Climate Scientists Steffen and Knorr defend Franzen's position

Fact Checking the Climate Crisis: Franzen vs. Facebook on False New. Wolfgang Knorr and Will Steffen. Feb. 19, 2020.

Critique of Climate Feedback fact checking exercise of an article by the writer Jonathan Franzen in the New Yorker, published September 8, 2019, by leading climate scientists Wolfgang Knorr and Will Steffen. 

Fact Checking the Climate Crisis: Franzen vs. Facebook on False News

Last year, the writer Jonathan Franzen, not known for mincing his words, took up an opportunity offered by the New Yorker to bring his very own perspective to the debate about the impending climate crisis. The headline read: What If We Stopped Pretending? The climate apocalypse is coming. To prepare for it, we need to admit that we can’t prevent it.

You would think that this was just another opinion piece on the climate catastrophe, this time by a writer from whom you would not expect new insights about the climate system, but maybe some other truths related to the general human predicament. If you happened to be a social media user, however, you might be in for some surprise. On February 15th, the moderators of the Facebook group Positive Deep Adaptation received a “partly false information” warning about Franzen’s New Yorker article having been posted in their group. The warning explicitly referred to an article by the fact checking site Climate Feedback (see image 1). Following considerable political pressure, fact checkers are now routinely used by social media sites to prevent the spread of “false news”. Sanctions for repeat offenders include reducing visibility for the group or site, or removing the ability to earn income.

This gives Climate Feedback’s verdict considerable weight. In this article, we will review the fact checking exercise itself, in order to see if the matter has been handled with the necessary care and level of responsibility. According to their web site Climate Feedback is a worldwide network of scientists sorting fact from fiction in climate change media coverage. As senior climate scientists, our goal is to help readers know which news to trust.

Fact checking the fact checkers

The first thing we noticed was that from the headline and the information below it, it was not possible to tell the exact claim that was being assessed. While the headline read “2°C is not known to be a ‘point of no return’, as Jonathan Franzen claims”, the actual claim by Franzen stated further down was “The consensus among scientists and policy-makers is that we’ll pass this point of no return if the global mean temperature rises by more than two degrees Celsius.” What was left out is some text that in the original article follows immediately after and is therefore an integral part of the claim made by Franzen: “(maybe a little more, but also maybe a little less)”. The verdict: incorrect.

There is an important distinction here: is the claim being reviewed that there is a consensus – which Climate Feedback easily refute because none of the scientist reviewers seems to subscribe to this supposed consensus – or is it rather about the existence of a supposed “point of no return”? In the following, we will discuss both possibilities.

Contrary to the claim by Climate Feedback, and the entire point made by the last reviewer, Marcus Fontela, there is indeed a scientific basis for Franzen’s article, even though he vastly overrates the degree of consensus or the level of scientific understanding of such a hypothesis.

All scientist reviewers – except for one who did not provide references at all – referred to an article led by one of us: Steffen and co-workers (2018) Trajectories of the Earth System in the Anthropocene1. It appears that the reviewers correctly identified the source of Franzen’s “point of no return”. The article in question hypothesizes the existence of mutually reinforcing tipping points, or positive feedback mechanisms, in the earth’s climate system. It does not provide definite proof of the existence of such a “tipping cascade”, nor does it say they will all suddenly happen when 2°C of warming is reached, but rather sketches out a plausible scenario. It also provides estimates for the degree of warming required for tipping to be triggered. “Tipping” here means irreversible changes that a return to a lower degree of warming will not be able to stop – or “point of no return”. According to our understanding, the following tipping elements might be affected at 2°C of warming:

1. West Antarctic Ice Sheet – likely tipped (i.e. irreversibly on the pathway to eventual collapse)2
2. Coral reefs – likely tipped (wide-spread destruction from heat stress and ocean acidification)
3. Arctic sea ice cover – likely tipped (irreversible situation arises from increasing heat provided by the darker ocean surface as summer ice disappears)4
4. Amazon rainforest - likely tipped taking into account current rates of human deforestation (i.e. the loss of forest itself decreases regional precipitation rates, thus increasing the overall reduction in rainfall, further influenced by a weakening AMOC – see next point)5,6
5. Atlantic meridional overturning circulation (AMOC, the ocean current that brings warm waters to western Europe) – probably not tipped, but significantly weakened7
6. Permafrost carbon stores – probably not tipped, but significant carbon emissions.3

The Special Report on 1.5°C Warming3 by the Intergovernmental Panel on Climate Change (IPCC) also backs up Franzen’s claim – to some degree. The IPCC assessed the probability of “large-scale singular events” at 2°C of warming and gave a rating of "moderate to high" (Summary for Policymakers, Figure 2). And a recent commentary in the journal Nature by Tim Lenton and co-workers8 shows evidence that some of those tipping elements may have already been activated.

Another point of potential confusion is that the reviewers do not provide a clear definition of what they believe was meant by “point of no return”. They only suggest that Franzen was talking about a sharp boundary at 2°C warming, for example Patrick Brown: “[...] There was never a scientific consensus that 2.0°C represented some well-defined bright line where impacts suddenly became much worse or feedbacks suddenly became completely self-perpetuating.” Franzen does indeed talk about one point being crossed, but it is not clear if he means a sharp and clearly defined boundary, or rather that somewhere around 2°C a tipping cascade will be triggered. And if a tipping cascade exists, then it will be triggered at a single point, so by definition there would have to be a clearly defined point of transition. It is only questionable if it could be characterised simply by degrees of warming. We must also note that Brown’s 2.0°C “bright line” is a rhetorical ploy designed to ridicule the author’s scientific understanding, where Franzen in fact concedes that his “point” may not be reached at exactly 2°C.

A charge repeated by several reviewers is that Franzen does not understand how climate models work. In fact, Amber Kerr and Charles Koven seem to base their criticism entirely on modelling results. This must be a clear misunderstanding: neither does the New Yorker article refer anywhere to computer modelling, nor has it ever been claimed that it was possible to reliably model tipping cascades. Instead, it is the reviewers who display a remarkable lack of apprehension for the limitations of modelling. For example when Amber Kerr writes: “He says that ‘As a non-scientist, I do my own kind of modelling,’ but he seems to be unaware that scientists have already carried out many qualitative and quantitative climate risk assessments, using policy changes and human behavior as variables.” But even the supporters of such models stress their limitations, while others argue that they are entirely unsuitable for the job. 

The most serious criticism brought forwards by the reviewers is that of Franzen’s fatalism, when he claims “that additional warming over 2°C doesn’t matter” (Amber Kerr). This is a criticism we share. We note, however, that Franzen's apparent misinterpretation of the science is based on a gross overstatement of scientific certainty, which is interesting given the reviewers’ own faith in modelling results. As climate scientists having worked with and developed computer models we rather share Franzen’s pessimism about what models can achieve. For very similar reasons, other scientists who have worked specifically on the possibility of catastrophic climate change have based their conclusions mostly on evidence from past climates, using only minimal modelling9.

The most balanced and objective review, in our opinion, is the one by Alexis Berg, whose main point is the speculative nature of the tipping cascade scenario. He is also more honest and cautious when referring to modeling (italics included by the authors): “Climate model simulations, for instance, which do include some of these feedbacks, do not suggest runaway climate change beyond 2°C.” It is true that the article by Steffen and co-workers is “intended to highlight [...] the high side of the risk distribution”. Franzen seems indeed to have ignored that he has based his whole essay on a hypothesis that was just that – a hypothesis and a warning of what could happen if we continue on the current path.

One more important question that the reviewers, and possibly Climate Feedback in general, should address is whether statements that are inherently about the future can even be fact checked. It is true that the review mostly talks about our current understanding of the climate system. But the event this refers to is in the future. We have therefore no way of empirically either proving or refuting the hypothesis of a discontinuity in the climate system. Hence the repeated reference of the reviewers to model results. What they don’t seem to realise, however, is that models are themselves nothing else than codified hypotheses.

Our recommendation

We believe that this New Yorker article – classified as a “cultural commentary” – should never have been fact checked in the first place. Other authors should of course be free to criticise it, as they have done repeatedly. But at the very least Climate Feedbacks should have clearly stated that the fact checking exercise does not concern the possibility of a tipping cascade being triggered – which can be neither proven nor refuted – but only the level of scientific consensus. For such cases, Climate Feedbacks’ methods provide for a set of well-defined categories that should be stated in the details section of the overall verdict:

Overstates scientific confidence: Presents a conclusion as conclusive while the hypothesis is still being investigated and there remains genuine scientific uncertainty about it.

We would have agreed with such a verdict. Instead, the details provided characterise Franzen’s article as “Misleading: While positive feedbacks exist that amplify temperature changes, scientists have not identified a ‘point of no return’ at 2°C.” 

This overall verdict is itself incorrect, or at least seriously misleading. Scientists have indeed identified the likelihood of such a point of no return, even though it is unlikely the point exists at a sharply defined temperature threshold. Identification does not imply there to be a definite, or even a strong scientific proof. And the overall presentation of the verdict is itself misleading for several reasons: it blatantly omits the final part of the sentence being criticised where the author concedes that he does not believe in a definite threshold value, the rhetorical use of “2.0°C”, the reference to climate models and the claim the author does not understand them, while creating the impression the reviewers don’t understand them either, and the lack of understanding that it is not possible to fact check assertions about the future.

In fact, if one is to “fact check” statements about the future, then the criterion for judgment should be whether there is enough credible evidence – be it from paleo studies, observational evidence, modelling results or, more appropriately, a synthesis of all such types of information – to make a well-reasoned case that the statement represents a plausible future. In other words, when dealing with future risks and events in general, the emphasis should be on whether or not the statement presents a plausible risk assessment rather than a “scientific fact”.

The elephant in the room

What most scientists commenting on his piece did not seem to have noticed is the value of Franzen’s article in naming the elephant in the room: that while emissions keep on rising inexorably, no political action even remotely strong enough to address the problem can be seen anywhere on the horizon. Let us listen to his words here:

As a non-scientist, I do my own kind of modelling. I run various future scenarios through my brain, apply the constraints of human psychology and political reality, take note of the relentless rise in global energy consumption [...], and count the scenarios in which collective action averts catastrophe. [...]

Vast sums of government money must be spent without wasting it and without lining the wrong pockets. Here it’s useful to recall the Kafkaesque joke of the European Union’s biofuel mandate, which served to accelerate the deforestation of Indonesia for palm-oil plantations, and the American subsidy of ethanol fuel, which turned out to benefit no one but corn farmers.

Anyone with even the faintest inkling of today’s political reality will agree that it is irrelevant whether we can still prevent major disruptions of the climate system in theory10, when the heads of state of the largest economy and of the country home to the largest tract of rainforest are outspoken climate change deniers. Needless to say, the so-called integrated assessment models used by the IPCC to run various scenarios contain no concept whatsoever of politics.

Fact checking and difficult truths

Our own view of the situation is somewhat different from Fanzen’s – it does not matter at precisely which level of warming tipping points will be reached, but it does matter how much planetary heating we will generate. As long as there is enough carbon11 contained in fossil-fuel reserves to triple even the current already high amounts of atmospheric CO2, and with no mechanism in sight that could guarantee us an end to fossil-fuel extraction, sooner or later the degree of global heating will reach a critical threshold that leads to major disruptions to human society. The question of whether it is already too late is not one that depends so much on the climate system, but on the ability of human society to rapidly change course. Most scientists, policy makers and even activists seem to tend towards optimism. Franzen disagrees, and we believe scientists should at least listen. 

Difficult truths are always hard to deal with, especially when they concern one’s own and all of humanity’s survival. Even if there was a sliver of truth in Franzen’s message, it would be a cause for great concern and anxiety. Since no scientist so far has said what Franzen does here, that it might not only be time to panic, but already too late, the predictable reaction of professional climate researchers has been almost universally negative. Someone known for his provocative writing style was treading on their turf!

We believe that fact checking is not a helpful approach to improve the debate about the climate crisis. It is not helpful when we need to confront a difficult truth that many, including scientists, find difficult to accept. It is also not helpful, because it supports a mistaken view of the sciences as being foremost about hard facts, and not about interpretation, debate, truth seeking and philosophical attitudes. And even more importantly – as George Marshall has amply demonstrated in his book “Don’t Even Talk about It” – opinions in the climate debate are very rarely swayed by facts. But users of social media have the right to know the position of the scientific community with respect to assertions made on matters of climate science. A much more helpful approach would be to offer this view in the form of reviews, critiques or other forms of essay, as it is done traditionally in the print media. Rather than being sent warnings with a threat of sanctions for repeat-offenders, publishers on social media could be required in certain cases to provide a suitable link so that the reader is helped to form his own opinion. In the case of the Franzen article, a simple disclaimer that his view of the climate system is seen as a low-probability but plausible scenario would surely have been appreciated. The climate crisis requires us to build bridges and not to exclude certain groups from the debate. 

Publisher's notes: 

On February 20th 2020 the "false news" warning is no longer showing on the admin panel of Positive Deep Adaptation facebook group. We will send this article to Facebook, Climate Feedback and the New Yorker for information and advice. 

An interview with the co-author of this article, climate scientist Dr Wolfgang Knorr, goes in to more detail about how climate scientists might often mislead audiences through typical modes of communication. An article by IFLAS founder explores the extent to which assumptions about social psychology underpin the motivations and arguments of scientists and activists to criticise people for publically considering the implications of worst case scenarios. 

Co-author of this article, climate scientist Dr Will Steffen recently called for more research, dialogue and action on the possibilities of societal breakdown or collapse due to climate impacts. 

Last year IFLAS released a compendium of recent research that indicates the worst case scenarios for societal disruption from climate change are now increasingly likely. This was produced by Professor Jem Bendell, the initiator of the deep adaptation framework to climate change response. To discuss the issues arising from this article and issue, consider the Narratives and Messaging group on the Deep Adaptation Forum.

References
1 Steffen, W., J. Rockström, K. Richardson, T. M. Lenton, C. Folke, D. Liverman, C. P. Summerhayes, A. D. Barnosky, S. E. Cornell, and M. Crucifix (2018), Trajectories of the Earth System in the Anthropocene, Proceedings of the National Academy of Sciences, 115(33), 8252-8259. (link)
2 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (IPCC, 2019). (link)
3 IPCC Special Report on Global Warming of 1.5°C (IPCC, 2018). (link)
4 Drijfhout, S., S. Bathiany, C. Beaulieu, V. Brovkin, M. Claussen, C. Huntingford, M. Scheffer, G. Sgubin, and D. Swingedouw (2015), Catalogue of abrupt shifts in Intergovernmental Panel on Climate Change climate models, Proceedings of the National Academy of Sciences, 112(43), E5777-E5786. (link)
5Lovejoy, T. E. A., and C. Nobre (2018), Amazon tipping point, Science Advances, 4(2), eaat2340. (link)
6 Brayshaw, D. J., T. Woollings, and M. Vellinga (2009), Tropical and extratropical responses of the North Atlantic atmospheric circulation to a sustained weakening of the MOC, J. Clim., 22(11), 3146-3155. (link)
7 Caesar, L., S. Rahmstorf, A. Robinson, G. Feulner, and V. Saba (2018), Observed fingerprint of a weakening Atlantic Ocean overturning circulation, Nature, 556(7700), 191-196. (link)
8 Lenton, T. M., J. Rockström, O. Gaffney, S. Rahmstorf, K. Richardson, W. Steffen, and H. J. Schellnhuber (2019), Climate tipping points—too risky to bet against, Nature, 575, 592-595. (link)
9 Hansen, J., M. Sato, G. Russell, and P. Kharecha (2013), Climate sensitivity, sea level and atmospheric carbon dioxide, Philosophical Transactions of the Royal Society A, 371, 20120294. (link)
10 Anderson, K. (2015). Duality in climate science. Nature Geoscience, 8, 898-900. (link)
11 Global Energy Assessment (2012), International Institute for Applied Systems Analysis, Laxenburg, Austria. (link)

We've been blithely betting against them for 30 years

Climate tipping points — too risky to bet against

The growing threat of abrupt and irreversible climate changes must compel political and economic action on emissions.

Timothy M. Lenton, Johan Rockström, Owen Gaffney, Stefan Rahmstorf, Katherine Richardson, Will Steffen & Hans Joachim Schellnhuber. Nature. November 27, 2019.



Politicians, economists and even some natural scientists have tended to assume that tipping points1 in the Earth system — such as the loss of the Amazon rainforest or the West Antarctic ice sheet — are of low probability and little understood. Yet evidence is mounting that these events could be more likely than was thought, have high impacts and are interconnected across different biophysical systems, potentially committing the world to long-term irreversible changes.


Here we summarize evidence on the threat of exceeding tipping points, identify knowledge gaps and suggest how these should be plugged. We explore the effects of such large-scale changes, how quickly they might unfold and whether we still have any control over them.


In our view, the consideration of tipping points helps to define that we are in a climate emergency and strengthens this year’s chorus of calls for urgent climate action — from schoolchildren to scientists, cities and countries.


The Intergovernmental Panel on Climate Change (IPCC) introduced the idea of tipping points two decades ago. At that time, these ‘large-scale discontinuities’ in the climate system were considered likely only if global warming exceeded 5 °C above pre-industrial levels. Information summarized in the two most recent IPCC Special Reports (published in 2018 and in September this year)2,3 suggests that tipping points could be exceeded even between 1 and 2 °C of warming (see ‘Too close for comfort’).



Source: IPCC


If current national pledges to reduce greenhouse-gas emissions are implemented — and that’s a big ‘if’ — they are likely to result in at least 3 °C of global warming. This is despite the goal of the 2015 Paris agreement to limit warming to well below 2 °C. Some economists, assuming that climate tipping points are of very low probability (even if they would be catastrophic), have suggested that 3 °C warming is optimal from a cost–benefit perspective. However, if tipping points are looking more likely, then the ‘optimal policy’ recommendation of simple cost–benefit climate-economy models4 aligns with those of the recent IPCC report2. In other words, warming must be limited to 1.5 °C. This requires an emergency response.


Ice collapse


We think that several cryosphere tipping points are dangerously close, but mitigating greenhouse-gas emissions could still slow down the inevitable accumulation of impacts and help us to adapt.


Research in the past decade has shown that the Amundsen Sea embayment of West Antarctica might have passed a tipping point3: the ‘grounding line’ where ice, ocean and bedrock meet is retreating irreversibly. A model study shows5 that when this sector collapses, it could destabilize the rest of the West Antarctic ice sheet like toppling dominoes — leading to about 3 metres of sea-level rise on a timescale of centuries to millennia. Palaeo-evidence shows that such widespread collapse of the West Antarctic ice sheet has occurred repeatedly in the past.


The latest data show that part of the East Antarctic ice sheet — the Wilkes Basin — might be similarly unstable3. Modelling work suggests that it could add another 3–4 m to sea level on timescales beyond a century.


The Greenland ice sheet is melting at an accelerating rate3. It could add a further 7 m to sea level over thousands of years if it passes a particular threshold. Beyond that, as the elevation of the ice sheet lowers, it melts further, exposing the surface to ever-warmer air. Models suggest that the Greenland ice sheet could be doomed at 1.5 °C of warming3, which could happen as soon as 2030.


Thus, we might already have committed future generations to living with sea-level rises of around 10 m over thousands of years3. But that timescale is still under our control. The rate of melting depends on the magnitude of warming above the tipping point. At 1.5 °C, it could take 10,000 years to unfold3; above 2 °C it could take less than 1,000 years6. Researchers need more observational data to establish whether ice sheets are reaching a tipping point, and require better models constrained by past and present data to resolve how soon and how fast the ice sheets could collapse.


Whatever those data show, action must be taken to slow sea-level rise. This will aid adaptation, including the eventual resettling of large, low-lying population centres.


A further key impetus to limit warming to 1.5 °C is that other tipping points could be triggered at low levels of global warming. The latest IPCC models projected a cluster of abrupt shifts7 between 1.5 °C and 2 °C, several of which involve sea ice. This ice is already shrinking rapidly in the Arctic, indicating that, at 2 °C of warming, the region has a 10–35% chance3 of becoming largely ice-free in summer.

Biosphere boundaries

Climate change and other human activities risk triggering biosphere tipping points across a range of ecosystems and scales (see ‘Raising the alarm’).




Source: T. M. Lenton et al.


Ocean heatwaves have led to mass coral bleaching and to the loss of half of the shallow-water corals on Australia’s Great Barrier Reef. A staggering 99% of tropical corals are projected2 to be lost if global average temperature rises by 2 °C, owing to interactions between warming, ocean acidification and pollution. This would represent a profound loss of marine biodiversity and human livelihoods.


As well as undermining our life-support system, biosphere tipping points can trigger abrupt carbon release back to the atmosphere. This can amplify climate change and reduce remaining emission budgets.


Deforestation and climate change are destabilizing the Amazon — the world’s largest rainforest, which is home to one in ten known species. Estimates of where an Amazon tipping point could lie range from 40% deforestation to just 20% forest-cover loss8. About 17% has been lost since 1970. The rate of deforestation varies with changes in policy. Finding the tipping point requires models that include deforestation and climate change as interacting drivers, and that incorporate fire and climate feedbacks as interacting tipping mechanisms across scales.


With the Arctic warming at least twice as quickly as the global average, the boreal forest in the subarctic is increasingly vulnerable. Already, warming has triggered large-scale insect disturbances and an increase in fires that have led to dieback of North American boreal forests, potentially turning some regions from a carbon sink to a carbon source9. Permafrost across the Arctic is beginning to irreversibly thaw and release carbon dioxide and methane — a greenhouse gas that is around 30 times more potent than CO2 over a 100-year period.


Researchers need to improve their understanding of these observed changes in major ecosystems, as well as where future tipping points might lie. Existing carbon stores and potential releases of CO2 and methane need better quantification.


The world’s remaining emissions budget for a 50:50 chance of staying within 1.5 °C of warming is only about 500 gigatonnes (Gt) of CO2. Permafrost emissions could take an estimated 20% (100 Gt CO2) off this budget10, and that’s without including methane from deep permafrost or undersea hydrates. If forests are close to tipping points, Amazon dieback could release another 90 Gt CO2 and boreal forests a further 110 Gt CO211. With global total CO2 emissions still at more than 40 Gt per year, the remaining budget could be all but erased already.




Bleached corals on a reef near the island of Moorea in French Polynesia in the South Pacific.Credit: Alexis Rosenfeld/Getty



Global cascade


In our view, the clearest emergency would be if we were approaching a global cascade of tipping points that led to a new, less habitable, ‘hothouse’ climate state11. Interactions could happen through ocean and atmospheric circulation or through feedbacks that increase greenhouse-gas levels and global temperature. Alternatively, strong cloud feedbacks could cause a global tipping point12,13.


We argue that cascading effects might be common. Research last year14 analysed 30 types of regime shift spanning physical climate and ecological systems, from collapse of the West Antarctic ice sheet to a switch from rainforest to savanna. This indicated that exceeding tipping points in one system can increase the risk of crossing them in others. Such links were found for 45% of possible interactions14.


In our view, examples are starting to be observed. For example, Arctic sea-ice loss is amplifying regional warming, and Arctic warming and Greenland melting are driving an influx of fresh water into the North Atlantic. This could have contributed to a 15% slowdown15 since the mid-twentieth century of the Atlantic Meridional Overturning Circulation (AMOC) , a key part of global heat and salt transport by the ocean3. Rapid melting of the Greenland ice sheet and further slowdown of the AMOC could destabilize the West African monsoon, triggering drought in Africa’s Sahel region. A slowdown in the AMOC could also dry the Amazon, disrupt the East Asian monsoon and cause heat to build up in the Southern Ocean, which could accelerate Antarctic ice loss.


The palaeo-record shows global tipping, such as the entry into ice-age cycles 2.6 million years ago and their switch in amplitude and frequency around one million years ago, which models are only just capable of simulating. Regional tipping occurred repeatedly within and at the end of the last ice age, between 80,000 and 10,000 years ago (the Dansgaard–Oeschger and Heinrich events). Although this is not directly applicable to the present interglacial period, it highlights that the Earth system has been unstable across multiple timescales before, under relatively weak forcing caused by changes in Earth’s orbit. Now we are strongly forcing the system, with atmospheric CO2 concentration and global temperature increasing at rates that are an order of magnitude higher than those during the most recent deglaciation.


Atmospheric CO2 is already at levels last seen around four million years ago, in the Pliocene epoch. It is rapidly heading towards levels last seen some 50 million years ago — in the Eocene — when temperatures were up to 14 °C higher than they were in pre-industrial times. It is challenging for climate models to simulate such past ‘hothouse’ Earth states. One possible explanation is that the models have been missing a key tipping point: a cloud-resolving model published this year suggests that the abrupt break-up of stratocumulus cloud above about 1,200 parts per million of CO2 could have resulted in roughly 8 °C of global warming12.


Some early results from the latest climate models — run for the IPCC’s sixth assessment report, due in 2021 — indicate a much larger climate sensitivity (defined as the temperature response to doubling of atmospheric CO2) than in previous models. Many more results are pending and further investigation is required, but to us, these preliminary results hint that a global tipping point is possible.


To address these issues, we need models that capture a richer suite of couplings and feedbacks in the Earth system, and we need more data — present and past — and better ways to use them. Improving the ability of models to capture known past abrupt climate changes and ‘hothouse’ climate states should increase confidence in their ability to forecast these.


Some scientists counter that the possibility of global tipping remains highly speculative. It is our position that, given its huge impact and irreversible nature, any serious risk assessment must consider the evidence, however limited our understanding might still be. To err on the side of danger is not a responsible option.


If damaging tipping cascades can occur and a global tipping point cannot be ruled out, then this is an existential threat to civilization. No amount of economic cost–benefit analysis is going to help us. We need to change our approach to the climate problem.

Act now


In our view, the evidence from tipping points alone suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute (see ‘Emergency: do the maths’).

EMERGENCY: DO THE MATHS

We define emergency (E) as the product of risk and urgency. Risk (R) is defined by insurers as probability (p) multiplied by damage (D). Urgency (U) is defined in emergency situations as reaction time to an alert (τ) divided by the intervention time left to avoid a bad outcome (T). Thus:

E = R × U = p × D × τ / T

The situation is an emergency if both risk and urgency are high. If reaction time is longer than the intervention time left (τ / T > 1), we have lost control.

We argue that the intervention time left to prevent tipping could already have shrunk towards zero, whereas the reaction time to achieve net zero emissions is 30 years at best. Hence we might already have lost control of whether tipping happens. A saving grace is that the rate at which damage accumulates from tipping — and hence the risk posed — could still be under our control to some extent.


The stability and resilience of our planet is in peril. International action — not just words — must reflect this.