The United Nations outlined Wednesday how the world might bring global temperatures back to 1.5C once the milestone target is breached in a first-of-its-kind assessment that drew criticism from climate activists.

UN Secretary General Antonio Guterres last year acknowledged the inevitability of overshooting 1.5C but this is the first time a UN agency has addressed this head-on in a dedicated report.

Scientists agree that warming above 1.5C relative to pre-industrial times poses grave risks and nearly 200 nations committed in 2015 to try and keep global temperature rises to this safer threshold.

But the world is expected to blow past 1.5C in a matter of years and attention has pivoted to making any "overshoot" as brief and contained as possible.

In a much-anticipated report, the UN Environment Programme (UNEP) stated unequivocally that 1.5C would be surpassed and "the challenge has shifted from avoiding overshoot to navigating it".

"It's a reality check. We have to live with this warmer world... but there's still a lot we can do to limit warming," said report co-author Richard Betts from the University of Exeter.

The report "was about responses rather than just sort of saying it is happening", he told journalists.

Fellow author Debra Roberts said it was important for policymakers "to realise the rules of the game are now changing".

"I think for me that's the clarion call... we now have to reconfigure our climate governance around dealing with an overshoot pathway. None of our plans are prepared for that," said Roberts from the University of KwaZulu-Natal.

Dismay
The report's stark language alarmed climate campaigners who have rallied around 1.5C since it was affirmed as the more ambitious goal of the Paris Agreement a decade ago.

"This is a heartbreaking moment, but overshoot is not the reason to give up and surrender 1.5C", said Jasper Inventor, Deputy Programme Director at Greenpeace International.

Bill Hare, a climate scientist and CEO of policy institute Climate Analytics, slammed the report and said it risked becoming "a call to apathy rather than a call to arms".

Small island nations threatened by rising seas have been particularly vocal defenders of the 1.5C target and have urged wealthier countries not to abandon the milestone even as it slips out of reach.

The world has already warmed roughly 1.4C since 1850-1900 mostly from the burning of fossil fuels and the last three years have been the hottest on record.

No good outcomes
Scientists say any overshoot of 1.5C threatens more extreme weather and pushes the planet closer to tipping points like the collapse of ice sheets and loss of coral reefs.

UNEP executive director Inger Andersen said there "are no good outcomes if we remain above 1.5C".

"Let's be very clear -- that 1.5C target is still the goal. But now, we need to approach it differently: from above," Andersen told journalists.

Guterres it was possible to bring warming back below 1.5C by the end of the century.

"The fight for 1.5 degrees is the fight for humanity," he said.

To achieve this, carbon dioxide removal -- the process of pulling carbon from the air and storing it -- would be essential alongside deep and rapid cuts to greenhouse gas emissions, the UNEP report said.

At present, carbon removal plays a vanishingly small part in addressing climate change and is strongly opposed by many activists pushing for a faster phase out of coal, oil and gas.

"The only way to minimise harm is to cut emissions now... not to delay action with speculative techno-fixes," said Lili Fuhr from campaign organisation CIEL.