The totemic 1.5°C climate target: Best ideas of the century

During the first decade of the 21st century, most scientists and policymakers considered 2°C to be the highest “safe” threshold for warming above pre-industrial levels. But new research was beginning to suggest that even that was too bad, threatening sea level rise that could wipe out low-lying islands. In response, some scientists have begun to study the benefits of keeping any temperature increase above 1.5°C.

Building on this research, the Alliance of Small Island States (AOSIS), a UN negotiating bloc, has called for the adoption of a global target to limit warming to 1.5°C, warning that a 2°C warming limit would “devastate many small island developing countries”.

James Fletcher, UN negotiator for the AOSIS bloc at the 2015 UN COP climate summit in Paris, says convincing other countries to adopt this much more ambitious global target has been an uphill battle. He remembers the head of a delegation from a low-income country cornering him at the end of a meeting in Paris: “He waved a finger in my face and said, ‘You small island states will have 1.5°C on my corpse.’ That’s how angry they were about it.”

Thanks to pressure from the European Union, tacit support from the United States and even the intervention of Pope Francis, the temperature of 1.5°C was enshrined in the 2015 Paris Agreement, which has had considerable influence. Yet without a formal assessment of what 1.5°C of warming would actually mean for the planet, climate scientists around the world set to work.

In 2018, the Intergovernmental Panel on Climate Change released its report on the 1.5°C target, confirming the relative benefits of keeping warming at a lower level and crystallizing a new global target to reach net zero emissions by 2050, in line with a 1.5°C trajectory.

These two goals have quickly become a rallying cry for governments and businesses around the world, and some countries, including the United Kingdom, have revised their national climate targets upwards to align them with the new, stricter target.

Piers Forster of the University of Leeds, UK, credits the 1.5°C target with pushing nations to commit to much tougher climate targets than they would have previously considered. “I think it created a sense of urgency,” he says.

The target’s legacy is mixed. Despite all the fanfare, global temperatures continue to rise and the world has achieved nothing like the emissions reductions needed to meet the promise of 1.5°C warming. The best scientific assessments now assume that the world will pass this warming threshold in just a few years.

Nevertheless, 1.5°C remains the central climate target against which global progress in reducing emissions is measured. The public and policymakers now focus much more on each fraction of a degree of temperature increase. “Exceeding” beyond 1.5°C is widely considered a risky future, and the idea that 2°C was ever considered a “safe” threshold for warming seems laughable.