Geoengineering refers to deliberate, often global-scale, manipulations of the climate system. In general, the goal of geoengineering would be to counteract the effect of human greenhouse gas emissions or their consequences. Geoengineering could potentially help lower greenhouse gas concentrations in the atmosphere; counteract the physical impact of increasing greenhouse gas concentrations; address specific climate change impacts; or offer desperation strategies in the event that abrupt, catastrophic, or otherwise unacceptable climate change impacts become evident.
Geoengineering could also create new sources of risk because attempts to engineer the earth system on a large scale could lead to unintended and adverse consequences. Notably, the complexity of the earth system (which couples numerous physical and biological systems and processes) and society’s relationship to the earth system (which involves further coupling with social institutions) makes it challenging for scientific research to fully identify and quantify the potential consequences associated with geoengineering. As a result, the potential impacts from geoengineering could inadvertently compound the dangers associated with climate change.
Even to the extent that potential consequences of geoengineering can be well characterized, those consequences would almost certainly differ among countries and individuals. This raises potentially complex legal, ethical, diplomatic, and national security concerns. Furthermore, the potential for geoengineering as a desperation strategy could distract from mitigation and adaptation efforts, which may have a higher probability of contributing positively to risk management.
Nevertheless, two categories of geoengineering are most prevalent within scientific and policy discussions: solar radiation management and carbon removal and sequestration.
The goal of solar radiation management is to increase the earth’s reflectivity to incoming solar energy (e.g., by injecting reflective particles into the atmosphere or increasing the brightness or distribution of certain types of cloud cover). This could, in principle, reflect incoming shortwave solar radiation by an amount that matches the increased heat trapping (i.e., longwave radiation) due to increased greenhouse gas concentrations in the atmosphere. Although shortwave and longwave radiation are likely not entirely interchangeable, this could reduce the magnitude of human disturbance of the overarching energy balance of the climate system.
Solar radiation management might be a relatively fast-acting option for quickly reversing some of the warming associated with increasing greenhouse gas concentrations. However, solar radiation management represents a substantial global-scale manipulation of the earth system that would be likely to have broad reaching impacts, some of which may be difficult to predict.
The goal of carbon removal and sequestration is to capture some of the increased carbon in the atmosphere that results from human activities and store that carbon away from the atmosphere, most likely in either the ocean or below ground (i.e., geologically). This could be challenging to do at a scale that matches current and expected greenhouse gas emissions. However, the risk of adverse impacts associated with sequestration is generally considered to be lower than for solar radiation management.
Finally, other large-scale interventions might be designed to reduce specific climate impacts. For example, the massive deployment of sea walls, efforts to protect continental ice sheets through snow making or preserving activities, or the use of assisted movement for biological systems might all be conducted at a sufficiently large scale to be considered geoengeering.
Notably, geoengineering likely wouldn’t address all potential impacts associated with greenhouse gas emissions. Solar radiation management, for example, will not reduce the amount of carbon dioxide in the air or the ocean and would therefore have no impact on ocean acidification or the direct effects of carbon dioxide enrichment on biological systems.
Policy options for geoengineering generally fall into five categories. We could conduct research and analysis in order to develop or vet potential options. We could study the impacts and potential unintended consequences. We could create punitive measures to discourage reckless for unilateral attempts to geoengineer. We could create policies that promote cooperation and transparency or help ensure that governance issues would be addressed. Of course, policies could also seek to implement geoengineering approaches.
To date, U.S. federal climate policy has rarely considered geoengineering explicitly. However, efforts to promote carbon capture and storage (CCS) and overcome the barriers to widespread deployment of CCS are widespread. Furthermore, at least one international treaty that the United States has ratified, the Environmental Modification Convention (ENMOD), may currently prohibit at least some forms of geoengineering, particularly solar radiation management.