Spratt on tipping points

Have tipping points already been passed for critical climate systems? (1) The basics. David Spratt, Climate Code Red. Jan. 18, 2022.

Tipping points and potential cascade effects

First in a series.

As global heating reduces the extent of floating Arctic sea-ice each summer, the heat-reflecting ice is replaced by heat-absorbing dark ocean water, adding energy to the Arctic system, driving more melting. This is a “positive feedback”, a self-reinforcing change. Examples abound in the climate system. On Greenland, for example, warming is reducing the height of the ice, and this lower elevation means it will melt more, because the temperature is higher at lower altitudes.

Sixteen years ago, James Hansen warned that “We live on a planet whose climate is dominated by positive feedbacks, which are capable of taking us to dramatically different conditions. The problem that we face now is that many feedbacks that came into play slowly in the past, driven by slowly changing forcings, will come into play rapidly now, at the pace of our human-made forcings, tempered a few decades by the oceans thermal response time."

Those feedbacks can drive non-linear (or abrupt) change that is difficult to forecast. That happened to Arctic sea-ice in the summer of 2007, when a collapse in the ice extent led one experienced glaciologist to exclaim that it was melting “100 years ahead of schedule”; actually, the scientific understanding was 100 years behind reality! The same thing is happening in Antarctica now, according to the new observations of the Thwaites Glacier.

At a glance...

Positive climate feedback: a process whereby an initial change in the climate system, for example warming generated by a climate forcing, causes a secondary change which in turn magnifies the initial effect and becomes self-reinforcing. 

Tipping point: A threshold at which a small change causes a larger, more critical change to be initiated, taking the climate system from one state to a discreetly different state. The change may be abrupt and irreversible on relevant time frames, possibly leading to cascading events.

Non-linear/abrupt change: Sudden change rather than smooth progress, often associated with a tipping point.

Cascading events: An unforeseen chain of events which may occur when one event in a system has a negative effect on other related components. For example, the mutual interaction of individual climate tipping points and/or abrupt, non-linear changes, which may lead to more profound changes to the system as a whole. 

Hothouse Earth: A planetary threshold in which a cascade of system changes makes warming self-sustaining, leading to conditions hotter than any experienced over the last few hundred thousand years by modern humans. 

Climate forcing: A physical process affecting the Earth’s climate through one or more forcing factors. For example at present, a perturbation (change) in the Earth’s energy system resulting from an imbalance between incoming and outgoing radiation caused by many factors but principally an increase in the “blanket” of human-caused greenhouse gases, with associated warming.

A group of eminent scientists point to “biosphere tipping points which can trigger abrupt carbon release back to the atmosphere… permafrost across the Arctic is beginning to irreversibly thaw and release carbon dioxide and methane… the boreal forest in the subarctic is increasingly vulnerable”. They say that other tipping points could be triggered at low levels of global warming with “a cluster of abrupt shifts between 1.5 °C and 2 °C...”

Feedbacks, with or without abrupt change, can drive a system past its tipping point, which is a critical threshold at which small change causes a larger, more critical change to be initiated, taking components of the Earth system from one state to a discreetly different state. In other words, the system has reached a point of fragility such that it will move to a different state due to its own internal dynamics, even if there is no further external forcing (such as additional warming).

An overview from Australia’s Centre of Excellence for Climate Extremes describes a number of key aspects of tipping points:

• The implications of tipping points are not thoroughly quantified in the major IPCC analyses.
• Some tipping point changes are irreversible on timescales of centuries to millennia.
• We do not know exactly how close we are to a tipping point, or even whether we have already passed it. We also do not always know if the changes are reversible, and if so, on what timescales.
• There are tipping points that while not yet triggered may already be fully committed to. For example, the warming required for the West Antarctic Ice Sheet to permanently melt might have already been reached.
• Climate models lack the mechanisms to robustly simulate many tipping points, and the interactions between tipping points that could lead to cascading impacts, and therefore our understanding of the risks is limited.
• Since the risk is hard to quantify, global negotiations around climate change have not appropriately taken into account the risks of initiating tipping points, which is essentially a gamble on the future of the Earth’s climate.

Tipping may be irreversible on relevant time frames, such as the span of a few human generations. For example, ice sheets can disintegrate abruptly — and drive up sea levels — much faster than they can gain mass. So whilst sea levels could rise two or three metres this century — and rates as high as five metres per century have been recorded in the past — it could take thousands of years to reset the ice and get sea levels back down because there are not abrupt negative feedbacks in play.

This is an example of hysteresis, or bifurcation of a system, where it may be more difficult, or impossible, to return to its previous state. Extinctions are an example of the latter. Carbon Brief explains: “In some cases, there is evidence that once the system has jumped to a different state, then if you remove the climate forcing, the climate system doesn’t just jump back to the original state – it stays in its changed state for some considerable time, or possibly even permanently.”

Major tipping points are interrelated and may cascade, as illustrated. Interactions between these climate systems could lower the critical temperature thresholds at which each tipping point is passed.

For example, Earth is approaching a temperature range above which photosynthesis rates decline and the storage of carbon in the terrestrial biosphere (the “land sink”) falls. This tipping point lies within the next 20–30 years and on a high-emissions trajectory a near halving of the land sink strength may result as early as 2040. This will accelerate the warming rate, trigger further sea-ice loss, more melting on Greenland and freshwater injection into the North Atlantic, helping to further slow the Atlantic Meridional Overturning Circulation (AMOC), often known as the “Gulf Stream”. This in turn would change rainfall patterns over the Amazon and further weaken its carbon stores and Earth’s land sink. And so it goes on.

Physical interactions among the Greenland and West Antarctic ice sheets, AMOC and the Amazon rainforest tend to destabilise the network of tipping elements. The polar sheets are often the initiators of these cascade events, with evidence that Greenland and West Antarctica have passed their tipping (in coming posts).

In 2012, James Hansen warned of scientists’ fear about the Arctic and the cascading of tipping points triggered in the Arctic: “Our greatest concern is that loss of Arctic sea ice creates a grave threat of passing two other tipping points – the potential instability of the Greenland ice sheet and methane hydrates… These latter two tipping points would have consequences that are practically irreversible on time scales of relevance to humanity.”

Cascading events may in turn lead to a “Hothouse Earth” scenario, in which climate system feedbacks and their mutual interaction drive the Earth System climate to a “point of no return”, whereby further warming would become self-sustaining (that is, without further human-caused perturbations). This planetary threshold could exist at a temperature rise as low as 2°C, possibly even in the 1.5°C–2°C range.

The problem, elaborated in a 2019 paper, "Climate tipping points — too risky to bet against", is that time is close to running out: "We argue that the intervention time left to prevent tipping could already have shrunk towards zero, whereas the reaction time to achieve net zero emissions is 30 years at best. Hence we might already have lost control of whether tipping happens. A saving grace is that the rate at which damage accumulates from tipping — and hence the risk posed — could still be under our control to some extent” (emphasis added).

Likewise, former UK Chief Scientist Sir David King warns that: “What global leaders do in the next three-to-five years will determine the future of humanity.”

Tipping point analyst Prof. Tim Lenton says that the evidence from tipping points alone “suggests that we are in a state of planetary emergency: both the risk and urgency of the situation are acute… If damaging tipping cascades can occur and a global tipping point cannot be ruled out, then this is an existential threat to civilization”.


Next post in this series: (2) West Antarctica: The “doomsday” glacier