- The IPCC AR6 acknowledges that many human-caused changes, especially to the ocean, ice sheets and global sea level, are high risk and irreversible for centuries to millennia – some of them involving tipping processes (see Background) – and that these changes are key to a comprehensive risk assessment.
- Significant destabilization of several key climate tipping elements is already being observed today.
- In many cases, the dominant driver of this destabilization is global warming. But direct human influence on land cover change, such as degradation and active deforestation of the Amazon rainforest, can play an equal or even stronger role.
- Some tipping elements, for example melting ice sheets and changes to ocean currents, but also deforestation of rainforests, influence each other. Recent research indicates that interactions among tipping elements can ultimately cause shifts to happen at lower levels of global warming than anticipated.
The IPCC has, over the past two decades, continually strengthened their risk assessment concerning so-called “large scale singular events”. Stronger than before, the most recent report (IPCC AR6 WG I) recognizes risks from non-linear changes in tipping elements in the climate system (see Background) as well as irreversible long-term commitments, as possible outcomes of anthropogenic climate change and direct human pressure. Changes in tipping elements are particularly afflicted with high uncertainties (in terms of likelihood or timing, or both), but also associated with large risks for societies and ecosystems. These high-impact, high uncertainty risks are termed “low-likelihood, high impact outcomes” in IPCC AR6 – even if, as the authors explicitly state, probabilities are not necessarily known to be low, but simply not well constrained. The consequences are large, long term and associated with existential risks for nature and societies.
Among these risks associated with selected tipping elements, as discussed in the Spotlight, are
- from an increased disintegration of the ice sheets: a catastrophic 2 metres of sea level rise by 2100 and even more devastating 5 metres by 2150 (which are explicitly not ruled out by the IPCC AR6 WGI, 22.214.171.124). In the long term, we are committed to a similar degree of sea-level rise, for instance of 2-6 metres in the two millenia following peak 2°C warming (IPCC AR6 WGI, 126.96.36.199 and Cross chapter Box 12.1);
- from a slowdown or shutdown of the major Atlantic Ocean circulation: abrupt changes in weather patterns, for example in Europe, which are currently highly dependent on the inflow of heat from the Atlantic Ocean circulation;
- from a shift in the Atlantic circulation: shifts of water cycles affecting several monsoon systems, and specifically a heat redistribution that alters precipitation patterns over the Amazon, although it remains unclear whether overall rainfall will be increased or reduced;
- from a climate regime shift in the Amazon: a feedback to regional climate conditions (loss of rainforest vegetation changes the moisture exchange with the air, for example), and a potential permanent loss of the Amazon basin as a major carbon sink.
A number of recently published research results confirm and refine the IPCC’s assessment, and furthermore indicate that several climate subsystems are already showing signs of losing stability today and are moving towards critical thresholds (tipping points, see Spotlight).
In addition to the risks from individual tipping processes, science has identified an overarching, additional layer of risk: tipping elements may be involved in domino-type cascades – with one tipping process (for example Greenland ice loss) triggering another one (in that case a slowdown of the major Atlantic Ocean circulation due to meltwater intrusion). Therefore, in addition to the risk from each tipping element alone, their interaction exacerbates the situation and compounds the overall risk – tipping could happen more easily (at lower global temperatures) than if they were separated.
The Greenland Ice Sheet is losing mass at accelerating rates, due to meltwater runoff and ice discharge at outlet glaciers. Surface melt will continue to increase with further atmospheric warming. While ice discharge is 14% greater now than during 1985-1999, the reasons for this increase differ from region to region, making it difficult to project future developments.
Observations and modelling have shown that there are several processes that could lead to self-enforcing ice loss and sea-level rise, once a weakening of the ice and/or a retreat of the line where the ice starts to float is initiated. In the past, meltwater from the Greenland Ice Sheet has raised global mean sea level, directly influencing Antarctic Ice Sheet retreat.
Atlantic Ocean circulation
Studies of prehistoric climate, combined with modern sea level and salinity observations, show that the main Atlantic Ocean circulation system has weakened significantly in the past decades and is at its weakest in at least a millennium. Recent statistical analyses of sea-surface temperature and salinity observations give rise to the concern that this decline is indeed a sign of an ongoing loss of stability of the circulation, rather than just a temporary weakening.
The Amazon rainforest is the world’s largest tropical rainforest and plays an important role in global carbon budget and water circulation. The south-eastern part of the Amazon basin has turned into a net source of carbon to the atmosphere, not even taking the effect of fires into account (Gatti et al., 2021; see more details in Insight 8). Observations show that changes in rainfall do alter vegetation types within the Amazon – but climate change alone will most likely not cause a basin-wide dieback. But in combination with human-caused forest degradation (at 17% of the Amazon basin, higher than previously estimated), and deforestation (at 18% already), climate change could trigger climate regime shifts in parts of the Amazon rainforests.
Global warming and direct human pressure impose significant changes to key components of the climate system. Next to gradual changes that are proportional to the level of global warming, some components face the risk of undergoing dramatic and non-linear transitions at varying timescales, often without a chance to turn back to normal for a long time. These parts of the climate system are called tipping elements.
The actual transition can unfold over centuries to millennia (when ice sheets melt or disintegrate), over decades to centuries (when ocean currents slow down or reshape) or years to decades (especially when direct human interference pushes a transition, like deforestation in the Amazon rainforest).
The challenge with tipping elements is that we don’t know exactly at what levels of global warming thresholds will be passed, given the complex nature of interactions. The danger is that once the thresholds are crossed there may be no realistic turning back. Even if levels of global temperature are brought back down again, self-reinforcing effects can drive further ice loss, forest die-back or other shifts, until the system can no longer be recognized.
Implications & Recommendations
Climate negotiators and decision makers on all levels – international, national and local, need to:
- be aware of high-impact risks from climate tipping elements, and incorporate the remaining uncertainties about likelihood and timing in their risk assessments;
- apply the precautionary principle and aim for more ambitious climate protection, rather than the reverse. This is analogous to the approach to other societal risks, like that of a catastrophic nuclear accident, where large safety margins are prudent.
At a local level, where direct human influence has the potential to initiate tipping processes, for instance in parts of the Amazon rainforest, actors must take all possible measures to avoid tipping risks.