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 and the United States, jointly responsible for about 38 percent of global emissions, ratified the pact on Saturday, on the eve of a meeting of G20 leaders meeting in Hangzhou, China -- considerably boosting efforts.

Until Beijing and Washington joined the club, only 24 nations emitting just over one percent of global gases had officially acceded, according to the UN climate body overseeing the deal to cap global warming at two degrees Celsius (3.6 degrees Fahrenheit) 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," said Pascal Canfin of environmental group WWF.

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

US President Barack Obama said the pact represented "the moment we finally decided to save our planet", and UN chief Ban Ki-moon said he was "optimistic" of its taking effect before the end of the year.

Patricia Espinosa, executive secretary of the UN Framework Convention on Climate Change, thanked the US and China for ratifying the pact she said held the key to a sustainable future for all.

"The earlier that Paris is ratified and implemented in full, the more secure that future will become," she said, and urged other nations "to join this wave of ambition and optimism towards a better and sustainable world."

The Hangzhou gathering brings together world leaders representing 85 percent of the world's GDP, two-thirds of its population, and some 75 percent of greenhouse gas emissions.

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

- Getting rid of coal -

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 (WRI), a climate think tank.

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

Greenpeace senior policy adviser Li Shuo said accession by the US and China brought the possibility of the Paris Agreement entering into force early much closer to reality.

"But this moment should be seen as a starting point, not the finale, of global action on climate," he said.

France, which hosted the UN huddle dubbed COP 21 (21st Conference of Parties) which yielded the climate pact, is pushing hard for it to enter into force before the next meet in Marrakech, Morocco from November 7 to 18.

"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. Neither the US or China signed up to it.

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 2 C pathway," said researcher Niklas Hohne of the NewClimate Institute.

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