Publications

Solar geoengineering with sulfuric acid aerosols in the lower stratosphere might be used to reduce the impacts of climate change. Almost all studies have assumed sulfur is transported aloft as sulfur dioxide; several other sulfur compounds are plausible. We evaluated elemental sulfur (S₈), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), carbon disulfide (CS₂), sulfur trioxide (SO₃), and sulfuric acid (H₂SO₄), examining the mechanisms of dispersal in liquid form, safety, and costs for materials supply and deployment with a 20 km altitude aircraft. Levelized, combined costs for material production and lofting at 100 ktonne/year scale range from $1140 to $5150/tonne-S. We find that the cost to loft H₂S is about half that of SO₂, but the production cost of H₂S is more uncertain and could be high enough to change this conclusion. The additional oxidation step required for H₂S adds <20% to the aerosol formation time scale, so H₂S and SO₂ should produce similar aerosol size distributions if new particle formation is negligible. H₂S and SO₂ have high and roughly equivalent toxicity. H₂S presents a further flammability risk that SO₂ does not. S₈ would be much safer than H₂S or SO₂, but it requires airborne combustor technology that is not yet developed. CS₂ is not preferred on any metric. SO₃ and H₂SO₄ are ∼4 times more expensive than H₂S to loft but offer additional control over the aerosol size distribution that will likely reduce some side effects of sulfuric acid aerosol geoengineering.

Lightweight nanofabricated structures could photophoretically loft payloads in near-space. Proposed structures range from microscale engineered aerosols¹, to centimetre-scale thin disks with variations in surface accommodation coefficients^2,3, to sandwich structures with nanoscale thickness^4,5 that might be extended to metre-scale width^6,7. Quantitative understanding of how structural and surface properties determine photophoretic lofting forces is necessary to develop a practical flying device. Here we focus on thermal transpiration as the most promising photophoretic mechanism for lofting large devices⁸ and present a hybrid analytical–numerical model of the lofting force on a structure that consists of two perforated membranes spaced a small distance apart. We identify optimal structural parameters, including device size, membrane perforation density and distribution of the vertical ligaments that connect the two membranes, each as a function of atmospheric altitude. Targeting these optimal parameters, we fabricate structures with a heterogeneous ligament distribution, which efficiently compromises between structural rigidity and photophoretic performance. We measure how lofting forces generated by these structures depend on pressure using gases with three different molecular weights. We observed photophoretic levitation of a 1-cm-wide structure at an air pressure of 26.7 Pa when illuminated by 750 W m⁻², about 55% the intensity of sunlight. Lastly, we describe the preliminary design of a 3-cm-radius device with 10-mg payload capacity at 75-km altitudes and discuss horizontal motion control, overnight settling, and applications in climate sensing, communications and Martian exploration.

Solar radiation modification (SRM) is increasingly discussed as a potential method to ameliorate some negative effects of climate change. However, unquantified uncertainties in physical and environmental impacts of SRM impede informed debate and decision making. Some uncertainties are due to lack of understanding of processes determining atmospheric effects of SRM and/or a lag in development of their representation in models, meaning even high-quality model intercomparisons will not necessarily reveal or address them. Although climate models at multiple scales are advancing in complexity, there are specific areas of uncertainty where additional model development (often requiring new observations) could significantly advance understanding of SRM’s effects, and improve our ability to assess and weigh potential risks against those of choosing to not use SRM. We convene expert panels in the areas of atmospheric science most critical to understanding the three most widely discussed forms of SRM. Each identifies three key modeling gaps relevant to either stratospheric aerosols, cirrus, or low-altitude marine clouds. Within each area, key challenges remain in capturing impacts due to complex interactions in aerosol physics, atmospheric chemistry/dynamics, and aerosol-cloud interactions. Across all three, in addition to arguing for more observations, the panels argue that model development work to either leverage different capabilities of existing models, bridge scales across which relevant processes operate, or address known modeling gaps could advance understanding. By focusing on these knowledge gaps we believe the modeling community could advance understanding of SRM’s physical risks and potential benefits, allowing better-informed decision-making about whether and how to use SRM.

Stratospheric aerosol injection, which would reflect a small fraction of sunlight away from the Earth to lower temperatures, involves many unanswered questions. One of these is, who could deploy it? We consider this with reference to a scenario in which global temperatures are reduced by 1°C by midcentury; we term this a ‘PLUS’ deployment—Planetary, Large-scale, Uninterrupted, and Speedy. The technical requirements of a PLUS deployment—a fleet of a hundred or more specialized aircraft—limit the number of capable actors to ten states. The geopolitical requirements—broad-spectrum capabilities sufficient to overcome external constraints—mean that only the US and China are capable of implementing unilaterally against strong opposition. As such, the US and China will be decisive in determining whether and how a PLUS-type deployment takes place. In particular, the degree of Sino-American alignment on this issue will strongly influence the likelihood of a PLUS deployment and its disruptive potential. We examine three cases in which activities with the potential to harm global commons were debated during the Cold War: scientific research in Antarctica, atmospheric testing of nuclear weapons, and experiments in outer space. Backed by evidence from these cases, we then consider several implications of our findings.

Stratospheric aerosol injection (SAI) could change the sky’s appearance. This could play a role in
shaping public perception of SAI. Noticeability depends strongly on tropospheric aerosol optical
depth (AOD) and the amount of SAI used. We aim to quantify the noticeability of changes in sky
color and brightness due to SAI. We use a 3D visible light radiative transfer package to generate
cloudless sky images during high sun, sunset, and twilight under SAI from the reference point of a
ground observer. We consider three aerosol types: H2SO4, CaCO3, and diamond. We consider
stratospheric aerosol loadings required to produce radiative forcings of −1, −2, and −4 W m−2.
We use population density and AOD data to compute the distribution of AODs people experience
and then simulate sky images for the 10th, 50th, and 90th percentiles of that distribution. We
compare the simulated changes in color and brightness to experimental measurements of
minimum thresholds humans can detect. The three aerosol types cause similar changes, except
most notably the diamond aerosol increases brightness of the solar aureole by roughly three to five
times less than do H2SO4 or CaCO3. During high sun, sky whitening from sulfate SAI at
−2 W m−2 is undetectable for roughly half of observers chosen randomly from the global
population. For the remainder of the population, we expect whitening to still be unnoticeable for
all but perhaps the most astute observers aided by color samples. Brightening and enlargement of
the solar aureole is the most visible feature during high sun for H2SO4 and CaCO3, while changes
near twilight would be the most noticeable impact of SAI. We cannot evaluate the fraction of the
population who would notice these changes.

Decisions about solar geoengineering (SG) entail risk–risk tradeoffs between the direct risks of SG and SG’s ability to reduce climate risks. Quantitative comparisons between these risks are needed to inform public policy. We evaluate idealized SG’s effectiveness in reducing deaths from warming using two climate models and an econometric analysis of temperature-attributable mortality. We find SG’s impact on temperature-attributable mortality is uneven with decreases for hotter, poorer regions and increases in cooler, richer regions. Relative to no SG, global mortality is reduced by over 400,000 deaths annually [90% CI: (−1.2 million,2.7 million)] for cooling of 1 °C from 2.5 °C above preindustrial in 2080. We find no evidence that mortality reduction achieved by SG is smaller than the reduction from equivalent cooling by emissions reductions. Combining our estimates with existing estimates of sulphate aerosol injection direct mortality risk from air quality and UV-attributable cancer enables the first quantitative risk-risk comparison of SG. We estimate with 61% probability that the mortality benefits of cooling outweigh these direct SG risks. We find the benefits outweigh these risks by 13 times for our central estimates, or 4 deaths per 100,000 per 1 °C per year [90% CI: (−11,23)]. This is not a comprehensive evaluation of the risk–risk tradeoffs around SG, yet by comparing some of the most consequential impacts on human welfare it is a useful first step. While these findings are robust to a variety of alternative assumptions, considerable uncertainties remain and require further investigation.

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).

Solar geoengineering, also called sunlight reflection or solar radiation modification (SRM), is a potential climate response that would cool the Earth’s surface and reduce many other climate changes by scattering on order 1% of incoming sunlight back to space. SRM can only imperfectly correct for elevated greenhouse gases, but it might complement other climate responses to reduce risks, while also bringing new risks and new challenges to global governance. As climate alarm and calls for effective near-term action mount, SRM is attracting sharply increased attention and controversy, with many calls for expanded research and governance consultations along with ongoing concerns about risks, misuse, or overreliance. We review SRM’s history, methods, potential uses and impacts, and governance needs, prioritizing the approach that is most prominent and promising, stratospheric aerosol injection. We identify several policy-relevant characteristics of SRM interventions and identify four narratives that capture current arguments over how SRM might be developed or used in socio-political context to either beneficial or destructive effect, with implications for near-term research, assessment, and governance activity.

The photophoretic force exerted on an illuminated membrane vanishes when its size is much larger than the molecular mean-free-path. Perforations in the membrane can increase the force, but they make modeling the gas flow more difficult. In the present letter, a new model of gas interaction with a perforated surface is proposed. The direct simulation Monte Carlo method is used to calculate the flow-field and photophoretic force exerted on a perforated membrane, showing a significant increase in the force at high rarefaction parameters. This effect could be used to levitate membranes of a few centimeters in size at mesospheric altitudes.