3.2.3.4 Solar geoengineering

Solar geoengineering or solar radiation modification (SRM) is a group of hypothetical and controversial methods that might help decrease global temperature by directly altering the Earth’s energy balance, typically by reflecting a small fraction (around 1 per cent) of the incoming sunlight (NASEM, 2021). The best-known suggestions are Stratospheric Aerosol Injection (SAI), which would involve creating a thin reflective cloud layer of reflective aerosol in the higher atmosphere, and Marine Cloud Brightening (MCB), which would involve making oceanic stratocumulus clouds more reflective by providing sea salt dust particles to increase the number of cloud droplets.

It has been suggested that solar geoengineering techniques might reduce the likelihood of crossing temperature-related tipping points, postpone their arrival, or, more speculatively, even reverse ongoing tipping processes (Heutel, Moreno-Cruz, and Shayegh, 2016; Felgenhauer et al., 2022). The latter possibility is ruled out by Lenton (2018). The linkages between different kinds of solar geoengineering and the drivers of tipping points are understudied and uncertain. Moreover, proposed techniques are currently hypothetical, and not practically available as options to contribute reliably to the prevention of ESTPs. There is already early consensus that geoengineering techniques would not offer an emergency response to anticipated tipping events (Horton, 2015; Lenton, 2018). However, assessment over whether they might provide pre-emptive measures to support prevention is ongoing, and heavily contested (Gupta et al., 2020).

Modelling studies on stratospheric aerosol injection suggest beneficial effects on particular tipping systems (e.g. delay), such as AMOC decline (Xie et al., 2022), Greenland ice loss (Moore et al., 2019), West Antarctic ice loss (Sutter et al., 2023) or permafrost thaw (Chen et al., 2023). However, in these studies geoengineering interventions typically appear less efficacious than GHG mitigation. This underscores that they could at most complement, but not replace, mitigation. Nevertheless, these studies come from modelling simplified or idealised deployment scenarios at the global scale, which suffer from model uncertainties and bracket out technical, social, ethical, political and economic considerations which would be crucial for the conditions of deployment (Corry, 2017; McLaren, 2018). For other, more regional or localised techniques – including marine cloud brightening and ice albedo modification (see Box 3.2.3) – even the direct effects remain uncertain (Diamond et al., 2022; Johnson et al., 2022; Webster and Warren, 2022).

All approaches are poorly researched with respect to outdoor experimentation, technology development, side-effects, justice and ethics, public acceptability, and governance frameworks. Furthermore, deployment would be accompanied by the risk of termination shock (Parker and Irvine, 2018) – a risk of rapid warming if deployment were to be abruptly halted – along with other challenges and uncertainties regarding effectiveness and the regional-to-global distribution of their effects on various environmental and social systems such as weather, agriculture, health and biodiversity.

The prospect of collaborative, effective and democratic international governance – particularly of the global SAI approach – faces many practical and political challenges (Szerszynski et al., 2013; Horton et al., 2018; Flegal et al., 2019; Gardiner and McKinnon, 2020). Expectations that solar geoengineering might be deployed to avoid tipping points would carry a risk of deterring or slowing mitigation efforts (Corner and Pidgeon, 2014; McLaren, 2016; Merk, Pönitzsch, and Rehdanz, 2016). Idealised deployment that would mirror idealised modelling studies is unlikely: actual deployment would be beset by significant ethical and distributional challenges (McLaren, 2018) and would need to be sustained for decades or centuries (Baur et al., 2023). Developing required long-term, stable governance institutions (Parker and Irvine, 2018) would be difficult and slow, reflecting challenges in global climate governance on historic and future responsibilities, unequal capacities, and loss and damage (Biermann et al., 2022). In their absence, unilateral, club-based, or even corporate efforts to deploy geoengineering would present challenges regarding accountability and liability.

The prospective value of solar geoengineering approaches is greatly disputed among scientists, with networks emerging around an international non-use agreement (Biermann et al., 2022) and calls for further research and funding (Doherty et al., 2023; Wieners et al., 2023). Recently, the Overshoot Commission called for a moratorium on SRM deployment and large-scale experiments combined with ‘exploration’ by appropriately governed research and governance dialogue. Without commenting on these strands of activity,

We strongly caution against reliance on solar geoengineering as a major tool for preventing tipping points, or the expectation that this kind of approach will be available and politically acceptable in the future to contribute to prevention efforts. Nor should SRM ever be considered a possible replacement for mitigation. 

Governments should therefore take measures on both international and national scales to prevent premature, uncoordinated, or self-interested actions on SRM, by means of an (at least temporary) international moratorium on SRM deployment and large-scale experiments, as well as a ban on commercial activities even at a small scale. Multilateral efforts should also be undertaken to govern research and enable timely public debate on SRM’s potential, limitations and risks, including its potential to reduce or possibly exacerbate ESTP risks and to interact with social tipping points. The provisions of the London Protocol, prohibiting ocean iron fertilisation, with exemptions for legitimate scientific research, may provide a starting point for drafting regulations to ensure that any exploration of SRM is conducted in a responsible, safe and inclusive manner.