We are talking, of course, about the Paris Agreement goal of capping Earth's average surface temperature at 1.5 degrees Celsius above levels in the late 19th century, when burning fossil fuels began to seriously heat up the planet.
At barely 1.2C above that threshold, the world today has already seen a crescendo of deadly and destructive extreme weather.
Fifteen years ago, a 1.5C limit on global warming -- championed by small island nations worried about sea level rise -- was rejected by most scientists as unrealistic and by most countries as unnecessary.
A 2C "guardrail" was assumed to be safe enough.
Today, the 1.5C target is enshrined in everything, everywhere, all at once. While technically no more than an "aspirational" goal, it has become the de facto North Star for UN climate talks, national climate plans and the business world.
From Apple and Facebook to Big Pharma and even Big Oil, multinationals have unveiled promises and plans to be "1.5C-aligned", even if most of those plans don't hold up very well under scrutiny.
You can draw a straight line from 1.5C to the science-base imperative to nearly halve global emissions by 2030 and achieve net zero around mid-century, meaning any residual carbon pollution must be offset by removals.
Both of these targets are set to be affirmed in a report summarising six years of climate science, released Monday by the UN's Intergovernmental Panel on Climate Change (IPCC).
- 2C not good enough -
This raises a perplexing question, according to Beatrice Cointe, a sociologist at France's National Centre for Scientific Research and co-author of a recent study on the history of the 1.5C target.
"How did an almost impossible target become the point of reference for climate action?" she asked.
And what will happen when the world experiences its first full year at or above 1.5C, which the IPCC says could easily happen within a decade, even under aggressive emissions reduction scenarios?
"The target appears increasingly unattainable," Cointe and co-author Helene Guillemot, a historian of science at the Centre Alexandre-Koyre, wrote in the journal WIREs Climate Change.
"And yet calls to 'Keep 1.5C Alive' have been growing louder."
The backstory of the 1.5C goal reveals an interplay of science and politics, with one driving and shaping the other.
Going into the 2015 climate negotiations that yielded the breakthrough Paris treaty, it seemed unlikely that 195 nations would significantly improve on the 2C target already set in stone.
But a scientific evaluation by a UN technical body delivered ahead of the December summit sounded an alarm about the dangers of a +2C world and suggested greater ambition might be wise.
"While science on the 1.5C limit is less robust, efforts should be made to push the defence line as low as possible," it concluded.
A growing coalition of developing nations, meanwhile, had gathered behind the 1.5C goal, eventually joined by the European Union and the United States.
Emerging giants and oil exporters baulked, fearful of the constraints on their fossil-fuel dependent economies.
"China was against it, India was against it, Saudi Arabia fought us tooth-and-nail to the very end," recalled Saleemul Huq, director of the International Centre for Climate Change and Development in Dhaka.
Even today, these nations remain lukewarm on the idea.
But in the end, nearly 200 nations committed to cap warming at "well below 2C", while "pursuing effort to limit the temperature increase to 1.5C."
- 'A moral target' -
It was a stunning diplomatic coup. Many scientists, however, were less than thrilled.
"It will be very hard -- if not impossible -- to keep warming below 1.5 C during the entire 21st century," Joeri Rogelj, a climate modeller currently at Imperial College London who played a key role in the technical report, told AFP at the time.
But because the target was part of the Paris Agreement, nations called on the IPCC -- which exists to brief policymakers on climate science -- for a "special report".
The resulting bombshell, delivered in October 2018, left no doubt as to the difference a half-a-degree makes: a 1.5C world will see deep change but remain liveable; a 2C world could tip the climate system into overdrive, outstripping our capacity to adapt, it warned.
Today, the IPCC -- including Rogelj, a lead author of the 2018 report -- insists that the 1.5C goal is technically feasible.
But that conclusion hangs by the thinnest of threads.
There is no scenario that avoids "overshooting" the target, and bringing temperatures back under the wire will require extracting billions of tonnes of CO2 from thin air, something we can't do yet at scale.
But whether the 1.5C target is feasible may be missing the point, say others.
"Getting 1.5 C into the agreement was a moral target," Huq told AFP not long after the Paris pact was inked.
"It's our leverage, the whip we will use to hit everybody on the back so they can go faster," he added.
"Whether we achieve it or not is going down a dark track. From now on, it's about raising ambition."
Piers Forster, director of the University of Leeds Priestley International Centre for Climate and a coordinating lead author for the IPCC, describes the 1.5C objective as a "huge, but not impossible, task".
"Hopefully the IPCC report can push the urgency," he told AFP. "If it's ignored, we would have to give up on 1.5C."
UN to deliver diagnosis, prescription for climate crisis
Paris (AFP) March 20, 2023 -
The United Nations was poised to release a capstone report Monday distilling nearly a decade of published science on the impacts and trajectory of global warming, and the tools available to prevent climate catastrophe.
The Intergovernmental Panel on Climate Change's 30-odd page "summary for policymakers" -- compressing 10,500 pages authored by more than 1,000 scientists -- is as dense as a black hole and will deliver a stark warning.
"We are nearing a point of no return," UN chief Antonio Guterres said last week as diplomats from 195 nations gathered in Interlaken, Switzerland, to hammer out the final wording, finalised on Sunday night by exhausted and sleep-deprived delegates two days behind schedule.
"For decades, the IPCC has put forward evidence on how people and planet are being rocked by climate destruction."
Since the last IPCC synthesis report in 2014, science has determined that devastating impacts are happening more quickly and at lower levels of warming than previously understood.
With Earth's average surface temperature 1.2 degrees Celsius above pre-industrial levels so far, the planet has seen a steady crescendo extreme weather, including tropical storms made worse by rising seas.
On current trends, the world is on track to warm by an additional 1.6 degrees.
In 2022, climate change quantifiably amplified deadly heatwaves in South America and South Asia, massive flooding in Nigeria and Pakistan, and record-breaking drought in Western Europe and the Unites States, according the World Weather Attribution consortium, which includes many IPCC authors.
Science in the last decade has also elevated the danger posed by so-called tipping points in Earth's climate system that could -- beyond certain temperature thresholds -- see tropical forests in the Amazon morph into savannah, and ice sheets in Greenland and West Antarctica shed enough water to lift oceans by metres.
- Global stocktake -
But most of the wrangling at the week-long IPCC meeting centred on potential solutions, especially on how to decarbonise the global economy quickly enough to avoid crippling impacts, according to participants.
Under the 2015 Paris Agreement, nations vowed to collectively cap warming at "well below" 2C, and at 1.5C if possible.
A 2018 IPCC special report made it alarmingly clear that the more ambitious aspirational goal -- since adopted by governments and business as a hard target -- was a better guarantee for a climate-safe world.
Some countries emphasise the need to rapidly phase out fossil fuel use and reduce consumer demand, and others the potential of technological solutions.
"Over time, IPCC meetings became more politicised as government representatives -- mainly, but not exclusively, from oil-producing states -- interfered in the scientists' discussions," the journal Nature said in a recent editorial.
In Interlaken, negotiators from Saudi Arabia, for example, fought hard to remove or dilute passages that emphasised the central role of fossil fuels in driving global warming.
They also insisted on balancing any mention of renewable solar and wind energy with technologies that reduce the carbon emissions from burning gas or coal, such as carbon capture and storage.
"Other countries were hiding behind them, but the Saudis were most vocal," said one participant at the closed-door deliberations.
The IPCC synthesis report will also feed into the next high-level round of UN climate talks this December in Dubai, which will see the first "global stocktake" of progress toward achieving the Paris treaty goals.
To be unveiled ahead of COP28 in Dubai, the stocktake will confront countries with the deep inadequacy of their Paris pledges to cut emissions.
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