Even if it is possible to reverse the rise of global temperatures after a temporary overshoot of 1.5°C, many climate damages triggered at peak warming – including rising sea levels – will be irreversible, according to a new study
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°C limit, before falling again.
The paper, which involved 30 scientists and is published today in Nature, highlights clear benefits to reversing warming by acting now to achieve net zero globally in order to minimise exceedance of the Paris Agreement’s limit, instead of relying on an ‘overshoot’ scenario.
‘Overshoot’ also relies on net-negative CO2 emissions being achieved to bring global temperatures down after they peak, and this may be very difficult to do especially if earth system feedbacks, which accelerate global warming, are triggered during the ‘overshoot’
Achieving long-term temperature decline could lower sea level rise in 2300 by about 40cm compared to a situation in which temperatures merely stopped rising.
“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°C,” said 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.”
Co-authors Professor Rachel Warren and Associate Professor Jeff Price, from the Tyndall Centre for Climate Change Research and School of Environment Sciences at UEA, explored the irreversible consequences of overshoot for biodiversity loss.
A detailed analysis based on the Wallace Initiative was led by Associate Professor Price, who explains that “a global warming of 3°C would place 55% of the biodiversity of Brazil, the most biodiverse country in the world, at risk of local extinction”.
“After an overshoot of global temperatures, the world does not return to where it started, even if the global temperature does,” said Professor Warren. “In the paper we explain that there would be irreversible damage to ecosystems if global temperatures temporarily exceed the Paris Agreement targets, even if were possible to reduce global temperatures again later in an ‘overshoot’ scenario.”
Co-author Professor Joeri Rogelj, of the Grantham Institute at Imperial College London, added: “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.
“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.”
The study emphasises that while there are still pathways open to limiting warming to 1.5°C 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,” said 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.
