One broad framing of geoengineering is that it could be used to try and avoid a ‘climate emergency’. Several types of ‘climate emergency’ have been implied in the literature, ranging from abrupt, non-linear changes in the climate system itself – referred to here as ‘climate tipping points’ – to sudden changes in social systems that might be triggered even by gradual climate change – referred to here as ‘impacts tipping points’.
In particular, it has been argued that potent methods of sunlight reflection, such as stratospheric sulphate aerosol injection, might be deployed in emergency if a part of the Earth system is seen to be approaching, or to have passed, a climate tipping point. For example, a recent report from the Bipartisan Policy Center considers “most climate remediation concepts” to be “inappropriate to pursue except as complementary or emergency measures—for example, if the climate system reaches a “tipping point” and swift remedial action is required.” This begs the question: by the time you realize you have reached a tipping point, can you actually go back?
The Royal Society offer a subtly different framing of “Solar Radiation Management techniques” that “because they act quickly, they could be useful in an emergency, for example to avoid reaching a climate ‘tipping point’.” This begs a different question: how do you know you are approaching a tipping point? And if an early warning is possible: can you find out early enough and act fast enough to avoid reaching a tipping point?
The fundamental problems with the ‘emergency-use’ framing of geoengineering are that the parts of the climate system which may pass a tipping point are lagging behind anthropogenic forcing, and that passing a tipping point can lead to irreversible change. So, by the time you detect that abrupt change is either imminent or underway, the tipping point may long since have been passed and the change simply cannot be reversed.
To illustrate irreversibility, consider the special case of a tipping point that is a ‘fold’ bifurcation in the equilibrium solutions of a system (Figure 1a). This could be the Atlantic ocean’s overturning circulation or a large ice sheet such as on Greenland. When it reaches a tipping point, the current state of the system loses its stability and it undergoes an abrupt transition to an alternative state. The timescale of this transition is set by the internal dynamics of the system in question and can range from years (e.g. past abrupt warming events linked to reorganizations of the Atlantic overturning circulation) to centuries (e.g. for the melt of large ice sheets). Once in this alternative state, the system has to be taken to a different and distant tipping point to trigger recovery. Even after that it is not back where it started.
Irreversibility is even stronger in the case of tipping points in ecological systems, for example dieback of the Amazon or boreal forests. If species become extinct that is final, and particular configurations of ecosystems may also be unique and unrecoverable. Impacts tipping points in social systems may also be irreversible, for example, if a low-lying coastal city is abandoned in response to steady but overwhelming sea level rise. Putting the coupling between ecosystems and human systems into the equation probably only adds to the irreversibility, for example, when an established agricultural system becomes unviable and this triggers social unrest or mass migration.
Now add the problem of lag (Figure 1b); sluggish systems, such as the ocean circulation and ice sheets, cannot keep up with the rate of anthropogenic climate change. This means they are no longer in equilibrium with the climate forcing and instead they are in a transient state, lagging behind it. Even ecosystems such as the Amazon rainforest may lag climate forcing by several decades. When they start to show signs of abrupt change, they will already have overshot a tipping point and are well into the ‘basin of attraction’ of an alternative state. Any geoengineering applied at this intervention point has to fight against the system’s own dynamics which are trying to take it into an alternative state.
If this all sounds a bit technical, picture Looney Tunes’ Wile E. Coyote chasing Roadrunner to the edge of a cliff but overshooting spectacularly – he hovers tantalizingly in the air, far above the ground below (the lagged response), but his fate is already sealed. Geoengineering to alter his fate (in this metaphor) amounts to trying to propel Wile E. back onto the cliff top before gravity takes him abruptly to the alternative state on the ground below.
Thus, if one waits to see abrupt climate change unfurling, we may long since have been committed to it, and “swift remedial action” is actually not swift at all, however quickly geoengineering can be deployed and take effect.
It may still be possible in principle to reverse a change that is underway, but it could demand a reduction in radiative forcing well below the pre-industrial level (Figure 1). For example, the Greenland ice sheet is thought to be a relic of the last ice age – if it is removed it will not re-grow under the pre-industrial climate. To reinstate it would require us to geoengineering a much cooler climate that would be undesirable for many other reasons.
Those suggesting emergency geoengineering as a remedial action once a tipping point is passed need to focus their attention on tipping points that have the greatest reversibility, for example, abrupt loss of summer Arctic sea-ice cover.
As for geoengineering to “avoid reaching a climate ‘tipping point’”, there is a glimmer of hope in that systems approaching bifurcations carry generic early warning signals, such as becoming more sluggish in their recovery from natural fluctuation. These warning signals were present prior to some abrupt climate changes in the paleo-record, and are also found in models being slowly forced towards tipping points. However, the lag problem is still pertinent; we are forcing many parts of the climate system so rapidly, relative to their internal dynamics, that warning signals may not be detectable in advance of reaching a tipping point.
So, those suggesting geoengineering to avoid reaching a tipping point would do well to focus their attention on fast-responding systems which should carry the best early warning prospects, for example monsoons, or (once again) the Arctic sea-ice. Interestingly, these may also be among the more reversible systems. However, monsoons are particularly sensitive to aerosol forcing and may actually be disrupted rather than protected by deliberate aerosol injections.
One could take a wider view of ‘early warning’ and note that threats from multiple tipping points are already recognized, and one does not need a direct early warning signal to act. ‘Pre-emptive’ geoengineering might then be considered as a means of avoiding what are thought to be dangerous magnitudes, rates, or gradients of climate change. But in that case, it seems questionable that the geo-political willingness could be generated to geoengineer on a pre-emptive basis. After all, we have not yet succeeded in pre-emptively (or retrospectively) reducing emissions of carbon dioxide and other warming agents.
Part of the reason for this failure to act may be uncertainty over the proximity of tipping points. There is experimental evidence to suggest that the fear of crossing a dangerous threshold could turn climate negotiations into a coordination game, making collective mitigation action to try and avoid the threshold virtually assured. But current uncertainty about the location of tipping points instead causes cooperative efforts to avoid dangerous climate change to fail.
The irony here is that the surest way to reduce uncertainty about the location of climate tipping points is to get closer to them, but by then effective cooperative action to mitigate greenhouse gases emissions is likely to be too late to avoid tipping points. Instead sunlight reflection methods might be the only viable avoidance option left on the table, and they are only viable for a subset of tipping points that are directly related to temperature change.
Whether adding pre-emptive geoengineering into the mix of policy options will help achieve cooperative action on climate change is unclear. Our own experimental economics research suggests that the presence of multiple options, such as mitigation and geoengineering, can contribute to inefficient failure to coordinate collective action to avoid dangerous climate change.
In summary, ‘emergency-deployment’ framings of geoengineering to avoid or reverse climate tipping points could be seriously flawed. It needs to be researched under what – if any – circumstances, geoengineering could actually work to avoid a tipping point or to reverse one that had been passed. Whilst this might be taken as an argument for ‘pre-emptive’ geoengineering to reduce the risk of reaching a tipping point, it is not at all clear that introducing this option into the policy mix will actually help trigger effective action to avoid dangerous climate change.
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