The 1.5-degree target has become a rallying point for nations attending the COP28 climate talks, despite rising certainty among scientists that the world will spill over that threshold, potentially within a decade. Temperatures have already risen between 1.1 and 1.3 degrees.

It may be possible to bring global temperatures back down again, using still-unproven technological means to draw carbon dioxide out of the atmosphere. But at least some overshoot is probably unavoidable, scientists said in the new report to the U.N.

The looming shadow of overshoot is one of 10 stark warnings the researchers presented Sunday in an annual report on top climate science insights from the past year. Launched in 2017, the series is coordinated by scientific organizations Future Earth and Earth League, alongside the World Climate Research Programme, whose scientific work helps inform national climate commitments worldwide. The report is presented each year to the U.N. during its annual climate conference.

This year’s report includes a variety of findings.

Mountain glaciers are swiftly shrinking. Natural landscapes, like forests and wetlands, may soak up less carbon dioxide as the planet warms, causing more pollution to linger in the atmosphere. Compound climate events — multiple extreme weather disasters happening at the same time or in rapid succession — are a growing threat.

The report also includes insights on the links between climate change and biodiversity loss, the role that food systems can play in reducing carbon emissions, the plight of global populations that lack resources to relocate in the face of worsening climate impacts, and the importance of just and equitable climate adaptation efforts.

But its findings on the 1.5-degree target are among its starkest conclusions.

Nations have not reduced greenhouse gas emissions quickly enough to stay on track, the report finds. The world can emit only a certain amount of carbon before the 1.5-degree target slips out of reach, and recent studies suggest that threshold will arrive in about six years if humans keep burning carbon at their current rates.

Avoiding overshoot could still be technically possible — but that would require “truly radical transformations,” the report cautions. The U.N. Intergovernmental Panel on Climate Change, the world’s top authority on climate science, says global emissions must fall by a whopping 42 percent within the next six years to keep the target alive. And they must spiral down to net zero by midcentury.

Even then success would be “only a maybe,” said Nico Wunderling, a scientist at the Potsdam Institute for Climate Impact Research in Germany and one of the report’s authors.

Many scientists have long concluded privately that the world will at least temporarily miss that target. But that likelihood has only recently begun to surface in high-profile reports.

“It was already kind of an elephant standing in the room that we may actually not hold 1.5 degrees without an overshoot,” Wunderling said.

Many experts now say the best case scenario is if nations can limit overshoot as much as possible — ideally capping it at fractions of a degree — and bring temperatures back down as swiftly as possible.

The consequences of global warming worsen with every incremental amount the planet warms, scientists say. And some climate impacts can’t be reversed once they’re set in motion, like sea level rise or plant and animal extinctions.

That means 1.5 degrees should remain a centerpiece of the Paris Agreement, Wunderling said. Keeping that target in focus can motivate world nations to limit overshoot as much as possible.

“Minimizing the magnitude of overshoot, but also the duration of overshoot, is what the best case scenario really is,” Wunderling said.

That means global efforts to remove carbon dioxide from the sky must rapidly expand, the report adds. These methods can include everything from natural strategies, like planting forests, to constructing giant carbon-guzzling machines that suck the pollution directly out of the air — assuming the technology can be advanced to work at a large enough scale.

The IPCC has concluded that at least some carbon removal is essential to achieving net zero emissions by midcentury. Some sectors of the economy likely can’t get off fossil fuels that quickly, and their greenhouse gas emissions would have to be offset by pulling equal amounts of carbon dioxide out of the air.

Eventually, some experts hope humans can also use carbon dioxide removal to lower global temperatures to safer levels. That means drawing out more carbon than goes into the atmosphere.

Carbon dioxide removal isn’t a substitute for rapidly and immediately reducing emissions, the report warns. But it does need to swiftly scale up in order to limit overshoot as much as possible.

Meanwhile, rapidly phasing out fossil fuels is also key to limiting overshoot.

The emissions associated with existing fossil fuel infrastructure alone would already put the Paris targets out of reach, the report finds. Yet governments, companies and investors continue to build more fossil fuel projects.

“Consequently, governments and financial institutions need to actively plan for and implement a fossil fuel phase-out while accelerating the phase-in of renewable energy, aiming for a comprehensive and coordinated energy transition,” the report states.

This year’s climate conference is expected to feature a major debate between countries calling for a phase-out of all fossil fuels versus countries that want to soften the language to a “phase-down.”

The weaker language could result in slower global efforts to reduce emissions, some experts argue. It could open up the possibility that nations continue to burn fossil fuels, relying on the promise of carbon capture or carbon removal technology to clean up afterward, said Ploy Achakulwisut, a researcher with the Stockholm Environment Institute Asia and one of the report’s authors, in an email.

Another author, Gregor Semieniuk, an assistant research professor at the University of Massachusetts Amherst, referred to COP28 when he said, “These documents shape narratives that then, in turn, shape investment decisions and markets to some extent.”

Weaker language doesn’t necessarily commit the world to missing the Paris targets, he noted. The world could still proceed with strong enough mitigation efforts and climate financing to phase out fossil fuels.

“But it matters for sentiments and discussions, and therefore I think ‘phase out’ is pushed for by those who take this really seriously,” he added.

Historically, U.N. climate talks have avoided mentions of fossil fuels in their final decisions. Fossil fuels appeared in a decision text for the first time in 2021, at the conference in Glasgow, Scotland, when nations agreed to phase down — not phase out — coal.

Last year’s conference in Egypt reiterated that commitment, despite a push from many countries to adopt a phaseout of all fossil fuels. But the rapid approach of the 1.5-degree threshold calls for higher ambitions, the new report suggests.

“Raising the ambition and quality of the commitment language around fossil fuel phase-out would be an important first step towards achieving a 1.5C-aligned, rapid, well-managed, and equitable energy transition,” Achakulwisut said.

A version of this report first ran in E&E News’ Climatewire. Get access to more comprehensive and in-depth reporting on the energy transition, natural resources, climate change and more in E&E News.