The term "tipping point" commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term "tipping element" to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.

Abstract. With progressing global warming, there is an increased risk that one or several tipping elements in the climate system might cross a critical threshold, resulting in severe consequences for the global climate, ecosystems and human societies. While the underlying processes are fairly well-understood, it is unclear how their interactions might impact the overall stability of the Earth's climate system. As of yet, this cannot be fully analysed with state-of-the-art Earth system models due to computational constraints as well as some missing and uncertain process representations of certain tipping elements. Here, we explicitly study the effects of known physical interactions among the Greenland and West Antarctic ice sheets, the Atlantic Meridional Overturning Circulation (AMOC) and the Amazon rainforest using a conceptual network approach. We analyse the risk of domino effects being triggered by each of the individual tipping elements under global warming in equilibrium experiments. In these experiments, we propagate the uncertainties in critical temperature thresholds, interaction strengths and interaction structure via large ensembles of simulations in a Monte Carlo approach. Overall, we find that the interactions tend to destabilise the network of tipping elements. Furthermore, our analysis reveals the qualitative role of each of the four tipping elements within the network, showing that the polar ice sheets on Greenland and West Antarctica are oftentimes the initiators of tipping cascades, while the AMOC acts as a mediator transmitting cascades. This indicates that the ice sheets, which are already at risk of transgressing their temperature thresholds within the Paris range of 1.5 to 2 ∘C, are of particular importance for the stability of the climate system as a whole.

The risk of points-of-no-return, which, once surpassed lock the world into new dynamics, have been discussed for decades. Recently, there have been warnings that some of these tipping points are coming closer and are too dangerous to be disregarded. In this paper we report that in the ESCIMO climate model the world is already past a point-of-no-return for global warming. In ESCIMO we observe self-sustained melting of the permafrost for hundreds of years, even if global society stops all emissions of man-made GHGs immediately. We encourage other model builders to explore our discovery in their (bigger) models, and report on their findings. The melting (in ESCIMO) is the result of a continuing self-sustained rise in the global temperature. This warming is the combined effect of three physical processes: (1) declining surface albedo (driven by melting of the Arctic ice cover), (2) increasing amounts of water vapour in the atmosphere (driven by higher temperatures), and (3) changes in the concentrations of the GHG in the atmosphere (driven by the absorption of CO₂ in biomass and oceans, and emission of carbon (CH₄ and CO₂) from melting permafrost). This self-sustained, in the sense of no further GHG emissions, melting process (in ESCIMO) is a causally determined, physical process that evolves over time. It starts with the man-made warming up to the 1950s, leading to a rise in the amount of water vapour in the atmosphere-further lifting the temperature, causing increasing release of carbon from melting permafrost, and simultaneously a decline in the surface albedo as the ice and snow covers melts. To stop the self-sustained warming in ESCIMO, enormous amounts of CO₂ have to be extracted from the atmosphere.

In climate science, a tipping point is a critical threshold that, when crossed, leads to large, accelerating and often irreversible changes in the climate In climate science, a tipping point is a critical threshold that, when crossed, leads to large, accelerating and often irreversible changes in the climate system. If tipping points are crossed, they are likely to have severe impacts on human society and may accelerate global warming. Tipping behavior is found across the climate system, for example in ice sheets, mountain glaciers, circulation patt In… The IPCC Sixth Assessment Report defines a tipping point as a "critical threshold beyond which a system reorganizes, often abruptly and/or irreversibly". It can be brought about by a small disturbance causing a disproportionately large change in the system. It can also be associated with self-reinforcing feedbacks, which could lead to changes in the climate system irreversible on a human timescale. For any particular climate component, the shift from one state to a new stable state may take many decades or centuries. The 2019 IPCC Special Report on the Ocean and Cryosphere in a Changing Climate defines a tipping point as: "A level of change in system properties beyond which a system reorganises, often in a non-linear manner, and does not return to the initial state even if the drivers of the change are abated. For the climate system, the term refers to a critical threshold at which global or regional climate changes from one stable state to another stable state." In ecosystems and in social systems, a tipping point can trigger a regime shift, a major systems reorganisation into a new stable state. Such regime shifts need not be harmful. In the context of the climate crisis, the tipping point metaphor is sometimes used in a positive sense, such as to refer to shifts in public opinion in favor of action to mitigate climate change, or the potential for minor policy changes to rapidly accelerate the transition to a green economy. The West Antarctic Ice Sheet (WAIS) is a large ice sheet in Antarctica; in places more than 4 kilometres (2.5 mi) thick. It sits on bedrock mostly below sea level, having formed a deep subglacial basin due to the weight of the ice sheet over millions of years. As such, it is in contact with the heat from the ocean which makes it vulnerable to fast and irreversible ice loss. A tipping point could be reached once the WAIS's grounding lines (the point at which ice no longer sits on rock and becomes floating ice shelves) retreat behind the edge of the subglacial basin, resulting in self-sustaining retreat in to the deeper basin - a process known as the Marine Ice Sheet Instability (MISI). Thinning and collapse of the WAIS's ice shelves is… The…