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In the first decade of the 21st century, most scientists and policy-makers were focused on 2°C as being the highest “safe” threshold for warming above pre-industrial levels. But emerging research was beginning to suggest that even this was too severe, threatening sea level rise that would wipe out low-lying islands. In response, some scientists .

Armed with this research, the Alliance of Small Island States (AOSIS), a United Nations negotiating bloc, called for the adoption of a global target to limit warming to 1.5°C, warning that a 2°C warming limit “”.

, a UN negotiator for the AOSIS bloc at the 2015 UN COP climate summit in Paris, says it was an uphill battle to convince other countries to adopt this much tougher global goal. He recalls the head of a lower-income nation’s delegation cornering him at the end of a meeting in Paris: “He was wagging his finger in my face and saying, ‘You small island states will get 1.5°C over my dead body’. That’s how incensed they were about this.”

With the help of pressure from the European Union, tacit support from the US and even an intervention from Pope Francis, 1.5°C made it into the hugely influential 2015 Paris Agreement. Yet, with no formal assessment of what 1.5°C of warming would really mean for the planet, the world’s climate scientists set to work.

In 2018, the Intergovernmental Panel on Climate Change published its  on the 1.5°C goal, confirming the relative advantages of holding warming to the lower level and crystallising a new global target to reach net-zero emissions by 2050, in line with a 1.5°C trajectory.

Both goals quickly became a rallying cry for governments and companies around the world, and some countries, including the UK, upgraded their national climate goals to be in line with the new, more stringent target.

at the University of Leeds, UK, credits the 1.5°C target for helping to push nations to commit to much tougher climate targets than they would have previously countenanced. “I think it has created a sense of urgency,” he says.

The target’s legacy is mixed. Despite the fanfare, global temperatures are still rising, and the world has delivered nothing like the emissions cuts needed to deliver on the 1.5°C promise. The best scientific assessments now assume the world will cross that warming threshold in just a few years’ time.

Nevertheless, 1.5°C remains the central climate goal against which global progress in reducing emissions is measured. The public and policy-makers are now much more focused on each fraction of a degree of temperature rise. “Overshooting” beyond 1.5°C is widely viewed as a risky future, and the idea that 2°C was ever seen as a “safe” threshold for warming seems laughable.