At midnight on Dec. 31, Earth ended its hottest year in recorded history, scientists said Friday. The previous hottest year was 2023. And the next one will soon be upon us: by continuing to burn huge amounts of coal, oil and gas, humanity has almost guaranteed it.

The planet’s record high average temperature last year reflects weeks of spring heat waves of 104 degrees Fahrenheit that closed schools in Bangladesh and India. This reflected the effects of bathtub-warm ocean waters that strengthened hurricanes in the Gulf of Mexico and cyclones in the Philippines. And it reflected the hot summer and fall conditions that led Los Angeles to the most devastating wildfires in its history this week.

“We are facing a very new climate and new challenges, challenges that our society is not ready for,” said Carlo Buontempo, director of the Copernicus Climate Change Service, the European Union’s monitoring agency.

But even within this progression of warmer years and intensifying risks to homes, communities and the environment, 2024 stood out in another unwelcome way. According to Copernicus, it was the first year in which global temperatures averaged more than 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above those experienced by the planet at the start of the industrial age.

In the last decade, the world has tried to avoid crossing this dangerous threshold. Countries incorporated the goal into the 2015 Paris Agreement to combat climate change. “Keep 1.5 Alive” was the mantra at United Nations summits.

Still, here we are. Global temperatures will fluctuate somewhat, as they always have, which is why scientists often look at warming averaged over longer periods rather than just one year.

But even by that standard, staying below 1.5 degrees seems more and more unattainable, he says researchers who crunched the numbers. Globally, despite hundreds of billions of dollars invested in clean energy technologies, carbon dioxide emissions hit a record high in 2024 and show no signs of abating.

One recent study published in the journal Nature concluded that the absolute best humanity can hope for now is a warming of about 1.6 degrees. To achieve this, nations had to start reducing emissions at a pace that would strain political, social and economic feasibility.

But what if we had started earlier?

“We were guaranteed to get to this point where the gap between reality and the trajectory we needed for 1.5 degrees was so big it was ridiculous,” said David Victor, a professor of public policy at the University of California, San Diego.

The question now is what, if anything, should replace 1.5 as the guiding star for nations’ climate aspirations.

“These top-level goals are at best a compass,” said Dr. Victor. “They are a reminder that significant climate impacts await us if we do not do more.”

The 1.5 degree threshold has never been the difference between safety and doom, between hope and despair. That was the number negotiated by governments trying to answer the big question: What is the biggest rise in global temperature — and the associated level of danger, whether heat waves or wildfires or melting glaciers — that our societies should strive to avoid?

The result, as codified in the Paris Agreement, was that nations would strive to keep warming to “well below” 2 degrees Celsius while “making efforts” to limit it to 1.5 degrees.

Even at the time, some experts called the latter goal unrealistic because it required such deep and rapid emissions reductions. Nevertheless, the United States, the European Union and other governments have accepted it as a roadmap for climate policy.

Christoph Bertram, an associate research professor at the University of Maryland’s Center for Global Sustainability, said the urgency of the 1.5 target has prompted companies of all kinds — auto makers, cement plants, power utilities — to start thinking hard about what it would mean to eliminate their emissions by mid-century. . “I think that led to serious measures,” said Dr. Bertram.

But the lofty aspiration of the 1.5 target has also exposed deep lines of cleavage between nations.

China and India never supported that goal, as it required them to limit their use of coal, gas and oil at a rate they said would hamper their development. It started with rich countries struggling to reduce their own emissions stifling funding in developing countries for fossil fuel projects that were economically beneficial. Some low-income countries felt that this was deeply unfair ask them for a sacrifice for the climate given that rich nations – not them – produced most of the greenhouse gases that are now warming the world.

“The 1.5 degree target has created a lot of tension between rich and poor countries,” said Vijaya Ramachandran, director of energy and development at the Breakthrough Institute, an environmental research organization.

Costa Samaras, a professor of environmental engineering at Carnegie Mellon University, compared the warming targets to health officials’ guidelines on, say, cholesterol. “We don’t set health goals according to what is realistic or possible,” said Dr. Samaras. “We say, ‘This is good for you. You won’t get sick this way.'”

“If we were to say, ‘Well, 1.5 is probably out of the question, let’s put it at 1.75,’ that gives people a false sense of security that 1.5 isn’t that important,” said Dr. Samaras, who helped shape America’s 2021-2024 climate policy in the White House Office of Science and Technology Policy. “It’s very important.”

Scientists assembled by the United Nations they concluded that limiting warming to 1.5 degrees instead of 2 would spare tens of millions of people from exposure to life-threatening heat waves, water shortages and coastal flooding. It could mean the difference between a world that has coral reefs and arctic sea ice in the summer and a world that doesn’t.

Every small increase in additional warming, whether it’s 1.6 degrees from 1.5 or 1.7 from 1.6, increases the risks. “Even if the world goes beyond 1.5 degrees, and the chances of that happening increase every day, we must continue to strive” to reduce emissions to zero as soon as possible, said Inger Anderson, executive director of the United Nations Environment Programme.

Officially, the sun has not yet set on the 1.5 target. The Paris accord remains in place, even though President-elect Donald J. Trump has vowed to withdraw the United States from it for a second time. At the UN climate talks, talk of 1.5 has become more muted compared to years past. But it’s hardly gone.

“With the right measures, 1.5 degrees Celsius is still achievable,” said Cedric Schuster, minister of natural resources and environment for the Pacific island nation of Samoa, at last year’s summit in Azerbaijan. Countries should “rise to the occasion with new, very ambitious” policies, he said.

dr. To Victor at UC San Diego, it’s strange but all too predictable that governments keep talking this way about what seems like an unattainable goal. “No mainstream political leader who wants to be taken seriously on climate is willing to stick their neck out and say, ‘1.5 degrees is not feasible. Let’s talk about more realistic goals,’ he said.

However, the world will eventually that discussion should be heldsaid dr. Victor. And it is unclear how it will go.

“It might be constructive when we start asking, ‘How much warming are we really in for?’ And how are we going to deal with it?’” he said. “Or it could look very toxic, with a lot of political finger-pointing.”