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The United Nations has warned world leaders must cut greenhouse gas emissions by a staggering 7.6 per cent every year for the next decade to meet the most ambitious goal of the Paris climate deal, or suffer the impacts of greater global warming.

In a report on Tuesday, the UN Environment Programme (UNEP) laid out the enormous scale of the challenge to meet the target of holding temperature rises to 1.5°C by the end of the century, following a decade when emissions have on average grown by 1.5 per cent a year.

“We haven’t succeeded in bending the global emissions curve,” says Anne Olhoff at the Technical University of Denmark, one of the report’s lead authors.

Including land use, annual emissions stand at 55.3 gigatonnes of carbon dioxide, but by 2030 must fall by 15 gigatonnes to meet Paris’s goal of holding temperature rises to 2°C at the worst, and 32 gigatonnes to hit 1.5°C. That means hitting 1.5°C requires ongoing 7.6 per cent annual cuts in emissions, or 2.7 per cent for 2°C, says UNEP.

The world’s top climate scientists the stark impact of overshooting 1.5°C and landing at 2°C, including wiping out the planet’s coral reefs, more droughts and extreme heat days and exposing hundreds of millions of people to climate-related risks.

Globally, emissions have never fallen before, though they plateaued during 2014 and 2016, and have plunged dramatically at a country level before, such as in Russia after the fall of the Soviet Union.

Despite the impossible-seeming cuts required, UNEP maintains it is still feasible to stay under 1.5°C. “How long can we keep 1.5°C alive? We haven’t killed it yet. Even if we don’t get to 1.5°C, 1.7°C is a hell of a lot better than 2.5°C, or the 3.2°C we’re looking at now. Every 0.1°C counts,” says Olhoff.

The report comes less than a week before international climate talks resume at a summit in Madrid. The COP25 conference sets the stage for next year, when countries are due to submit bolder carbon-curbing plans under the Paris agreement – around 70 have said they will, including China and the EU. The US, which is quitting the Paris accord, will not.

UNEP says existing Paris pledges will see 3 to 3.2°C of warming, and the plans will have to be five times stronger to hit 1.5°C, and three times stronger for 2°C.

One bright spot is the decline of coal power, which is set for a 3 per cent fall in 2019 – the biggest drop on record – . UNEP cites protests by schoolchildren and the falling costs of green technologies as other reasons for hope, but concedes there is “no sign of greenhouse gas emissions peaking in the next few years”.

Each year of postponement leaves the world with an increasingly quixotic task. If emissions keep rising to 2025, Olhoff calculates we would then need annual emissions cuts of 15.7 per cent to meet 1.5°C. “By then it’s definitely too late,” she says.