LE BOURGET, France — The Paris climate-change conference was supposed to be about the needs of big countries and what they are willing to do to slow the warming of Earth’s atmosphere. But in the end, the two weeks of sometimes round-the-clock negotiations have focused at least as much on some of the smallest, most defenseless nations whose very existence could hinge on the outcome of the talks.

The result could be a tougher set of policy goals than anyone originally thought could emerge from the conference. While the ultimate agreement is expected to embrace a goal of limiting global warming to less than two degrees Celsius, it also is likely to recognize a far more challenging and aspirational goal of 1.5 degrees Celsius.

That tougher language might not be legally binding for countries such as the United States, but the fact it is in the running is testament to the tireless work of delegations from remote countries facing an urgent threat from the rising seas of a warmer Earth.

The growing momentum behind 1.5 degrees is a story of fast-breaking science, savvy politics and a change in tone in the climate debate – one that, pushed by Pope Francis, has focused increasing attention on the needs of the most vulnerable countries. (The Vatican on Thursday came out in favor of the 1.5-degree target.)

“The small guys have managed to push the big guys, and that is a big story,” Monica Araya, founder and executive director of the Costa Rican nongovernmental organization Nivela and special adviser to the Climate Vulnerable Forum, said at the conference Friday.

PROGRESS WITH SOME STUMBLES

The ongoing debate over temperature targets came as delegates from 195 nations met in closed sessions to try to resolve remaining obstacles to a climate treaty that could be approved as early as Saturday. Diplomats remained divided over a handful of thorny issues, including financial aid to developing countries hit hard by climate change, as well as rules and procedures for judging whether countries are honoring their commitments to cut pollution.

Secretary of State John F. Kerry, in Paris to help push for a deal, said there had been “a lot of progress” but also a few snags during late-night bargaining sessions Thursday and early Friday.

“I’m hopeful,” he told reporters. “I think there is a way to go forward, that there’s a reasonableness.”

For many years, small island nations such as the Maldives – joined more recently by a broader group of climate-vulnerable countries in Africa, Asia and Latin America – have pushed to make the world recognize tougher climate goals. It has been a long-shot fight because of the massive effort required to meet even the less stringent goal of restricting warming to less than two degrees Celsius above preindustrial temperatures, and also because of their relative lack of political and economic power.

“Maldives itself has over 3,000 years of history,” said Ahmed Sareer, the Maldives’ permanent representative to the United Nations and its ambassador to the United States. “The location, the culture, the language, the traditions, the history, all this would be wiped off” if sea levels are allowed to rise high enough.

Nonetheless, holding warming to 1.5 degrees hardly seemed realistic. With the world already at about one degrees Celsius over preindustrial levels and current national emissions pledges well off target even for 2 degrees, how would 1.5 ever happen?

Still, small island nations brought their case to Paris. Their message was epitomized by a poster at the Wider Caribbean Pavilion at the vast Le Bourget conference center. The poster shows a young girl up to her neck in ocean water. Behind her, the now-submerged beach she’s standing on sports a drowned sign: “1.5 to stay alive,” it reads.

PLEADING THE ISLAND CASE

The talks in Paris were barely getting underway last week when representatives from Antigua and Barbuda made a series of impassioned pleas to the nations gathered to negotiate a climate treaty. In speeches and in a written appeal, officials from the islands warned that their homeland was literally in danger of being swept away by rising sea levels.

Even if all countries honored their current promises to cut greenhouse-gas emissions, global temperatures would rise by 2.7 degrees Celsius – and “that would be too much,” the delegation said, summarizing its view on its official Twitter account.

“The ministers came to the [talks] so that we might escape a world of plus-3 degrees, and for refusing to sign the death warrant of certain countries,” the message read. “It seems that this promise is forgotten.”

Similar appeals have been made for years, but in Paris the islanders acquired new allies: African nations, Europeans, even some Americans expressed sympathy, the diplomat said.

As the Paris meeting unfolded, the 1.5 target received more and more acknowledgment from major economies such as France, Canada and the United States. Then, a near-final draft agreement released Thursday enshrined it as the aspirational climate goal of the entire world. Countries, the draft said, will take steps to “hold the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C.”

Something could still change in how the final text treats 1.5 degrees..

Still, observers say that the moral appeal of small-islanders has merged with a growing body of troubling science suggesting that their temperature target turns out to be a meaningful one.

But the talk of 1.5 degrees brings with it deeply sobering implications that, until now, many in the climate debate largely managed to avoid or ignore.

MASSIVE CARBON REMOVAL?

Increasing talk about this target also opens up, more than ever, a troubling discussion about “negative emissions” technologies that do not exist on any mass scale at present but theoretically would be able to pull carbon dioxide out of the air. Perhaps the most popular of them is bioenergy with carbon capture and storage, or BECCS, which would involve burning plants for power and then storing the carbon released in the ground.

These technologies will be needed, scientists say, for 1.5 to be possible. It may be that the only way to land the planet at 1.5 degrees is to temporarily overshoot that target and then cool things back down again through massive carbon removal from the air, according to scientists.

Criticisms of “negative emissions” technologies are mounting. Recently, a large group of scientists said it would be “extremely risky” to rely on such technologies rather than simply cutting carbon emissions sharply, because they all have major trade-offs (BECCS, for instance, would require a huge amount of land). But nonetheless, they’ve become a part of the debate out of necessity.

Thus, the powerful moral case made by small island nations and other climate-vulnerable countries now runs head on into the extraordinarily complex math of the global carbon budget, with a little science fiction thrown in to boot.

But even if humans cannot manage to keep the planet from warming more than 1.5 degrees Celsius, there could be a benefit to the effort.

“Having aimed for 1.5 in the first place,” Rogelj said, “if we are not lucky, if some technologies don’t turn out, then maybe we will be safe enough to stay below 2 degrees.”