1: Earth System Tipping Points

Section lead coordinating authors: David I. Armstrong McKay, Sina Loriani
Reviewers: John Dearing, Carl Folke, Matthew T. Huber, Charles Koven, Caroline Muller, Timothy Naish, Stefan Rahmstorf, Ashwin K. Seshadri, Simon Willcock

Key messages

We identify more than 25 parts of the Earth system that have tipping points, based on evidence from palaeoclimate records, observations, theory and complex computer models, including:
- In the cryosphere, evidence exists for large-scale tipping points in Greenland and Antarctic ice sheets, and for localised tipping in glaciers and permafrost.
- In the biosphere, tipping points are present in a variety of ecosystems, including Amazon forest dieback, savanna and dryland degradation, lake eutrophication, coral reef and mangrove die-offs, and the collapse of some fisheries.
- In ocean-atmosphere circulations, there is evidence for tipping points in Atlantic and Southern Oceans overturning, as well as for the West African monsoon.
Multiple drivers are destabilising these systems. Climate change is a key driver for most, as well as habitat loss (e.g. deforestation), exploitation (e.g. overfishing), and pollution (e.g. aerosols or nutrients) particularly in the biosphere.
Some Earth system tipping points could be very close already. Coral reefs and some ice sheets could tip at current warming levels, and other systems’ thresholds will soon be reached on current warming trends. Complex co-drivers, interactions, and feedbacks, as well as limited observational records, can make tipping thresholds difficult to assess for other systems, particularly in the biosphere.
Some climate tipping systems closely interact, and most interactions tend towards destabilising, making tipping ‘cascades’ possible. There are large uncertainties around these cascades, but warming is approaching levels where they are becoming possible.
‘Early warning signals’ can sometimes indicate that a system is losing resilience and so may be approaching a tipping point. Parts of the Greenland ice sheet, Atlantic Meridional Overturning Circulation, and the Amazon rainforest show such early warning signals, which is consistent with these systems approaching tipping points. However, these signals don’t show for certain if or when a tipping point will occur.

Recommendations

Prevent destabilisation of the Earth’s tipping systems through urgent and ambitious elimination of greenhouse gas emissions and reduction of other pressures such as deforestation, black carbon emissions and nutrient pollution.
Reduce deep uncertainties, for example related to key processes and feedbacks like marine ice cliff instabilities, ecosystem responses to increasing extreme events and fine-scale ocean mixing, through further research and model intercomparison. Co-design research, bringing together the natural and social sciences, scholars across the Global South and North and multiple knowledge systems including Indigenous and traditional ecological knowledge.
Improve risk assessments of potential tipping cascades through: i) representing more tipping system interactions in Earth system models, ii) large model ensembles to allow less common events to emerge, iii) studying possible cascades in ancient climate records, and iv) a fresh elicitation of expert knowledge to identify missed interactions.
Support development of novel and improved early warning techniques (such as using machine learning) to detect declining resilience and other potential signs of tipping. Expand remote sensing capabilities and palaeorecords to improve datasets for early warning detection. Foster international data sharing and collaboration, and improve observational coverage in under-monitored regions such as Africa and Asia.

Summary

Tipping points exist across the Earth system – the interconnected systems that support life on this planet, including the cryosphere (ice-bound domains), biosphere (the living world), ocean and atmosphere. It is often assumed that environmental systems respond relatively linearly to human-driven pressures (such as climate change, habitat destruction and pollution). However, in some systems, pressure beyond a threshold causes them to shift to a very different state, often abruptly or irreversibly, as a result of self-sustaining feedbacks – they pass a tipping point.

In this section we compile evidence for tipping dynamics across the Earth system, noting where certainty and confidence is low and more research is needed. We also review the potential for interactions between climate tipping points to trigger tipping cascades, and the scope for detecting early warning signals before tipping points in monitoring data.

There is evidence for tipping points across the Earth system, including in major ice sheets, which could lock in multiple metres of sea-level rise, in ecosystems like the Amazon rainforest, which could die back to a degraded state, and in major ocean circulation patterns, which could abruptly shut down. Monitoring and early warning signals suggest some of these systems are already destabilising, indicating that tipping points could be approaching. Interactions between climate tipping points are destabilising in most (but not all) cases, and could lead to tipping cascades that destabilise wider parts of the climate system.

However, how close some Earth system tipping points may be is uncertain, with threshold estimates often spanning a large range. Established models, though capturing past and current climate trends well, often have a limited resolution of some processes that are key for making more accurate tipping dynamics estimates. However, they do hold evidence for potential tipping, which is strongly supported by conceptual models and palaeo reconstructions, which show that certain systems likely tipped in the past.

Given that tipping is possible, and that human-driven emissions are rapidly pushing the Earth to a climate unseen in at least the past 120,000 years, this provides strong motivation for rapidly reducing human-driven pressures on the Earth system (see Sections 3 and 4). It lays the foundation for preparing adaptation plans for the societal impacts of Earth system tipping points that cannot be avoided (see Section 2). Even if some tipping points are reached, mitigation to prevent further tipping points remains critically important.