Humanity has less than three years to halt the rise of planet-warming carbon emissions and less than a decade to slash them by nearly half, UN climate experts said Monday, warning the world faced a last-gasp race to ensure a "livable future".

That daunting task is still -- only just -- possible, but current policies are leading the planet towards catastrophic temperature rises, the UN's Intergovernmental Panel on Climate Change (IPCC) made clear.

The world's nations, they said, are taking our future right to the wire.

The 2,800-page report -- by far the most comprehensive assessment of how to halt global heating ever produced -- documents "a litany of broken climate promises", said UN chief Antonio Guterres in a blistering judgement of governments and industry.

"Some government and business leaders are saying one thing -- but doing another. Simply put, they are lying. And the results will be catastrophic," Guterres said.

In recent months, the IPCC has published the first two installments in a trilogy of mammoth scientific assessments covering how greenhouse gas emissions are heating the planet and what that means for life on Earth.

This third report outlines what we can do about it.

"We are at a crossroads," said IPCC chief Hoesung Lee. "The decisions we make now can secure a livable future. We have the tools and know-how required to limit warming."

US Secretary of State Antony Blinken said these tools "are firmly within our grasp": "Nations of the world must be brave enough to use them."

The solutions touch on virtually all aspects of modern life, require significant investment and need "immediate action", the IPCC said.

The very first item on the global to-do list is to stop greenhouse gas emissions from rising any further.

That must be done before 2025 to have a hope of keeping within even the Paris Agreement's less ambitious warming target of two degrees Celsius above preindustrial levels.

Barely 1.1C of warming so far has ushered in a surge of deadly extreme weather across the globe.

The report makes clear that investments to cut emissions will be far less expensive than the cost of failing to limit warming.

Scientists warn that any rise above 1.5C risks the collapse of ecosystems and the triggering of irreversible shifts in the climate system.

To achieve that target, the report said carbon emissions need to drop 43 percent by 2030 and 84 percent by mid-century.

"It's now or never, if we want to limit global warming to 1.5C," said Jim Skea, a professor at Imperial College London and co-chair of the working group behind the report.

"Without immediate and deep emissions reductions across all sectors, it will be impossible."

- Slashing coal, oil, gas -

To do that the world must radically reduce the fossil fuels behind the lion's share of emissions.

Nations should stop burning coal completely and cut oil and gas use by 60 and 70 percent respectively to keep within the Paris goals, the IPCC said, noting that both solar and wind were now cheaper than fossil fuels in many places.

But cutting emissions is no longer enough, the IPCC said. Technologies to suck CO2 out of the atmosphere -- not yet operating to scale -- will need to be ramped up enormously.

While government policies, investments and regulations will propel emissions cuts, the IPCC made clear that individuals can also make a big difference.

Cutting back on long-haul flights, switching to plant-based diets, climate-proofing buildings and other ways of cutting the consumption that drives energy demand could reduce greenhouse gas emissions 40 to 70 percent by 2050.

Those with the most, also pollute the most, the report said.

Households whose income is in the top 10 percent globally -- two thirds of whom are in developed countries -- emit up to 45 percent of carbon pollution.

"Individuals with high socio-economic status contribute disproportionately to emissions and have the highest potential for emissions reductions -- as citizens, investors, consumers, role models and professionals," the IPCC said.

- Fuel for war -

For 2019, if energy consumption is included, industry accounted for 34 percent of human-caused greenhouse gas emissions; agriculture, forestry and land use was 22 percent; transport 15 percent; buildings 16 percent; and the energy supply sector 12 percent.

The report's finding will feed into high-level UN political negotiations, which resume in November in Egypt at COP 27.

Recently updated national climate pledges emerging from these talks still put the 1.5C target "beyond reach", the report warned.

With war in Ukraine spurring efforts to transition away from Russian oil and gas in the West, observers said the report should sharpen nations' focus on climate commitments.

"It is heart-breaking for me, as a Ukrainian climate activist, to be living through a war which has fossil fuel money at its core," said Olha Boiko, an activist from the Climate Action Network, based in Ukraine.

"The money, that we begged not to invest in dirty energy, is now flying over our heads in the form of bombs."