• "It's now or never, if we want to limit global warming to 1.5°C," IPCC Working Group III co-chair Jim Skea said.
• The 1.5 degrees Celsius goal is the aspirational temperature threshold ascribed in the landmark 2015 Paris Agreement.
• The IPCC's latest report follows a series of mind-bending extreme weather events worldwide.
• For instance, in just the last few weeks, an ice shelf the size of New York City collapsed in East Antarctica following record high temperatures and heavy rains deluged Australia's east coast, submerging entire towns.

The fight to keep global heating under 1.5 degrees Celsius has reached "now or never" territory, according to a new report released Monday by the world's leading climate scientists.

The highly anticipated report, delayed slightly due to last-minute disputes over the exact wording of the document, says curbing global heating to 1.5 degrees Celsius above pre-industrial levels would require greenhouse gas emissions to peak before 2025 at the latest.

At the same time, methane, a potent greenhouse gas, would also need to be reduced by roughly one-third.

The U.N.'s Intergovernmental Panel on Climate Change said it is "almost inevitable" that humanity will briefly surpass the critical temperature threshold of 1.5 degrees in this scenario, but it could return below this level by the end of the century.

"It's now or never, if we want to limit global warming to 1.5°C," IPCC Working Group III co-chair Jim Skea said in a statement accompanying the report. "Without immediate and deep emissions reductions across all sectors, it will be impossible."

The 1.5 degrees Celsius goal is the aspirational temperature threshold ascribed in the landmark 2015 Paris Agreement. It is recognized as a crucial global target because beyond this level, so-called tipping points become more likely. These are thresholds at which small changes can lead to dramatic shifts in Earth's entire life support system.

It has been feared that Russia's unprovoked onslaught in Ukraine may eclipse the findings, despite the fact that the report could be the last comprehensive assessment of climate science while there is still time to secure a liveable future.

The report marks the third installment from the IPCC in less than eight months, with one additional document scheduled for later in the year. The previous volumes have assessed the causes and impacts of the climate emergency but this one focused on mitigation.

"First thing is, we're on the wrong track," Julia Steinberger, ecological economist and professor from Switzerland's University of Lausanne, told CNBC via telephone. "In terms of a trajectory and also in terms of policies, we are just not on track for 1.5 or even 2 degrees."

Steinberger, a lead author on the IPCC's latest report, described the warning that global emissions must peak by 2025 at the latest as "a bit of a bombshell" given how little time there is to prevent the worst of what the climate crisis has in store.

"We're not talking about transition anymore. That ship has sailed — or, more like, failed to sail. Instead, the report is very much focused on transformation," Steinberger said.

"I really think the report contains elements of a positive turn. For the first time in human history, we have the technologies available to us that allow us to live comfortable lives without consuming ginormous amounts of energy," she added. "It's almost the first time that we can plausibly think about pathways to get beyond the age of combustion — and wouldn't that be exciting?"

The IPCC's latest report follows a series of mind-bending extreme weather events worldwide. For instance, in just the last few weeks, an ice shelf the size of New York City collapsed in East Antarctica following record high temperatures and heavy rains deluged Australia's east coast, submerging entire towns.

The catastrophic scenario prompted the largest mass youth climate strike since 2019 last month, with hundreds of thousands of environmental activists from 93 countries on all continents marching under the banner of "#PeopleNotProfit." The Fridays For Future movement called for climate reparations and justice.

What does the report say?

IPCC Chair Hoesung Lee said the report shows humanity is "at a crossroads," but the tools and know-how required to limit global heating are available.

"I am encouraged by climate action being taken in many countries," Lee said. "There are policies, regulations and market instruments that are proving effective. If these are scaled up and applied more widely and equitably, they can support deep emissions reductions and stimulate innovation."

The U.N. climate panel has said that to keep rising global temperatures under 1.5 degrees Celsius this century, emissions from warming gases must be halved by the end of the decade.

IPCC scientists also repeated calls for a substantial reduction in fossil fuel use to curb global heating, now at 1.1 degrees Celsius above pre-industrial levels.

Reacting to the IPCC's findings on Monday, U.S. Special Presidential Envoy for Climate John Kerry said it "represents a defining moment for our planet."

"The stakes are clear," Kerry said. "Complacency will be met by irreversible and unthinkable impacts from climate change."

The report says the average annual global greenhouse gas emissions were at their highest levels in human history from 2010 through to 2019, but the rate of growth has since slowed. This has coincided with increasing evidence of climate action, but the report warns that limiting global heating to 1.5 degrees will not be possible without immediate and deep emissions reductions across all sectors.

These reductions will require major transitions in the energy sector, the report says, including a massive reduction in fossil fuel use, widespread electrification, improved energy efficiency and the use of alternative fuels — such as hydrogen.

Cities are thought to offer significant opportunities for emissions reductions. The report says this can be achieved through lower energy consumption, electrification of transport combined with low-emission energy sources and enhanced carbon uptake and storage using nature.

"Climate change is the result of more than a century of unsustainable energy and land use, lifestyles and patterns of consumption and production," the IPCC's Skea said. "This report shows how taking action now can move us towards a fairer, more sustainable world."

What is the IPCC?

The IPCC is a U.N. body of 195 member states that assesses the science related to the climate crisis on behalf of governments every few years. It is currently in its sixth assessment cycle, having published its first major report in 1990.

The first installment of the IPCC's Sixth Assessment Report, published in August, focused on the physical science basis of climate change. The findings made it clear that limiting global heating to 1.5 degrees Celsius would soon be beyond reach without immediate and large-scale reductions in greenhouse gas emissions.

The second major report examined climate impacts and found that about half of the world's population is now acutely vulnerable to disasters stemming from the burning of fossil fuels. Published in late February, it warned that any further delay in concerted global action "will miss a brief and rapidly closing window to secure a livable future."

The final part of the IPCC's sixth assessment cycle is the so-called "Synthesis Report," which combines the findings of each of the three previous installments. This is scheduled to be published in September.