- Irreversible and catastrophic environmental tipping points could still be avoided, even if we exceed global emission reduction targets — provided the world is able to reverse overshoot quickly, according to researchers.
- Simple mathematical models of four earth system tipping elements reveal a lag between overshooting the threshold and irreversible change. “Slow-onset” elements like icecap melt operate on century-long timescales, while Amazon dieback could pass a point of no return in just decades.
- However, experts warn that the models fail to take interactions between different tipping elements into account, which could shorten the amount of time a threshold can be overshot. Many of these interactions are poorly understood, making them difficult to include in climate models.
- Researchers say these results show there is still good reason to take action to mitigate global warming, even if we do overshoot the Paris Agreement target of 1.5°C. Some warn the study results could be used as an excuse to tolerate further delays on global climate action.
Catastrophic irreversible environmental tipping points — such as the melting of polar icecaps — could be avoided even if we exceed global climate emissions reduction targets, provided we are able to reverse that overshoot quickly, according to a study published in the journal Nature last week.
The 2009 Planetary Boundaries framework proposed nine key Earth System processes and described tipping points brought on by human exploitation beyond which the system could shift irreversibly into a new climatic state — one in which human civilizations would find it difficult to survive.
In the present study, researchers at the University of Exeter and the U.K. Centre for Ecology and Hydrology, both in the United Kingdom, developed simple mathematical models of four environmental elements in the Earth System that are fairly well understood: melting of polar ice caps, disruption of the Atlantic Meridional Ocean Circulation (AMOC), Amazon forest dieback, and disruption of India’s summer monsoon cycle.
Scientists found that crossing a climate change threshold would not immediately trigger irreversible change, provided that the duration of the overshoot was relatively short compared to that tipping element’s recovery time. For example, models of icecap melt and the AMOC showed about a 50-year lag between the world passing the theoretical climate threshold and the start of irreversible changes; the authors dubbed these tipping elements as “slow-onset.”
“Slow-onset tipping points take place over a timescale of many centuries and — depending on the level of warming — this would give us more time [than previously predicted] to act,” said lead author Paul Ritchie, a Research Fellow at the University of Exeter’s Global Systems Institute. On the other hand, Amazon forest dieback and India’s summer monsoon are “fast-onset” tipping elements, the authors say, because transgressing the threshold could trigger rapid and irreversible change in a matter of decades.
“Fortunately, the tipping points that are believed to be closest are slow-onset tipping points. This may give us a lifeline to avoid dangerous climate change.” said study co-author Joe Clarke, a PhD student at the University of Exeter.
Searching for a safe overshoot zone
The team combined their models to map out the “safe zone” of how much, and how long, we could overshoot 1.5°C warming without triggering any irreversible changes, (at least in the four tipping elements modelled). “Both the amount of overshoot and the period of overshoot are important,” explained Peter Cox, Professor of Climate System Dynamics at the University of Exeter and a co-author on the new study. But “tipping is a higher risk if we overshoot a threshold for a longer-time.”
“I support the use of simple models to help understand nonlinear dynamics such as tipping,” said Steven Lade, a Stockholm Resilience Centre researcher not involved in the present study. However, he expressed concerns that the new study’s “models are not sufficiently complex to distinguish whether a lagged response is reversible or irreversible.” For example, internal feedback processes may have already locked some Antarctic ice sheets into an irreversible melting state, even if those effects won’t be seen for centuries. “This is a subtle but important distinction that could substantially change their findings,” Lade said.
“It is true that the models that we use are deliberately simple. This allows us to demonstrate clearly how the timescale of a tipping element affects the ‘safe’ overshoot,” Cox responded. “We believe it is general enough to also apply to tipping points in more complex models, but this still needs to be confirmed”.
In addition, interactions between different elements — for example, a biome-shift from rainforest-to-degraded-savanna in the Amazon basin, exacerbating climate change and biodiversity loss — “could shorten the amount of time a threshold could be overshot,” warned Lade. Some of these interactions have been studied in depth but many more are still quite poorly understood, meaning they’re difficult or impossible to include in mathematical models of climate tipping points. Nevertheless, “neglecting interactions means any estimate will likely be optimistic,” Lade noted.
“It is possible to imagine cascades of tipping points,” agreed Cox. “This is another possible avenue for future research.”
The Paris Agreement’s target of keeping average global climatic warming within 1.5° degrees Celsius (2.7° Fahrenheit) of pre-industrial levels was selected so as to prevent triggering catastrophic tipping points like these. But with countries’ progress towards hitting that target disastrously slow (almost no nation is on target to meet their promised emission cuts, which are inadequate), it seems increasingly likely that the Earth will exceed 1.5°C warming — for a short while, at least — before any climate change mitigation strategies we implement over the coming decades start to take effect.
“We are quite likely to overshoot some tipping point thresholds temporarily,” said Cox. But these results show that “this is not a reason for despair, but instead a reason for stronger action to slow climate change.”
The study could offer some hope for humanity and the world. Dire reports detailing our looming proximity to several climate tipping points “may lead some to get dispirited about attempts to slow climate change,” said Cox, but these model outputs indicate “it is possible to overshoot tipping point thresholds without leading to an abrupt and permanent climate change… this means that there is always value in trying to slow and reverse climate change,” he said.
However, Lade cautioned that these results could also spark a more apathetic reaction. “The paper could easily be misread to say that we don’t need to take action on climate for a century or more yet. While this is not their message, I am worried about how their results could be misinterpreted,” he said.
Ritchie, P. D. L., Clarke, J. J., Cox, P. M., & Huntingford, C. (2021). Overshooting Tipping Point Thresholds in A Changing Climate. doi: 10.1038/s41586-021-03263-2