by David Spratt
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| Image: MaxPixel |
Reflecting on the evidence presented in this tipping point series, a number of conclusions may be drawn:
- At just 1.2°C of warming, tipping points have been passed for several large Earth systems. At just 1.2°C of global average warming, tipping points have been passed for several large Earth systems. These include Arctic sea ice, the Greenland Ice Sheet, The Amundsen Sea glaciers in West Antarctica, the eastern Amazonian rainforest, and the world’s coral systems. The world will warm to 1.5°C by around 2030, with additional warming well beyond 1.5°C in the system after that. Yet even at the current level of warming, these systems will continue to move to qualitatively different states. In most cases, strong positive feedbacks are driving abrupt change. At higher levels of warming, the rate of change will quicken. The meme that “we have eight years to avoid 1.5°C and tipping points” should be deleted from the climate advocacy vocabulary. It is simply wrong.
- System-level change is happening faster than forecast. In each case surveyed above, abrupt change is happening earlier and/or faster than projected only two decades ago. The 2007 Arctic sea-ice collapse was “100 years ahead of schedule”; in 2014 the tipping point for Amundsen Basin glaciers was one that “none of us thought would pass so quickly”. It was said that the guardrail for coral reefs was warming under 2°C, then 1.5°C; it is now clear that it is under 0.5°C. In 1995, the IPCC projected “little change in the extent of the Greenland and Antarctic ice sheets… over the next 50-100 years”. The 2001 IPCC report suggested that the Greenland and the West Antarctic ice sheets would not lose significant mass by 2100. Both have now passed their tipping points. The effect of the permafrost carbon feedback has not been included in the IPCC scenarios, including the 2014 report. And on it goes.
- Climate models don’t incorporate key processes and are not reliable. 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, including the stalling of the Gulf Stream, the melting of polar ice and the uptick in extreme weather events. One example is the failure to predict the intensity of extreme heat and flood events in Europe and North America in 2021. Jason Box has described in detail the feedbacks on Greenland not represented in models. Thus Earth system and Integrated Assessment Model projections, and their use in determining carbon budgets, are not to be relied upon. It is important that observations, semi-empirical models, expert elicitations, and lessons from past climates are given more weight, given current model deficiencies.
- The whole Earth climate system is undergoing abrupt change. It is not just that individual elements of the climate system are tipping and/or changing rapidly. Abrupt change is cascading. The Arctic is warming four times faster than the global average, summer sea-ice is diminishing fast, adding a further forcing to Greenland’s instability. It is inconceivable that Greenland could remain stable in Arctic summer blue-water conditions. Greenland melt-water injection into the north Atlantic is one factor slowing the AMOC, which in turn is negatively affecting precipitation over Amazonia and impacting the diminution of its carbon stores. Arctic warming is destabilising the polar Jet Stream into a more "wavy" pattern, bringing record-breaking heat into the Arctic and triggering huge fires, with impacts on the mobilisation of permafrost carbon stores; the Jet Steam destabilisation combined with global warming is also bringing new extremes over Europe, Asia and North America, including unprecedented rains and floods.
Tipping points and potential cascade effects - Cascades and accelerated warming may trigger Hothouse conditions. These chains of events are consistent with the cascade of system changes that may drive Earth past the “Hothouse Earth” threshold. This is not to say that this scenario is already locked into the system, but scientists have warned that it may become active in the 1.5–2°C threshold, and that is where we are heading now, likely at an accelerated rate of warming over the next two decades. Current models, reported in the 2021 IPCC report, show around 0.3°C warming between 2020 and 2030. The next 25 years are projected to warm at a rate of 0.25–0.35°C per decade; and there are warnings that the rate of global warming over the next 25 years could be double what it was in the previous 50 years. Even with greater emission-reduction ambition than at Glasgow, warming will likely exceed 2°C sometime in the 2040s. Short-lived atmospheric sulfate aerosols are a by-product of burning fossil fuel and have a cooling effect which has been hiding up to 1°C of global warming. Reduced emissions of greenhouse gases (and clean air policies) will also reduce this aerosol cooling, so there is little prospect that decarbonisation policies will significantly bend down the temperature curve over the next two decades. This appears poorly appreciated by many climate action advocates whose strategy is built around decarbonisation only, rather than the “three levers” of decarbonisation, drawdown and active cooling.
- Risks have been underestimated. Because abrupt system change is happening faster than forecast, we are ill-prepared for what may happen. Ice-sheet loss and rising sea levels are a delayed response to warming. Australian scientists from the University of New South Wales report that: “An equilibrium climate under current temperatures would have a sea level several metres higher than what we have today (likely 5–10 metres higher). We also know that an equilibrium climate under current carbon dioxide (CO2) concentrations would have a sea level 5–25 metres higher” (emphasis added). Is this widely understood? Due to model limitations, we will not know exactly how the climate crisis will unfold until it’s too late. Is there a risk that the Hothouse scenario has already been initiated? Scientists have answered “yes”, thus: “We might already have lost control of whether tipping happens” (emphasis added). What actions would be required to mitigate this outcome, or the risk it may soon be triggered, depending on human actions over the next decade? These questions are barely being asked, let alone answered, yet the consequences of not having a sound response may be existential for human civilisation.
- Decarbonisation is not enough. Even sharp reductions in emissions will not be enough to avoid crossing the 1.5°C threshold, and likely the 2°C threshold, given record-breaking use of fossil fuels in 2021 and the forecasts for 2022-24. 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.
This is the concluding part of this 7-part series.
