Scientists caution no guarantees when it comes to overshooting 1.5°C

9 October 2024, by Climate Analytics / CLICCS

Photo: UHH/CEN/T.Wasilewski

Even if it is possible to reverse the rise of global temperatures after a temporary overshoot of 1.5°C some climate damages triggered at peak warming, including rising sea levels, will be irreversible, according to a new study published today in Nature.

• New paper published in Nature from 30 scientists analyses future scenarios in which we temporarily exceed 1.5°C and bring temperatures back down in the long run
• The authors stress that such an overshoot comes with irreversible consequences and rapid near-term emission reductions are essential to reducing climate risks
• Bringing temperatures down again will limit long-term damages from impacts like sea level rise
• Environmentally sustainable carbon removal capacity is needed to hedge against the climate system warming more than expected

The study is the culmination of a three-and-a-half-year project, backed by the European innovation fund HORIZON2020, looking at so-called ‘overshoot’ scenarios, where temperatures temporarily exceed the Paris Agreement’s 1.5 degree Celsius limit, before descending again by achieving net-negative CO₂ emissions. Prof. Jana Sillmann from the Cluster of Excellence CLICCS at the University of Hamburg was also part of the team of authors.

“We are not on track to limit global warming to 1.5 degree Celsius and assumptions are that if we overshoot this threshold, we could bring temperatures down again by technologies such as carbon dioxide removal employed at large scales”, says Jana Sillmann, a geo-ecologist and an expert for climate extremes at CLICCS. “Besides significant uncertainties around the feasibility of the latter, this article shows also that societal-relevant impacts occurring due to the overshoot will be irreversible and also regional patterns in precipitation and temperature extremes will be different before and after the overshoot. Hence, overshoot will bring us into unchartered territory and challenges adaptation planning.”

It matters how high and for how long we let temperatures rise

“This paper does away with any notion that overshoot would deliver a similar climate outcome to a future in which we had done more, earlier, to ensure to limit peak warming to 1.5 degree Celsius,” commented Dr Carl-Friedrich Schleussner, head of integrated climate impacts group at the International Institute of Applied Systems Analysis and scientific advisor at Climate Analytics, who led the study. “Only by doing much more in this critical decade to bring emissions down and peak temperatures as low as possible, can we effectively limit damages,” Schleussner said.

The paper highlights that if we were to exceed 1.5 degree Celsius there are clear benefits to reversing warming by acting to achieve net negative emissions globally. Achieving long-term temperature decline could lower sea level rise in 2300 by about 40 centimeters compared to a situation in which temperatures merely stopped rising.

“Until we get to net zero, warming will continue. The earlier we can get to net zero, the lower peak warming will be, and the smaller the risks of irreversible impacts,” commented co-author Prof. Joeri Rogelj, professor of climate science and policy and director of research of the Grantham Institute at Imperial College London. “This underscores the importance of countries submitting ambitious new reduction pledges, or so-called ‘NDCs’, well ahead of next year’s climate summit in Brazil,” Rogelj said.

Full carbon dioxide removal capacity needed to hedge against higher warming

The study emphasises that while there are still pathways open to limiting warming to 1.5 degree Celsius or lower in the long run, there is a need to ‘hedge’ against higher warming outcomes if the climate system warms more than median estimates. To do this, ambitious emissions reductions need to go hand in hand with scaled and environmentally sustainable carbon dioxide removal technologies. A ‘preventive capacity’ of several hundred gigatonnes of net removals might be required. 

“There’s no way to rule out the need for large amounts of net negative emissions capabilities, so we really need to minimise our residual emissions. We cannot squander carbon dioxide removal on offsetting emissions we have the ability to avoid,” commented Gaurav Ganti, research analyst at Climate Analytics and another author on the study. 

“Our work reinforces the urgency of governments acting to reduce our emissions now, and not later down the line. The race to net zero needs to be seen for what it is – a sprint,” Dr Schleussner concluded.