The decision by China and the US, the world’s two biggest greenhouse gas emitters, to ratify the landmark Paris accord on climate change heralds a new era of global cooperation on limiting emissions.

Only a few years ago, such a commitment looked like a pipe dream. Today, with the two most powerful nations on board, the United Nations can celebrate a victory on this world-changing issue that has been more than 20 years in the making.

President Obama’s commitment is particularly significant in its timing. The presidential contender Donald Trump has vowed to unpick the Paris accord if elected, but if it were to come into force before he came into office, that would become difficult if not impossible under US law and political conventions.

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But vital though the joint declaration is, it is not yet enough to bring the Paris agreement into force, nor halt the impacts of human activities on the climate. Making this work in practice will take much more.

First is the task of getting more nations to ratify. The US and China make up about 39% of global emissions. Under the foundation treaty, the 1992 UN Framework Convention on Climate Change, at least 55 countries must ratify for the agreement to enter force, and countries accounting for at least 55% of global greenhouse gas emissions must be represented.

With China, the US and a host of smaller countries signed up, the biggest emitter left outstanding is the EU, which negotiated the agreement as a bloc. The EU is unlikely to be able to ratify the accord any time soon, because of the mechanics of getting legal surety from its 28 member states.

There is a way around this. Nicholas Stern, chairman of the Grantham Institute on Climate Change and author of the landmark 2006 report on the economics of climate change, called on EU member states and the UK to ratify the agreement individually, through their national parliaments, to speed up the process. EU members are legally parties to the accord at a national as well as a bloc level, so if enough major countries – including the UK and Germany – were to enact the necessary processes then the accord could pass the final hurdle.

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“This is a tremendous opportunity,” Stern told the Guardian. “It’s very important for the credibility of the process [of gaining global agreement on climate action through the UN] to get the treaty ratified this year. EU countries can and should ratify as soon as possible. It’s not sensible to hold back, when they could make a big difference.”

The second, and more important, problem is that the agreement itself – the first that is both legally binding and likely to cover all countries – is just the beginning. The document, reached at a historic conference last December after 20 years of tortuous stop-start talks, commits countries to holding global warming to no more than 2C above pre-industrial levels, which scientists regard as the limit of safety, beyond which climate change is likely to have catastrophic and irreversible effects.

It does not specify what actions countries must take. Nearly all signatories set out targets on cutting their future greenhouse gas emissions beyond 2020, when current commitments expire. But the cuts in carbon dioxide implied by those targets will not be enough to hold to the 2C threshold, let alone limit the world to the 1.5C of warming included as an “aspiration” and required to save the globe’s most low-lying areas.

In addition, though the framework is legally binding, the commitments on emissions are not. National governments will be relied upon to flesh out their targets themselves, through policies that could be controversial and perhaps expensive in the short term, such as pursuing renewable energy and leaving fossil fuels unexploited. That process has barely begun for many.

Green campaigners called on national governments to act urgently both to fulfil the legal pledges of the Paris accord, and the moral promises made to their people to halt dangerous climate change.

Li Shuo, senior policy advisor for east Asia at Greenpeace, said: “Political ambition must keep up with rising sea levels faced by vulnerable communities. Touting the Paris ‘triumph’ while handing out money to the fossil fuel industry is simply not compatible with the Paris agreement. Governments must keep coal, oil and gas in the ground, and urgently focus on a just transition to renewables. The G20 meeting must be the moment when leaders lay out a timetable for eliminating fossil fuel subsidies, a practice which the Paris agreement has made all the more hypocritical.”

Mike Childs, head of policy at Friends of the Earth, added: “Countries need to commit to even deeper cuts than they promised in Paris and they need to enact policies to turn warm words into meaningful action.”

Early ratification would give governments the spur needed to formulate new policies on limiting carbon, and give businesses a clear signal that they should invest in reducing their impact on the climate, said Nick Mabey, chief executive of the E3G thinktank. “[The US and China announcement] sends a strong signal to investors and businesses that they can continue investing on the assumption that the Paris [commitments] will be delivered,” he told the Guardian. “At a time of increasing geopolitical turbulence this sends a welcome signal that cooperation to tackling climate change is resilient to worsening relations among the world’s superpowers.”

Jake Schmidt, director at the US-based Natural Resources Defense Council, added that other governments must take note. “The US and China are sending a powerful signal to other countries that the time for action is now,” he said. “It shows to countries and investors that the world’s two largest economies and biggest polluters are preparing to help lead the shift to a low-carbon economy. Only a few short years ago, countries were trying to drag the US and China into the global climate agreements, and now these two are among the first to join the new legal regime. That is an important shift, which reflects the reality that countries now realise acting on climate change is in their own self-interest and failing to act is no longer a smart choice.”

The UK’s position, as the government negotiates exit from the EU, is of particular concern, as UK support over the course of more than two decades of UN climate negotiations has been a key influence.

Childs told the Guardian that the prime minister should not delay, saying: “It’s up to Theresa May to make sure that the UK ratifies the agreement too, and quickly. Otherwise the UK will go from being leader to laggard on climate change.”