Human activity is changing the Earth’s climate in ways “unprecedented” in thousands or hundreds of thousands of years, with some of the changes now inevitable and “irreversible”, climate scientists have warned.

Within the next two decades, temperatures are likely to rise by more than 1.5C above pre-industrial levels, breaching the ambition of the 2015 Paris climate agreement, and bringing widespread devastation and extreme weather.

Only rapid and drastic reductions in greenhouse gases in this decade can prevent such climate breakdown, with every fraction of a degree of further heating likely to compound the accelerating effects, according to the International Panel on Climate Change, the world’s leading authority on climate science.

The comprehensive assessment of climate science published on Monday, the sixth such report from the IPCC since 1988, has been eight years in the making, marshalling the work of hundreds of experts and peer-review studies. It represents the world’s full knowledge to date of the physical basis of climate change, and found that human activity was “unequivocally” the cause of rapid changes to the climate, including sea level rises, melting polar ice and glaciers, heatwaves, floods and droughts.

World leaders said the stark findings must force new policy measures as a matter of urgency, to shift the global economy to a low-carbon footing. Governments from 197 countries will meet this November in Glasgow for vital UN climate talks, called Cop26.

Each nation is asked to come to Cop26 with fresh plans to reduce greenhouse gas emissions to a level that will limit global heating to no more than 1.5C above pre-industrial levels, the ambition of the Paris climate agreement and a goal the IPCC emphasised was still possible, but only just.

Antonio Guterres, the UN secretary general, warned: “[This report] is a code red for humanity. The alarm bells are deafening, and the evidence is irrefutable: greenhouse gas emissions from fossil fuel burning and deforestation are choking our planet and putting billions of people at immediate risk.”

He called for an end to new coal plants and to new fossil fuel exploration and development, and for governments, investors and businesses to pour all their efforts into a low-carbon future. “This report must sound a death knell for coal and fossil fuels, before they destroy our planet,” he said.

Boris Johnson, prime minister of the UK, hosts of Cop26, said: “Today’s report makes for sobering reading, and it is clear that the next decade is going to be pivotal to securing the future of our planet … I hope today’s report will be a wake-up call for the world to take action now, before we meet in Glasgow in November for the critical Cop26 summit.”

John Kerry, special envoy to US president Joe Biden, said: “The IPCC report underscores the overwhelming urgency of this moment. The world must come together before the ability to limit global warming to 1.5C is out of reach … Glasgow must be a turning point in this crisis.”

Temperatures have now risen by about 1.1C since the period 1850 to 1900, but stabilising the climate at 1.5C was still possible, the IPCC said. That level of heating would still result in increasing heatwaves, more intense storms, and more serious droughts and floods, but would represent a much smaller risk than 2C.

Richard Allan, a professor of climate science at University of Reading, and an IPCC lead author, said each fraction of a degree of warming was crucial. “You are promoting moderate extreme weather events to the premier league of extreme events [with further temperature rises],” he said.

Civil society groups urged governments to act without delay. Doug Parr, chief scientist at Greenpeace UK, said: “This is not the first generation of world leaders to be warned by scientists about the gravity of the climate crisis, but they’re the last that can afford to ignore them. The increasing frequency, scale and intensity of climate disasters that have scorched and flooded many parts of the world in recent months is the result of past inaction. Unless world leaders finally start to act on these warnings, things will get much, much worse.”

Stephen Cornelius, chief adviser on climate change at WWF, added: “This is a stark assessment of the frightening future that awaits us if we fail to act. With the world on the brink of irreversible harm, every fraction of a degree of warming matters to limit the dangers.”

Even if the world manages to limit warming to 1.5C, some long-term impacts of warming already in train are likely to be inevitable and irreversible. These include sea level rises, the melting of Arctic ice, and the warming and acidification of the oceans. Drastic reductions in emissions can stave off worse climate change, according to IPCC scientists, but will not return the world to the more moderate weather patterns of the past.

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Ed Hawkins, a professor of climate science at the University of Reading, and a lead author for the IPCC, said: “We are already experiencing climate change, including more frequent and extreme weather events, and for many of these impacts there is no going back.”

This report is likely to be the last report from the IPCC while there is still time to stay below 1.5C, added Joeri Rogelj, director of research at the Grantham Institute, Imperial College London, and an IPCC lead author. “This report shows the closer we can keep to 1.5C, the more desirable the climate we will be living in, and it shows we can stay within 1.5C but only just – only if we cut emissions in the next decade,” he said. “If we don’t, by the time of the next IPCC report at the end of this decade, 1.5C will be out the window.”

Monday’s report will be followed next year by two further instalments: part two will focus on the impacts of the climate crisis; and the third will detail the potential solutions. Work on the report has been hampered by the Covid-19 pandemic, which delayed publication by some months, and forced scientists to collaborate mainly online and through video conferencing.