Professor Joeri Rogelj, Director of Research at the Grantham Institute – Climate Change and the Environment, argues that the risks of climate overshoot are dangerous and underappreciated. Image by WWF.
Climate action is far from what is required to keep global warming below the safety limits agreed under the Paris Agreement. Despite a recent wave of ambitious net-zero targets (1), implementation is lagging. Many long-term targets lack credible implementation plans (2).
Unless climate ambitions are strengthened and implemented, exceeding 1.5°C of global warming becomes an increasingly likely prospect (2, 3). All current evidence taken together, we are heading for warming well beyond 2°C by the end of the century with a central estimate of 2.4°C and a 1-in-10 chance that it exceeds 3°C (2).
The current failure to reduce emissions in line with 1.5°C has reinvigorated the idea of a temperature ‘overshoot’. The idea goes as follows: a temperature safety limit is temporarily exceeded before warming is returned below it at a later stage.
The sluggishness in the pace at which global emissions are reduced to net zero determines by how much the safety limit is exceeded. The net rate at which carbon dioxide can be removed from the atmosphere thereafter determines the length of the overshoot (4).
I argue that the ease with which this risky overshoot idea is being socialized is deeply concerning. Indeed, much of the overshoot discussion is characterized by stark overconfidence.
Here I look at three areas of overconfidence in the overshoot discourse: overconfidence in the geophysical characteristics of overshoot, overconfidence in effectively deploying technologies required to carry out an overshoot, and overconfidence in the resilience of institutions and societies in such a world. Let’s look at each of these in turn.
Professor Joeri Rogelj, photographed by Ivan Boll.
Physics don’t lie, when you dare to look them in the eye
From Earth and climate sciences, we understand the physics that tell us how to halt and potentially reverse warming. Warming is determined by the total cumulative amount of CO2 ever emitted into the atmosphere, and reaching net zero CO2 emission would halt further warming (5). Equally, achieving net negative CO2 emissions would result in a reduction in global temperatures (6).
These high-level insights, however, represent central estimates. When seeing overshoot pathways only through the narrow lens of central estimates, any interpretation of their implied risks and geophysical achievability becomes highly overconfident.
Large likelihood of larger warming
For example, the precise magnitude of how much the planet warms per ton of CO2 emitted is uncertain (6). This uncertainty means that even if we follow a future emissions path that the Intergovernmental Panel on Climate Change (IPCC) labels as delivering “no or limited overshoot” (7), unpleasant surprises are far from excluded. Pathways with “no or limited overshoot” are defined as exceeding 1.5°C by no more than 0.1°C (8). However, this is for a central (median) estimate of their warming outcome. They simultaneously imply about a 1-in-6 chance that maximum warming still reaches 2°C.
Even “no or limited overshoot” pathways can plausible lead to 0.5°C of excessive warming above 1.5°C instead of the anticipated limited exceedance by 0.1°C.
What does that mean for CO2 removal? Instead of requiring 220 billion tons of CO2 (GtCO2) to be effectively removed from the atmosphere, we’re now looking at a whopping 1100 GtCO2 to get back to 1.5°C.
Reversing warming might be very hard
Another dimension of geophysical overconfidence is the assumption that temperature will decline when CO2 is removed from the atmosphere (6). Also this is a central estimate. However, potential additional warming after net zero CO2 emissions are reached (9–11) mean this return to lower temperatures might well be much more challenging than currently imagined. Also here, there is a 1-in-6 chance that warming continues to increase beyond 1.5°C by more than 0.2°C after net zero CO2. That’s another additional commitment for CO2 removal – even to simply halt warming let alone reversing it.
Besides overconfidence in the geophysical characteristics of overshoot, there is also overconfidence in the delivery of CO2 removal technologies. Reversing warming requires the active large-scale removal of CO2 from the atmosphere.
Removing CO2 for overshoot is highly uncertain
CO2 removal can be broadly categorized in two groups: land-based removal and engineering removals. Land-based removals use nature to remove and store carbon, for example, through afforestation or reforestation. Engineering removals use a specific technology such as chemical filters to remove CO2 from the atmosphere and typically store it in geological reservoirs.
Current central estimates indicate that reversing 0.1°C of global warming requires 220 GtCO2 of net CO2 removal. This value, however, masks much larger amounts of gross CO2 removal, as not all emission sources can be avoided immediately (8).
This reliance on CO2 removal is highly at odds with current development, scale-up and reliability of available measures (12).
Land-based removals are subject to natural disturbances such as forest fires, which are projected to increase in a warmer world. The long-term reliability of natural systems as carbon stores is therefore in doubt and the confidence with which these removals are assumed in overshoot pathways is unwarranted (13).
Engineering removals have the potential to remove CO2 more permanently but are expensive, have historically failed to meet scale-up targets, and are not always accepted societally (12).
All CO2 removal methods have the potential to generate important sustainability trade-offs be it for biodiversity, food security or water security (13).
In short, at present all evidence indicates that the magnitude and plausibility of an effective contribution of CO2 removal to mitigation strategies is highly uncertain.
Overshoot pathways, however, assume the contrary.
What should people do during an overshoot?
A last dimension in which the societal conversation is overconfident in its discussion of overshoot is its resilience during the overshoot period. If extreme events, distributional impacts, and impacts on the overall economy are projected to disproportionally increase between 1.5°C and 2°C of warming (14), assuming society can pull off a massive turnaround and achieve global scale CO2 removal is very uncertain.
Overshooting key societal safety limits begs the question how society should plan for what happens during the overshoot period. A priori trusting that the world will manage to soon stabilise, and then reverse warming would be very naïve. The reason why overshoots are being discussed in the first place is because the world fails to act and achieve its objectives.
This means that adaptation planners will have to continue to entertain the possibility of ending up in a permanently warmer world.
A proposal for an honest way to speak about our situation
To conclude, the overconfidence with which the possibility of a temperature overshoot of 1.5°C is being discussed is deeply worrying. It hides behinds central estimates and turns a blind eye to the deep risks involved.
Instead of discussing an overshoot, I argue that it is more responsible to speak about peak and decline pathways. Such framing helps to formulate strategies that do not by design aim to miss the target they intend to reach, as is the case with overshoot pathways. Instead, peak and decline strategies set an explicit maximum warming level (and this might be higher than 1.5°C), focus on achieving net zero CO2 emissions consistent with a peak at this specified level, and work towards a sustainable level of net CO2 removal thereafter (4).
The consequences for the near term are clear: reach global net zero emissions as soon as possible in line with the peak warming target while adapting to risks than cannot be excluded, and prepare for long-term CO2 removal without that this distracts from the urgency of emission reductions now.
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