Global warming will likely exceed the most ambitious target set by the Paris climate agreement by around 2040, according to a draft United Nations (UN) report.

In its strongest warning yet about the dangers of climate change, the UN’s Intergovernmental Panel on Climate Change (IPCC) warned that only “rapid and far-reaching” changes to the world economy would prevent temperatures from rising 1.5C above pre-industrial times.

The report was obtained by Reuters, ahead of the publication of the final version in October after it has been revised and approved by governments.

Campaigners say “the moment of truth” has come for world leaders, who will have to take quick and decisive action in order to avoid the most harmful effects of climate change.

“If emissions continue at their present rate, human-induced warming will exceed 1.5C by around 2040,” according to the report, which broadly reaffirms findings in an earlier draft in January but is more robust, after 25,000 comments from experts and a wider pool of scientific literature.

The Paris climate agreement, adopted by almost 200 nations in 2015, set a goal of limiting warming to “well below” a rise of 2C above pre-industrial times while “pursuing efforts” for the tougher 1.5C goal.

Scientists broadly agree that a 1.5C goal is required to avoid many of the most destructive effects of climate change.

The deal has been weakened after US president Donald Trump decided last year to pull out and promote US fossil fuels.

Temperatures are already up about 1C and are rising at a rate of about 0.2C a decade, according to the draft, which was requested by world leaders as part of the Paris agreement.

“Economic growth is projected to be lower at 2C warming than at 1.5C for many developed and developing countries,” it says, drained by impacts such as floods or droughts that can undermine crop growth, or an increase in human deaths from heatwaves.

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In a plus 1.5C world, for instance, sea level rise would be 10 centimetres less than with 2C, exposing about 10million fewer people in coastal areas to risks such as floods, storm surges or salt spray damaging crops.

The report says current government pledges in the Paris agreement are too weak to limit warming to 1.5C.

IPCC spokesman Jonathan Lynn said it did not comment on the contents of draft reports while work was still ongoing.

“It’s all a bit punchier,” said one official with access to the report, who said it seemed slightly less pessimistic about prospects of limiting a rise in global temperatures that will affect the poorest nations hardest.

The report outlines one new scenario to stay below 1.5C, in which technological innovations and changes in lifestyles could mean sharply lower energy demand by 2050 even with rising economic growth.

And there is no sign that the draft has been watered down by Mr Trump’s doubts that climate change is driven by manmade greenhouse gases.

The draft says renewable energies, such as wind, solar and hydro power, would have to surge by 60 per cent from 2020 levels by 2050 to stay below 1.5C, “while primary energy from coal decreases by two-thirds”.

By 2050, that would mean renewables supplying between 49 and 67 per cent of primary energy.

The report says governments may have to find ways to extract vast amounts of carbon from the air, for instance by planting massive forests, to turn down the global thermostat if warming overshoots the 1.5C target.

It omits radical geoengineering fixes such as spraying chemicals high into the atmosphere to dim sunlight, saying such measures “face large uncertainties and knowledge gaps”.

“As was clear from a number of scientific studies that were published recently, there is now a lot of evidence that the risks of climate change are significantly lower when temperature increase is limited to 1.5C, compared to an increase of 2C,” said Bert Metz, fellow at the European Climate Foundation and former IPCC co-chair.

“This holds for the overall economic damage, loss of biodiversity, health impacts of fossil fuel associated air pollution, sea level rise – particularly over the next hundreds of years – and many other aspects of society and nature.”

“The moment of truth has come for our leaders,” said Greenpeace International executive director Jennifer Morgan.

“They can achieve their Paris commitment of capping temperature rise below 1.5C with ‘rapid and far-reaching’ actions.

“The feasibility of achieving the Paris goals is a political choice. This choice must be driven by a moral obligation that binds us together. Through international cooperation, real ambition and working with communities, our leaders still have the time to do what they must.”