Climate change talks next year will be make or break for international efforts to curb global warming, with the credibility of the UN-backed process at stake, the outgoing EU climate chief, Connie Hedegaard, has warned.

World leaders are expected to sign an agreement to limit greenhouse gas emissions from 2020 onwards at a Paris conference in December 2015. It could be pivotal in climate negotiations, if China, the US and Europe agree to hold global warming within what scientists say are safe limits.

But the risks are great, according to Hedegaard, who recently left the post of European commissioner for climate action and hosted the Copenhagen climate talks in 2009. “Say Paris could not deliver,” she said to the Guardian. “Who would believe the UN process would have credibility after that? That is what [we need] to make leaders understand – it’s now.”

At preparatory talks earlier this month in Peru, a framework was agreed for the coming year. Three of the world’s major economic blocs – Europe, China and the US – came forward with proposals on curbing emissions, but Hedegaard was adamant that these must be strictly scrutinised to ensure that no country shirks its responsibilities. “We need to know who is bringing what [commitments] in order to have time [before] Paris to see how all this adds up? Does it bring us closer to bridging the gap [between the world’s current emissions and the cuts needed to prevent more than 2C of warming], or are we more distant from it? Is it fair, is it not fair? That’s a difficult exercise.”

For Paris to succeed,the world’s major economies would need to be ambitious about greenhouse gas emissions, she said. There are reasons to be optimistic. China’s decision in November to set a peak year on its emissions has brightened the prospects for a global agreement.

“It is good that China accepts that their emissions [are] of interest to the whole world. I remember when [asking] the Chinese when their emissions would peak destroyed the good atmosphere in a room. I hope it will make India [and others] reconsider their strategy, and I see signs that the new Modi government is doing exactly this.”

She also praised the US, which set new post-2020 carbon targets, that were announced jointly by President Barack Obama and China’s Xi Jinping. “In 2009 [at the Copenhagen summit] China and the US played ‘after you, sir’ – none of them really moving. It makes a huge difference that they now move forward hand in hand and that they recognise their huge responsibility [as the world’s biggest emitters and economies],” she said.

But Hedegaard, who played the lead role in getting governments to agree to meet in Paris, also had stern warnings. “2030 is a very late peak year for China,” she said. The later the peak year, the higher emissions would be before that. She also questioned the value of the US promise to cut emissions by 26-28%, compared with 2005 levels, by 2025. “It is not certain how the US will deliver the 2025 targets – one year after Paris, Obama will be out of office.”

Hedegaard presided over the Copenhagen summit in 2009, which was attended by leaders including Barack Obama, Wen Jiabao, Gordon Brown and Angela Merkel. Photograph: Attila Kisbenedek/AFP/Getty Images

Hedegaard knows how hard it will be to succeed. She presided over the Copenhagen summit, arguably the most important talks in 20 years of climate negotiations, which was attended by leaders from nearly every country on the globe, including Barack Obama, Wen Jiabao, Gordon Brown and Angela Merkel.

Copenhagen ended with the most significant agreement on climate change to date – the first time developed and major developing countries had acquiesced to curbing their greenhouse gas emissions: a concord between the US, China, India, Europe and all the major emerging economies had been unthinkable only a few years before. But it was marred by scenes of chaos, with some leaders railing against one another. NGOs also criticised the outcome for falling far short of expectations.

If Copenhagen was a “nightmare”, in Hedegaard’s words, two years later she came back with an audacious plan. Without Hedegaard, there would be no Paris conference. The process of forging a post-2020 climate agreement started with her extraordinary single-woman stand at the climate talks in Durban in 2011, where in the face of fierce opposition she pushed through a timetable for new negotiations that would culminate in 2015 with the signing of a global pact to come into force in 2020. She crafted a grand alliance of most of the world’s developing countries, and the tacit support of the developed world. By the end, only China and India refused to countenance the timetable, and the exasperation of those two countries was clear.

It was a close-run thing. Even into the final minutes, after 48 hours of non-stop talks, all of the main participants were quietly briefing the Guardian that Hedegaard would cave. If she had done, the world would be without a way of tackling climate change. She did not. China and India backed down and Durban was a watershed.

Hedegaard will not be part of the EU negotiating team in Paris – “I was never too keen, too fond of UN conferences” – and although she will be chairing a foundation with 1bn Danish kroner to spend on climate and the economy, it is unlikely to be her only new job.

“I left politics as a 29-year-old – I was spokesperson in parliament for the prime minister’s party and I left – people couldn’t understand that. Then I went into journalism and I wrote, then I was head of radio news, then I was a minister, then a European commissioner. I’ve had the experience that yes, you say goodbye to something but always it has also been fantastic what came after,” she said. “I’ve been looking forward so much to a new chapter in my life.”