‘Overshoot’: As planet crosses 1.5°C global warming limit, can we still reverse course?

A milestone has been reached: 2024 was the warmest year ever recorded and the past two years were the first to exceed a 1.5°C temperature increase compared with pre-industrial levels, the European Union's Copernicus climate observatory announced on January 10. 

The news is symbolic. Since 2015, the Paris Agreement has aimed to keep rising global temperatures below this limit in order to reduce the impacts of human-driven climate change.

In the treaty, such an increase is referred to as a long-term climate trend – the average temperature would have to remain above the 1.5°C threshold for 20 to 30 years for the limit to be officially considered exceeded. But the figures for 2024 – which recorded a 1.6°C increase overall – have raised fears that the target may now be altogether unattainable.

“Today the 1.5 degree goal is practically ancient history already,” said Jochem Marotzke, climatologist and oceanographer at the Max-Planck-Institute for Meteorology in Hamburg, Germany. “Anyone who continues to say the world can stay below that limit is delusional. We have to face reality and adapt to increasing warming.”

But, is there a way to turn back the clock and return to a more liveable climate? Faced with the failure to keep global temperatures below 1.5°C, is it possible to overstep that limit and then reverse the trend?

This idea of reversible overheating, known as an “overshoot”, is increasingly referenced by some politicians and scientists. It is even part of scenarios modelled by the UN’s Intergovernmental Panel on Climate Change (IPCC).

But as reassuring as the concept may sound “it would entail many risks and its implementation today remains highly uncertain”, says Carl-Friedrich Schleussner, researcher at the International Institute for Applied Systems Analysis, and author of a study evaluating short- and long-term overshoot scenarios.

Billions of tonnes of CO2 

The first issue is that to bring down the global thermostat, billions of tonnes of CO2 would have to be captured in the atmosphere. For the world to even achieve carbon neutrality – a perfect balance between the carbon that is emitted and absorbed – it would first have to counteract unavoidable residual emissions, such as those generated by agriculture, and historical emissions. 

To reduce the global temperature by 0.1°C the study estimates that at least 150 billion tonnes of CO2 would need to be absorbed, equivalent to four years of global emissions.

But such quantities would be extremely difficult to achieve. Aside from extensive reforestation to recreate natural carbon sinks, it would only be possible through geoengineering such as carbon capturing plants or even the much-maligned solar geoengineering – a method of cooling the earth by reflecting sunlight back into space.

Such methods are still developing, often contested and expensive. They require large reserves of energy, water and land and could have unexpected long-term effects. 

For comparison, today only 2 billion tonnes of CO2 are eliminated per year by human efforts, mainly reforestation, 2 million of which are permanently eliminated by more technological solutions.

The quantities of CO2 that need to be stored could also turn out to be even greater than predicted. Scientists regularly warn of the risk of “hidden” warming, a scenario in which temperatures would continue to rise even after carbon neutrality had been achieved.

“And even if we could achieve this huge technological challenge, it still might not be enough,” says Schleussner. “Currently, there is no certainty about how carbon sinks would behave in a trajectory of decreasing CO2, but some data shows that the ocean could, initially, start releasing carbon.”

In short, returning to temperatures below the 1.5°C limit seems like a long shot, and the longer the temperature stays above the limit, the more CO2 will have to be absorbed to try and reverse the trend.

No going back

Even in a scenario where the carbon capturing technology worked perfectly, “the climate and the world would never be the same as before”, says Schleussner.

Some of the impacts of global warming are irreversible and will become more serious the longer the temperature stays above the limit and the higher it climbs, his study says. People will have died in heatwaves, countries will have had their economies devastated, some species of animals and plants will have gone extinct, glaciers, peat bogs and permafrost will have disappeared. 

“Sea levels will continue to rise for centuries to millennia even if long-term temperatures decline,” the study says. 

Changes specific to different regions could also persist, for example temperature increases in the Antarctic Ocean could continue as could droughts and desertification in West Africa.

“All this without factoring in that with every extra tenth of a degree, the risk increases of triggering tipping points – phenomena such as the collapse of sea ice and glaciers or the death of coral reefs and the Amazon, which, if they occur, would have a disastrous runaway effect on the climate and human life,” Schleussner adds.

In short, returning to temperatures below the 1.5°C mark seems like a long shot, and the longer that temperatures exceed 1.5°C, the more CO2 will have to be absorbed to try and reverse the trend.

‘Into uncharted territory’

So what next for our ever-warming planet? “We must be very clear about what science today knows and does not know about these ‘overshoot’ scenarios, and make reasonable plans based on this knowledge,” Schleussner says.

“Warming above 1.5°C takes us into uncharted territory,” he adds. “And we know that will have immense consequences for the planet and humans, especially in vulnerable countries that are not primarily responsible for climate deregulation.” 

Schleussner believes it is a good idea to develop CO2 capturing technologies “as a preventative measure”. 

But “we can't afford to waste this capacity on emissions that could be avoided in the first place … The only way to limit the damage is drastically and immediately reducing greenhouse gas emissions to reach carbon neutrality.”

This article has been translated from the original in French.