Context & Scale
An industrial process for large-scale capture of atmospheric CO₂ (DAC) serves two roles. First, as a source of CO₂ for making carbon-neutral hydrocarbon fuels, enabling carbon-free energy to be converted into high-energy-density fuels. Solar fuels, for example, may be produced at high-insolation low-cost locations from DAC-CO₂ and electrolytic hydrogen using gas-to-liquids technology enabling decarbonization of difficult-to-electrify sectors such as aviation. And second, DAC with CO₂ sequestration allows carbon removal.

The feasibility of DAC has been disputed, in part, because publications have not provided sufficient engineering detail to allow independent evaluation of costs. We provide an engineering cost basis for a commercial DAC system for which all major components are either drawn from well-established commercial heritage or described in sufficient detail to allow assessment by third parties. This design reflects roughly 100 person-years of development by Carbon Engineering.

Summary
We describe a process for capturing CO₂ from the atmosphere in an industrial plant. The design captures ∼1 Mt-CO₂/year in a continuous process using an aqueous KOH sorbent coupled to a calcium caustic recovery loop. We describe the design rationale, summarize performance of the major unit operations, and provide a capital cost breakdown developed with an independent consulting engineering firm. We report results from a pilot plant that provides data on performance of the major unit operations. We summarize the energy and material balance computed using an Aspen process simulation. When CO₂ is delivered at 15 MPa, the design requires either 8.81 GJ of natural gas, or 5.25 GJ of gas and 366 kWhr of electricity, per ton of CO₂ captured. Depending on financial assumptions, energy costs, and the specific choice of inputs and outputs, the levelized cost per ton CO₂ captured from the atmosphere ranges from 94 to 232 $/t-CO₂.

Counter-geoengineering is the idea that a country might seek or threaten to counteract the cooling effect of solar geoengineering through technical means. Although this concept has been mentioned with increasing frequency in commentary on geoengineering, it has received little scholarly attention. We offer a preliminary analysis. We begin by distinguishing two kinds of counter-geoengineering: countervailing with a warming agent, and neutralising with a physical disruption. Based on this distinction, we review prior suggestions and describe novel methods by which either method might be accomplished, within the constraints imposed by deep technical uncertainties and substantial technical challenges. We then reflect on the strategic requirements and motivations for developing counter geoengineering and use a simple game-theoretic framework to demonstrate how counter-geoengineering might interact with the free-driver dynamic of solar geoengineering to shape climate geopolitics. We find that any state that could credibly threaten counter-geoengineering would effectively have a veto over the use of solar geoengineering, which could reduce the prospects of unilateral deployment. Alternatively, the development of geoengineering and countergeoengineering capabilities could lead to dangerous brinkmanship. We conclude that the development of counter-geoengineering would face considerable practical obstacles and would signal continuing political failure to manage climate risks on a cooperative basis.

Solar geoengineering is no substitute for cutting emissions, but could nevertheless help reduce the atmospheric carbon burden. In the extreme, if solar geoengineering were used to hold radiative forcing constant under RCP8.5, the carbon burden may be reduced by ~100 GTC, equivalent to 12–26% of twenty-first-century emissions at a cost of under US$0.5 per tCO₂.

Global warming potentials (GWPs) have become an essential element of climate policy and are built into legal structures that regulate greenhouse gas emissions. This is in spite of a well-known shortcoming: GWP hides trade-offs between short- and long-term policy objectives inside a single time scale of 100 or 20 years (1). The most common form, GWP₁₀₀, focuses on the climate impact of a pulse emission over 100 years, diluting near-term effects and misleadingly implying that short-lived climate pollutants exert forcings in the long-term, long after they are removed from the atmosphere (2). Meanwhile, GWP₂₀ ignores climate effects after 20 years. We propose that these time scales be ubiquitously reported as an inseparable pair, much like systolic-diastolic blood pressure and city-highway vehicle fuel economy, to make the climate effect of using one or the other time scale explicit. Policy-makers often treat a GWP as a value-neutral measure, but the time-scale choice is central to achieving specific objectives (2–4).

We offer a hypothesis that if solar geoengineering (SG) were deployed to offset half of the increase in global-mean temperature from the date of deployment using a technology and deployment method chosen to approximate a reduction in the solar constant then, over the 21st century, it would (a) substantially reduce the global aggregate risks of climate change, (b) without making any country worse off, and (c) with the aggregate risks from side-effects being small in comparison to the reduction in climate risks. We do not set out to demonstrate this hypothesis; rather we propose it with the goal of stimulating a strategic engagement of the SG research community with policy-relevant questions. We elaborate seven sub-hypotheses on the effects of our scenario for key risks of climate change that could be assessed in future modeling work. As an example, we provide a defence of one of our sub-hypotheses, that our scenario of SG would reduce the risk of drought in dry regions, but also identify issues that may undermine this sub-hypothesis and how future work could resolve this question. SG cannot substitute for emissions mitigation but it may be a useful supplement. It is our hope that scientific and technical research over the next decade focuses more closely on well-articulated variants of the key policy-relevant question: could SG be designed and deployed in such a way that it could substantially and equitably reduce climate risks?

Injecting sulfate aerosol into the stratosphere, the most frequently analyzed proposal for solar geoengineering, may reduce some climate risks, but it would also entail new risks, including ozone loss and heating of the lower tropical stratosphere, which, in turn, would increase water vapor concentration causing additional ozone loss and surface warming. We propose a method for stratospheric aerosol climate modification that uses a solid aerosol composed of alkaline metal salts that will convert hydrogen halides and nitric and sulfuric acids into stable salts to enable stratospheric geoengineering while reducing or reversing ozone depletion. Rather than minimizing reactive effects by reducing surface area using high refractive index materials, this method tailors the chemical reactivity. Specifically, we calculate that injection of calcite (CaCO₃) aerosol particles might reduce net radiative forcing while simultaneously increasing column ozone toward its preanthropogenic baseline. A radiative forcing of −1 W⋅m⁻², for example, might be achieved with a simultaneous 3.8% increase in column ozone using 2.1 Tg⋅y⁻¹ of 275-nm radius calcite aerosol. Moreover, the radiative heating of the lower stratosphere would be roughly 10-fold less than if that same radiative forcing had been produced using sulfate aerosol. Although solar geoengineering cannot substitute for emissions cuts, it may supplement them by reducing some of the risks of climate change. Further research on this and similar methods could lead to reductions in risks and improved efficacy of solar geoengineering methods.