Halving warming with stratospheric aerosol geoengineering moderates policy-relevant climate hazards

Keith, David, and Peter Irvine. “Halving warming with stratospheric aerosol geoengineering moderates policy-relevant climate hazards.” Environmental Research Letters 15, no. 4 (2020). Publisher's Version Abstract

irvine_2020_environ._res._lett._15_044011.pdf

Stratospheric aerosol geoengineering is a proposal to artificially thicken the layer of reflective aerosols in the stratosphere and it is hoped that this may offer a means of reducing average climate changes. However, previous work has shown that it could not perfectly offset the effects of climate change and there is a concern that it may worsen climate impacts in some regions. One approach to evaluating this concern is to test whether the absolute magnitude of climate change at each location is significantly increased (exacerbated) or decreased (moderated) relative to the period just preceding deployment. In prior work it was found that halving warming with an idealized solar constant reduction would substantially reduce climate change overall, exacerbating change in a small fraction of places. Here, we test if this result holds for a more realistic representation of stratospheric aerosol geoengineering using the data from the geoengineering large ensemble (GLENS). Using a linearized scaling of GLENS we find that halving warming with stratospheric aerosols moderates important climate hazards in almost all regions. Only 1.3% of land area sees exacerbation of change in water availability, and regions that are exacerbated see wetting not drying contradicting the common assumption that solar geoengineering leads to drying in general. These results suggest that halving warming with stratospheric aerosol geoengineering could potentially reduce key climate hazards substantially while avoiding some problems associated with fully offsetting warming.

Technical characteristics of a solar geoengineering deployment and implications for governance

MacMartin, Douglas, Peter Irvine, Ben Kravitz, and Joshua Horton. “Technical characteristics of a solar geoengineering deployment and implications for governance.” Climate Policy 19, no. 10 (2019): 1325-1339. Publisher's Version Abstract

technical_characteristics_of_a_solar_geoengineering_deployment_and_implications_for_governance.pdf

Consideration of solar geoengineering as a potential response to climate change will demand complex decisions. These include not only the choice of whether to deploy solar engineering, but decisions regarding how to deploy, and ongoing decisionmaking throughout deployment. Research on the governance of solar geoengineering to date has primarily engaged only with the question of whether to deploy. We examine the science of solar geoengineering in order to clarify the technical dimensions of decisions about deployment – both strategic and operational – and how these might influence governance considerations, while consciously refraining from making specific recommendations. The focus here is on a hypothetical deployment rather than governance of the research itself. We first consider the complexity surrounding the design of a deployment scheme, in particular the complicated and difficult decision of what its objective(s) would be, given that different choices for how to deploy will lead to different climate outcomes. Next, we discuss the on-going decisions across multiple timescales, from the sub-annual to the multi-decadal. For example, feedback approaches might effectively manage some uncertainties, but would require frequent adjustments to the solar geoengineering deployment in response to observations. Other decisions would be tied to the inherently slow process of detection and attribution of climate effects in the presence of natural variability. Both of these present challenges to decision-making. These considerations point toward particular governance requirements, including an important role for technical experts – with all the challenges that entails.

Quantifying the effects of solar geoengineering on vegetation

Dagon, Katherine, and Daniel Schrag. “Quantifying the effects of solar geoengineering on vegetation.” Climatic Change 152, no. 1-2 (2019): 235–251. Publisher's Version Abstract

dagon-schrag2019_article_quantifyingtheeffectsofsolarge.pdf

Climate change will have significant impacts on vegetation and biodiversity. Solar geoengineering has potential to reduce the climate effects of greenhouse gas emissions through albedo modification, yet more research is needed to better understand how these techniques might impact terrestrial ecosystems. Here, we utilize the fully coupled version of the Community Earth System Model to run transient solar geoengineering simulations designed to stabilize radiative forcing starting mid-century, relative to the Representative Concentration Pathway 6 (RCP6) scenario. Using results from 100-year simulations, we analyze model output through the lens of ecosystem-relevant metrics. We find that solar geoengineering improves the conservation outlook under climate change, but there are still potential impacts on terrestrial vegetation. We show that rates of warming and the climate velocity of temperature are minimized globally under solar geoengineering by the end of the century, while trends persist over land in the Northern Hemisphere. Moisture is an additional constraint on vegetation, and in the tropics the climate velocity of precipitation dominates over that of temperature. Shifts in the amplitude of temperature and precipitation seasonal cycles have implications for vegetation phenology. Different metrics for vegetation productivity also show decreases under solar geoengineering relative to RCP6, but could be related to the model parameterization of nutrient cycling. The coupling of water and carbon cycles is found to be an important mechanism for understanding changes in ecosystems under solar geoengineering.

Belfer Technology Factsheet Series: Solar Geoengineering

Burns, Lizzie, David Keith, Peter Irvine, and Joshua Horton. “Belfer Technology Factsheet Series: Solar Geoengineering” (2019).Abstract

belfer_factsheet_on_solar_geoengineering.pdf

Solar geoengineering refers to a set of emerging technologies that could alter the Earth’s radiative balance— perhaps through injecting aerosols into the stratosphere, where they would reflect a small fraction of sunlight back into space—reducing the amount of climate change caused by greenhouse gases. It could not replace reducing emissions (mitigation), coping with a changing climate (adaptation), or carbon dioxide removal (CDR). Yet it does have the potential to supplement these efforts, and it might provide reductions in climate risk that are unachievable by other means.

Multilateral parametric climate risk insurance: a tool to facilitate agreement about deployment of solar geoengineering?

Keith, David, and Joshua Horton. “Multilateral parametric climate risk insurance: a tool to facilitate agreement about deployment of solar geoengineering?” Climate Policy (2019). Publisher's Version Abstract

horton_and_keith_climate_policy_2019.pdf

States will disagree about deployment of solar geoengineering, technologies that would reflect a small portion of incoming sunlight to reduce risks of climate change, and most disagreements will be grounded in conflicting interests. States that object to deployment will have many options to oppose it, so states favouring deployment will have a powerful incentive to meet their objections. Objections rooted in opposition to the anticipated unequal consequences of deployment may be met through compensation, yet climate policy is inhospitable to compensation via liability. We propose that multilateral parametric climate risk insurance might be a useful tool to facilitate agreement on solar geoengineering deployment. With parametric insurance, predetermined payouts are triggered when climate indices deviate from set ranges. We suggest that states favouring deployment could underwrite reduced-rate parametric climate insurance. This mechanism would be particularly suited to resolving disagreements based on divergent judgments about the outcomes of proposed implementation. This would be especially relevant in cases where disagreements are rooted in varying levels of trust in climate model predictions of solar geoengineering effectiveness and risks. Negotiations over the pricing and terms of a parametric risk pool would make divergent judgments explicit and quantitative. Reduced-rate insurance would provide a way for states that favour implementation to demonstrate their confidence in solar geoengineering by underwriting risk transfer and ensuring compensation without the need for attribution. This would offer a powerful incentive for states opposing implementation to moderate their opposition.

Exploring accumulation-mode H2SO4 versus SO2 stratospheric sulfate geoengineering in a sectional aerosol–chemistry–climate model

Vattioni, Sandro, Debra Weisenstein, David Keith, Aryeh Feinberg, Thomas Peter, and Andrea Stenke. “Exploring accumulation-mode H2SO4 versus SO2 stratospheric sulfate geoengineering in a sectional aerosol–chemistry–climate model.” Atmospheric Chemistry and Physics 19 (2019). Publisher's Version Abstract

acp-19-4877-2019.pdf

Stratospheric sulfate geoengineering (SSG) could contribute to avoiding some of the adverse impacts of climate change. We used the SOCOL-AER global aerosol–chemistry–climate model to investigate 21 different SSG scenarios, each with 1.83 Mt S yr⁻¹ injected either in the form of accumulation-mode H₂SO₄ droplets (AM H₂SO₄), gas-phase SO₂ or as combinations of both. For most scenarios, the sulfur was continuously emitted at an altitude of 50 hPa (≈20 km) in the tropics and subtropics. We assumed emissions to be zonally and latitudinally symmetric around the Equator. The spread of emissions ranged from 3.75^∘ S–3.75^∘ N to 30^∘ S–30^∘ N. In the SO₂ emission scenarios, continuous production of tiny nucleation-mode particles results in increased coagulation, which together with gaseous H₂SO₄ condensation, produces coarse-mode particles. These large particles are less effective for backscattering solar radiation and have a shorter stratospheric residence time than AM H₂SO₄ particles. On average, the stratospheric aerosol burden and corresponding all-sky shortwave radiative forcing for the AM H₂SO₄ scenarios are about 37 % larger than for the SO₂ scenarios. The simulated stratospheric aerosol burdens show a weak dependence on the latitudinal spread of emissions. Emitting at 30^∘ N–30^∘ S instead of 10^∘ N–10^∘ S only decreases stratospheric burdens by about 10 %. This is because a decrease in coagulation and the resulting smaller particle size is roughly balanced by faster removal through stratosphere-to-troposphere transport via tropopause folds. Increasing the injection altitude is also ineffective, although it generates a larger stratospheric burden, because enhanced condensation and/or coagulation leads to larger particles, which are less effective scatterers. In the case of gaseous SO₂ emissions, limiting the sulfur injections spatially and temporally in the form of point and pulsed emissions reduces the total global annual nucleation, leading to less coagulation and thus smaller particles with increased stratospheric residence times. Pulse or point emissions of AM H₂SO₄ have the opposite effect: they decrease the stratospheric aerosol burden by increasing coagulation and only slightly decrease clear-sky radiative forcing. This study shows that direct emission of AM H₂SO₄ results in higher radiative forcing for the same sulfur equivalent mass injection strength than SO₂ emissions, and that the sensitivity to different injection strategies varies for different forms of injected sulfur.

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Geoengineering refers to a set of emerging technologies that could manipulate the environment and partially offset some of the impacts of climate change.

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Frank Keutsch — Heterogeneous Chemistry and Ageing of Designer Aerosol Particles to Assess the Risk of Solar Geoengineering

Robert Stavins — Governance of Solar Geoengineering: Advancing Understanding and Action