Climate activists and small nations at risk of global warming's direst consequences, welcomed Saturday's agreement by Chinese and US leaders on a global pact to curb planet-harming carbon emissions.
The agreement by President Xi Jinping and US President Barack Obama - representing the world's two largest green-house gas emitters - brought the hard-fought agreement, concluded in Paris in December a major step closer to tak-ing legal effect.
It is "the strongest signal yet that what we agreed in Paris, will soon have the force of law", said Mattlan Zackhras, minister-in-assistance to the president of the Republic of the Marshall Islands, which faces the threat of climate-change-induced sea-level rise.
"With the two biggest emitters ready to lead, the transition to a low emissions, climate resilient global economy is now irreversible."
The previous international effort to curb reliance on plan-et-harming fossil fuels, the Kyoto Protocol, had excluded China and other developing nations, while the United States refused to sign up.
"It's remarkable that in a few short years the world's two leading climate antagonists have become the world's two leading climate champions," said Bob Perciasepe, president of the Center for Climate and Energy Solutions, a US-based think tank.
"The US can no longer claim that China's inaction is an excuse to do nothing, and vice versa. With both again committing themselves to a low carbon future, the two countries are setting an example the rest of the world can hardly ignore."
Observers urged others to follow suit, while stressing that Saturday's agreement was not enough to meet climate change goals.
The Paris pact has so far 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.
Depending on their constitutions, for many countries this means passing domestic legislation, but in the US some things may be done by executive presidential order.
China and the US, jointly responsible for about 38 per-cent of global emissions, ratified the Paris agreement on the eve of a meeting of G20 leaders meeting in Hangzhou, China, where all eyes will now be on other major economies to follow suit.
Until Beijing and Washing-ton joined the club, 24 nations emitting just over one percent of global gases had officially acceded to the deal to cap global warming at 2 C above pre-Industrial Revolution levels.
This must be achieved by replacing atmosphere-polluting fossil fuels with renewable sources - an ambitious goal toward which most UN nations have already pledged emissions curbs.
On current country pledges, scientists expect the world to warm by 3 C or more, and more drastic measures are needed to effect a large-scale shift toward wind, solar and other sustainable energies.
"Now, other countries must act swiftly to ratify the deal, and to reduce their emissions in line with the Paris Agreement's long-term goals," said Lo Sze Ping of environmental group WWF-China.
"The fight against climate change remains difficult and urgent, but having heavy-hitters like China and the US on your side is extremely heartening," said Erik Solheim, from the UN Environment Programme.
Agence-France Presse