Prof Bill McGuire, University College London, summarises the disturbing evidence on tipping points in the climate system.
Article from Responsible Science journal, no.6; advance online publication: 7 December 2023
“Prediction is very difficult, especially about the future.” This quote is particularly pertinent to the future of our planet's climate, which anthropogenic global heating is transforming possibly more rapidly – notwithstanding transient cataclysms like asteroid impacts – than at any time in our world's 4.6-billion-year history. What the end game will be is still anyone's guess, and despite the best efforts of climate scientists, there are so many imponderables that we really have no idea where our world will end up.
Projections of what our thermally-challenged world will be like in the decades and centuries ahead are based largely upon computer modelling. Climate scientists are always very careful about what they put into a model, so that the output is as reliable and as accurate as it can be. Even with the best will in the world, however, the climate system – and the manifold responses of society and the economy to global heating – are so complex and interwoven, that getting the model input right is far from straightforward. The real flies in the ointment, however, are so-called 'tipping points', which can see dangerous elements of our changing climate locked onto courses that are impossible to turn around, at least on a scale of the human life-span, and which are notoriously difficult to model.
Defining climate tipping points
There are plenty of definitions for 'tipping point' out there, but the most apposite – in the context of climate breakdown – is that provided by the American Heritage Dictionary of the English Language, which describes it as “the point at which a slow, reversible change becomes irreversible, often with dramatic consequences”. This really does hit the nail on the head, and describes exactly what will happen if we don't rein in greenhouse gas emissions immediately – although even this may no longer be enough. In broad terms, ‘climate tipping points’ (CTPs) mark thresholds beyond which negative feedback effects, which act to maintain stability, are overwhelmed by positive feedbacks, which drive and reinforce self-perpetuating change. The consequences of crossing a tipping point may be obvious within decades or even years, or it may take centuries for the full ramifications to become apparent.
Our understanding of CTPs has changed significantly over the last couple of decades. Twenty years ago, they were recognised as serious threats only if and when unmitigated global heating raised the average temperature of the planet (compared to pre-industrial times) by 4°C or more. Now, however, we know that critical components of the climate system could tip following a hike in global temperature of little more than 1°C [1]. Given that this year is (as of November) 1.43°C hotter than the 1850-1900 average [2], with the temperature rise even breaching the 2°C mark for a few days in November [3], this is a huge cause for concern.
As the global average temperature rise closes in on the permanent breaching of the 1.5°C mark – widely trailed as the ‘dangerous climate change guardrail’ – so the risk of crossing a number of tipping points, which will have dire consequences for our world and our civilisation, becomes more serious by the year [4].
Those parts of the overall climate system that are capable of tipping have been identified through the analysis of past episodes of climate change, and they are known as ‘climate tipping elements’ (CTEs) [1]. There are plenty to choose from, with nine global and seven regional CTEs having been identified as being critical to how our world will look in the future [4].
CTEs can be grouped together according to those parts of the climate system they relate to. ‘Cryosphere tipping elements’, for example, involve large-scale changes to the cryosphere, which is the catch-all term for all the planet's frozen water, including ice sheets, ice caps and glaciers. The two attracting most concern involve the collapse of the Greenland and West Antarctic ice sheets which, together, would raise global sea-level by 10 m-12 m. A third involves the wholesale, abrupt thawing of permafrost across Alaska, northern Canada and Siberia. If this happens, colossal quantities of methane – a greenhouse gas far more potent, at shorter times-scales, than carbon dioxide – would be released into the atmosphere, accelerating the rate of global heating.
Tipping points that affect the biosphere include the loss of the Amazon Rainforest and the high-latitude boreal (coniferous) forests. Ocean currents can have tipping points too, and particular attention is focused on the Gulf Stream and associated North Atlantic currents that make up what is known as the Atlantic Meridional Overturning Circulation (AMOC), and its potential future shutdown.
How soon will we reach them?
Many countries are coalescing around the goal of achieving net zero emissions by 2050 [5], not for any solid scientific reason, but because it is a nice round number and far enough away to justify inaction in the near term. The global average temperature rise is predicted to climb permanently above 1.5°C by between 2026 and 2042, with a central estimate of 2032, while business as usual will see the 2°C breached by 2050 or very soon after [6]. This means that by mid-century a number of tipping points may already have tipped [4], ensuring a major transformation of our world from which there is no return. Neither rapid cuts in emissions nor the direct removal of carbon from the atmosphere will turn back the clock.
Tipping points that may have been crossed by mid-century include collapse of the system of rotating currents that make up the so-called Sub-polar Gyre in the northernmost Atlantic. This could be instigated as a consequence of a global average temperature rise of as little as 1.1°C and, once begun, collapse could happen in as short a time-frame as five years. The consequences of this would be severe, including a fall of as much as 3°C in temperatures across the North Atlantic region, elevated levels of extreme weather in Europe and serious knock-on effects across the world [6]. There is also the possibility that the AMOC itself could shut down at any point upwards of a global average temperature rise of 1.4°C. Indeed, a recent study [7] proposes that AMOC collapse could happen at any time from 2025 to 2090, with a central estimate of 2050. This is projected to result in regional cooling of anything from 4°C to 10°C.
The fates of both the Greenland and West Antarctic ice sheets could also be sealed long before 2050. The estimated threshold temperature for the unstoppable collapse of these great ice masses is 1.5°C, but could be 1°C or even less, suggesting that it could already be too late to prevent wholesale melting and – as mentioned earlier – an ultimate sea-level rise of 10m-12m.
Climate elements having regional rather than global consequences, which could tip at 1.5°C or less, include the abrupt thaw of high-latitude Northern Hemisphere permafrost, the decay of mountain glaciers, and the sudden loss of Barents Sea winter ice.
A 2°C global average temperature hike, easily possible by 2050, could see the irreversible dieback of the Amazon Rainforest, resulting in the addition of colossal amounts of carbon dioxide to the atmosphere. Unstoppable melting of parts of the prodigious East Antarctic Ice Sheet could also be initiated at a threshold as low as 2°C, augmenting sea-level rise due to collapse of the Greenland and West Antarctic ice sheets.
As if all this wasn't bad enough, an additional major concern is that climate elements might well tip in clusters rather than in isolation. This is because the knock-on effects of one tipped element may lead to conditions that bring forward the timing of another – and so on – resulting in a cascade or domino effect that could have devastating consequences for society and the economy [8, 9]. For example, tipping of the Greenland Ice Sheet significantly increases the likelihood of AMOC shutdown, which would have global ramifications for weather and climate. This, in turn, could lead to an intensification of the Pacific El Niño – Southern Oscillation (ENSO), bringing further unwelcome changes to the world's weather [8, 9]. Detailed discussion of the impacts of cascading tipping elements, including how these differ according to the order with which elements tip, is beyond the scope of this article, and readers who wish to know more are referred to the 2016 Nature paper by Cai et al. [8], and the comprehensive 2023 Global Tipping Points report by Lenton et al. [9].
The fact that there is no linear relationship between the level of global heating and geophysical responses such as ice sheet melting, permafrost thaw, and ocean current shutdown, means that the time-frame of climate breakdown is hard to pin down, which in turn makes it more perilous. Consequently, any overshoot of the 1.5°C ‘dangerous climate change guardrail’ is extremely risky. Bringing temperatures down to below this mark, at a later stage, by sucking carbon out of the air will do nothing to reverse tipping elements that have already tipped. The corollary of this is that a net zero target of 2050 is far too late. The longer we delay action to slash emissions as the science demands, the more likely it becomes that we will push one or more climate tipping elements beyond the point of no return, locking in changes to our world with the potential to threaten the very existence of our civilisation.
Bill McGuire is Professor Emeritus of Geophysical & Climate Hazards at UCL, and a Patron of Scientists for Global Responsibility. His latest book is Hothouse Earth: an Inhabitant's Guide.
Image credit: Antarctic ice sheet by Pixabay photographer 12019.
References
[1] Lenton (2021). Tipping points in the climate system. Royal Meteorological Society. https://rmets.onlinelibrary.wiley.com/doi/10.1002/wea.4058
[2] Copernicus (2023). October 2023 - exceptional temperature anomalies; 2023 virtually certain to be warmest year on record. https://climate.copernicus.eu/copernicus-october-2023-exceptional-temperature-anomalies-2023-virtually-certain-be-warmest-year
[3] Copernicus (2023). Global temperature exceeds 2°C above pre-industrial average on 17 November. https://climate.copernicus.eu/global-temperature-exceeds-2degc-above-pre-industrial-average-17-november
[4] McKay et al. (2022). Exceeding 1.5°C global warming could trigger multiple climate tipping points. Science, 377 (6611). https://www.science.org/doi/10.1126/science.abn7950
[5] Climate Action Tracker (2023). CAT net zero target evaluations. https://climateactiontracker.org/global/cat-net-zero-target-evaluations/
[6] Carbon Brief (2020). Analysis: When might the world exceed 1.5C and 2C of global warming? https://www.carbonbrief.org/analysis-when-might-the-world-exceed-1-5c-and-2c-of-global-warming/
[7] Ditlevsen & Ditlevsen (2023). Warning of a forthcoming collapse of the Atlantic meridional overturning circulation. Nature Communications, 14 (4254). https://www.nature.com/articles/s41467-023-39810-w
[8] Cai et al. (2016). Risk of multiple interacting tipping points should encourage rapid CO2 emission reduction. Nature Climate Change, 6 (5). https://www.researchgate.net/publication/298914472_Risk_of_multiple_interacting_tipping_points_should_encourage_rapid_CO2_emission_reduction
[9] Lenton et al (2023). Global Tipping Points Report. University of Exeter et al. https://global-tipping-points.org/
