Ed. note: Part 1 of this series is published on Resilience.org here.
The first part in this series looked at:
- Emissions trends,
- The 1.5°C target,
- Overshooting and cooling back to 1.5°C, and
- The likelihood of achieving the 2°C target.
This post looks at system stability at 2°C, warming at 3°C, and feedbacks and cascades.
5 2°C degrees is not a point of system stability
Yet it is a big mistake to think we can stabilise or “park” the Earth System at around 2°C and expect it to stay there, says Will Steffen. Earth’s climate history shows 2°C is not a point of system stability, but a signpost on a road to a hotter planet.
When projections in late 2021 showed future warming of around 2.7°C, Potsdam Institute Director Johan Rockström responded:
“I barely even want to talk about 2.7°C… If we go beyond 2°C, it’s very likely that we have caused so many tipping points that you have probably added another degree just through self-reinforcing changes. And that’s without even talking about extreme events.”
Similarly Hans Joachim Schellnhuber told an audience:
“If the [climate system] tipping elements interact and cascades develop, then the heating could become independent [i.e. self sustaining] at 2°C. Whether that is the case is perhaps the most important question of science right now because it would mean the end of our civilisation.”
Earlier, in a landmark paper, scientists had pointed 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” and concluded that “other tipping points could be triggered at low levels of global warming… a cluster of abrupt shifts between 1.5°C and 2°C…”
6 We are heading towards 3°C or more
With emissions peaking by 2030 only a possibility, and future emission reductions likely far from those needed to hold warming to 2°C, and with self-reinforcing warming processes already in play, it is clear the world is now heading towards 3°C, or perhaps more, warming. [Some models suggest a little less than 3°C, but these are not able to quantify all relevant mechanisms, feedbacks, cascades and non-linearities.]
In December 2022, in “World Scientists’ Warning of a Climate Emergency 2022”, a group of 12 scientists concluded that “current policies are taking the planet to around 3°C warming by 2100, a temperature level that Earth has not experienced over the past three million years. The consequences of global heating are becoming increasingly extreme, and outcomes such as global societal collapse are plausible and dangerously underexplored.” In November 2021, Nature journal reported on a survey which found that six in ten climate scientists expect the world to warm by at least 3°C by the end of the century.
US security analysts say 3°C could result in a world of “outright chaos”. Chatham House’s scenario in its Climate Change Risk Assessment 2021 had a mean warming of 2.7°C, and a plausible worst-case scenario of warming greater than 3.5°C. The assessment warned that the world is “dangerously off track” to meet the Paris Agreement goals, that the risks are compounding, and that “without immediate action the impacts will be devastating” in the coming decades, especially for food security.
The report concluded that impacts likely to be locked in for the period 2040–2050 unless emissions rapidly decline include a global average 30% drop in crop yields by 2050, and that more severe and extensive droughts will contribute to cascading climate impacts that will “drive political instability and greater national insecurity, and fuel regional and international conflict”.
7 System-level change and tipping points are happening faster than forecast
Major elements of Earth’s climate system are now increasingly influenced by self-reinforcing warming processes — or positive feedbacks — due to climate change caused by human greenhouse gas emissions. A “tipping point” or threshold exists where a 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. Tipping point thresholds for the Arctic, Greenland, West Antarctica and coral systems, and for land sinks such as eastern Amazonia, have been reached before or at the current level of warming of 1.2°C, with an overview of the key research in the 2022 Climate Dominoes survey of key systems.
In September 2022, McKay et al. concluded that even global warming of 1°C, a threshold that we already have passed, puts us at risk by triggering some tipping points. New evidence on tipping points presented in 2022 includes:
East Antarctica: Denman Glacier, in Australia’s Antarctic Territory, was identified in 2022 as susceptible to collapse of its ice shelf and inundation of the glacier itself, which sits on a retrograde (below sea level) base: “The Denman Glacier is potentially at risk of unstable retreat triggered by transport of warm water to the ice shelf cavity.” In 2020, the Washington Post had reported NASA scientist Virginia Brancato as saying: “If I have to look at East Antarctica as a whole, this [Denman Glacier] is the most vulnerable spot in the area.”
West Antarctica: On 13 December 2021, scientists announced that the Thwaites Glacier ice shelf in the Amundsen Sea area was fracturing and is likely to break apart in the next five years or so, resulting in a speeding up of the glacier’s flow and ice discharge, possibly heralding the collapse of the glacier itself, and triggering similar increases across the Amundsen Sea glaciers; “the final collapse of Thwaites Glacier’s last remaining ice shelf may be initiated … within as little as five years” (emphasis added). A 2022 study showed that the the Pope, Smith and Kohler glaciers in the Amundsen Sea embayment of West Antarctica have experienced enhanced ocean-induced ice-shelf melt, glacier acceleration, ice thinning and grounding-line retreat coincident with high melt rates of ungrounded ice in the past 30 years. The retreat rates are faster than anticipated by numerical models.
The State of the Cryosphere report, released in November 2022, concluded that more than four metres of additional sea level rise was locked in “with sections of the West Antarctic ice sheet potentially collapsing even without any further emissions over the coming centuries”. And in work just published, an ingenious look at the genetic history of Turquet’s octopus to establish when different populations were moving and mixing together across Antarctica in past warm periods led researchers to conclude that “even under global heating of 1.5°C – the most ambitious goal under the global Paris climate agreement – the West Antarctic Ice Sheet could be consigned to collapse”.
Greenland: In late 2022, scientists reported that Greenland Ice Sheet (GIS) glaciers are melting 100 times faster than previously calculated, according to a new model that takes into account the unique interaction between ice and water at the island’s fjords, whilst in August researchers showed that the Arctic has warmed nearly four times faster than the globe since 1979 and concluded it is likely climate models systematically tend to underestimate this amplification.
A number of feedback mechanisms are driving ice mass loss, including algal blooms darkening ice, the change in reflectivity resulting in greater heat absorption and enhanced melting of the surface ice by as much as 20%:
“Ice algae have started to colonise larger parts of Greenland. They’ve become an x-factor in the melting process,” says Prof. Jason Box. At the end of 2021, Box said that GIS has passed a tipping point/point of system viability: “Technically, now [at 1.2°C] Greenland is beyond its viability threshold… 1.5°C [of warming would] mean the ‘beyond the threshold’ state is enhanced and the loss [of ice mass] becomes a complex, non-linear, amplified response guaranteeing the ice sheet remains beyond its viability threshold. [We are documenting] several physical processes and amplifiers that guarantee more rapid response of the ice than is currently encoded in climate models that project sea-level rise… we cannot yet rely on ice sheets models for credible sea level projections.
Permafrost: Permafrost carbon emissions and the dangerous climate feedback loops they will set off are not accounted for in most Earth system models or Integrated Assessment Models, including those which informed the IPCC’s special report on global warming of 1.5°C, nor are they fully accounted for in global emissions budgets. If carbon-cycle feedbacks are accounted for, “such as tipping points in forest ecosystems and abrupt permafrost thaw, the estimated remaining budget could disappear altogether”.
Sea levels: Events at both poles are not properly incorporated into current climate models. The evidence suggests that sea-level rises this century will be greater than currently considered feasible by policymakers. Evidence from climate history suggests the current global average temperature increase is enough for 5–10 metres of sea-level rise in the longer term, inundating small island states, agriculturally rich alluvial deltas and vulnerable coastal cities.
Amazon: In ground-breaking research published in 2021, Katharyn Duffy and colleagues mapped the relationship between increasing temperatures and carbon uptake in Amazon forests by analyzing more than 20 years of data from 250 sites that measure the transfer of CO2 between plants, land and the atmosphere. They found that in recent hot periods the thermal maximum for photosynthesis had been exceeded. The land sink is now approaching a tipping point, and the sink could halve in as soon as two decades: “We show that the mean temperature of the warmest quarter (3-month period) passed the thermal maximum for photosynthesis during the past decade. At higher temperatures, respiration rates continue to rise in contrast to sharply declining rates of photosynthesis.” Under business-as-usual emissions, this divergence elicits a near halving of the land sink strength by as early as 2040.
This post looks at cascading risks, climate extremes and necessary actions.
8 Risks are cascading, and underestimated
Climate system feedbacks can drive abrupt, non-linear change that is difficult to model and forecast, with the Earth moving to dramatically different conditions. Such changes may be irreversible on relevant time frames, such as the span of a few human generations. Major tipping points are interrelated and may cascade, so that interactions between them lower the critical temperature thresholds at which each tipping point is passed.
Climate models do not yet incorporate key processes, and therefore are deficient, especially when projecting abrupt change, system cascades, and changes in the cryosphere and in the carbon cycle. Whether it be permafrost, Greenland or West Antarctica (and hence sea-level rises), the story is the same. Current climate models are not capturing all the risks, such as the stalling of the Gulf Stream, polar ice melt and the uptick in extreme weather events. Thus Earth system and Integrated Assessment Model projections, and their use in determining carbon budgets, are not reliable. It is important that observations, semi-empirical models, expert elicitations, and lessons from past climates are given more weight, given current model deficiencies.
In addition, the range of extremes being experienced today are greater than forecast in many instances. Prof. Andy Pitman, Director of the ARC Centre of Excellence for Climate Extremes notes that global mean warming is badly understood. He says as a general rule of thumb, global average warming of 4°C (covering land and ocean) is consistent with 6°C over land, and 8°C in the average warming over mid-latitude land. That risks 10°C in the summer average, or perhaps 12°C in heatwaves. Western Sydney has already reached 48°C. If you add 12°C to the 48°C you get summer heatwaves of 60°C.
It is well established that a tipping point may be abrupt and irreversible on relevant time frames, possibly leading to cascading events, even driving the system towards a “Hothouse Earth”. In 2022, Will Steffen explained that:
“The current trajectory is accelerating the [Earth] system towards [a point of] bifurcation, with the increasing risk that our pressures will push the system onto the ‘Hothouse Earth’ trajectory. The critical point here is that there is a point beyond which we lose control of the system and its own internal feedbacks drive it past a global threshold and irreversibly into a much hotter state… “
In 2022, Nico Wunderling et al. published “Global warming overshoots increase risks of climate tipping cascades in a network model”, which found that overshooting climate targets could significantly increase risk for tipping cascades. Wunderling explained: “Even if we would manage to limit global warming to 1.5°C after an overshoot of more than 2°C, this would not be enough as the risk of triggering one or more global tipping points would still be more than 50% percent. With more warming in the long-term, the risks increase dramatically.” Jonathan Donges added: “To effectively prevent all tipping risks, the global mean temperature increase would need to be limited to no more than one degree – we are currently already at about 1.2°C.”
Many cascades are well established, for example, that Arctic sea-ice loss driving enhanced Greenland deglaciation, which contributes to a slow of the AMOC, which in turn is decreasing rainfall over parts of the Amazon, and enhancing carbon losses. Now new research establishes a link between climate changes in the Amazon and the Tibetan Plateau. The researchers explain:
“Our research confirms that Earth system tipping elements are indeed inter-linked even over long distances, and the Amazon is one key example how this could play out… When it’s getting warmer in the Amazon, it also does so in Tibet, hence for temperature there’s a positive correlation.”
The importance of a paper published last year by ten authors, including Steffen, cannot be underestimated in bringing together a high-level analysis on climate risks and the need for climate research to focus on the worse-case, high-end possibilities. The paper is “Climate Endgame: Exploring catastrophic climate change scenarios” and its key findings include:
- “Prudent risk management requires consideration of the bad-to-worst-case scenarios” because low-probability, high-impact extreme outcomes have damages are so large, as to perhaps be unquantifiable. Large uncertainties about dangerous surprises “are reasons to prioritize rather than neglect them”.
- “Climate damages are likely to be nonlinear” and result in an even larger risk tail, with feedbacks in the carbon cycle and potential tipping points that could generate high greenhouse concentrations that are often missing from models. There are even more uncertain feedbacks, which, in a very worst case, might amplify to an irreversible transition into a “Hothouse Earth” state including “recent simulations suggest that stratocumulus cloud decks might abruptly be lost at CO₂ concentrations that could be approached by the end of the century, causing an additional ∼8 °C global warming. Large uncertainties about dangerous surprises are reasons to prioritize rather than neglect them.”
- Declining emissions does not rule out extreme climate change due to feedbacks in the carbon cycle and potential tipping points that could generate high greenhouse concentrations that are often missing from models. Examples include Arctic permafrost thawing that releases methane and CO2, carbon loss due to intense droughts and fires in the Amazon, and the apparent slowing of dampening feedbacks such as natural carbon sink capacity. These are likely to not be proportional to warming; instead, abrupt and/or irreversible changes may be triggered at a temperature threshold. Particularly worrying is a “tipping cascade” in which multiple tipping elements interact in such a way that tipping one threshold increases the likelihood of tipping another.
9 New climate extremes recorded in 2022
2022 was a big year for breaking-record, extreme climate events, including:
- Large parts of the northern hemisphere were exceptionally hot and dry. Record breaking heatwaves were observed in China, Europe, North and South America.
- Europe experienced its hottest summer ever recorded, with prolonged and intense heatwaves affecting western and northern Europe, and persistent low levels of rainfall leading to widespread drought conditions, and wildfires especially in Spain and Portugal.
- The United Kingdom saw off-the-charts temperatures shattered records kept nearly as far back as William Shakespeare’s time, with a new national record on 19 July when the temperature topped more than 40°C for the first time.
- In East Africa, rainfall has been below average in four consecutive wet seasons, the longest in 40 years.
- A large area around northern Argentina, southern Bolivia, central Chile, and most of Paraguay and Uruguay experienced record-breaking temperatures during two consecutive heatwaves in November–December 2022.
- Prolonged heatwave conditions affected Pakistan and northern India in spring. In May, temperature exceeded 50°C (122°F) in Jacobabad, Pakistan, and more than a billion people in South Asia endured several months of almost uninterrupted temperatures above 100°F.
- China had the most extensive and long-lasting heatwave since national records began and the second-driest summer on record. Heat waves and drought stretched over eight weeks and dried up parts of the Yangtze River to the lowest level since at least 1865.
- In July and August, Pakistan saw record-breaking rainfall leading to large-scale flooding over one-third of the country causing widespread destruction and loss of life, with at least 1700 deaths and 33 million people affected. 7.9 million people displaced.
- At Vostok station, in the interior of East Antarctica, the reported temperature reached -17.7°C, the warmest ever measured in its 65-year record.
- 2022 took an exceptionally heavy toll on glaciers in the European Alps, with initial indications of record-shattering melt.
- The Greenland ice sheet lost mass for the 26th consecutive year and it rained (rather than snowed) on the summit for the first time in September.
Sources for extremes are here and here.
10 Conclusion
In summary, emissions still have not peaked and are unlikely to be significantly lower in 2030 than 2020; warming of 1.5°C is likely this decade; the emissions trend and reduction commitments are currently nowhere near keeping warming to 2°C; and once the full range of feedbacks, non-linearities and cascades are taking into account, warming may well exceed 3°C this century, a level of warming that will likely result in climate-driven collapse of ecological and social systems. The contradiction is stark: the world will sail past 1.5°C, but 1.5°C may be enough to trigger ‘Hothouse Earth’ cascades; indeed, it is evident that some tipping points have already been passed, and some cascading events are occurring already.
So what to do? Last year, in the concluding section of Climate Dominoes, I wrote the following. It seems just as apt today:
Decarbonisation is not enough. Even sharp reductions in emissions will not be enough to avoid crossing the 1.5°C threshold, and very likely the 2°C threshold, given record-breaking use of fossil fuels. It is a big mistake to think we can “park” the Earth System at any given temperature rise – say 2°C – and expect it to stay there. 2°C may not be a point of system stability. Reducing the level of atmospheric CO₂ by carbon drawdown is vital, but the drawdown impact is relatively slow. The more damaging impacts, and risk of triggering non-linear events — associated with a higher level of warming for several decades in overshoot scenarios — are understated or ignored. The need to cool the planet in order to avoid cascade/collapse/”Hothouse” scenarios needs to be taken seriously. There are proposals for more direct cooling of threatened systems — as advocated, for example, by the Climate Crisis Advisory Group and the Cambridge Centre for Climate Repair for the Arctic with marine cloud brightening — or of the planet as a whole, whether by mirrors or sulfates. Whilst not yet proven to be of net benefit, and/or cost effective, such proposals seem vital if Earth is to be kept below a level of warming where more system tipping points are activated and cascade into an avalanche of warming and system feedbacks that human actions will no longer have the capacity to rein in.
