A New Analysis of Tipping Points

A new meta-analysis of tipping points[1] was just published that evaluates the 10 most significant tipping elements, and for each tipping element, the study assesses how well we understand the processes involved, the associated time scales, and how large the climate impact may be. The paper is currently behind a paywall, and while the authors have said they're working on making the paper open access, you can still view a prepub version of the paper[2] on line for free.

The concept of a tipping point can be easily misunderstood. It's often described (at least in climate discussions) as a point beyond which catastrophic, runaway global warming occurs, taking global climate irreversibly into an inhospitable state (sometimes compared to runaway warming on Venus). This paper defines "tipping elements" and "tipping points" consistent with a paper by Kopp et al 2016. For this paper, "'tipping elements'...refer to any systems capable of committed nonlinear shifts between states—rapid or gradual—resulting from small changes in forcing. Such committed changes are irreversible on non-geologic timescales, even following subsequent reduction or alleviation in the magnitude of the forcing.... The term 'tipping points' would then refer strictly to critical thresholds beyond which tipping elements undergo more rapid 'Gladwellian' state shifts, with little temporal lag separating commitment and realization." Systems with thresholds that are not "Gladwellian" (that do not have small temporal lags separating commitment and realization), would not have "tipping points" by this definition.

In simple terms, a tipping point simply refers to the point beyond which a set of physical processes destabilize such that the system must change rapidly to some new stable equilibrium point. If you think of a weighted ball at the center of a seesaw, the system is stable. If  you push that ball a little in either direction, the weight of the ball will push down on the seesaw, and gravity will ensure that the ball will not stop until it rolls off the edge of the seesaw. In climate, there are many tipping elements that, if a tipping point is exceeded, those processes will irreversibly shift to a new equilibrium position.  That new position may or may not have a large impact on the climate system as a whole, so each system needs to be studied individually for its impact on the climate system. This paper examines ten of the most significant tipping elements:

1. Atlantic Meridional Overturning Circulation (AMOC) - whether it slows or stops
2. Methane release from Marine methane hydrate deposits 
3. Ice mass loss from polar ice sheets (Greenland & Antarctica) and associated SLR
4. Carbon release from permafrost soils 
5. Boreal forests 
6. Tropical seasonal monsoons
7. Stratocumulus cloud decks 
8. Tropical coral reefs 
9. Amazon rainforest 
10. Diminishing arctic sea ice 

These "tipping elements" should be thought of a feature of the climate system that has the potential to destabilize beyond a tipping point. For each of these tipping elements, the paper examines what is influencing the tipping element and how likely it is to exceed a tipping point with uncertainties and future projections.

There is significant variability in the severity of impact, the time scale for impacts to manifest and the uncertainties associated with the earth system components. There is a significant near-term risk of a die off of shallow tropical stony corals, a prevalence of sea-ice free Arctic summers, and a shift from rainforest to savanna in the Amazon. Long-term there is significant risk associated with shifts in boreal forests and carbon release from decomposing permafrost and the collapse of the West Antarctic Ice Sheet, though risk of a long-term collapse of the East Antarctic ice sheet is limited to high emissions scenarios following 2200 or so.  And while there is a significant probability of AMOC slowing down (it has likely already begun), there is a large degree of uncertainty about a full collapse of AMOC. There are also large uncertainties regarding carbon releases from marine methane hydrates and tropical monsoons.

Above I described a "tipping point" with the analogy of a weighted ball centered on a seesaw - nudging the ball creates a scenario that rapidly results in the ball rolling off the seesaw entirely. If climate tipping points are severe enough, perhaps these tipping points would combine to produce sudden and severe irreversible impacts - the authors of the paper describe this as "cliff-like" impacts. But if they are not as severe, they may be analogous what the authors say is more like a steep "slope." It's hard but not impossible to climb back up the slope. The authors conclude that tipping elements are more like a steep slope but not like a cliff. They are serious and may well exacerbate warming, but they are not so severe that at some point mitigation will do no good.

We can summarize the major conclusions of this paper with:

1. Reductions of our carbon emissions improve the likelihood that we will avoid risks associated with crossing many of these tipping points, especially with regards to permafrost soils, accelerating SLR, and shifts in boreal forests. 
2. Some tipping elements may have significant changes even if warming levels remain low, especially with reductions in summer Arctic sea ice, tropical coral reefs, and the Amazon rainforest.
3. Some tipping elements have a low to no risk of crossing a tipping point. For instance, summer Arctic sea ice likely scales linearly with Arctic temperatures.
4. Some of these elements may be influenced by other human activities beyond AGW. For instance, The Amazon rainforest is affected by deforestation as well as by climate-induced droughts).
5. The time scales associated with these tipping elements vary significantly, and some impacts will only be felt on century to longer time scales. The have impacts now, but the effects will be long-lasting.
6. Shifts in the climate system from these tipping elements are not fully accounted for in models and are meaningful feedbacks that may show that warming may exceed model projections. High latitude impacts from permafrost thaw and boreal forest shifts are positive feedbacks that may produce more warming than model projections.
7. However, while climate shifts from these tipping elements are too small to put us a risk for runaway global warming. This is good news. It means that we can still be successful in avoiding the worst impacts of AGW if we eliminate our carbon emissions.

The chief take away for me is that while tipping points are a serious risk that should be avoided, they do not have the potential to make catastrophic, runaway warming inevitable. In fact, runaway warming is not likely at all in any scenario, and mitigation of our carbon emissions can always be helpful in limiting our risk of future negative impacts of AGW. So while this paper may add a note of seriousness to the need to mitigate, it does not at all mean that at some point mitigation will do no good. We can always limit future warming with a rapid reduction of our carbon emissions.

References:

[1] Wang, S., Foster, A., Lenz, E. A., Kessler, J. D., Stroeve, J. C., Anderson, L. O., et al. (2023). Mechanisms and impacts of Earth system tipping elements. Reviews of Geophysics, 61, e2021RG000757. https://doi.org/10.1029/2021RG000757

[2] Prepublished version is here. https://www.authorea.com/doi/full/10.1002/essoar.10507834.1