Second in a 3-part series | Part 1 | Part 3
by David Spratt
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.
.... to be concluded.
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