Climate Tipping Points Fact-Check series: Introduction

In the last year there has been a surge in interest in and news on climate change, inspired in part by high-impact reports like the IPCC’s special report on keeping global warming within 1.5°C (which gave a limit of only 12 years for significant emission cuts) and the Hothouse Earth paper, along with an intense summer wildfire season and the fifth year of record-breaking temperatures. This has been accompanied by a wave of new climate activism seeking to initiate greater action on achieving decarbonisation to limit climate change, such as the School Strike for Climate movement started by Greta Thunberg in Sweden. 2018 also saw the start of the Extinction Rebellion (XR) movement in the UK, who have organised non-violent civil disobedience inspired by their popular Heading for Extinction speaking tour in dozens of towns in the UK.

But some activists, journalists, and academics have recently made widely seen and circulated claims that several climate tipping points may have already been triggered and that devastating warming is now almost inevitable. Commonly discussed scenarios include:

  • An ice-free Arctic will happen within the next few years and trigger a “Blue Ocean Event”, leading to a sudden and devastating increase in global warming or massive crop failures
  • A “Methane Bomb” scenario, in which vast amounts of the potent greenhouse gas methane is released soon from the Arctic, triggering sudden catastrophic warming
  • That even if we stopped emitting greenhouse gases now, we’re already committed to catastrophic warming far worse than the 1.5°C or 2°C Paris targets
  • That “Global Dimming” from aerosol pollution is masking a large amount of warming, so a cut in emissions would actually trigger sudden dangerous warming
  • That there are no significant stabilising negative feedbacks that counter the self-reinforcing positive feedbacks in the climate system, leading to runaway warming
  • The events of the Permian-Triassic Extinction 252 million years ago – in which 90-95% of all species died out in the “Great Dying” – could soon happen again

Many of these scenarios rely on the premise that many climate tipping points are on the verge of being triggered, and these will cause immediate and severe climate consequences rather than acting as gradual irreversible feedbacks.

Extreme climate scenarios have been posited before by fringe activists and commentators, but they are now entering into more mainstream activist, political, and media discussion. There has also been growing awareness and interest in the scientific (and not-so-scientific) discussion of climate tipping points and feedbacks, and has led to a surge in hits for by people wanting to find out more.

While this increase in interest in climate tipping points and their implications is welcome, some scientists are worried that the attention-grabbing but extremely unlikely catastrophic scenarios are dominating discussions. Many articles take the worst-case scenario (or beyond) and assume it is likely or even certain, when in reality they have a low – but importantly not insignificant – chance. And although climate change in some cases (like sea level or sea ice loss) is happening faster than the IPCC originally projected, and many models do not yet fully include the tipping points and feedbacks we know of, this doesn’t mean that the IPCC is unreliable and that the worst-case scenario is now inevitable.

On the other hand, there are also many commentators and politicians who are still sceptical that serious climate change exists at all. In trying to be clear about the realities and uncertainties of climate tipping points we should not give false comfort and suggest that they are not a problem. Climate change is a matter of uncertain risks, with tipping points making higher warming scenarios more likely and ‘lukewarm’ scenarios less likely.

For example, some recent analyses have found that on current emission trajectories we have a 50% chance of exceeding 3.2°C and a 5% chance of exceeding 4.9°C by 2100 (and conversely only a 5% chance of keeping below the 2°C Paris target). However, adding some possible climate tipping points and feedbacks moves the 50% value up to 4.1°C and the upper 5% value up to 6.5°C, values well into the realms of what many believe represents an existential risk. A 5% chance seems low, but it represents a one-in-twenty chance of an unacceptable outcome. Discussions of the extreme possibilities should be clear that they are still unlikely, but still likely enough to make these scenarios well worth avoiding by keeping below 1.5-2°C as a safe precaution. Climate tipping points load the dice a bit more against us, but are not yet a foregone conclusion.

In this new Fact-Check series, aims to clear up some confusion about climate tipping points, so that people can be clear about how best to act as a result. The conversation about climate tipping points is important and needs to be well informed in both what we know and what we don’t know, otherwise we risk the discussion becoming mired with misinformation and its warnings unheard.

Click each link below to find out more about each claim and the reality behind it (links will be updated as the series is published):

Fact-Check: will an ice-free Arctic trigger a climate catastrophe?

Claim: A summer ice-free Arctic (called by some the “Blue Ocean Event”) will happen within the next few years and will cause an abrupt worsening of climate change and possible runaway feedbacks.

Reality: A summer ice-free Arctic will probably happen within the next few decades, but the exact year will depend on unpredictable natural variability. A summer ice-free Arctic would worsen regional warming and impacts, but would not cause a big or sudden increase in global temperatures.

Fact-Check: do tipping points and feedbacks commit us to rapid catastrophic warming?

Claim: More than 3°C of warming is already locked in over the next ~10 years even if we reduced or stopped emissions now, making catastrophic warming inevitable.

Reality: By 2030 we’ll likely reach an average of ~1.3oC. If all carbon emissions ceased now we’d reach ~1.7°C by 2100 (and more with reduced aerosol emissions), but our current trajectory is for far higher. Many of the values in committed warming lists are excessive, double-counted, or happen far slower than claimed.

Fact-Check: is an Arctic “Methane Bomb” about to go off?

Claim: A huge amount of methane is trapped in permafrost and methane hydrates in the Arctic and is starting to leak out, and even a partial release could at any time trigger a sudden shock increase in global warming of up to 5°C within 5 years.

Reality: Methane levels have recently increased but so far have a mainly tropical or fossil fuel source. Methane release from permafrost and hydrates will happen as a gradual chronic leak acting as an unwelcome but modest feedback on warming, rather than being a sudden, catastrophic release.

Fact-Check: is “global dimming” shielding us from catastrophe?

Claim: Global dimming (due to cooling by aerosols, as opposed to global warming from greenhouse gases) is masking a large amount of warming (0.7-1.5°C), so if we stopped carbon emissions now we’d get a catastrophic jump in global warming.

Reality: Global dimming is masking around 0.6°C of anthropogenic warming. There are many aerosol sources – including some that cause warming – and so shutting down the worst carbon emitters (like coal power stations) now would not lead to all aerosols disappearing immediately or a sudden, dramatic warming.

Fact-Check: will 2°C of global warming trigger rapid runaway feedbacks?

Claim: Once global warming reaches 2°C (which we’ve nearly reached already, but scientists are downplaying it), positive feedback loops and tipping points will trigger rapid runaway warming and guarantee apocalyptic climate change in the next few decades.

Reality: The risk of tipping points grows significantly above 2°C, but this is an uncertain precautionary boundary and not a sharp definite threshold. Most feedbacks are long-term, committing to a Hothouse by the year ~3000 rather than 2100. Current warming is 1.1°C above the 1850-1900 baseline, not ~2°C. The 1.5°C & 2°C targets are still geophysically possible, and reduce the risk of passing more tipping points.

Fact-Check: is more than 2°C of global warming already locked in?

Claim: The lag between CO2 emissions and warming means ~0.7°C of warming is yet to come and aerosols are masking another ~0.7°C, meaning warming of more than 2°C is already locked in even if we stopped all emissions now.

Reality: If emissions stopped now, falling greenhouse gas concentrations would reduce the effects of the warming lag from ~0.6°C to ~0.1°C. Stopping aerosol emissions would cause a warming boost of ~0.2°C, but a slower partial phase-out can reduce it and spread it out. If we stopped all emissions now (including methane) there’d even be an overall cooling by 2100.

Fact-Check: are there no negative feedbacks to stop runaway warming?

Claim: There are lots of amplifying positive feedbacks in the climate system and no significant stabilising negative feedbacks, making runaway climate change likely.

Reality: Outgoing longwave radiation acts as the main major negative feedback, as hot things radiate more heat away. Positive feedbacks do not inevitably lead to runaway warming, as negative feedbacks will eventually counter them – if there were no negative feedbacks Earth would have become as hot as Venus long ago.

Fact-Check: are we re-triggering the Permian-Triassic Extinction?

Claim: Four out of five previous mass extinction events were caused by massive CO2 increases triggering methane releases, stagnant oceans, and crashing oxygen levels and led to 95% of life dying, and human warming is on the verge of doing the same.

Reality: Only two past extinctions definitely featured CO2-driven warming (PTX & TJX), with another two featuring CO2-linked cooling. The Permian-Triassic extinction featured far more extreme emissions and was amplified by many other factors that are not the case now. A total plankton die-off and low oxygen is nigh-on impossible.

Fact-Check: are climate tipping points always sudden and dramatic?

Claim: Tipping points are always abrupt events with big, immediate impacts. Passing a climate tipping point like with Arctic sea-ice or methane release will result sudden jumps in warming within only a few years.

Reality: The most important feature of tipping points is irreversibility, even if they only happen gradually. Once a process passes a tipping point it will keep on going even if you remove the initial driver. For example, Arctic methane may start leaking out even if we cut back emissions, but it’ll come out over centuries rather than years.


This post was written by Dr. David A. McKay, currently a Postdoctoral Researcher at Stockholm Resilience Centre (Stockholm University), where he is part of the Earth Resilience in the Anthropocene Project (funded by the European Research Council) and is researching non-linear climate-biosphere feedbacks. This post was written in his spare time with no funding support for this site, and was proofread and edited by Dr. Rachael Avery.

Update log: 7/8/2020 to update some links.


  1. Regarding the following:

    “Only two past extinctions definitely featured CO2-driven warming (PTX & TJX), with another two featuring CO2-linked cooling. The Permian-Triassic extinction featured far more extreme emissions and was amplified by many other factors that are not the case now. A total plankton die-off and low oxygen is nigh-on impossible.”

    Could you please expand upon this? I’ve seen people claim otherwise without really providing any real evidence or citation, so it’d be very much appreciated


    1. Hi! I’ve been meaning to get round to that post for a while, but I’ve been a bit too swamped by regular work recently to get round to it… But briefly, both the Permian-Triassic ( and Triassic-Jurassic ( extinctions were associated with Large Igneous Provinces (huge & long-winded volcanic eruptions, the latter of which started the opening of the Atlantic Ocean) releasing vast amounts of greenhouse gases and other nasties like sulphur dioxide, while the older Ordovician-Silurian ( & Late Devonian ( extinctions appear to be associated with falling greenhouse gas concentrations (possibly from plant- or volcanism-induced weathering) triggering rapid pulses of glaciation and shifting ocean currents, although as they’re older events it’s harder to be sure on the exact causes.

      Re. the PTX being far worse, carbon release was something like 8500 GtC (we’re at 1500ish so far) and temperatures likely went up by ~8C, and the past factors that made it worse are that there was a whole bunch of other nasties emitted (carbon monoxide, SO2, H2S) that messed with the ozone layer, the world was arranged as a supercontinent made much more land susceptible to extreme aridity & erosion and one big ocean basin prone to becoming stagnant, and the absence then of calcareous plankton (plankton with chalky shells which evolved ~150 million years ago) meant the ocean carbon cycle had far less buffering capacity to resist big carbon inputs than now.

      I’ve so far never found any basis for the total plankton die-off claim that occasionally goes around, as even in a extreme warming scenario there’d still be tropical-adapted plankton able to expand polewards, as they’ve done before. Even if there was a hypothetical total die-off atmospheric oxygen would take millions of years to decline – it’s true that plankton do make around half of the oxygen input, but that total input represents a tiny trickle into a vast reservoir that took millions/billions of years to build up, and so turning the trickle off wouldn’t cause a sudden mass draining. There’s also a key negative feedback that resists oxygen falling too low, as once it falls below ~15% (from current ~21%) stuff struggles to catch fire any more, and so more plants survive to pump more oxygen out (conversely, oxygen above ~30% means *everything* easily catches fire, leading to fewer plants – this happened in the Carboniferous ~300 million years ago due to all the swamp-jungles that so much coal is from).


  2. What about eco-social (or socio-ecological) tipping points? eg As I published in 2016:

    6. Eco-Social Tipping Points
    The final concept to be introduced is “eco-social tipping points”. There is an increasing literature about environmental (or ecological) tipping points [45] and it is intuitively obvious to some that sufficient environmental change will inevitably induce, or be accompanied by, large-scale social change. However, despite this intuitive attraction, there is comparatively little recent scientific literature on this subject, particularly in association with the Anthropocene, Earth system science, Planetary Boundaries, and Limits to Growth, though it is implicit. For example, the subtitle of the Planetary Boundaries articles is a “safe operating space” not for the Earth system, but for “humanity” [50,51]. If the environment changes sufficiently, can society remain largely unaffected?

    This reticence appears in part be a reaction against “environmental determinism”, the largely
    shunned idea, first formulated in the 18th century, that many social and historical phenomena are influenced (even “controlled”) by environmental factors and changes [89]. It is thus important to stress that social changes in response to major environmental shifts are not inevitable, but may occur.

    The concept is of importance if Limits to Growth and Planetary Boundaries are valid, because it means, beyond thresholds, such changes increase the risk to society, to health and, especially, to those who are poor and vulnerable. This will be explained in the next section.
    Extreme interpretations in the other direction (i.e., that environmental determinants are irrelevant to human well-being) not only violate the precautionary principle but appear ideological. But, of course, relationships are not simple, nor always inevitable. Eco-social tipping points may exist, but they can be avoided.


    1. Hi Colin, socio-ecological tipping points are indeed an important topic that I’m interested in too (I work at Stockholm Resilience Centre in the Planetary Boundaries group, and there’s lots of socio-ecological systems work here – I also work with Prof. Tim Lenton, who’s currently getting in to triggering ‘positive’ socio-ecological tipping points for sustainable transformations: To keep things concise though I’ve focused this site specifically on biophysical climate feedbacks and tipping points, which is what I know most about and is currently generating a lot of concern. But I agree that socio-ecological tipping deserves more investigation!


      1. David, thank you for troubling to reply. In 2013 an editor at Science encouraged to me write a review about these socio-ecological tipping points, but alas I have never been able to finish it, though I have published quite a bit that is relevant, in low-impact journals. As you likely know there is a powerful school of thought denying links between environmental change and social consequences (almost of any kind), even denigrating it as “environmental determinism”. Critics often assert that analysts underestimate the role of “politics” in phenomena such as conflict, forced migration and famine. However, very often, politics is itself entangled with environmental factors. We have argued that the denial of any environmental component for many social phenomenon risks the opposite sin, of “social determinism”, in correspondence to Nature in 2018 (Vol 555, p 587). For me, with my background in public health, the social consequences of environmental change are the most important, and these are likely to become apparent far earlier than a person who is optimistic about the human future might think. For example the war in Syria arose well before there were absolute shortages of resources.


  3. I have a question about ocean acidification which is related to questions about plankton and PTX.

    And is there an acidification trajectory (pessimistic scenarios included) where the ocean is so acidic that marine life disappears? For example, by making it impossible to manufacture certain types of shells or exoskeletons.
    In short, a dead ocean.


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