Urgent UN talks on tackling the climate emergency are still not addressing the true scale of the crisis, one of the world’s leading climate scientists has warned, as high-ranking ministers from governments around the world began to arrive in Madrid for the final days of negotiations.

Talks are focusing on some of the rules for implementing the 2015 Paris agreement, but the overriding issue of how fast the world needs to cut greenhouse gas emissions has received little official attention.

“We are at risk of getting so bogged down in incremental technicalities at these negotiations that we forget to see the forest for the trees,” said Johan Rockström, joint director of the Potsdam Institute for Climate Impact Research. “There is a risk of disappointment in the UN process because of the inability to recognise that there is an emergency.”

In the next few days, environment and finance ministers from more than 190 governments will begin the “high-level segment” of the UN talks, which began on 2 December, and will finish on Friday. Over the weekend, negotiators produced the latest draft of a key text on carbon markets, which still does not have the consensus needed to pass.

The stately pace of negotiations was in stark contrast with the scenes outside the conference in Madrid, where on Friday evening more than 500,000 people marched through the Spanish capital led by the Swedish school striker Greta Thunberg. Protests continued through the weekend, with Extinction Rebellion and groups from across the world. On Monday, Thunberg and other youth activists will hold meetings with officials inside the conference.

Rockström said the UN conference must grapple urgently with reversing emissions of greenhouse gases, which are still on the rise despite repeated scientific warnings over three decades and multiple resolutions by governments to tackle the problem.

“We must bend the curve next year,” he told the Guardian, citing stark warnings from the Intergovernmental Panel on Climate Change. “Next year is the year of truth. The year when we must move decisively to an economy that really starts to reduce investments in fossil fuels.”

Even the coal-fired power plants currently planned or in construction are enough to produce double the amount of carbon that can safely be put in the atmosphere for the next century, Rockström said.

The situation was so dire that governments should be starting to consider geoengineering technology, he said. Such projects could use a combination of natural and artificial means, from seeding clouds to erecting reflectors in space.

“Geoengineering has to be assessed, maybe even piloted already in case we need to deploy it,” he said. “It makes me very nervous. That is really playing with biological processes that might kick back in very unexpected ways. But I don’t think we should rule anything out – an emergency is an emergency.”

As the UN conference enters its final stages, the role of the UK is likely to come under much greater scrutiny. Britain will play host to next year’s conference at which world leaders must pledge much greater cuts in emissions than have yet been made, if the 2015 Paris accord is to succeed.

Claire O’Neill, the former Tory climate minister designated to lead next year’s conference, is in Madrid but cannot make official announcements because of the “purdah” rules surrounding political announcements in the run-up to the general election.

However, the UK’s plans were rated as “insufficient” in a key independent analysis called the Climate Action Tracker. Despite the government’s eye-catching commitment last summer to cut carbon emissions to net zero by 2050 – one of the first major economies to make such a pledge – few measures are in place to keep pace with the target.

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“There has been a dearth of new significant climate policies in recent years which, if left unaddressed, will leave the UK missing its medium and long-term targets,” concluded the analysis of global emissions-cutting plans.

That would damage the host nation’s credibility at next year’s crucial talks in Glasgow, campaigners said.

Dr Bill Hare, a climate scientist and the chief executive of Climate Analytics, which carried out the study, said it was clear which of the two biggest parties had the better plans on the issue before this week’s general election.

“While both major political parties have proposed further climate action, the Conservatives have not put sufficient proposals on the table to close this gap, whereas [our analysis shows] the Labour’s £250bn could easily close that gap and push on towards a 1.5C pathway,” Hare said.