As the clock ticks down to the start of Cop26 the challenges facing efforts to deliver action to avert dangerous climate change are coming into sharp focus.

The summit, taking place in Glasgow from Sunday, has no global treaty to agree – unlike the last major UN climate conference in Paris in 2015.

But it is being described as the conference that must deliver on the Paris Agreement and its key pledge to limit global warming to well below 2C or 1.5C to avoid the worst impacts of global warming.

The talks come in the wake of “code red” scientific warnings on the climate crisis and the risk of going over 1.5C of warming, and severe climate-related weather extremes from heatwaves in the US to deadly flooding in Europe and China.

Earlier this week, UN scientists warned the build-up of planet-warming greenhouse gases in the atmosphere had sped up and hit record levels in 2020.

The UN has also warned that the latest national action plans and pledges from countries to tackle emissions up to 2030 are far off what is needed to limit dangerous warming, and put the world on track for temperature rises of 2.7C.

So a key factor in making Glasgow a “success” is keeping the pathway to 1.5C within reach by increasing climate action in this decade, with more national efforts from countries, progress on areas such as coal and saving forests, and a process to ensure countries take more action in the next few years.

It is a goal made arguably harder by the latest climate plan submitted by China on Thursday, which does not increase its ambition beyond existing announcements.

There is also the question of the long-promised 100 billion US dollars a year from 2020-2025 for poorer nations to develop cleanly and adapt to the changing climate, and negotiations to finalise the last bits of how the Paris Agreement will work.

It has emerged developed countries will not deliver on the 100 billion dollar goal until 2023, although around 500 billion dollars of climate finance will be provided over five years to 2025.

Peter Betts, former lead negotiator for the EU, and the UK, at UN climate talks, said: “Defining success in Glasgow is harder than it was in Paris, where we had a treaty to wave at the end.

“Success in Glasgow is more about implementation and action.”

He said that on efforts to curb warming “we need very substantial step-ups in countries’ emission reduction plans, coupled with serious sectoral coalitions on issues like finance and coal, and a road map to more ambition raising in the coming years.

“On finance it means the 100 billion dollar goal being met, with better delivery for the most vulnerable including on adaptation,” adding he expected developing countries to press for discussion on the delay to the delivery of the promised funding in Glasgow.

Importantly, he said, the Paris Cop21 was successful – despite insufficient commitments – because people saw governments were serious and that drove action.

“We need the same boost to momentum from Glasgow”.

Nick Mabey, chief executive and co-founder of E3G climate think tank, said the biggest hurdle to success would be whether enough had been put on the table – particularly from China given its disappointing new action plan – to give confidence there had been enough progress towards 1.5C.

And whatever is committed by countries and sectoral deals, it will not be enough, so a process is needed to come back sooner than expected to raise efforts in the face of starker scientific evidence on climate dangers, he said.

“It’s about the pace of change. Yes, we are changing, that’s very clear, but we have to go faster because the climate risks are worse,” he said.

He added it was right of the UK to push for keeping the 1.5C goal within reach at the Glasgow talks, and it was up to countries to decide if they wanted to pursue it.

World leaders will kick start the summit by setting out the domestic action they are taking, although key players will be missing including China’s Xi Jinping and Russia’s Vladimir Putin.

But a year’s delay to the talks as a result of the pandemic has allowed for the election of a new US president, Joe Biden, who has announced a net zero goal, and a significant boost to climate finance – and will attend the summit.

All G7 leading industrialised nations have targets to hit net zero emissions – needed by mid-century globally to meet the 1.5C goal – along with major polluters such as China, and in the past few days from the likes of Saudi Arabia and Australia, though critics question how robust these targets are.

And they do not solve the problem of the need for far greater action in the 2020s – seen as the “decisive decade” for putting the world on the right track to 1.5C or even “well below” 2C – with the UK the only major economy judged as being close to having plans in line with what is needed.

Prime Minister Boris Johnson has admitted it is “touch and go” whether key goals of the summit could be met.

Statements from blocs of countries ahead of the talks suggests fault lines that will be familiar to anyone who has attended UN climate conferences in the past – along questions over which countries should take what action and how it should be paid for.

G20 countries which have not brought forward new or updated plans – or ones that are insufficient – are under pressure from others to increase action.

But leading emerging economies including India and China have pushed back against efforts to get all countries to adopt net zero targets for 2050, saying developed countries should do more.

Meanwhile countries most threatened by climate change want a “climate emergency pact” that includes finance and a process that sees countries – particularly big emitters – come forward every year with additional ambition.

Then there are the negotiations on the final – thorny – issues of carbon markets and transparency under the Paris Agreement, not to mention the logistical challenges of holding a global summit of 25,000 people while the pandemic still rages.

Incoming Cop26 President Alok Sharma has said that all countries he has spoken to want the conference to be a success, but what the outcome of the talks will be – and whether it can be considered successful – remains to be seen.