The 1.5C temperature limit to be discussed by world leaders at critical meetings this weekend is a vital physical threshold for the planet’s climate, and not an arbitrary political construct that can be haggled over, leading climate scientists have warned.

World leaders are meeting in Rome and Glasgow over the next four days to thrash out a common approach aimed at holding global temperature rises to 1.5C above pre-industrial levels, the lower of two limits set out in the 2015 Paris climate agreement.

But some countries are unwilling to peg their emissions plans to the tougher goal, as it would require more urgent efforts. They prefer to consider long-term goals such as net zero by 2050.

Johan Rockström, the director of the Potsdam Institute for Climate Impact Research and one of the world’s foremost climate scientists, warned that the 1.5C target was not like other political negotiations, which can be haggled over or compromised on.

“A rise of 1.5C is not an arbitrary number, it is not a political number. It is a planetary boundary,” he told the Guardian in an interview. “Every fraction of a degree more is dangerous.”

Allowing temperatures to rise by more than 1.5C would vastly increase the risk of irreversible changes to the climate, he said. For instance, it would raise the risk of the Arctic losing its summer ice, with dire knock-on effects on the rest of the climate as the loss of reflective ice increases the amount of heat the water absorbs, in a feedback loop that could rapidly raise temperatures further.

The Greenland ice sheet, the melting of which would raise sea level rises, could also be tipped into a state of irreversible decline beyond 1.5C.

A rise of more than 1.5C would also threaten changes to the Gulf Stream, which could also become irreversible. It could result in catastrophe for biodiversity hotspots, damage agriculture across swathes of the globe, and could inundate small islands and low-lying coastal areas.

Thousands of protesters from around the world arrived in Glasgow on Saturday to demand urgent action on the escalating ecological emergency before the two-week Cop26 climate conference.

Campaigners from scores of environmental justice, indigenous and civil society groups are converging on Scotland’s biggest city to forge alliances and pressure political leaders.

Among the activists to arrive in Glasgow on Saturday evening was Greta Thunberg who was mobbed by supporters at Glasgow Central train station. Climate protesters held a demonstration at the station ahead of her arrival and the teenage activist was greeted by large crowds of supporters.

Protests – from marches to strikes, and occupations to roadblocks – are being planned and activists say their campaigns of peaceful civil disobedience will be crucial to the outcome of the talks. “It is the protests which give me hope,” said Cat Scothorne, 18, an activist with Glasgow Calls Out Polluters.

“It is a chance to foreground the voices of those people on the frontline of the climate crisis and push back against the influence and ‘green washing’ of corporations at this Cop – a chance to tell people what is really happening, especially in the global south.”

Campaigners from Europe, Africa and Asia joined UK activists on Saturdayon the streets as protests and civil society events got under way. On the banks of the Clyde, overlooking the Cop26 conference centre, activists from Ocean Rebellion dressed as mermaids to highlight the huge impact industrial fishing has on greenhouse emissions.

Delegates from the Minga Indigena collective, representing indigenous communities in North and South America, were welcomed to the city by Scotland’s first minister Nicola Sturgeon. They mixed water from Scotland and the Andes before calling for climate justice to be a “unifying demand of Cop26”.

In central Glasgow, activists who had walked to Cop26 from across the UK and Europe arrived in the city demanding justice for those on the frontline of the climate crisis.

Alex Cochrane, from Extinction Rebellion Glasgow, which helped organise the “pilgrims’ procession”, said it was time for governments to “walk the walk for the global south”.

Cochrane added: “Cop26 must end a growing crime against humanity by wealthy governments where the global south are sacrificed to bear the brunt of the global north’s affluent, carbon-intensive lifestyles.”

Protester numbers were due to grow on Saturday eveningwith the arrival of a “climate train” carrying hundreds of activists from across Europe at Glasgow Central station.

Federico Pastoris, a climate justice activist and campaigner with the Stop Cambo group, which is campaigning to prevent drilling in the Cambo oilfield in the North Sea, said the next two weeks were as much about building links between environment campaigners in Europe and frontline communities in the global south as they were about influencing what goes on inside the conference centre.

“It is summed up by the idea of climate justice… there is a realisation that the Cop process is ineffective so we need to build grassroots collaborations and solidarity to find new ways of addressing this crisis. That is why people have made such an effort to get here.”

Over the next fortnight, campaigners are planning a series of protests and civil disobedience actions. On Friday Thunberg is planning to join a school strike in Glasgow, and on Saturday a global day of action will see large-scale marches in both Glasgow and London, with campaigners promising spin-off civil disobedience protests. Activists say there will also be smaller, more disruptive actions throughout the two weeks of the conference.

Many campaigners and civil society groups from the global south have had severe difficulty getting to Glasgow because of problems with visas and the changing Covid-19 travel restrictions.

However, by Saturday some had made it. Patience Nabukalu, 24, had travelled from Uganda as part of the Mapa – “most affected people and areas” – organisation, representing communities disproportionately affected by climate change.

“This is an opportunity for people like us, who live in areas that are really facing the climate effects right now, to raise our voices,” she said.

Nabukalu, who was speaking from a coach that was expected to arrive in Glasgow on Saturday evening, said she had grown up facing ever-more-regular and extreme flooding, which had had dire consequences for her family and the wider community.

She added: “The only thing I want to hear [from world leaders] are climate solutions and climate action. I am tired of promises and pledges because promises keep getting made but nothing actually happens and we are running out of time.”

“This is real science – it is a real number. Now we can say that with a high degree of confidence,” he said, as 1.5C indicated a physical limit to the warming the planet can safely absorb.

Rockström added: “[Staying within] 1.5C is achievable. It is absolutely what we should be going for.”

The leaders of the G20 group of the world’s biggest economies – developed and developing – are meeting on Saturday in Rome. They will fly to Glasgow for Monday morning, where they will be joined by more than 100 leaders from the rest of the world for the UN Cop26 climate summit.

The UK, as host of Cop26, has set the aim of “keeping 1.5C alive”, but some countries – including China, Saudi Arabia and Russia – have been reluctant to agree to focus on the 1.5C limit, preferring to point out that the Paris agreement states the world must hold temperatures “well below” 2C while “pursuing efforts” to stay within 1.5C.

However, scientific research since the Paris agreement was signed has added to a compelling body of global science showing that if temperatures are allowed to rise by more than 1.5C, the consequences will be severely damaging and many are likely to be irreversible.

Other leading climate scientists echoed Rockström’s warnings. Mark Maslin, a professor of Earth systems science at University College London, said: “The report by the Intergovernmental Panel on Climate Change (IPCC) published in 2018 made the science very clear: there are significant climate impacts all round the world even if we limit warming to 1.5C.

“The report also showed there were significant increases to impacts and damages if we overshoot this target … These results were fully supported by the very latest 2021 IPCC science report [published in August]. This is the science and these agreed climate targets set by the Paris agreement are non-negotiable and have been agreed already by all 197 countries of the UN.”

Joeri Rogelj, the director of research at the Grantham institute, Imperial College London, said: “Science tells us that climate change risks increase rapidly between 1.5C and 2C of warming. Looking at the last years, during which we experienced some of the impacts of a 1.2C warmer world [such as heatwaves, flooding and extreme weather] – one would be hard pressed to call this safe.

Matthew Taylor

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