Geoengineering Explained

Solar Radiation Management

Stratospheric aerosol injection (SAI) would mimic volcanic eruptions by dispersing reflective sulfate particles in the upper atmosphere. Models suggest injecting 5 to 10 million tonnes of sulfur dioxide annually could offset 1 to 2 degrees of warming within months. The approach is technically feasible and relatively inexpensive (estimated 2 to 10 billion dollars annually), making it potentially attractive but dangerously easy to deploy unilaterally.

Marine cloud brightening would spray sea salt particles into low marine clouds to increase their reflectivity. This approach is more regional and potentially reversible than SAI but faces challenges in predicting cloud responses and could alter precipitation patterns. Space-based sunshades positioned at the Lagrange point between Earth and Sun remain purely conceptual due to enormous scale and cost requirements.

Risks of Solar Radiation Management

SRM does not address ocean acidification since atmospheric CO2 remains elevated. If deployment were suddenly terminated (a scenario called "termination shock"), temperatures would rise rapidly as the masking effect disappeared while greenhouse gases remained high. This rapid warming could be more damaging than gradual warming because ecosystems and societies would have less time to adapt.

Regional side effects include potential disruption of monsoon patterns affecting billions of people, ozone depletion from stratospheric chemistry changes, altered diffuse-to-direct sunlight ratios affecting photosynthesis and solar power, and uneven cooling that could benefit some regions while harming others. These distributional effects create geopolitical tensions about deployment decisions.

Carbon Dioxide Removal

CDR approaches address the root cause by reducing atmospheric CO2. Nature-based methods include afforestation (planting new forests), reforestation (restoring degraded forests), soil carbon sequestration through agricultural practices, and coastal blue carbon (restoring mangroves, seagrasses, salt marshes). These approaches offer co-benefits for biodiversity and are available now but face land-use competition and permanence concerns.

Technological CDR includes direct air capture (DAC), which uses chemical sorbents to extract CO2 from ambient air for geological storage. Current costs range from 400 to 1,000 dollars per tonne, though proponents project reductions to 100-200 dollars at scale. Bioenergy with carbon capture and storage (BECCS) grows biomass that absorbs CO2, burns it for energy, and captures the resulting emissions for underground storage. Enhanced weathering accelerates natural rock weathering by spreading crusite silicate minerals on agricultural land.

Governance Challenges

No international governance framework exists for geoengineering deployment. SRM raises particularly acute concerns because it could be deployed cheaply by a single nation or wealthy individual, affecting the entire planet. The lack of consensus about acceptable climate targets, risk tolerance, and equitable distribution of effects makes governance extremely challenging.

Some researchers argue that even outdoor experiments should be governed carefully to prevent a slippery slope toward deployment. Others argue that research is essential to understand options in case mitigation proves insufficient. The scientific community generally agrees that geoengineering cannot substitute for emission reductions but may complement them if carefully governed.