Executive Summary
As climate risks intensify and emissions reductions may not proceed fast enough to limit near-term warming, sunlight reflection technology (SRT), also referred to as solar radiation management (SRM), is receiving growing attention as a possible supplementary intervention method. The most widely discussed SRM approach is stratospheric aerosol injection (SAI), in which submicron aerosols or aerosol precursors are injected into the stratosphere to increase solar flux reflection. If it can be developed and implemented safely, SAI may help reduce near-term warming and some of its associated impacts while buying time for emissions reductions and other long-term climate responses.
The main goal of Stardust’s R&D is to develop the technology required to build a complete SAI system, allowing a safe, controllable, and practical SAI implementation. This is crucial for enabling governments to make informed decisions on the development and possible implementation of such systems.
Our approach differs from most earlier work in two major respects:
1
We begin with the safety and controllability requirements that any SRM system should meet in order to be considered as a viable option for implementation. Our R&D is focused on designing an SAI system that meets these requirements and allows a practical implementation.
2
We develop the technology enabling a complete end-to-end system, including particle fabrication, lofting, and dispersion, and operational monitoring.
In keeping with our commitment to transparency and open peer review of our R&D results (see our guiding principles), we are publishing a series of papers (all available on our website) that describe the current status of our R&D efforts. We present specific designs for the particle structure and its production process, an initial design for the dispersion system, and the results of our laboratory experiments. Although the experimental program is ongoing, its current results, combined with theoretical work, imply that our technology will enable a complete, practical SAI system that meets safety and controllability requirements and provides a (negative) Radiative Forcing (RF) of ~1% solar, comparable to the excess (positive) RF due to GHG gases accumulated from 1850. Below, we briefly describe the main content of the papers.
Our proposal for this set of requirements is presented in a dedicated white paper. They are intended to address risks of adverse or unintended effects within three categories: on humans, ecosystems, and the environment; on atmospheric chemistry and composition; and on the climate system. The latter are addressed through requirements on the predictability, controllability, and monitoring ability of the induced RF. Validating predictability requires, in particular, the ability to conduct small-scale experiments with negligible induced RF (which is not possible for sulfate-based SAI due to the large stratospheric sulfate background).
Establishing regulatory frameworks for SAI deployment, defining safety and controllability requirements, and ensuring SRM systems meet them are the responsibility of governments and require international cooperation. The purpose of the white paper is to present an initial proposal for the SAI-based SRM system requirements that reflects our current understanding and guides Stardust’s SAI system R&D efforts. We hope it will also serve as a basis for further discussion and consideration.
A set of functionality requirements on the particles’ properties - optical properties, stratospheric residence time, scalable manufacturing compatibility, and aerial dispersion compatibility- that, if met, ensure the feasibility of practical implementation, providing reflection of ∼ 1% of the solar flux, is presented in a companion overview paper the tables in the appendix of this paper summarize the requirements on particle properties derived from safety, controllability and functionality requirements). The requirements on particle functionality properties are closely tied to the planned implementation route, which we consider to be via aerial dispersion by a fleet of airplanes flying in the lower stratosphere. We assume a fleet of several hundred planes capable of lifting 10 Mt per year to ~60kf altitude.
The combined requirements do not identify a unique solution, but favor sub-micron particles with tightly controlled size distributions and stable properties over their stratospheric lifetime, and motivate a composite design where the bulk core composition is selected primarily for optimal radiative properties, and the outer shell surface is engineered to control atmospheric chemistry and aging, and enhance aerial dispersion compatibility. We present in the overview paper two specific safe and biodegradable particle designs and a production process that meet all requirements: amorphous silica spheres and calcium-carbonate cores surrounded by spherical silica shells (both with appropriate surface treatments). The former is at an advanced stage of experimental verification of meeting all requirements, provides a practical platform for surface engineering, and will enable reaching a substantial fraction of ∼ 1% solar flux reflection (limited by the requirement to avoid excess stratospheric heating). The latter is under development and will enable reaching > 1% reflection.
The particles are made of materials that are structural materials of natural creatures and are already part of the natural cycle. These materials are also used in products that serve all of us in our daily lives, from toothpaste to food additives. Rather than grinding down larger material, the particles are grown from the molecular level up, which gives precise control over their size, shape, and surface chemistry, and lets each batch be tagged with a unique signature during manufacture.
In the overview paper, we also provide a description of the experimental program intended to verify compliance with all requirements, as well as a brief summary of the current results obtained from this program, which are described in more detail in the rest of the papers.
1
Size & shape control
Consistent synthesis was demonstrated (see companion publication) of amorphous silica spheres with tight control over size distribution and morphology: The particles are small enough to reflect sunlight efficiently and to stay aloft in the stratosphere for about a year.
2
Coating durability
The coating that protects the particles from chemical alteration in the stratosphere holds up under lab tests designed to mimic a full year of exposure to stratospheric ultraviolet flux and to reactive gases, including ozone (see companion publication).
3
Stratospheric wetting avoidance
Treated particles are highly hydrophobic and strongly repel water and sulfuric acid (see companion publication). This suggests a low propensity for wetting, for nucleation of water- or sulfate-rich surface layers under stratospheric conditions, and for PSC nucleation.
4
Heterogeneous interaction minimization
The particles interact minimally with the gases that drive ozone chemistry, implying that any contribution to ozone depletion would be negligible. Our publication reports results from direct measurements of the composite particle performance, using methods validated in a companion publication that presents a comparison across a range of particle compositions.
5
Airborne dispersion feasibility
Dispersion of 10 Mt per year by a fleet of a few hundred planes requires a dispersion rate of a few tons/hr per airplane during operation. Avoiding major particle agglomeration under such conditions is a major challenge for sub-micron particles. It is addressed by combining a surface treatment that reduces adhesion with an appropriately designed dispersion system. In the companion study, we present results from experiments conducted in a dedicated 10-meter indoor spray tunnel using our prototype dispersion system, demonstrating that efficient deagglomeration can be achieved under pressurized airflow conditions compatible with feasible aircraft platforms. This experimentally validated building block is now being incorporated into our full aircraft-compatible dispersion system design.
6
Effectiveness and scenario parametrizations
Atmospheric modeling reported in a companion publication evaluates the dispersion rates required for offsetting chosen fractions of the global warming due to excess GHG accumulation (and the associated stratospheric heating). It is shown that dispersing the engineered particles at a rate of ~1 million tons per year would offset warming due to excess GHG accumulation over roughly the past decade, while dispersion at a rate of ~10 million tons/year would offset the effect due to GHG accumulation over the past 50 years.
7
Manufacturing at scale
Stardust's manufacturing analysis (see companion paper) finds no major technological or supply chain showstoppers to producing the engineered particles at a rate compatible with the deployment rate required to achieve ~0.1-1% solar flux reflection, at an affordable price, using industrial chemistry processes that are in use today.
8
Particle tagging
The ability to determine the origin (time and location) of a particle observed at a given (space and time) point is essential for validating the predictability of the SAI system and for carrying out small-scale experiments. It would also be highly useful for governance purposes, as described in our dedicated study, which suggests technological means to enable future multilateral collaboration.
In upcoming publications, we will outline the dedicated monitoring and control system that enables a gradual, evidence-based ramp-up. This system is designed to allow operators and governments to verify, adjust, or halt a deployment in real time and safely. It consists of two layers: first, tracking individual batches of particles through the atmosphere using their chemical fingerprints; and second, directly measuring the resulting radiative effects. This enables real-time attribution and feedback control over what is dispersed, where, and when.