Publications

Stratospheric aerosol injection (SAI) aims to reflect solar radiation by increasing the stratospheric aerosol burden. To understand how the background circulation influences stratospheric transport of injected particles, we use a Lagrangian trajectory model (lacking numerical diffusion) to quantify particles’ number, flux, lifetime, and tropospheric sinks from a SAI injection strategy under present-day conditions. While particles are being injected, stratospheric particle number increases until reaching a steady-state. During the steady-state, the time series of particle number shows a dominant period of ~2 years (rather than a 1-year cycle), suggesting modulation by the quasi-biannual oscillation. More than half of particles, injected in the tropical lower stratosphere (15° S to 15° N, 65 hPa), undergo quasi-horizontal transport to the midlatitude. We find a zonal asymmetry of particles’ tropospheric sinks that are co-located with tropopause folding beneath the midlatitude jet stream, which can help predict tropospheric impacts of SAI (e.g., cirrus cloud thinning).

For half a century, climate researchers have considered the possibility of injecting small particles into the stratosphere to counteract some aspects of climate change. The idea is that by reflecting a small fraction of sunlight back to space, these particles could partially offset the energy imbalance caused by accumulating carbon dioxide, thereby reducing warming as well as extreme storms and many other climate risks.

Debates about this idea, a form of solar geoengineering called stratospheric aerosol injection (SAI), commonly focus either on small-scale outdoor research that seeks to understand the physical processes involved or on deployment at a climate-altering scale. The gulf between these is gigantic: an experiment might use mere kilograms of aerosol material whereas deployment that could substantially slow or even reverse warming would involve millions of metric tons per year—a billionfold difference in scale. Appreciably cooling the planet via SAI would also require a purpose-built fleet of high-altitude aircraft, which could take one or two decades to assemble. This long lead time encourages policymakers to ignore the hard decisions about regulating deployment of SAI.

Stratospheric Aerosol Injection (SAI) aims to offset some climate hazards by injecting aerosols into the stratosphere to reflect solar radiation. The lifetime of injected particles influences SAI’s radiative efficacy—the ratio of radiative forcing to particle mass flux. We employ a Lagrangian trajectory model with particle sedimentation to simulate how background circulations influence the transport of passive particles (without microphysical growth) in the stratosphere and quantify sensitivities of particle lifetime to injection locations. At 20 km, optimizing injection locations can increase particle lifetime by >40%. Injection strategies can be constrained to maintain an interhemispheric balance of particle lifetime without significantly decreasing total lifetime. Generally, increasing injection altitude increases particle lifetime while also increasing costs and environmental impacts of deployment aircraft. Optimizing injection latitude and longitude can relax this altitude-lifetime trade-off by increasing lifetime without needing to increase altitude, which warrants further testing in global climate models with aerosol microphysics.

Temperature-attributable mortality is a major risk of climate change. We analyze the capacity of solar geoengineering (SG) to reduce this risk and compare it to the impact of equivalent cooling from CO2 emissions reductions. We use the Forecast-Oriented Low Ocean Resolution model to simulate climate response to SG. Using empirical estimates of the historical relationship between temperature and mortality from Carleton et al. (2022), we project global and regional temperature-attributable mortality, find that SG reduces it globally, and provide evidence that this impact is larger than for equivalent cooling from emissions reductions. At a regional scale, SG moderates the risk in a majority of regions but not everywhere. Finally, we find that the benefits of reduced temperature-attributable mortality considerably outweigh the direct human mortality risk of sulfate aerosol injection. These findings are robust to a variety of alternative assumptions about socioeconomics, adaptation, and SG implementation.

We introduce a class of ultra-light and ultra-stiff sandwich panels designed for use in photophoretic levitation applications and investigate their mechanical behavior using both computational analyses and micro-mechanical testing. The sandwich panels consist of two face sheets connected with a core that consists of hollow cylindrical ligaments arranged in a honeycomb-based hexagonal pattern. Computational modeling shows that the panels have superior bending stiffness and buckling resistance compared to similar panels with a basketweave core, and that their behavior is well described by Uflyand-Mindlin plate theory. By optimizing the ratio of the face sheet thickness to the ligament wall thickness, panels maybe obtained that have a bending stiffness that is more than five orders of magnitude larger than that of a solid plate with the same area density. Using a scalable microfabrication process, we demonstrate that panels as large as 3 × 3 cm² with a volumetric density of 20 kg/m³ and corresponding area density of 2 g/m² can be made in a few hours. Micro-mechanical testing of the panels is performed by deflecting microfabricated cantilevered panels using a nanoindenter. The experimentally measured bending stiffness of the cantilevered panels is in very good agreement with the computational results, demonstrating exquisite control over the dimensions, form, and properties of the microfabricated panels.

Photophoretic forces could levitate thin 10 centimeter-scale structures in Earth’s stratosphere indefinitely. We develop analytical models of the thermal transpiration force on a bilayer sandwich structure in the stratosphere. Lofting is maximized when the layers are separated by an air gap equal to the mean free path (MFP), when about half of the layers’ surfaces consist of holes with radii < MFP, and when the top layer is solar-transmissive and infrared-emissive while the bottom layer is solar-absorptive and infrared-transmissive. We use the models to design a 10 cm diameter device with sufficient strength to withstand forces that might be encountered in transport, deployment, and flight. The device has a payload of about 285 mg at an altitude of 25 km; enough to support bidirectional radio communication at over 10 Mb/s and limited navigation. Such devices could be useful for atmospheric science or telecommunications on Earth and Mars. Structures a few times larger might have payloads of a few grams.

Solar geoengineering (SG) might be able to reduce climate risks if used to supplement emissions cuts and carbon removal. Yet, the wisdom of proceeding with research to reduce its uncertainties is disputed. Here, we use an integrated assessment model to estimate that the value of information that reduces uncertainty about SG efficacy. We find the value of reducing uncertainty by one-third by 2030 is around $4.5 trillion, most of which comes from reduced climate damages rather than reduced mitigation costs. Reducing uncertainty about SG efficacy is similar in value to reducing uncertainty about climate sensitivity. We analyse the cost of over-confidence about SG that causes too little emissions cuts and too much SG. Consistent with concerns about SG’s moral hazard problem, we find an over-confident bias is a serious and costly concern; but, we also find under-confidence that prematurely rules out SG can be roughly as costly. Biased judgments are costly in both directions. A coin has two sides. Our analysis quantitatively demonstrates the risk-risk trade-off around SG and reinforces the value of research that can reduce uncertainty. Key policy insights The value of reducing uncertainty about solar geoengineering is comparable to the value of reducing uncertainty about other key climate factors, such as equilibrium climate sensitivity. The benefits of research that reduces uncertainty about solar geoengineering may be more than a thousand times larger than the cost of a large-scale research programme. Under-confidence in solar geoengineering’s effectiveness can be as costly as over-confidence. The majority of the benefits of reduced uncertainty come from reducing climate damages rather than from slowing emissions reductions.

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Stratospheric emissions from aircraft or rockets are important sources of chemical perturbations. Small-radius high-aspect-ratio plumes from stratospheric emissions are smaller than global Eulerian models’ grid cells. To help global Eulerian models resolve subgrid plumes in the stratosphere, a Lagrangian plume model, comprising a Lagrangian trajectory model and an adaptive-grid plume model with a sequence of plume cross-section representations (from a highly resolved 2-D grid to a simplified 1-D grid based on a tradeoff between the accuracy and computational cost), is created and embedded into a global Eulerian (i.e., GEOS-Chem) model to establish a multiscale Plume-in-Grid (PiG) model. We compare this PiG model to the GEOS-Chem model based on a 1-month simulation of continuous inert tracer emissions by aircraft in the stratosphere. In the PiG results, the final injected tracer is more concentrated and approximately 1/3 of the tracer is at concentrations 2–4 orders of magnitude larger compared to the GEOS-Chem results. The entropy of injected tracer in the PiG results is 6% lower than the GEOS-Chem results, indicating less tracer mixing. The total product mass from a hypothetical second-order process (applied to the injected tracer) in the PiG results is 2 orders of magnitude larger than the GEOS-Chem results. Increasing the GEOS-Chem model’s horizontal resolution 4-fold is insufficient to resolve this product difference, while requiring over seven times the computational resources of the PiG model. This paper describes the PiG model framework and parameterization of plume physical processes. Chemical and aerosol processes will be introduced in the future.

Studies of stratospheric solar geoengineering have tended to focus on modification of the sulfuric acid aerosol layer, and almost all climate model experiments that mechanistically increase the sulfuric acid aerosol burden assume injection of SO₂. A key finding from these model studies is that the radiative forcing would increase sublinearly with increasing SO₂ injection because most of the added sulfur increases the mass of existing particles, resulting in shorter aerosol residence times and aerosols that are above the optimal size for scattering. Injection of SO₃ or H₂SO₄ from an aircraft in stratospheric flight is expected to produce particles predominantly in the accumulation-mode size range following microphysical processing within an expanding plume, and such injection may result in a smaller average stratospheric particle size, allowing a given injection of sulfur to produce more radiative forcing. We report the first multi-model intercomparison to evaluate this approach, which we label AM-H₂SO₄ injection. A coordinated multi-model experiment designed to represent this SO₃- or H₂SO₄-driven geoengineering scenario was carried out with three interactive stratospheric aerosol microphysics models: the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM2) with the Whole Atmosphere Community Climate Model (WACCM) atmospheric configuration, the Max-Planck Institute’s middle atmosphere version of ECHAM5 with the HAM microphysical module (MAECHAM5-HAM) and ETH’s SOlar Climate Ozone Links with AER microphysics (SOCOL-AER) coordinated as a test-bed experiment within the Geoengineering Model Intercomparison Project (GeoMIP). The intercomparison explores how the injection of new accumulation-mode particles changes the large-scale particle size distribution and thus the overall radiative and dynamical response to stratospheric sulfur injection. Each model used the same injection scenarios testing AM-H₂SO₄ and SO₂ injections at 5 and 25 Tg(S) yr⁻¹ to test linearity and climate response sensitivity. All three models find that AM-H₂SO₄ injection increases the radiative efficacy, defined as the radiative forcing per unit of sulfur injected, relative to SO₂ injection. Increased radiative efficacy means that when compared to the use of SO₂ to produce the same radiative forcing, AM-H₂SO₄ emissions would reduce side effects of sulfuric acid aerosol geoengineering that are proportional to mass burden. The model studies were carried out with two different idealized geographical distributions of injection mass representing deployment scenarios with different objectives, one designed to force mainly the midlatitudes by injecting into two grid points at 30° N and 30° S, and the other designed to maximize aerosol residence time by injecting uniformly in the region between 30° S and 30° N. Analysis of aerosol size distributions in the perturbed stratosphere of the models shows that particle sizes evolve differently in response to concentrated versus dispersed injections depending on the form of the injected sulfur (SO₂ gas or AM-H₂SO₄ particulate) and suggests that prior model results for concentrated injection of SO₂ may be strongly dependent on model resolution. Differences among models arise from differences in aerosol formulation and differences in model dynamics, factors whose interplay cannot be easily untangled by this intercomparison.