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13 maltatoday | WEDNESDAY • 12 OCTOBER 2022 OPINION David Armstrong McKay David Armstrong McKay is Researcher in Earth System Resilience, Stockholm University CONTINUED greenhouse gas emissions risk triggering cli- mate tipping points. These are self-sustaining shifts in the cli- mate system that would lock-in devastating changes, like sea-lev- el rise, even if all emissions end- ed. The first major assessment in 2008 identified nine parts of the climate system that are sensitive to tipping, including ice sheets, ocean currents and major for- ests. Since then, huge advances in climate modelling and a flood of new observations and records of ancient climate change have giv- en scientists a far better picture of these tipping elements. Extra ones have also been proposed, like permafrost around the Arc- tic (permanently frozen ground that could unleash more carbon if thawed). Estimates of the warming levels at which these elements could tip have fallen since 2008. The collapse of the west Antarctic ice sheet was once thought to be a risk when warming reached 3°C-5°C above Earth's pre-indus- trial average temperature. Now it's thought to be possible at cur- rent warming levels. In our new assessment of the past 15 years of research, myself and colleagues found that we can't rule out five tipping points being triggered right now when global warming stands at roughly 1.2°C. Four of these five become more likely as global warming ex- ceeds 1.5°C. These are sobering conclu- sions. Not all of the news cover- age captured the nuance of our study, though. So here's what our findings actually mean. Uncertain thresholds We synthesised the results of more than 200 studies to esti- mate warming thresholds for each tipping element. The best estimate was either one that multiple studies converged on or which a study judged to be particularly reliable reported. For example, records of when ice sheets had retreated in the past and modelling stud- ies indicate the Greenland ice sheet is likely to collapse be- yond 1.5°C. We also estimated the minimum and maximum thresholds at which collapse is possible: model estimates for Greenland range between 0.8°C and 3.0°C. Within this range, tipping be- comes more likely as warming increases. We defined tipping as possible (but not yet like- ly) when warming is above the minimum but below the best estimate, and likely above the best estimate. We also judged how confident we are with each estimate. For example, we are more confident in our esti- mates for Greenland's ice sheet collapse than those for abrupt permafrost thaw. This uncertainty means that we do not expect four climate tipping points to be triggered the first year global tempera- tures reach 1.5°C (which cli- mate scientists suggest is pos- sible in the next five years), or even when temperatures averaged over several years reach 1.5°C sometime in the next couple of decades. In- stead, every fraction of a degree makes tipping more likely, but we can't be sure exactly when tipping becomes inevitable. This is especially true for the Greenland and west Antarctic ice sheets. While our assess- ment suggests their collapse becomes likely beyond 1.5°C, ice sheets are so massive that they change very slowly. Col- lapse would take thousands of years, and the processes driving it require warming to remain beyond the threshold for sever- al decades. If warming returned below the threshold before tip- ping kicked in, it may be possi- ble for ice sheets to temporar- ily overshoot their thresholds without collapsing. For some other tipping points, change is likely to be more dis- persed. We estimate that both tropical coral reef death and abrupt permafrost thaw are possible at the current warm- ing level. But thresholds vary between reefs and patches of permafrost. Both are already happening in some places, but in our assessment, these chang- es become much more wide- spread at a similar time beyond 1.5°C. Elsewhere, small patches of the Amazon and northern for- ests might tip and transition to a savannah-like state first, bypassing a more catastrophic dieback across the whole forest. Model results that are yet to be published suggest that Amazon tipping might occur in several regions at varying warming lev- els rather than as one big event. There may also be no well-de- fined threshold for some tip- ping elements. Ancient climate records suggest ocean cur- rents in the North Atlantic can dramatically flip from being strong, as they are now, to weak as a result of both warming and melting freshwater from Greenland disrupting circu- lation. Recent modelling sug- gests that the threshold for the collapse of Atlantic circulation depends on how fast warming increases alongside other hard- to-measure factors, making it highly uncertain. Into the danger zone There are signs that some tipping points are already ap- proaching. Degradation and drought have caused parts of the Amazon to become less re- silient to disturbances like fire and emit more carbon than they absorb. The front edge of some re- treating west Antarctic glaciers are only kilometres away from the unstoppable retreat. Ear- ly warning signals in climate monitoring data (such as bigger and longer swings in how much glaciers melt each year) suggest that parts of the Greenland ice sheet and Atlantic circulation are also destabilising. These signals can't tell us exactly how close we are to tipping points, only that dest- abilisation is underway and a tipping point may be approach- ing. The most we can be sure of is that every fraction of further warming will destabilise these tipping elements more and make the initiation of self-sus- taining changes more likely. This strengthens the case for ambitious emissions cuts in line with the Paris agree- ment's aim of halting warming at 1.5°C. This would reduce the chances of triggering multiple climate tipping points – even if we can't rule out some being reached soon. Climate tipping points could lock in unstoppable changes to the planet – how close are they? Greenland's ice sheet is showing signs of destabilising at current warming levels (Photo: David Dennis/Shutterstock)