1.1 Introduction

In this section we scan different parts of the Earth system for evidence of tipping dynamics and assess whether they are likely to be tipping systems or not, providing confidence levels for each proposed tipping point and identifying knowledge gaps to be targeted with further research. We focus on the biophysical aspects of Earth system tipping points, with the societal impacts of these tipping points and adaptation to them explored in more depth in Section 2, and ways to govern the prevention of, and adaptation to, them examined in Section 3. We also consider how Earth system tipping points interact and potentially ‘cascade’ (where one tipping point triggers another, and so on) and assess the extent to which observations of these systems could give early warnings of impending tipping points.

Figure: 1.1.1
Figure 1.1.1: Illustration of the Earth system, showing the different ‘spheres’. The shown systems are a selected subset of the many components making up the Earth system.

The Earth system describes the interconnected complex system at the surface of the planet that sustains life (Figure 1.1.1). It is comprised of multiple subsystems (or spheres), including the cryosphere (ice-related systems, including ice sheets, sea ice, glaciers and permafrost), biosphere (global ecosystems), atmosphere, hydrosphere (water-based systems, including oceans, rivers and lakes) and the lithosphere (the Earth’s solid surface) (Kump, Kasting, and Crane, 1999; Lenton, 2016). Together these subsystems and their interactions – referred to by the IPCC as the ‘climate system’ – determine the climate (the average long-term weather conditions at a place or across the Earth, usually measured over 30 years) (IPCC AR6 WG1 Annex VII). 

At a smaller scale, ecosystems describe the complex systems composed of assemblages of living organisms and their physical environment in a particular location (e.g. a patch of rainforest in the Brazilian state of Amazonas), which at a larger scale form ecoregions, biomes and, ultimately, the whole global biosphere (Dinerstein et al., 2017; Keith et al., 2022). Humans, too, are a part of the biosphere, forming ‘socio-ecological systems’ in which social and ecological dynamics have been inextricably long intertwined (Folke et al., 2016, 2021; Ellis et al., 2021).

Evidence from modelling, observations, theory based on understanding of fundamental biophysical processes, and geological records of ancient climate change (referred to as palaeorecords) suggests some of the Earth’s systems can exhibit tipping points and associated dynamics (Lenton et al., 2008; Armstrong McKay et al., 2022; Wang et al., 2023). For example, there are multiple self-reinforcing feedback processes in ice sheets that not only amplify the effects of human-caused global warming, but may also lead to self-sustained melting beyond a critical warming threshold (Robinson et al., 2012; Garbe et al., 2020). Palaeorecords show that such collapses have happened before (Christ et al., 2021; Turney et al.,, 2020), and evidence from models and contemporary observations suggest some of these systems show increasing proximity to or may even be beyond tipping points (Feldmann and Levermann, 2015; Waibel et al., 2018; Rignot et al., 2014; Joughin et al., 2014; Boers and Rypdal, 2021). Similar evidence for tipping points and destabilisation exists for ocean currents – such as the Atlantic Meridional Overturning Circulation (AMOC) (Böhm et al., 2015; Boers, 2021; Ditlevsen and Ditlevsen, 2023) – and ecosystems (Scheffer et al., 2009; Staal et al., 2020; Boulton et al., 2022). Tipping is often relatively rapid and irreversible, and has far-reaching implications for the climate, ecosystems and humans.

In this section we use the following tipping point definition to categorise proposed tipping systems, with key terms (defined in the Glossary) italicised:


Our Earth system tipping point (ESTP) definition

Tipping points occur when change in a tipping system (also known as a tipping element) becomes self-sustaining once a forcing threshold is passed, leading to a qualitative state change (e.g. an ecological regime shift) driven by one or more positive/amplifying feedback loops

Climate tipping points, for example, occur when parts of the climate system reach global warming thresholds beyond which positive/amplifying feedbacks propel a shift to a totally different state, such as the inevitable collapse of an ice sheet or shutdown of a deep ocean convection site (Figure 1.1.2). Recent research suggests that five such tipping points may become likely beyond 1.5℃ warming, including Greenland and West Antarctic ice sheet collapse, warm-water coral reef die-offs, overturning circulation collapse in the North Atlantic Subpolar Gyre, and widespread localised abrupt thaw in permafrost (Armstrong McKay et al., 2022). Earth system tipping points can occur due to a wider set of environmental drivers, including for example deforestation or nutrient pollution, as well as climate change.

Figure: 1.1.2
Figure 1.1.2: Self-sustaining change due to strong positive/amplifying feedbacks. Left shows one exemplary positive/amplifying feedback loop, the melt-elevation feedback that exists, e.g. in ice sheets. An increase in local surface temperatures leads to increasing melt, such that the melting ice sheet gets to lower heights. Since it gets warmer with lower altitude (atmospheric lapse rate), the local surface temperature is increased, restarting this circle. Such positive feedbacks amplify the initial change – however, if there is a critical threshold, beyond which the amplification leads to a change that is as large as the initial change, this leads to a vicious circle, self-sustaining the change.

We consider potential Earth system tipping systems in Chapters 1.2, 1.3, and 1.4 based on the scientific literature. In the cryosphere chapter (Chapter 1.2) we assess the ice sheets on Greenland and Antarctica, as well as sea ice in the Arctic and Southern Oceans, glaciers outside of polar regions, and permafrost. In the biosphere chapter (Chapter 1.3), on land we consider forests in tropical, temperate and boreal zones, as well as savannas, drylands and freshwater systems (lakes and rivers), and in the ocean we consider coral reefs, coastal and open ocean ecosystems. In the ocean and atmosphere circulation chapter (Chapter 1.4), we assess circulation in the North Atlantic and Southern Oceans, as well as atmosphere systems including monsoons, climate oscillations like the El Niño Southern Oscillation (ENSO), mid-latitude weather patterns like jet stream changes, as well as climate sensitivity and circulation linked to tropical clouds.

In many cases, the consequences of passing one tipping point make other connected tipping systems more or less likely to tip as a result. If passing one tipping point makes another tipping point more likely, then tipping points could cascade, with a chain of tipping points triggering each other. In Chapter 1.5 we present what is known about tipping point interactions in the climate system, including between the AMOC and ice sheets, Amazon and Arctic sea ice and between ENSO and coral reefs, Amazon and the West Antarctic ice sheet, and present some palaeoclimate case studies.

It may sometimes be possible to detect tipping points before they happen. Theory suggests that before some types of tipping point, subtle changes may be observable in the statistical properties of monitoring data, known as early warning signals (EWS). The most common type of EWS is critical slowing down, where natural fluctuations in an observed property of a system (such as temperature or tree cover) become bigger and longer, leading to larger values of variability (e.g. in variance) or self-similarity to recent values (e.g. in autocorrelation). In Chapter 1.6 we present different techniques and some case studies of detecting EWS before Earth system tipping points, and discuss both their limitations and future opportunities.

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