals.

With some 30 other nations having indicated their intent to formally adopt the pact, Royal may very well get her way.

"Our assessment is that 55 parties are likely to ratify this year, representing 58 percent" of emissions," said the WRI's David Waskow.

"It is a much more rapid process... than we have seen in the past for climate or any international regime of this type."

By comparison, it took eight years for the Kyoto Protocol, which preceded the Paris agreement, to enter into force.

The G20 meeting will be closely watched for progress.

"The G20 countries generate about 75 percent of global greenhouse gas emissions," said Christoph Bals of the Germanwatch pressure group.

"On that account they play a decisive role for the implementation of the ambitious Paris targets."

More important even than ratification, observers agree, is cutting fossil fuel subsidies and other funding.

"If G20 countries were to rid themselves of their reliance on coal, this would significantly impact their ability to increase their climate pledges and get their emissions trajectories on a below 2C pathway," said researcher Niklas Hohne of the NewClimate Institute.

On current pledges, the planet will warm by a dangerous 3 C, according to scientists.

But there has been progress too: China's declining coal use, an 18-percent increase in renewable energy since 2008, and the declining price of solar energy, NGOs say.