Achieving a global climate deal in Glasgow in the next three weeks will be harder than signing the Paris agreement of 2015, the UK president-designate of the Cop26 talks has said.

Alok Sharma, the cabinet minister in charge of the UK-hosted talks, just over a week away, said the task would be to get nearly 200 countries to implement stringent cuts to their greenhouse gas emissions, in line with holding global temperature increases to within 1.5C of pre-industrial levels – a goal fast receding as global carbon output continues to climb.

“What we’re trying to do here in Glasgow is actually really tough,” he said. “It was brilliant what they did in Paris, it was a framework agreement, [but] a lot of the detailed rules were left for the future.

“It’s like, we’ve got to the end of the exam paper and the most difficult questions are left and you’re running out of time, the exam’s over in half an hour and you go, ‘How are we going to answer this one?'”

Preparations for Cop26, which opens on Sunday 31 October after being postponed by a year, have been hampered by the Covid-19 pandemic. More than 120 world leaders and at least 25,000 delegates and are expected to attend, vaccines, testing and quarantine arrangements for which will be a big logistical challenge.

“This is definitely harder than Paris on lots of levels,” said Sharma, who took charge of the summit in February 2020, weeks before the first lockdown. “[But] what we have going for us is that there is an understanding that we need to deal with this [climate crisis].”

In August, the Intergovernmental Panel on Climate Change, made up of the world’s leading climate scientists, delivered its starkest warning yet of “irreversible” climate disaster. “The IPCC report, while it was very alarming, was quite helpful in helping to focus minds,” said Sharma. “The question is whether or not countries are willing in Glasgow to go forward and commit to consensus on keeping 1.5C alive – that’s where the challenge will be.”

Under the Paris agreement, 197 nations agreed to limit global heating to “well below” 2C above pre-industrial levels and to “pursue efforts” to remain within 1.5C. But the commitments they came up with, called nationally determined contributions (NDCs), were inadequate, and would lead to a catastrophic 3C rise, while new scientific advice has strengthened the case for the lower limit of 1.5C.

The accord contains a ratchet mechanism requiring governments to return to the negotiating table every five years with more ambitious plans, a timetable stretched to this year by the Covid-19 postponement of Cop26.

Prospects for the talks were boosted this week as the UK government produced its long-awaited net zero strategy, aimed at generating $90bn of investment and 440,000 green jobs this decade. While the strategy came under fire from some environmental campaigners for gaps and insufficient funding, the existence of a substantial plan was vital for the hosts’ credibility at the talks.

Measuring all countries’ national climate plans against the 1.5C goal would be a key part of Cop26, said Sharma. “This is going to be a big ask. What we’re potentially saying to countries is that if your NDC isn’t good enough, you’re going to have to come back to the table. This is something we’ve got to try and fix over the next few years.”

He warned that the world would judge harshly any country seen to damage the chances of meeting the 1.5C goal. “What nobody will want at the end of this process is to have the finger pointed at them to say, ‘You, country A, B, C or D, are the ones who ultimately ensured that Glasgow wasn’t seen as producing a credible result.'”

But Glasgow did not represent the last chance of meeting the 1.5C aim, he added. Countries could still strengthen their commitments in future years. “If there is a gap, and there may well be a gap between [NDCs] and where you need to be in terms of 1.5C, we need to find a way of how we address that gap.”

Geopolitics have also changed markedly in the six years since the Paris talks, with relations between the US and China now at a low ebb. “The stars were perhaps differently aligned going into Paris,” concedes Sharma.

Sharma said he was still awaiting an NDC from China, the world’s biggest emitter. “They signed up to the communique in July that we negotiated in Naples, that all the G20 would come up with enhanced NDCs before COP – I reminded them they needed to deliver on that.”

There have been questions over China’s commitment to climate action, as the government signalled it could increase coal production in response to high energy prices.

Chinese experts downplayed these fears, pointing to the country’s increases in renewable energy generation, but doubts remain over whether China will stick to a relatively unambitious goal of causing emissions to peak by 2030, which scientists fear would be too late to allow the world to stick to 1.5C.

China’s president, Xi Jinping, and Russia’s Vladimir Putin are almost certain to miss the talks, though Joe Biden of the US will be there “with bells on”. The UK and the UN have downplayed the significance of the absences, as the countries will still field high-level negotiating teams.

As Cop president, the UK must act as a broker and referee at the talks rather than a leading player. But while remaining neutral, Sharma will insist on the voice of smaller countries being heard. “I see myself as a champion of developing countries and the climate-vulnerable [countries]. Some of them will be under water at 1.5C,” he said.

“All of these people are pretty adamant that what has to emerge from Glasgow is for us to be able to say we’ve kept 1.5 alive.”

In the long term, the direction of travel for the world is clear, he added. “If someone says to me, ‘Do you think we will ever get to the point where we have a net zero global economy?’, my answer is ‘Yes, I’m pretty sure we will.’ The issue is, will it be fast enough to deal with the challenges that we face right now?”