PARIS — Even as some experts are questioning the extent to which it’s even possible, more and more voices at the U.N. climate change conference here are standing up for at least trying to keep global warming below 1.5 degrees Celsius above pre-industrial levels, rather than the more commonly cited 2 degrees C.

It’s a dizzying goal: The world is already at about 1 degrees Celsius of warming, with about 400 parts per million of carbon dioxide in the atmosphere and concentrations growing by around two parts per million per year on average. And recent research suggests that to limit global warming to 1.5 degrees C, concentrations couldn’t exceed about 420 to 440 parts per million by 2100.

The position that 1.5 degrees should be our planetary temperature target has long been held by small island nations and a growing number of developing countries – and has now been supported by 108 countries in total, according to the Climate Vulnerable Forum. But what’s been striking in Paris is that major developing country emitters have also started voicing at least limited support as well.

The latest line for these countries appears to be that warming should be held as much below 2 degrees C as possible, and that if it can be below 1.5 as well, that’s even better.

Politically speaking, the number 1.5 degrees signals the “existential threat that islands are facing,” explains Jennifer Morgan, climate program director at the World Resources Institute. So “when the developed countries move towards it, [it] represents that they understand that existential threat.”

And that’s just what appears to be happening, albeit cautiously in some cases. In his opening speech here in Paris, for instance, French President Francois Hollande stated that “we need to define and mark out a credible path that will enable us to contain global warming below 2°C or even 1.5°C, if possible.”  Other recent supporters of either a 1.5 C target or at least language citing 1.5 C include AustraliaCanada — and now, the U.S.

Special envoy on climate change Todd Stern told press on Monday that “the goal is to hold temperatures to I think as far as possible below two degrees or something like that.” But he then added that “we are working with other countries on some formulation that would include reference to the 1.5 degrees, all as part of a somewhat larger, broader sentence.” And John Kerry Tuesday in Paris made similar remarks, saying of 1.5 degrees that “I think we should embrace it as a legitimate aspiration,” although he added that the official target should remain 2 degrees.

“It’s not new that some parties are trying to keep the reference to 1.5,” says Kyle Ash, senior legislative representative for Greenpeace in the U.S. “What’s new is that there are more countries making it the priority.”

“The 1.5 degrees C coalition is growing,” said Dr. James Fletcher, the minister of Public Service, Sustainable Development, Energy, Science and Technology  for the Caribbean island nation of St. Lucia, on Tuesday. Fletcher is an influential negotiator who is co-chairing a key group here charged with determining the level of ambition to be embodied in the final climate agreement, and has been a central figure involved in expanding support for the 1.5 degree target.

Throughout the Caribbean, Fletcher says, the slogan “1.5 to stay alive” has caught on, capturing the fear that with any more warming than that, sea level rise will swamp much of the territory of low-lying island nations.

“I left with a mandate that I cannot come back without 1.5 degrees Celsius,” says Fletcher.

The draft agreement that emerged here on Saturday, which will form the basis of the final text, currently contains just two options when it comes to the temperature target, both of which are fairly ambitious. The parties to the agreement, it currently reads, will aim “to hold the increase in the global average temperature [below 1.5 °C] [or] [well below 2 °C] above preindustrial levels.”

Either option would go farther than the 2009 Copenhagen accord, which acknowledged “the scientific view that the increase in global temperature should be below 2 degrees Celsius” (though it also suggested there should be a scientific reassessment of whether 1.5 degrees might be a better target).

What has changed since then, of course, is the science — dismaying climate news has been coming fast and furious in the past two years, particularly with respect to the planet’s polar ice sheets, which seem to be showing troubling signs of destabilization. Scientists have also become increasingly vocal and critical of the 2 degrees C target, including in a paper published Monday in Nature Geoscience contending that “no scientific assessment has clearly justified or defended the 2° C target as a safe level of warming, and indeed, this is not a problem that science alone can address.”

Granted, not all countries appear to be on board at this point.

One media outlet charged recently that Saudi Arabia and India have sought to “block attempts” to make reference, in the final Paris agreement, to a U.N. report that explored the issue of holding warming to 1.5 degrees C, and noted that “limiting global warming to below 1.5 °C would come with several advantages in terms of coming closer to a safer ‘guardrail.’”

“A final decision has not been made on that yet, but there was some very strong resistance from some countries” to citing the document, Fletcher says.

The Saudi Arabian delegation did not respond to a press question about its stance on 1.5 versus 2 degrees, citing the large number of press inquiries it is receiving. In a recent statement, Saudi Arabia’s minister of petroleum and mineral resources, Ali Ibrahim Al Naimi, said that a Paris agreement “must strike a fair balance between greenhouse gas emissions reduction requirements and adaptation to the adverse effects of climate change, and must cover all sectors instead of focusing exclusively on the energy sector.”

It’s not hard to understand how a country whose economy is very dependent on fossil fuels might be concerned at the suggestion of tougher targets, explains Wael Hmaidan, director of the Climate Action Network International. “They’re concerned if we move into 1.5, that means even more radical action,” he said of Saudi Arabia. “How will they cope with needing to change completely their economy in the next 35 years?”

The deepest problem is that even as 1.5 degrees is gaining momentum in the Paris discussion, at least as a number that might be mentioned in the final agreement text alongside 2 degrees C, the grim math behind the carbon problem raises severe doubts about how achievable the target really is.

One study earlier this year, for instance, found that while keeping warming from ever exceeding 1.5 degrees may not be possible, initially overshooting that number and then coming back down again might be, assuming future technologies that can remove carbon dioxide from the air at a massive scale. Another scientific group, Climate Analytics, has also acknowledged that 1.5 degrees C will require such “negative emissions,” but pointedly observes that the same is true for the 2 C limit.  “Because of the delay in substantial global reductions to date, both limits now rely on negative emissions,” the group notes.

However, on Monday a group of scientists strongly critiqued the hope of relying on such “negative emissions” technologies, noting that the leading idea, bioenergy combined with carbon capture and sequestration or BECCS, would require a staggering amount of land if used at a scale sufficient to really make a difference. Other negative emissions technologies also come with major tradeoffs, the report noted.

But then, as noted before, many 2 degrees C scenarios also require negative emissions. “What we’ve seen is that the technology to get us to 2 degrees Celsius is the same technology that will get us to 1.5 degrees, the only difference is the deployment,” says Fletcher. “You will need much more rapid deployment.”

One thing seems clear — the more the world seriously considers a target of 1.5 degrees Celsius, the more likely it also is that it will actually stay under 2.

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