2. Climate system tipping points and the need for urgent climate action

This chapter draws on contributions to the Horizontal Project carried out under the responsibility of the Environment Policy Committee.

The Intergovernmental Panel on Climate Change (IPCC) delivered a stark warning of the extreme urgency of the climate challenge in its Sixth Assessment Report, projecting that severe climate impacts could occur in many regions of the world at lower levels of warming than previously thought (IPCC, 2021[1]). Climate change will lead to an increased frequency and intensity of extreme weather events as well as “slow onset” effects such as sea-level rise and changes in precipitation. A less well-understood impact of climate change are “tipping points” in the climate system and the potential for the thresholds of these critical points to be crossed.

The IPCC defines a tipping point as “a critical threshold beyond which a system reorganises, often abruptly and/or irreversibly”. A tipping element is “a component of the Earth system that is susceptible to a tipping point” (Chen et al., 2021[2]). Climate system tipping points may lead the global or regional climate to change from one stable state to another, or result in changes that occur non-linearly and faster than the rate of change expected from climate forcing (Alley et al., 2003[3]; Lee, 2021[4]).1 Such abrupt and/or irreversible changes are particularly dangerous because they can occur in timeframes short enough to defy the ability and capacity of human societies to adapt. As such, the impacts of crossing climate tipping point thresholds would be severe and widespread, with potentially catastrophic consequences for human and natural systems.

Tipping elements have been identified in three types of climate sub-systems: the cryosphere (ice bodies); circulations of the oceans and the atmosphere (circulation patterns); and the biosphere. Key examples include the collapse of the West Antarctic and Greenland ice sheets and the melting of the Arctic permafrost (cryosphere); the slowdown or collapse of the Atlantic Meridional Overturning Circulation (circulation patterns); and the dieback of the Amazon rainforest and destruction of coral reefs (biosphere). (For more detail on major tipping elements see Box 2.1.)

The issue of climate tipping points was first introduced by the IPCC over two decades ago, when they were projected to possibly occur in “the next few centuries if greenhouse gas concentrations continue to increase” (IPCC, 2001[9]). More recent IPCC reports recognise the risk of crossing tipping point thresholds at much lower levels of warming and therefore within considerably shorter timescales (IPCC, 2018[10]; IPCC, 2019[11]). Indeed, the latest IPCC report recognises that the possibility of crossing the thresholds of climate tipping points cannot be ruled out this century and must be an integral part of risk management strategies (IPCC, 2021[1]).

A recent synthesis of the most up-to-date evidence on tipping points shows that current global warming of ~1.1°C could already be within the lower end of the uncertainty range of five climate system tipping points, including the collapse of the Greenland and West Antarctic ice sheets, die-off of low-latitude coral reefs, and widespread abrupt permafrost thaw (McKay et al., 2022[12]). This means that crossing the thresholds of these tipping points is already “possible” (Ibid). The same study shows that within the Paris Agreement range of 1.5 to < 2°C warming, these climate tipping points and two others (abrupt Barents Sea ice loss and Labrador-Irminger Seas/SPG convection collapse) become “likely”. While not yet scientific consensus, these findings challenge the previously well-accepted assumption that climate system tipping points are low likelihood outcomes (OECD, 2022[13]).

To restate: the most recent findings thus suggest that the thresholds of many climate tipping points could be crossed with a considerably higher probability and at much lower levels of warming than previously thought. Scientific advances increasingly and systematically point to the potentially catastrophic impacts of continued warming, with evidence that irreversible tipping elements in Earth systems could already be triggered this century. This will have long-lasting effects over a timeframe of centuries to millennia (Lee, 2021[4]).

Figure 2.1 and Table 2.1 show the temperature range at which a number of tipping elements may be triggered – that is, ranges of temperature within which change in these tipping elements is strong enough to self-propel them.2 Despite significant scientific advances in reducing these ranges over past decades, uncertainty remains. Considering that larger uncertainties can potentially lead to a larger range of potential climate risks, they should amplify – not weaken – the case for strong climate action (OECD, 2021[5]).

Evidence that climate tipping points may be approaching is increasing and has led scientists to declare a climate and ecological emergency (Lenton et al., 2019[14]; Ripple, 2020[15]).. For example, irreversible loss of part of the West Antarctic ice sheet may have already begun (Good et al., 2018[16]) and the Greenland ice sheet may also reach a tipping point whereby irreversible loss begins at 1.5°C of warming (Lenton et al., 2019[14]). Ocean ecosystems are already experiencing large-scale changes. For example, ocean heatwaves and acidification are causing mass bleaching of warm-water coral reefs; at warming above 2°C, 99% of coral reefs are projected to be lost (Lenton et al., 2019[14]). In addition, the Atlantic Meridional Overturning Circulation (AMOC) has been slowing over the past two decades (Good et al., 2018[16]) and is at its weakest for over a millennium (Caesar et al., 2021[17]; Boers, 2021[18]). Recent evidence that deforestation – itself a key contributor to climate change – combined with a warming climate, raises the probability that the Amazon will already shift from a humid to dry state during the 21st century (Lenton et al., 2019[14]; Arias et al., 2021[19]). If triggered, these tipping points would lead to often abrupt and irreversible impacts with potentially cascading global implications, including triggering of further tipping points, with dramatic effects on human and natural systems (Lenton et al., 2019[14]).

If triggered, climate system tipping points may lead to changes in the regional or global climate. At a regional level, individual tipping points are associated with potentially severe local impacts such as extreme temperatures and higher frequency of droughts, forest fires and unprecedented weather. At the global level, due to atmospheric and ocean circulation, tipping elements are not isolated systems; rather, they interact. This means that the tipping of one element has the potential to trigger others (Lenton et al., 2019[14]; Wunderling et al., 2021[8]). Such cascading effects – when crossing the threshold of one tipping point triggers further tipping elements – could lead to a “hothouse” global climate that would be less suitable for human existence (Steffen et al., 2018[20]; Lenton et al., 2019[14]). The potential impacts of selected climate tipping points are summarised in Table 2.2.

The impacts of crossing tipping point thresholds can also cascade through socio-economic and ecological systems, often rapidly. They would intensify a range of climate hazards such as droughts, floods and other extreme weather events, causing direct damage to infrastructure and impacts on ecosystems, and water and food systems. These would, in turn, affect socio economic systems with impacts that could then propagate across sectors and international borders via global trade, financial flows and supply networks. Socio-economic responses to such impacts may themselves be non-linear, tipping socio-economic subsystems into a different state, inducing migration or political instability. Socio-economic responses can also result in positive or negative feedback effects with the climate system, either increasing emissions or accelerating mitigation action, thus potentially affecting further tipping elements.

Socio-economic systems also exhibit non-linearities and their own potential tipping points when subjected to worsening climate impacts. Yet models that underpin most economic analyses of climate change rarely include the possibility of abrupt changes to climate or economic systems (Rose, 2022[29]).

A comprehensive review of the literature highlights that economic modelling that includes tipping points has not informed climate policy in a substantive way (Kopits, Marten and Wolverton, 2013[30]). This is largely because modelling efforts that included large-scale singular events were based on Integrated Assessment Models (IAMs) that used ad hoc parameters without empirical bases and typically without considering the multi-decade time horizons at which such large-scale events could unfold. Recent efforts have attempted to more accurately incorporate physical science into economic models and better capture the dynamics of Earth systems and their associated uncertainties (Nordhaus, 2019[31]; Yumashev et al., 2019[32]; Kikstra et al., 2021[33]). As an interesting example, Dietz et al. (2021[34]) provide estimations for the impacts of different tipping points on the social cost of carbon (SCC),4 proposing a meta-analytic IAM that includes eight climate tipping points under a unified framework. They find that, taken collectively, tipping points increase the SCC by around 25% and, importantly, that they increase global economic risk.

Efforts have also been made to take interactions between tipping points into account. Models wherein the crossing of one tipping point threshold increases the probability of another tipping point result in substantially increased SCC (Cai, Lenton and Lontzek, 2016[35]) (Lemoine and Traeger, 2016[36]). Given the uncertainties in the timing and biophysical effects of tipping points in the scientific literature, factoring uncertainty into economic models is also essential. Models that include the uncertainty of economic and climate risks also result in large effects on the social cost of carbon (Cai and Lontzek, 2019[37]) (Cai, Lenton and Lontzek, 2016[35]).

To better understand the cost of climate system tipping points, economic models of climate tipping points should account not only for changes in global mean temperature but also for other physical climate or weather elements (e.g. water cycle, radical changes in the seasonality of extreme events, etc.). There is also a need for improved tipping metrics that consider, for instance, some interval of temperatures. Finally, to improve the coupling of economic and geophysical models, researchers should start with a recursive/simulation model and only then move to optimisation models. There seems to be merit in performing simulation, not optimisation-based models, to achieve a better sense of expected damages.

Finally, climate impacts including tipping points are also inextricably linked to socio-economic systems. There is a clear need to better understand these interlinkages. This includes, for example, modelling the impacts of climate change on financial stability (Kiley, 2021[38]) (Lamperti et al., 2020[39]) (Lamperti et al., 2019[40]); the macroeconomic implications of the transition to net zero (Pisani-Ferry, 2021[41]); regional differences in macroeconomic impacts (Batten, forthcoming[42]); and interactions between impacts across different economic sectors.

Global policy efforts and actions that explicitly target risks associated with the triggering of climate system tipping points and their potential cascading effects remain intangible and highly insufficient. It is imperative to assess how the latest knowledge on climate tipping points can inform risk-management strategies today, including the mitigation of greenhouse gas emissions and net-zero transition strategies; adaptation to climate change and building resilience to accelerating potential climate impacts; and technological development and innovation.

Fully integrating the risks associated with climate tipping points into climate risk management strategies requires a precautionary approach to mitigation. Considering that some tipping points may already be triggered between 1.5 and 2°C of warming, this means limiting the temperature increase to 1.5°C, with no or very limited overshoot. Indeed, an overshoot of the 1.5°C limit could result in a considerably higher risk of crossing climate tipping-point thresholds even if temperatures return to 1.5 °C levels by the end of this century (OECD, 2022[28]).

The estimated possibility of climate system tipping points effectively limits the number and shape of emissions pathways towards 1.5°C of warming and renders lenient interpretations of the Paris Agreement goal significantly more dangerous. Limiting warming to 1.5°C with no or very limited overshoot requires urgent acceleration of near-term action to reach net-zero CO2 emissions. It is therefore no longer “only” about achieving net-zero emissions by mid-century but how this can be achieved: rapid and deep emissions cuts must be made already this decade. As is, current policies in line with meeting countries’ Nationally Determined Contributions (NDCs) will not limit warming to 1.5°C without overshoot. It is critical that NDCs are strengthened before 2025, and that commensurate policies are implemented at relevant timescales to meet revised targets (OECD, 2022[28]).

It is also important to note that, if triggered, some climate tipping points would effectively reduce the remaining carbon budget for reaching temperature objectives. Indeed, loss of sea ice and ice sheets leads to a decrease of solar radiation reflection, raising surface temperatures. If these tipping points are crossed, emissions reductions would need to be even larger than previously thought to meet stated temperature targets. In addition, permafrost carbon emissions have already lowered the estimated remaining carbon budgets for achieving the 1.5°C and 2°C warming objectives, even without having reached the threshold for an abrupt tipping point (Canadell et al., 2021[25]). As most of these tipping elements are likely to be tipped already within the 1.5 to 2°C range, temperature feedback loops further stress how crucial it is to avoid or limit overshooting 1.5°C.

Moreover, for some tipping elements, climate and human disturbances outside GHG emissions can potentially interact with global warming and contribute to surpassing critical thresholds. For example, land use and hydrological changes alongside climate change could lead to widespread dieback of the Amazon rainforest in the near term. In the Arctic, wildfires are increasingly contributing to abrupt permafrost thaw and carbon release from boreal forests.

Collective ambitious action to keep warming below 1.5°C is the safest and most cost-effective way to mitigate climate change and minimise the risk of triggering climate tipping points. The latest economic assessments show that when faced with the risk of cascading tipping points and their non-linear and irreversible impacts, the additional costs of implementing stringent climate policies earlier are worth paying. A rise of 1.5°C can still be achieved with very limited or no overshoot, although this requires a very rapid transformation of economies and societies, and a focus on a resilient and orderly transition (see Part II of this report). As emissions continue to rise, however, this window of opportunity is closing swiftly.

Given the lower levels of warming at which the thresholds of climate system tipping points may be crossed, adaptation planning must account for the possibility of climate tipping points and their cascading impacts. The sheer magnitude of these possible impacts, coupled with their possibly abrupt and non-linear nature, requires more than just incremental adaptation efforts. To remain resilient, policy makers need to consider transformational adaptation actions. Even if the Paris Agreement’s temperature target is met, it is likely that some transformational adaptation will be needed (Ara Begum, 2022[43]). If mitigation efforts fall short of the 1.5°C target, transformational adaptation will be all the more important as impacts become more severe, and the risk of crossing tipping points increases (Ara Begum, 2022[43]).

Transformational adaptation means changing the fundamental characteristics of human and natural systems to increase their capacity to cope with potential hazards (IPCC, 2022[44]). In light of the threat of climate system tipping points, transformational adaptation could necessitate stringent and even drastic measures to reduce impacts and avoid losses. Such drastic measures, potentially transforming whole communities and economic sectors, will undoubtedly result in short-term disruptions. However, considering the immense cost of inaction in the face of tipping point impacts, transformational adaptation is worthwhile.

Transformational adaptation measures can be technological, for example, implementing water capture and storage solutions in areas at risk of drought. They can also be behavioural or include fundamental changes in institutional arrangements, priorities, and norms (Kates, Travis and Wilbanks, 2012[45]). They can also target the spatial development of human activities, such as managed retreat of communities and settlements, relocation of assets and infrastructure to less at-risk areas, long term spatial planning, and urban and agricultural zoning. Other examples include international co-operation among governments to manage migrations, the deployment of nature-based solutions (NbS), and livelihood transformation in land systems (New, 2022[46]).

A specific example is adapting energy planning and zoning in the Amazon region, which is heavily reliant on large-scale hydropower, to anticipate the impacts on the region’s hydrological cycle should the Amazon turn from rainforest to savannah. This includes decentralisation of energy production, diversification of energy sources focusing on small-scale hydropower and solar power, and investments in energy saving (Lapola et al., 2018[47]). A transformational approach to adaptation in the Amazon would also mean livelihood transformation, i.e. encouraging farmers to switch to crop varieties and livestock suitable for drier conditions in order to protect the agricultural sector (ibid).

Transformational adaptation measures that are beneficial or low-cost even if tipping points do not occur are “no-regret” policies (Heltberg, Siegel and Jorgensen, 2009[48]). Some may come with large co-benefits by contributing to reductions in GHG, supporting the advancement of other sustainable development goals (e.g. energy and water access), and building the resilience of systems and populations to future climate shocks. This is especially the case for investment in rehabilitating ecosystems and ecosystems services and the implementation of nature-based solutions, which are also important for accelerating mitigation efforts (Schipper et al., 2022[49]). (NbS are discussed further in Chapter 12.)

Many current adaptation initiatives prioritise near-term climate risk reduction and may even clash with efforts to bring about transformational adaptation (IPCC, 2022[37]). For example, building flood defences may lock in infrastructure development in areas that would be untenable if tipping points were crossed. Instead, to ensure societies’ resilience to climate-change impacts, especially those brought about by the crossing of tipping-point thresholds, transformational adaptation efforts must be pursued alongside stringent mitigation efforts to limit climate risk. Implementing transformative actions that support sustainable development goals is a key opportunity for climate resilient development.

Technological developments and innovation can also help to reduce and manage the risks associated with climate tipping points.

First, technologies for better monitoring and modelling of the climate system are essential to assess how climate-related hazards resulting from tipping points may evolve over time. In addition, it is difficult to predict the thresholds at which tipping points may be crossed, as the parameters that induce a shift often show only incremental changes before the system makes a sudden or persistent transition. Climate modelling and remote sensing are improving in their ability to detect early warning signals, but these new findings are not yet feeding into the risk management strategies informing policy makers. Greater efforts to make this information readily accessible is essential to addressing the threat of climate tipping points.

Second, technologies can help develop and implement ways to reduce and manage the risks of climate system tipping points. Carbon-dioxide removal (CDR) technologies are key, as scenarios that limit warming to 1.5°C with no or very limited overshoot all include some deployment of CDR. It is essential to note that CDR technologies are needed to accelerate early and deep emissions reductions and balance out harder to-abate sectors that continue to produce residual emissions during the first half of the century, but CDR technologies should not justify delayed mitigation action (Riahi et al., 2021[50]).

Moreover, there are legitimate concerns regarding CDR technologies, for example the immense demand for land and resulting implications for land-use practices that bioenergy with carbon capture and storage (BECCS) requires (Creutzig et al., 2021[51]). Questions also remain over the scalability of CDR. The potential trade-offs between the risks of employing CDR technologies – in particular BECCS – and the risks of triggering climate system tipping points if CDR technologies are not employed are poorly understood. Investments are needed to better understand these trade-offs and evaluate the risks associated with scaled-up use of CDR technologies or failure to employ them altogether.

Mounting evidence that climate tipping point thresholds may be crossed sooner and at lower warming levels than previously thought means that every effort must be made to avoid this scenario. This requires an immediate acceleration of climate mitigation efforts, as even the most optimistic projections of current policy commitments are insufficient. Focusing solely on the goal of net-zero emissions by 2050 is not enough. Even if net-zero emissions by are achieved by 2050, it is still possible that certain tipping point thresholds will be crossed this century, even more so if the target of 1.5°C warming is overshot. Drastic, transformational adaptation efforts must be considered to ensure the resilience of climate policies in the face of future disruptions.

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Notes

← 1. Climate or radiative forcing is the change in energy flux in the atmosphere caused by natural or anthropogenic factors of climate change, such as greenhouse gas emissions or increased water vapour. It is a direct measure of the amount that the Earth’s energy budget is out of balance due to external drivers of change.

← 2. The latest IPCC report does not provide temperature thresholds for tipping per tipping element, but summarises levels of change according to different levels of temperature increase. For example, the report estimates that it is likely that under temperature levels of 1.5°C, 2.0°C or 3.0°C relative to 1850–1900, the Atlantic Meridional Overturning Circulation (AMOC) will continue to weaken for several decades by about 15%, 20% and 30% of its strength. In addition, at sustained warming levels between 2°C and 3°C, there is evidence, albeit limited, that the Greenland and West Antarctic ice sheets will be lost almost completely and irreversibly over multiple millennia. The probability of their complete loss and the rate of mass loss increases with higher surface temperatures (high confidence) (Arias et al., 2021[19])

← 3. This sub-section draws partially on the results of the OECD Expert workshop on Economic Modelling of Climate and Related Tipping Points held under the Net Zero+ project on 18-19 October 2021: https://www.oecd.org/env/indicators-modelling-outlooks/Workshop-Tipping-Points-Summary-Report.pdf.

← 4. The social cost of carbon (SCC) is the central concept for the inclusion of climate change damages in the cost-benefit analysis of public policy and public investments. It measures the present value in monetary terms of the damages incurred when an additional tonne of carbon (or any other greenhouse gas) is released into the atmosphere.

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