April 21, 2021

expert reaction to review of climate tipping points

A review published in Nature looks at overshooting tipping point thresholds in a changing climate.

Prof Grant Allen, Professor of Atmospheric Physics, University of Manchester, said:

“Tipping points are often thought of as sudden points of no return in the climate system, where climate-critical elements such as ocean circulation (e.g. the Gulf Stream), ice sheet loss, and Amazon rainforest dieback become suddenly unstoppable past some critical threshold of climate warming. This study shows that it may not be accurate to think of such shifts as being sudden and irreversible – at least over short timescales (of several years). It tells us that dramatic wholesale shifts in climate dynamics such as permanent changes in ocean circulation may not become immediately irreversible if climate warming exceeds some critical temperature threshold temporarily. This may offer a very tiny bit of wiggle room in our expectations of when such dire climate shifts might become permanent and extremely damaging as Earth continues to warm. But we also need to be extremely clear about what this study means, and does not in any way suggest otherwise – which is that global warming remains a clear and present danger, is already causing extreme damage, and continues at pace.

“Tipping points are just one aspect of the climate emergency and a brief delay to some of its more catastrophic consequences does not mean that it is any less dangerous now.  It is absolutely by no means a reason to take our foot off the pedal in dramatically cutting carbon emissions and pursuing carbon capture as a matter of utmost urgency. Rather, this study tells us that the very worst permanent impacts of climate change may not be as sudden as we might have previously expected, and offers a glimmer of hope that we may yet avoid some of the more sudden and catastrophic impacts if we act urgently to reduce carbon in the atmosphere and meet our climate targets. We do not have any more luxury for delay than we had before.”

Prof Richard Betts, Head of Climate Impacts Research at the Met Office Hadley Centre, said:

“This research suggests that even if global warming breaches 1.5°C, it will still be important to keep up urgent action to reduce our influence on climate. The warming level itself is not a hard threshold but provides a benchmark for the danger zone – the longer that warming stays at higher levels, the greater the chances of triggering major changes in the climate system.”

Prof Myles Allen, Professor of Geosystem Science, University of Oxford, said:

“The paper suggests that a peak global warming of 4°C would be in the “safe zone” for the global climate tipping points if temperatures were subsequently reduced to 1.5°C within 100 years. Reducing global temperatures by 2.5°C in 100 years would require over 6 trillion tonnes of carbon dioxide to be removed from the atmosphere over the course of a century, which is 50% faster than we are currently putting it in. The authors acknowledge this “may not be possible with available technologies”, but even if it is possible at all, such a rate of carbon dioxide removal cannot be considered “safe” – and relying on it, even less so.”

Prof Tim Palmer, Royal Society Research Professor, University of Oxford, said:

“Based on results from simplified idealised climate models, the authors show that it may be possible to reverse a slow-onset tipping point if action is taken quickly enough. This work is manifestly relevant to policy on climate mitigation: depending on the timescale of onset, negative carbon emissions (taking CO2 out of the air) made after a tipping point is reached will have limited impact on mitigating human-induced climate change. The authors acknowledge that “further work is required to demonstrate that similar behaviour is present in more complex models”.  However, current-generation comprehensive climate models are not reliable guides to the likelihood of occurrence of either slow- or fast-onset tipping points:  the key nonlinear processes driving such tipping points, whether related to ice sheets, rainforests or ocean currents, are too small scale for current climate models to represent accurately. Consistent with the conclusions of this study, it is becoming increasingly urgent that we develop a complete “digital twin” of the Earth System which is fully accurate in all such small-scale nonlinear processes.”

Prof William Collins, Professor of Meteorology, University of Reading, said:

“This paper illustrates that the most important goal is to achieve a long-term stabilisation of climate. It shows that the 1.5 degree limit isn’t an unbreakable barrier, but if we do exceed it we need to reduce temperatures quickly. The timescales of some of these tipping points means that every step taken by us to reduce warming is vital as this will decrease the time spent “overshooting”. The study shows that if temperatures overshoot the tipping points for too long we may set in place irreversible changes in the climate system, such as dying of the Amazon rainforest.”

Prof Hannah Cloke, a natural hazards researcher at the University of Reading, said:

“Taking action to slow and reverse global warming can only be a good thing. Although this study is encouraging in suggesting we can avoid irreversible damage to the planet, we should not look at climate tipping points like a see saw. By definition, once a tipping point is surpassed there is no coming back. What this research does confirm is that by acting quickly to curb global warming we can give ourselves more time to change course and avoid surpassing the point of no return.

“Adequate action relies on governments around the world making genuine commitments to urgently reduce carbon emissions, and making changes to the way of life we have grown accustomed to. The US President hosting a summit of world leaders to discuss action on climate change, and the UK Prime Minister announcing more ambitious carbon cutting targets, are both welcome developments this week, but actions speak louder than words. Decisive policies and large-scale changes must follow to give us hope for the future.”

Prof Andrew Shepherd, Professor of Earth Observation at the University of Leeds, said:

“Parts of Earth’s cryosphere have already changed beyond recognition in our lifetime – for example the succession of Antarctic Peninsula ice shelves that have collapsed after surviving for thousands of years and the glacier imbalance that has spread across a quarter of West Antarctica since the 1990’s – and so it’s important to understand how easily our climate can be tipped into new states. But the problem is that we still don’t know how to reliably predict these abrupt changes, and so there is work to be done before we can be sure when tipping points might be crossed in future and indeed whether they can or cannot be easily reversed.”

Prof Valerio Lucarini, Professor of Statistical Mechanics at the University of Reading, said:

“This study provides an interesting new take on the theory of tipping points in the natural world. It clearly shows another important aspect of the agency by humans in the context of the ongoing climate crisis by emphasising the role of competing timescales of the climate system and of our actions in terms of climate change mitigation. A rapid realignment of climate policies towards more sustainable levels can avoid irreversible changes in the climate system.

“The natural world is a delicate balance of systems, and dramatically altering one can cause others to fall like a house of cards. Research to estimate where these tipping points lie is improving all the time, but it remains difficult to pinpoint them.”

Dr Karsten Haustein, Climate Service Center Germany, said:

“The new paper has been revising a number of simple empirical models of tipping elements and compares their temporal behaviour (namely Amazon forest dieback, Indian summer monsoon, polar ice caps and Atlantic Meridional Overturning Circulation aka AMOC). Based on the results, the authors suggest that virtually all four elements under consideration are reversible if – what they call safe – temperature thresholds are not exceeded, and temperatures are brought back to 1.5°C warming afterwards (by means of negative emission technologies). The key point is, that these thresholds appear to be higher than what we currently consider safe, i.e. >3°C vs the upper target of the Paris agreement which sits at 2°C global warming above pre-industrial levels (we are currently at 1.2°C warming). 

“In addition, the authors also point out that the timescale matters. Longer overshoot duration (with lower peak warming) might damage the polar ice caps irreversibly, whereas a shorter overshoot duration (with higher peak warming) has a high potential to destroy the Amazon rainforest irreversibly. Such result certainly makes intuitive sense from a physical perspective.

“While I have no expertise in tipping point modelling, I’m aware of a few studies that have been making similar points with regards to reversibility of the four climate elements in question at higher temperatures than the upper Paris target. Yet I am in no position to judge whether the choice of their model parameters is robust. But I guess the point of the paper is less to determine where the exact safe level of warming for those four elements lies, rather than highlighting that tipping points are likely less sensitive to permanent collapse than widely assumed. What the paper does best in my view, is that it condenses the information in a very smart and understandable way. Not only do they compare the multiple tipping elements in one plot as function of peak warming and return time to 1.5°C warming afterwards, but they also illustrate neatly how the time-dependency affects the tipping point behaviour.

“The authors point out the caveats and clearly state that their quantitative estimates are not the final word. Overall, they present a counter-point to the doomsday scenario presented in the Steffen et al. study from 2018. While it is great to point out that there is hope and that the climate system is likely less prone to sudden collapse (as far as the four tipping elements in question are concerned), my fear is that the paper also incentivises delayed action, as it is highly suggestive of a scenario where critical climate elements are going to be fine, as long as we return to 1.5°C later. In my view, it is rather unlikely that negative emission technologies are going to be employed at the scale necessary to return to truly safe levels, especially if we end up in a world which has warmed by 3°C or more.”

‘Overshooting tipping point thresholds in a changing climate’ by Paul Ritchie et al. was published in Nature at 4pm UK TIME on Wednesday 21 April 2021.

https://doi.org/10.1038/s41586-021-03263-2

Declared interests

Prof Grant Allen: “holds UKRI funding to study the global methane budget and other research funding associated with quantifying carbon emissions from natural and manmade sources.”

None others received.