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Global warming overshoots increase risks of climate tipping cascades in a network model

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Nico Wunderling
Nico Wunderling
Nico Wunderling
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Ricarda Winkelmann
Ricarda Winkelmann
Ricarda Winkelmann
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Johan Rockström at Stockholm University
Johan Rockström
Sina Loriani at Potsdam Institute for Climate Impact Research
Sina Loriani
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Abstract and Figures

Current policies and actions make it very likely, at least temporarily, to overshoot the Paris climate targets of 1.5–<2.0 °C above pre-industrial levels. If this global warming range is exceeded, potential tipping elements such as the Greenland Ice Sheet and Amazon rainforest may be at increasing risk of crossing critical thresholds. This raises the question of how much this risk is amplified by increasing overshoot magnitude and duration. Here we investigate the danger for tipping under a range of temperature overshoot scenarios using a stylized network model of four interacting climate tipping elements. Our model analysis reveals that temporary overshoots can increase tipping risks by up to 72% compared with non-overshoot scenarios, even when the long-term equilibrium temperature stabilizes within the Paris range. Our results suggest that avoiding high-end climate risks is possible only for low-temperature overshoots and if long-term temperatures stabilize at or below today’s levels of global warming. Temporarily exceeding temperature targets could increase risk of crossing tipping-element thresholds. This study considers a range of overshoot scenarios in a stylized network model and shows that overshoots increase tipping risks by up to 72% compared with remaining within targets.
Interacting climate tipping elements a, Exemplary global warming overshoot scenario with a peak temperature of TPeak = 2.5 °C, a convergence temperature of TConv = 2.0 °C above pre-industrial and a time to convergence to 2.0 °C of tConv = 400 yr. This scenario is applied to a set of four interacting climate tipping elements with an exemplary draw of critical thresholds from their full uncertainty ranges (Supplementary Table 1). b, The effect of the overshoot trajectory shown in a: the Greenland Ice Sheet, the West Antarctic Ice Sheet and the AMOC tip. The grey shaded areas depict the two possible states, either not tipped (baseline regime) or tipped (transitioned) state. c, Map of the four interacting climate tipping elements. Each arrow represents a physical interaction mechanism between a pair of tipping elements, which can be destabilizing (denoted as +), stabilizing (denoted as −) or unclear (denoted as ±). AMAZ, Amazon rainforest; GIS, Greenland Ice Sheet; WAIS, West Antarctic Ice Sheet. The underlying map in c has been created with cartopy⁶¹.
Interacting climate tipping elements a, Exemplary global warming overshoot scenario with a peak temperature of TPeak = 2.5 °C, a convergence temperature of TConv = 2.0 °C above pre-industrial and a time to convergence to 2.0 °C of tConv = 400 yr. This scenario is applied to a set of four interacting climate tipping elements with an exemplary draw of critical thresholds from their full uncertainty ranges (Supplementary Table 1). b, The effect of the overshoot trajectory shown in a: the Greenland Ice Sheet, the West Antarctic Ice Sheet and the AMOC tip. The grey shaded areas depict the two possible states, either not tipped (baseline regime) or tipped (transitioned) state. c, Map of the four interacting climate tipping elements. Each arrow represents a physical interaction mechanism between a pair of tipping elements, which can be destabilizing (denoted as +), stabilizing (denoted as −) or unclear (denoted as ±). AMAZ, Amazon rainforest; GIS, Greenland Ice Sheet; WAIS, West Antarctic Ice Sheet. The underlying map in c has been created with cartopy⁶¹.
… 
Effect of overshoot peak temperature a, Number of tipped elements crossing tipping points due to additional forcing at overshoot peak temperatures of 2.0–4.0 °C above pre-industrial levels. Colours from yellow to dark red denote the number of tipped elements (0, 1, 2, 3, or 4). b, Risk for the individual climate tipping elements of transitioning into the undesired state crossing tipping points at overshoot peak temperatures of 2.0–4.0 °C. The bar colours denote the respective colour of the individual climate tipping element as in Fig. 1a,b. We depict the average of the equilibrium run (long-term tipping after 50,000 simulation years) over the entire ensemble as the bar height, and the error bars show the standard deviation. High-end overshoot peak temperatures up to 6.0 °C above pre-industrial levels and transition times (after 100 yr, after 1,000 yr and in equilibrium) are shown in Extended Data Fig. 2.
Effect of overshoot peak temperature a, Number of tipped elements crossing tipping points due to additional forcing at overshoot peak temperatures of 2.0–4.0 °C above pre-industrial levels. Colours from yellow to dark red denote the number of tipped elements (0, 1, 2, 3, or 4). b, Risk for the individual climate tipping elements of transitioning into the undesired state crossing tipping points at overshoot peak temperatures of 2.0–4.0 °C. The bar colours denote the respective colour of the individual climate tipping element as in Fig. 1a,b. We depict the average of the equilibrium run (long-term tipping after 50,000 simulation years) over the entire ensemble as the bar height, and the error bars show the standard deviation. High-end overshoot peak temperatures up to 6.0 °C above pre-industrial levels and transition times (after 100 yr, after 1,000 yr and in equilibrium) are shown in Extended Data Fig. 2.
… 
Expected number and risk of tipping events at different convergence temperatures a, Number of tipped elements averaged over the entire ensemble for all investigated convergence times tConv and peak temperatures TPeak at a convergence temperature of TConv = 1.0 °C above pre-industrial levels. The white lines show the conditions at which 0.5, 1.0 and 1.5 elements are tipped on average. <#>tipped,min is the average number of tipped elements at tConv = 100 yr and TPeak = 2.0 °C. b,c, As in a but for convergence temperatures of 1.5 °C (b) and 2.0 °C (c). d, The risk that at least one tipping element transitions to its alternative state in equilibrium (after 50,000 simulation years) for a convergence temperature of 1.0 °C. The equipotential line in red indicates the high climate-risk zone (tipping risk is equal to 33%). <Risk>tipping,min is the average risk of at least one element being tipped at tConv = 100 yr and TPeak = 2.0 °C. e,f, As in d but for convergence temperatures of 1.5 °C (e) and 2.0 °C (f). The simulations for TConv = 0.0 °C (return to pre-industrial temperatures) and TConv = 0.5 °C can be found in Extended Data Fig. 4. High-end scenarios with TPeak = 4.0–6.0 °C are added in Extended Data Figs. 5 and 6.
Expected number and risk of tipping events at different convergence temperatures a, Number of tipped elements averaged over the entire ensemble for all investigated convergence times tConv and peak temperatures TPeak at a convergence temperature of TConv = 1.0 °C above pre-industrial levels. The white lines show the conditions at which 0.5, 1.0 and 1.5 elements are tipped on average. <#>tipped,min is the average number of tipped elements at tConv = 100 yr and TPeak = 2.0 °C. b,c, As in a but for convergence temperatures of 1.5 °C (b) and 2.0 °C (c). d, The risk that at least one tipping element transitions to its alternative state in equilibrium (after 50,000 simulation years) for a convergence temperature of 1.0 °C. The equipotential line in red indicates the high climate-risk zone (tipping risk is equal to 33%). <Risk>tipping,min is the average risk of at least one element being tipped at tConv = 100 yr and TPeak = 2.0 °C. e,f, As in d but for convergence temperatures of 1.5 °C (e) and 2.0 °C (f). The simulations for TConv = 0.0 °C (return to pre-industrial temperatures) and TConv = 0.5 °C can be found in Extended Data Fig. 4. High-end scenarios with TPeak = 4.0–6.0 °C are added in Extended Data Figs. 5 and 6.
… 
Timing and mechanisms of tipping events following temperature overshoots Tipping risk with respect to overshoot scenarios of 2.0–4.0 °C and convergence temperatures within the Paris range of 1.5–2.0 °C above pre-industrial levels. The pie charts split the tipping events into the timescale when they occur: after 100 simulation years (dark red), after 1,000 simulation years (light red) or in equilibrium simulations (after 50,000 simulation years, orange). The size of the pie chart indicates the overall tipping risk (for example, 67.4% at TConv = 1.5 °C and TPeak = 2.5 °C). The bar chart directly below the pie chart indicates the ratio between the two possible tipping mechanisms: (1) due to the convergence temperature being above the critical temperature for one or several tipping elements (baseline tipping, for an example see Greenland Ice Sheet in Extended Data Fig. 1d,e) and (2) due to the overshoot trajectory (overshoot tipping, for an example see AMOC in Extended Data Fig. 1c). a,b, Scenario where global mean temperature converges to 1.5 °C (a) or to 2.0 °C (b). c, Expected warming in 2100 after the COP26 pledges and targets (orange vertical line: 1.7–2.6 °C) and the policies and action (dark red vertical line: 2.0–3.6 °C) together with the current warming of 1.2 °C and the Paris temperature target (blue vertical line: 1.5–2.0 °C). Note that the vertical axes are nonlinear due to visibility. The data for the vertical lines have been compiled from the November 2021 update by Climate Action Tracker¹⁴. The scenarios with lower convergence temperatures of 0, 0.5 and 1.0 °C above pre-industrial are depicted in Extended Data Fig. 7. High-end climate scenarios and overshoots for peak temperatures between 4.5 and 6.0 °C are shown in Extended Data Fig. 8.
Timing and mechanisms of tipping events following temperature overshoots Tipping risk with respect to overshoot scenarios of 2.0–4.0 °C and convergence temperatures within the Paris range of 1.5–2.0 °C above pre-industrial levels. The pie charts split the tipping events into the timescale when they occur: after 100 simulation years (dark red), after 1,000 simulation years (light red) or in equilibrium simulations (after 50,000 simulation years, orange). The size of the pie chart indicates the overall tipping risk (for example, 67.4% at TConv = 1.5 °C and TPeak = 2.5 °C). The bar chart directly below the pie chart indicates the ratio between the two possible tipping mechanisms: (1) due to the convergence temperature being above the critical temperature for one or several tipping elements (baseline tipping, for an example see Greenland Ice Sheet in Extended Data Fig. 1d,e) and (2) due to the overshoot trajectory (overshoot tipping, for an example see AMOC in Extended Data Fig. 1c). a,b, Scenario where global mean temperature converges to 1.5 °C (a) or to 2.0 °C (b). c, Expected warming in 2100 after the COP26 pledges and targets (orange vertical line: 1.7–2.6 °C) and the policies and action (dark red vertical line: 2.0–3.6 °C) together with the current warming of 1.2 °C and the Paris temperature target (blue vertical line: 1.5–2.0 °C). Note that the vertical axes are nonlinear due to visibility. The data for the vertical lines have been compiled from the November 2021 update by Climate Action Tracker¹⁴. The scenarios with lower convergence temperatures of 0, 0.5 and 1.0 °C above pre-industrial are depicted in Extended Data Fig. 7. High-end climate scenarios and overshoots for peak temperatures between 4.5 and 6.0 °C are shown in Extended Data Fig. 8.
… 
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Nature Climate Change | Volume 13 | January 2023 | 75–82 75
nature climate change
https://doi.org/10.1038/s41558-022-01545-9
Article
Global warming overshoots increase risks of
climate tipping cascades in a network model
Nico Wunderling   1,2,3 , Ricarda Winkelmann   1,4, Johan Rockström   1,2,
Sina Loriani   1, David I. Armstrong McKay   2,5,6, Paul D. L. Ritchie   5,
Boris Sakschewski   1 & Jonathan F. Donges   1,2,3
Current policies and actions make it very likely, at least temporarily, to
overshoot the Paris climate targets of 1.5–<2.0 °C above pre-industrial levels.
If this global warming range is exceeded, potential tipping elements such
as the Greenland Ice Sheet and Amazon rainforest may be at increasing risk
of crossing critical thresholds. This raises the question of how much this
risk is amplied by increasing overshoot magnitude and duration. Here we
investigate the danger for tipping under a range of temperature overshoot
scenarios using a stylized network model of four interacting climate tipping
elements. Our model analysis reveals that temporary overshoots can
increase tipping risks by up to 72% compared with non-overshoot scenarios,
even when the long-term equilibrium temperature stabilizes within the Paris
range. Our results suggest that avoiding high-end climate risks is possible
only for low-temperature overshoots and if long-term temperatures
stabilize at or below today’s levels of global warming.
It has long been proposed that important continental-scale subsystems
of Earth’s climate system possess nonlinear behaviour
1,2
. The defining
property of these tipping elements is their self-perpetuating feedbacks
once a critical threshold is transgressed
3
such as the melt–elevation
feedback for the Greenland Ice Sheet4 and the moisture recycling
feedback for the Amazon rainforest5. The global mean surface
temperature has been identified as the driving parameter for the state
of the climate tipping elements1,6,7, which include, among others,
systems such as the large ice sheets on Greenland and Antarctica, the
Atlantic meridional overturning circulation (AMOC) and the Amazon
rainforest8–11.
Besides further amplifying anthropogenic global warming
3
, the
disintegration of such climate tipping elements individually would have
large consequences for the biosphere and human societies, including
large-scale sea-level rise or biome collapses. Since the first mapping of
climate tipping elements in 20081, the scientific focus has increased,
with a 2019 warning that 9 of the 15 known climate tipping elements are
showing signs of instability12, followed by a listing of all known climate
tipping elements with expert judgements of tipping-point confidence
levels in Working Group I’s contribution to the Sixth Assessment Report
of the IPCC13. While the uncertainty for crossing tipping points is still
stated as medium to high, the IPCC concludes that crossing them trig-
gering potentially abrupt changes cannot be excluded from projected
future global warming trajectories
13
. As this science has advanced over
the past two decades, potential temperature thresholds have been
corrected downwards several times
12
. The most recent scientific assess-
ment places the critical threshold temperatures of triggering tipping
points at 1–5 °C, with moderate risks already at 1.5–2.0 °C for several
systems, such as the Greenland and West Antarctic ice sheets6. In this
sense, tipping-elements research provides even further scientific sup-
port to hold global mean surface temperatures within the Paris range of
well below 2 °C while at the same time emphasizing that tipping-point
risks cannot be ruled out even at this lower temperature range
6,7
. There
is thus a triple dilemma emerging here. First, insufficient policies and
actions mean that the world is following a trajectory well beyond 2 °C by
the end of this century14. Second, essentially all IPCC scenarios that hold
Received: 4 March 2022
Accepted: 3 November 2022
Published online: 22 December 2022
Check for updates
1FutureLab Earth Resilience in the Anthropocene, Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, Potsdam,
Germany. 2Stockholm Resilience Centre, Stockholm University, Stockholm, Sweden. 3High Meadows Environmental Institute, Princeton University,
Princeton, NJ, USA. 4Institute of Physics and Astronomy, University of Potsdam, Potsdam, Germany. 5Faculty of Environment, Science and Economy,
University of Exeter, Exeter, UK. 6Georesilience Analytics, Leatherhead, UK. e-mail: nico.wunderling@pik-potsdam.de; jonathan.donges@pik-potsdam.de
Content courtesy of Springer Nature, terms of use apply. Rights reserved
... Within the climate system, interactions between several large-scale tipping elements including the AMOC and the Greenland Ice Sheet as well as the West Antarctic Ice Sheet and the Amazon rainforest have been described (Kriegler et al., 2009;Gaucherel and Moron, 2017), and the arising dynamics may involve cascades (Lenton et al., 2019;Rocha et al., 2018). The interactions between these four key climate tipping elements tend to be overall destabilizing under ongoing warming as suggested by integrating expert knowledge and including uncertainties of critical temperature thresholds and interaction strengths into a risk analysis approach for these interacting tipping elements (Wunderling et al., 2023(Wunderling et al., , 2021(Wunderling et al., , 2020a. Employing physically motivated but still conceptual models, it was demonstrated that the intensification of ENSO, which is associated with growing oscillations of eastern Pacific sea surface temperatures after the crossing of a Hopf bifurcation, may be initiated by an AMOC collapse (Dekker et al., 2018). ...
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Full-text available
  • Aug 2022
Tipping elements are nonlinear subsystems of the Earth system that have the potential to abruptly shift to another state if environmental change occurs close to a critical threshold with large consequences for human societies and ecosystems. Among these tipping elements may be the Amazon rainforest, which has been undergoing intensive anthropogenic activities and increasingly frequent droughts. Here, we assess how extreme deviations from climatological rainfall regimes may cause local forest collapse that cascades through the coupled forest–climate system. We develop a conceptual dynamic network model to isolate and uncover the role of atmospheric moisture recycling in such tipping cascades. We account for heterogeneity in critical thresholds of the forest caused by adaptation to local climatic conditions. Our results reveal that, despite this adaptation, a future climate characterized by permanent drought conditions could trigger a transition to an open canopy state particularly in the southern Amazon. The loss of atmospheric moisture recycling contributes to one-third of the tipping events. Thus, by exceeding local thresholds in forest adaptive capacity, local climate change impacts may propagate to other regions of the Amazon basin, causing a risk of forest shifts even in regions where critical thresholds have not been crossed locally.
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Climate Endgame: Exploring catastrophic climate change scenarios
Article
Full-text available
  • Aug 2022
Prudent risk management requires consideration of bad-to-worst-case scenarios. Yet, for climate change, such potential futures are poorly understood. Could anthropogenic climate change result in worldwide societal collapse or even eventual human extinction? At present, this is a dangerously underexplored topic. Yet there are ample reasons to suspect that climate change could result in a global catastrophe. Analyzing the mechanisms for these extreme consequences could help galvanize action, improve resilience, and inform policy, including emergency responses. We outline current knowledge about the likelihood of extreme climate change, discuss why understanding bad-to-worst cases is vital, articulate reasons for concern about catastrophic outcomes, define key terms, and put forward a research agenda. The proposed agenda covers four main questions: 1) What is the potential for climate change to drive mass extinction events? 2) What are the mechanisms that could result in human mass mortality and morbidity? 3) What are human societies' vulnerabilities to climate-triggered risk cascades, such as from conflict, political instability, and systemic financial risk? 4) How can these multiple strands of evidence—together with other global dangers—be usefully synthesized into an “integrated catastrophe assessment”? It is time for the scientific community to grapple with the challenge of better understanding catastrophic climate change.
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An emission pathway classification reflecting the Paris Agreement climate objectives
Article
Full-text available
  • Jun 2022
The 2015 Paris Agreement sets the objectives of global climate ambition as expressed in its long-term temperature goal and mitigation goal. The scientific community has explored the characteristics of greenhouse gas emission reduction pathways in line with the Paris Agreement. However, when categorizing such pathways, the focus has been put on the temperature outcome and not on emission reduction objectives. Here we propose a pathway classification that aims to comprehensively reflect the climate criteria set out in the Paris Agreement. We show how such an approach allows for a fully consistent interpretation of the Agreement. For Paris Agreement compatible pathways, we report net zero CO2 and greenhouse gas emissions around 2050 and 2065, respectively. We illustrate how pathway design criteria not rooted in the Paris Agreement, such as the 2100 temperature level, result in scenario outcomes wherein about 6 - 24% higher deployment (interquartile range) of carbon dioxide removal is observed. Greenhouse gas emissions pathways compatible with the Paris Agreement simultaneously pursue warming of less than 1.5 °C, very likely never exceed 2 °C and achieve net zero greenhouse gas emissions by the second half of the century. They can require smaller amounts of carbon dioxide removal.
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Focus of the IPCC Assessment Reports Has Shifted to Lower Temperatures
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  • May 2022
We focus on how different global temperature increases represented in IPCC reports have shifted over time. While the first four assessment reports had a roughly equal focus on temperatures above and below 2°C, the more recent fifth and sixth assessment reports have a considerably stronger focus on warming below 2°C. This is concerning as warming above 2°C is more likely given current emissions trajectories and is more influential on climate risk assessments.
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Realization of Paris Agreement pledges may limit warming just below 2 °C
Article
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  • Apr 2022
  • NATURE
Over the last five years prior to the Glasgow Climate Pact¹, 154 Parties have submitted new or updated 2030 mitigation goals in their nationally determined contributions and 76 have put forward longer-term pledges. Quantifications of the pledges before the 2021 United Nations Climate Change Conference (COP26) suggested a less than 50 per cent chance of keeping warming below 2 degrees Celsius2–5. Here we show that warming can be kept just below 2 degrees Celsius if all conditional and unconditional pledges are implemented in full and on time. Peak warming could be limited to 1.9–2.0 degrees Celsius (5%–95% range 1.4–2.8 °C) in the full implementation case—building on a probabilistic characterization of Earth system uncertainties in line with the Working Group I contribution to the Sixth Assessment Report⁶ of the Intergovernmental Panel on Climate Change (IPCC). We retrospectively project twenty-first-century warming to show how the aggregate level of ambition changed from 2015 to 2021. Our results rely on the extrapolation of time-limited targets beyond 2030 or 2050, characteristics of the IPCC 1.5 °C Special Report (SR1.5) scenario database⁷ and the full implementation of pledges. More pessimistic assumptions on these factors would lead to higher temperature projections. A second, independent emissions modelling framework projected peak warming of 1.8 degrees Celsius, supporting the finding that realized pledges could limit warming to just below 2 degrees Celsius. Limiting warming not only to ‘just below’ but to ‘well below’ 2 degrees Celsius or 1.5 degrees Celsius urgently requires policies and actions to bring about steep emission reductions this decade, aligned with mid-century global net-zero CO2 emissions.
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Fragmented tipping in a spatially heterogeneous world
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Many climate subsystems are thought to be susceptible to tipping—and some might be close to a tipping point. The general belief and intuition, based on simple conceptual models of tipping elements, is that tipping leads to reorganization of the full (sub)system. Here, we explore tipping in conceptual, but spatially extended and spatially heterogenous models. These are extensions of conceptual models taken from all sorts of climate system components on multiple spatial scales. By analysis of the bifurcation structure of such systems, special stable equilibrium states are revealed: coexistence states with part of the spatial domain in one state, and part in another, with a spatial interface between these regions. These coexistence states critically depend on the size and the spatial heterogeneity of the (sub)system. In particular, in these systems the crossing of a tipping point not necessarily leads to a full reorganization of the system. Instead, it might lead to a reorganization of only part of the spatial domain, limiting the impact of these events on the system’s functioning.
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Pronounced loss of Amazon rainforest resilience since the early 2000s
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  • Mar 2022
  • Nat. Clim. Change.
The resilience of the Amazon rainforest to climate and land-use change is crucial for biodiversity, regional climate and the global carbon cycle. Deforestation and climate change, via increasing dry-season length and drought frequency, may already have pushed the Amazon close to a critical threshold of rainforest dieback. Here, we quantify changes of Amazon resilience by applying established indicators (for example, measuring lag-1 autocorrelation) to remotely sensed vegetation data with a focus on vegetation optical depth (1991–2016). We find that more than three-quarters of the Amazon rainforest has been losing resilience since the early 2000s, consistent with the approach to a critical transition. Resilience is being lost faster in regions with less rainfall and in parts of the rainforest that are closer to human activity. We provide direct empirical evidence that the Amazon rainforest is losing resilience, risking dieback with profound implications for biodiversity, carbon storage and climate change at a global scale.
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Cost and attainability of meeting stringent climate targets without overshoot
Article
Full-text available
  • Dec 2021
  • Nat. Clim. Change.
Global emissions scenarios play a critical role in the assessment of strategies to mitigate climate change. The current scenarios, however, are criticized because they feature strategies with pronounced overshoot of the global temperature goal, requiring a long-term repair phase to draw temperatures down again through net-negative emissions. Some impacts might not be reversible. Hence, we explore a new set of net-zero CO2 emissions scenarios with limited overshoot. We show that upfront investments are needed in the near term for limiting temperature overshoot but that these would bring long-term economic gains. Our study further identifies alternative configurations of net-zero CO2 emissions systems and the roles of different sectors and regions for balancing sources and sinks. Even without net-negative emissions, CO2 removal is important for accelerating near-term reductions and for providing an anthropogenic sink that can offset the residual emissions in sectors that are hard to abate.
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Exceeding 1.5°C global warming could trigger multiple climate tipping points
Article
  • Sep 2022
  • SCIENCE
Climate tipping points occur when change in a part of the climate system becomes self-perpetuating beyond a warming threshold, leading to substantial Earth system impacts. Synthesizing paleoclimate, observational, and model-based studies, we provide a revised shortlist of global "core" tipping elements and regional "impact" tipping elements and their temperature thresholds. Current global warming of ~1.1°C above preindustrial temperatures already lies within the lower end of some tipping point uncertainty ranges. Several tipping points may be triggered in the Paris Agreement range of 1.5 to <2°C global warming, with many more likely at the 2 to 3°C of warming expected on current policy trajectories. This strengthens the evidence base for urgent action to mitigate climate change and to develop improved tipping point risk assessment, early warning capability, and adaptation strategies.
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