An architecture for monitoring SG deployment could be based on models from regimes that require the use of wide-area environmental detection methods to verify state compliance. A particularly relevant example is the Comprehensive Nuclear-Test-Ban Treaty (CTBT), which seeks to establish a global prohibition on nuclear weapons testing in the atmosphere, underwa-ter, and underground.
The CTBT has a particularly well-developed information collection and sharing structure. It is supported by the International Monitoring System (IMS), a network of seismic, infrasound, hydroacoustic, and radionuclide monitoring stations run by member states that provides capa-bility for detecting and attributing nuclear tests all over the world (Preparatory Commission for the CTBTO 2018). In cases where the data indicate a possible nuclear explosion but are incon-clusive, the treaty provides for an on-site inspection mechanism: a defined area is searched and additional evidence is gathered to rule on whether or not a nuclear explosion has taken place.
Surprisingly, the IMS is functional even though the CTBT has not yet entered into force.
In practice, data acquired by IMS stations are transferred continuously to the CTBT organiza-tion’s International Data Center in Vienna for storage and analysis, and are openly shared with member states. These data are not directly available to the public, however. The overlapping coverage of stations operated by different states allows for cross-correlation and independent verification of the information collected. This architecture is also robust in the sense that states that have not joined or that decide to leave the treaty in the future would still be monitored. For example, IMS data have been used to assess all North Korean nuclear tests, despite the fact that North Korea is not a party to the CTBT.
In addition to this multilateral detection capability, some countries also deploy so-called national technical means of detection. For example, the United States operates space-based sensors placed on U.S. Global Positioning Systems (GPS) satellites that can detect optical, electromagnetic, and x-ray signals from nuclear explosions in space, as well as special-purpose aircraft to collect samples from the atmosphere to detect and identify nuclear explosions. This dual architecture of national and international technical means of detection could also emerge for SG governance.
The states that may be most interested in SG deployment are likely to seek national means of observing aerosol concentrations in the atmosphere, not only to guide their own deployment actions but also to respond to other states’ possible deployment activities. Implementing these monitoring capabilities could require significant financial resources and access to technologies, and it is not clear that global coverage could be achieved by a single state.
The detection and attribution of unilateral action will require a monitoring system that is capable of locating injection points, perhaps by using a combination of aerosol concentration measure-ments and atmospheric transport modeling. Assuming that the cheapest and easiest mode of injection is via direct releases to the atmosphere from airplanes (Smith et al. 2018), techniques for attribution could be based on correlating the location of injection points (if they can be determined) with radar and transponder information about the past positions of identified or unidentified aircraft. One interesting question is whether particular environmental conditions at the time of injection, combined with knowledge of the different injection processes involved, could constitute a unique forensic signature for attribution. If so, an inspection mechanism may be required to compare samples taken in-situ in the atmosphere to samples retrieved from incriminated airplanes, if they can be detected and accessed.
Conclusion
The sharing of monitoring and verification capabilities could provide important benefits for SG governance: it could reduce costs associated with data acquisition, and it could provide public and trusted information for coordinating deployment, or for enforcing a moratorium. One can imagine a state having sufficient resources to deploy SG but no access to space for deploying monitoring equipment with sufficient coverage. Because the atmosphere is a global public good, it also seems fundamental that any information related to large-scale alterations of the atmo-sphere should be made public.
An important attribute for transnational information acquisition and sharing systems is the ability to demonstrate that the data collected by various sensors or sources are genuine and can be trusted by all participating states. For sensors, existing tamper-resistant hardware and public-key cryptographic protocols, such as those used, for example, by the CTBT-IMS stations, are sufficient if a recognized centralized body can establish secure communications with all sensors in the network. Absent a trusted third party – or central authority – to certify information, however, new, decentralized information-sharing protocols would need to be developed to effi-ciently collect, record, and share data that can be trusted by all relevant stakeholders.
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