Host nation the UK opened the United Nations' COP26 climate summit in Glasgow on Sunday by urging global leaders to come together and act now to steer the world away from the worst effects of human-induced climate change.

Delayed by 12 months because of the Covid-19 pandemic, COP26 is seeking to keep alive the 2015 Paris Agreement to limit global warming to 1.5 degrees Celsius above pre-industrial levels.

The UN summit is aiming to secure deeper national commitments — known as Nationally Determined Contributions, or NDCs — to curb greenhouse gas emissions and make progress in areas such as financing to help poorer economies achieve the goals of the agreement reached in Paris six years ago, which was backed by almost 200 signatory countries.

It is also looking to finalise the so-called "Paris Rulebook" to implement the 2015 accord as some elements, particularly in the areas of emissions reporting and carbon trading, remain unresolved.

But weak commitments from the likes of China and India, and tame pledges to achieve net zero carbon emissions by Australia and Saudi Arabia, have cast a shadow over the two-week summit, leaving its ability to succeed in the balance.

Dampening expectations, Russian President Vladimir Putin and Chinese President Xi Jinping will not be in attendance.

COP26 president Alok Sharma told delegates in his opening address that the summit is the world’s “last, best hope to keep 1.5C in reach”.

“If we act now and we act together, we can protect our precious planet. So, let's come together over these two weeks and ensure that where Paris promised, Glasgow delivers,” he said.

A United Nations report in August sounded a dire warning that the world is dangerously close to runaway warming and that humans are "unequivocally" to blame due to the burning of fossil fuels, including oil and gas.

The Intergovernmental Panel on Climate Change (IPCC) report said that a massive shift away from fossil fuels is needed to avoid the most dangerous increases in global temperatures, and this will require unprecedented global co-operation.

'Wake-up call'

Sharma told the conference: “As you know, we postponed COP26 by a year. But during that year, climate change did not take time off.

“And the IPCC report in August was a wake-up call for all of us. It made clear that the lights are flashing red on the climate dashboard.

“That report, agreed by 195 governments, makes clear that human activity is unequivocally the cause of global warming, and we know that the window to keep 1.5 degrees within reach is closing.”

Also addressing delegates, Patricia Espinosa, executive secretary of the UN Framework Convention on Climate Change (UNFCC), said: "We stand at a pivotal point in history. We either choose to achieve rapid and large-scale reductions of emissions to keep the goal of limiting global warming to 1.5C — or we accept that humanity faces a bleak future on this planet."

Climate analysis suggests that, before the Paris Agreement, the world was heading towards catastrophic global warming of 6 degrees Celsius.

Since Paris, pledges to cut emissions would see the planet's average temperature rise 2.7 degrees Celsius this century, but this would still supercharge the damage that climate change is already causing by intensifying storms, exposing more people to deadly heat and floods, killing coral reefs and destroying natural habitats.

The return of the US, the world's biggest economy, to UN climate talks will be a boon to the conference, after a four-year absence under President Donald Trump.

But, like many world leaders, US President Joe Biden will arrive at COP26 without firm legislation in place to deliver his own climate pledge as Congress wrangles over how to finance it and amid new uncertainty about whether US agencies can even regulate greenhouse gas emissions.

Earlier on Sunday, Sharma told the BBC’s Andrew Marr: "This is on leaders ... they need to come forward, and collectively we need to agree.

"Every country I've spoken to understands that climate change does not recognise borders.

“This is a huge challenge that we face collectively.

“I would argue that, actually, what we're trying to achieve here is tougher than what was achieved at Paris.

“Paris was a great, historic agreement. But it was the framework agreement and a lot of the detailed rules were left to later COPs.

“And now, after six years, we still have to resolve some of those issues. And of course, the geopolitics is also more challenging than at the time.”

Geopolitical challenges

Adding to the challenging geopolitical backdrop, a global energy crunch has prompted China to turn to highly polluting coal to avert power shortages, and left Europe seeking more gas, another fossil fuel.

Ultimately, negotiations will boil down to questions of fairness and trust between rich countries whose greenhouse gas emissions caused climate change, and poor countries being asked to decarbonise their economies with insufficient financial support.

Covid-19 has exacerbated the divide between rich and poor. A lack of vaccines and travel curbs mean some representatives from the poorest countries cannot attend the meeting.

Other obstacles — not least, sky-high hotel rates in Glasgow during the conference — have stoked concerns that civil society groups from the poorest nations, which are also most at risk from global warming, will be under-represented.

Covid-19 will make this UN climate conference different from any other, as more than 39,000 delegates from governments, companies, civil society, indigenous peoples, and the media will fill Glasgow's cavernous Scottish Event Campus.

All must wear masks, socially distance and produce a negative Covid-19 test to enter each day — meaning the final-hour "huddles" of negotiators that clinched deals at past climate talks are off the table.

World leaders will kick start COP26 on Monday with two days of speeches that could include some new emissions-cutting pledges, before technical negotiators lock horns over the Paris Agreement rules.

Any deal is likely to be struck hours or even days after the event's 12 November finish date.

Outside, tens of thousands of protesters are expected to take to the streets to demand urgent climate action.

On Saturday, climate activist Greta Thunberg was mobbed by supporters at Glasgow Central Station after arriving in Scotland’s largest city by rail.

Thunberg and hundreds of other activists, politicians and officials from countries including the Netherlands, Belgium and Germany travelled on a so-called "climate train" earlier arriving in London by Eurostar.

Assessing progress of COP26 will be complex, observers said.

Unlike past climate summits, the event will not deliver a new treaty or a big "win" but seeks to secure smaller but vital victories on emission-cutting pledges, climate finance and investment.

Ultimately, success will be judged on whether those deals add up to enough progress to keep the 1.5 degrees Celsius goal alive.

To meet it, global emissions must plummet 45% by 2030 from 2010 levels, and reach net zero by 2050 — requiring huge changes to countries' systems of transport, energy production, manufacturing and farming.

Countries' current pledges would see global emissions soar by 16% by 2030.

Pressed on Cambo

Closer to home, the COP26 president was also pressed by Andrew Marr about whether the UK would “set a good example” and “show moral authority” in tackling climate change if it granted Siccar Point Energy and Shell permission to develop the controversial Cambo oil and gas field in the west of Shetland area.

Sharma said a decision has not yet been taken on Cambo and any new oil and gas projects would have to be compatible with the UK's legally binding pledge to become a carbon neutral economy by 2050.

Sharma said: “It's worth setting out where we've actually got to. And I want to put [Cambo] in context — [the UK] as a country has decarbonised the economy faster than any other G20 nation over the past years.

“And in terms of oil and gas we have been very clear — we have said that in terms of granting any future licences there will be a climate compatibility checkpoint and any licences that are granted will have to be compatible with our legal requirement to be net zero by 2050.”

Last week, Ben van Beurden, chief executive of Shell, which holds a 30% interest in Cambo, said he believed the UK should give the go-ahead to the project, which has faced fierce protests from environmental groups.

Van Beurden said: “The decision to develop Cambo or not ... is a national decision. If indeed, the government of the UK wants to supply at least part of its energy needs with domestic resources, then it should develop projects like Cambo. If not, it should just import more.”

He added the North Sea was “one of the leading, if not the leading, basins in the world when it comes to the energy transition and carbon intensity”, particularly since the announcement of the North Sea Transition Deal, agreed between government and industry in March.

“So think again. If you say, ‘We are not going to develop our national resources in a basin that actually has one of the most progressive regulations when it comes to energy transition, done by a company that is going to use some of that cash flow to actually drive the energy transition in other parts of the energy system'. [If you say], 'We don't want any of that, let us just import it from elsewhere’, how does that make sense?” Van Beurden asked.

“It is not going to be our decision. It is going to be a decision taken in Westminster, and we will await that decision. But I think I know where I stand when it comes to global logic.”

The first of its kind for a G7 country, the UK's transition deal committed industry to emissions reductions from offshore production of 10% by 2025; 25% by 2027; and 50% by 2030 — against a 2018 baseline — before hitting net zero by mid-century.

Van Beurden also revealed Shell will be formally absent at COP26 after the company was told "it would not be welcome".

"We are not represented," Ben van Beurden told journalists last week. "That's probably all there is to it."

(Reuters contributed to this article.)