Supervolcanic eruptions induced abrupt global cooling (roughly at a rate of ~1ºC/year lasting for years to decades), such as the prehistoric Yellowstone eruption released, by some estimates, SO2 about 100 times higher than the 1991 Mt. Pinatubo eruption. An abrupt global cooling of several ºC, even if only lasting a few years, would present immediate and drastic stress on biodiversity and food production - posing a global catastrophic risk to human society. Using a simple climate model, this paper discusses the possibility of counteracting supervolcanic cooling with the intentional release of greenhouse gases. Although well-known longer-lived compounds such as CO2 and CH₄ are found to be unsuitable for this purpose, select fluorinated gases (F-gases), either individually or in combinations, may be released at gigaton scale to offset most of the supervolcanic cooling. We identify candidate F-gases (viz. C4F6 and CH3F) and derive radiative and chemical properties of ‘ideal’ compounds matching specific cooling events. Geophysical constraints on manufacturing and stockpiling due to mineral availability are considered alongside technical and economic implications based on present-day market assumptions. The consequences of F-gas release in perturbing atmospheric chemistry are discussed in the context of those due to the supervolcanic eruption itself. The conceptual analysis here suggests the possibility of mitigating certain global catastrophic risks via intentional intervention.

Solar Radiation Management (SRM) geoengineering is a proposed response to anthropogenic global warming (AGW)1. Stratospheric aerosol injection (SAI) is one proposed method, reliant on lofting particles into the stratosphere. Engineering reviews related to this technology approach have been sparse, with most major primary analyses now at least five years old. We attempt to bridge this gap – with a short, mainly qualitative review of recent developments in fields of engineering with potential applicability to stratospheric aerosol injection. Our analysis shows that a new conventional aircraft design is still likely to be the most dependable and affordable technology solution, with hybrid airships a potential challenger. Rockets, gas guns and MAGLEV/coilguns show some potential, although they lack the level flight capability preferred for direct aerosol distribution, without substantial additional engineering. Should very high-altitude access be required, rockets, hybrid rockets, and light-gas guns offer the required capability. Costs and performance for tethered balloons remain highly uncertain. No other methods are found to be promising. Nevertheless, the extreme accessibility of disposable balloons suggests that this method may be used primarily for reasons of political leverage, as opposed to being an optimal engineering solution.

Geoengineering, the deliberate modification of the climate system, is a proposed set of techniques to counter some of the effects of Anthropogenic Global Warming (AGW; Shepherd, 2009). Geoengineering includes Carbon Dioxide Removal (CDR) and Solar Radiation Management (SRM; Council, 2015). Global modelling studies of geoengineering have typically considered idealized scenarios, to understand the physical processes of interventions and their general impacts. These scenarios are not necessarily policy-relevant, and are often physically implausible (such as instantaneous quadrupling of CO2). The climatic and ecological impacts of politically-relevant and potentially plausible solutions have rarely been modeled and assessed. Nevertheless, commentators and policymakers often falsely assume that idealized or extreme scenarios are proposed as solutions to the AGW problem. This paper discusses various scenarios, which appear to be broadly plausible from both a political and Earth-system standpoint. These fall into the following categories: 1) Well-designed intervention strategies, including combined approaches, intended to meet specific climate targets 2) Global emergency response strategies, in case of unexpected natural or social events 3) Regional intervention strategies (with potential global consequences) 4) Miscellaneous and long-term intervention strategies We propose relevant, non-prioritized model experiments, corresponding to these scenarios. Some may be performed with existing setups of global climate models; others need to be first designed in detail. Developing and running these experiments, and assessing likely resulting impacts on society and ecosystems, would help to inform the public debate on the real-world feasibility of CDR and SRM geoengineering.