The UN said on Wednesday that the world was set to breach the warming threshold of 1.5°C above pre-industrial levels, agreed to under the 2015 Paris climate accord, within the next few years.
It warned that governments must now work to limit the rise and bring temperatures back down.
In an assessment drawing on the work of dozens of scientists, the UN Environment Programme said global action to curb greenhouse gas emissions had fallen short, increasing the risks from extreme weather, rising seas and ecosystem loss.
“This is the legacy of our own inaction,” Unep executive director Inger Andersen said. “Extreme heatwaves are already proving that climate impacts will strike faster, hit harder and last longer, costing more lives and causing deeper disruptions.”
Above 1.5°C, extreme weather, glacier melting, ecosystem loss and sea-level rises would intensify, placing greater pressure on human health, food and water security, cities and infrastructure, Ms Andersen said.
“This is a matter of survival, not a matter of negotiation,” she told reporters at UN headquarters.
Unep said it remained possible to bring temperatures back below the threshold, although significant uncertainty surrounds whether this could be achieved.
Even under the most optimistic scenario, temperatures would peak at about 1.8°C before potentially falling back to 1.5°C later this century, the report said.
Getting them back under that level would require rapid and deep cuts in greenhouse gas emissions alongside carbon dioxide removal on a vast scale, through measures ranging from restoring forests to technology that extracts carbon directly from the atmosphere.
“Now, the question is, of course, whether those with the larger emissions are willing to lean in. We are seeing a good number of them doing so," Ms Andersen said. "We are seeing that renewable energy is much lower price in most markets than hydrocarbons.
"We are just not moving fast enough … if we were to bypass 1.8, then getting back down to 1.5 will be extremely difficult, with consequences that will have fundamental and calamitous impact for humanity."
UN Secretary General Antonio Guterres said last year that global temperatures were expected to temporarily rise above the 1.5°C threshold, compared with pre-industrial levels.
