It is the big surprise of the Paris talks: the growing acceptance of a call from small nations most vulnerable to climate change for the conference to declare warming should be halted at 1.5 degrees Celsius.

Even a few months ago, this seemed unimaginable. Two degrees was the only target on the table. But here it has gained momentum with more than 100 nations, including the United States and the European Union, agreeing that it should be in the final agreement. With more than a day of talks remaining, inclusion is far from a done deal. But it has strong support.

“1.5 is alive,” said Monica Araya, a delighted policy analyst from Costa Rica, on Wednesday night. “Something is shifting. It is a new music.”

Both science and diplomacy have been pointing toward a 1.5-degree target. Policymakers often say that a two-degree limit is a scientific judgment of what is safe. But it was never so. The origins of the two-degree target lie with European policy analysts a decade ago. Scientists here now say that 1.5 makes more sense for the planet.

One of them is Johan Rockström, director of the Stockholm Resilience Institute and pioneer in the science of establishing safe “boundaries” for human activity on the planet. Two degrees, he says, “contains significant risks for societies everywhere.” It could also trigger potentially dangerous feedbacks that accelerate warming by melting Arctic ice, parching rainforests, and releasing methane from tundra. “So 1.5 looks much more scientifically justifiable,” Rockström says.

“When it comes to food and farming, a 1.5-degree C warmer world is vastly different than a 2-degree C warmer world,” writes agricultural research Bruce Campbell of the International Center for Tropical Agriculture in a new blog. “Tropical maize, a staple in many parts of Africa, shows the biggest sensitivity to the extra half degree, with yield declining on average more than 50 percent.”

The people who have shouted the loudest for 1.5 are the leaders of small island states, such as Anote Tong, president of Kiribati, a Pacific Ocean nation made up of atolls where no land is more than two meters above sea level. Already washed by cyclones, they fear that two degrees would doom them, as warming gradually melts ice caps and pours ever more water into the oceans.

Michiel Schaeffer, of Climate Analytics, calculates that by 2300, two degrees would deliver sea-level rise of 2.7 meters, enough to drown most atoll islands, while 1.5 degrees would limit the rise to 1.5 meters, enough to save many.

“It is great to see 1.5 degrees in the text,” says Myles Allen, a climate analyst at Oxford University. It shows rising ambition among world leaders, he believes. But he warns that delivering 1.5 degrees will be twice as hard as delivering two degrees.

Since we are already at one degree of warming, “the maths is simple,” he says. “To limit warming to 2 degrees, CO2 emissions need to fall, on average, by 10 percent for every tenth of a degree of warming from now on. To limit warming to 1.5 degrees, CO2 emissions need to fall, on average, by 20 percent for every tenth of a degree of warming.” With the world warming by a tenth of a degree in less than a decade, that is a big ask.

Time is short to achieve such a goal. “For 1.5 degrees, carbon emissions must reach zero by 2050,” says Bill Hare, of Climate Analytics, a European think tank. No more new coal, for certain – but also widespread early shutdowns of oil fields and coal mines and conversion of the technologies that burn their products to clean fuels.

In reality, most believe that reaching a target of 1.5 degrees would, in practice, involve an overshoot and then clawing back by “negative emissions”. One way to create negative emissions would be to burn biomass in power stations and then capture and bury the resulting carbon emissions. That would deliver carbon-neutral energy. And when new biomass grows on the land created by the biomass harvest, it would absorb atmospheric CO2, creating negative emissions. Another route would be more exotic geoengineering to suck up CO2 or shade the planet.

There is no mention of resorting to geoengineering in the draft text being worked on here. Hare argues that it would be contrary to the climate convention itself, which prohibits all “dangerous” meddling with the climate.

But all this sounds like wild conjecture, since the pledges submitted by governments here set the world on a path for three degrees, rather than 1.5. Some say there is more cynicism than ambition in the emergence of the 1.5-degree target. They believe that it is being used as a pawn in a wider game to attract small island states and other vulnerable nations into different camps on other issues. They note that even India, despite its plans for a massive expansion of coal burning, claims to support the 1.5 target.

Pessimists fear it could discredit the entire agreement if the target were included without any means for achieving it.

But optimists believe that, even if 1.5 is unachievable, declaring it a long-term goal would strengthen the message to industrialists and investors that a low-carbon world is unstoppable. It would also encourage governments to “put in place policies that drive innovation and investment,” says Michael Jacobs, of the New Climate Economy Project and an advisor to the British prime minister Gordon Brown at the Copenhagen climate conference in 2009.

That’s politics. But it is also science. The bottom line is that stabilizing temperatures at any level — whether 1.5 or 2 or even 4 or 6 degrees — will eventually require bringing the global economy to zero emissions. The sooner we get there, the less the warming. But without zero emissions, temperatures will keep on rising. So, many here are asking, why not aim for 1.5 degrees?