The following “technologies” are mostly hypothetical proposals advanced by various geoengineering advocates. Nonetheless, we are keeping close track of each one.
Geoengineering technologies can be categorized by different approaches (solar radiation management, carbon dioxide removal, weather modification), or by where they seek to intervene in the planetary ecosystem (land, air, water). For more background, see: What is Geoengineering? and Reasons to Oppose.
Stratospheric Aerosol Injection (SAI)
Type: Solar Radiation Management Location: Upper atmosphere Impacts: Air, Land Proposal: Spray sulphites or other particles into the stratosphere to block the sun
SAI proposes to spray large quantities of sulphur particles (e.g. sulphur dioxide) into the stratosphere (the upper layer of the atmosphere) to act as a reflective barrier against incoming sunlight. Proposals range from shooting particles from artillery guns, using large hoses to the sky or emptying particles from the back of aircraft. The design of self-levitating particles, as well as the use of particles of other reflective minerals (e.g. titanium or aluminum) have also been considered.
Type: Solar Radiation Management Location: Farms and plantations Impacts: Land Proposal: Genetically modify crops to make them reflect more sunlight back into space
Various proposals suggest that growing crops that reflect more light (either new crops, or high-albedo varieties of existing crops) could cool the atmosphere reflecting more solar radiation back into space.
Snow Forest Clearance
Type: Solar Radiation Management Location: Forests Impacts: Land Proposal: Cut down trees, brush to reflect more light into space
Others suggest clearing forests that exist in areas that are snow-covered for a large part of the year, increasing the amount of light reflected back into space by the flatter, brighter snow.
Marine Cloud Brightening (MCB)
Type: Solar Radiation Management Location: Clouds Impacts: Land, Air Proposal: Spray salt water into clouds so that they reflect more sunlight
MCB proposals aim to increase the whiteness of clouds in order to reflect more sunlight back into space. One proposal involves spraying a fine mist (of seawater, for example) into low-lying marine clouds that would create more cloud condensation nuclei (the particles that provide a surface for vapour to condense upon, forming clouds), potentially making the cloud whiter. Others propose seeding clouds with biological or other nuclei.
Type: Solar Radiation Management Location: Ocean Impacts: Ocean Proposal: Millions of tiny air bubbles reflect light off of ocean
Microbubble proposals suggest that by generating millions of tiny air bubbles in the ocean, large areas could be made to reflect more sunlight back into space.
Cirrus Cloud Thinning
Type: Earth Radiation Management Location: Atmosphere Impacts: Air Proposal: Drones spray substances that dissipate cirrus clouds
By thinning cirrus clouds (wispy, elongated clouds at high altitudes), some researchers have proposed that more heat could be allowed to escape into space, creating an overall cooling of the climate.
Carbon Capture and Storage (CCS)
Type: Carbon Dioxide Removal Location: Land Impacts: Land, Air Proposal: Filter out carbon at the smokestack, bury it
CCS usually refers to the mechanical capture of CO2 emissions from power plants or other industrial sources. The CO2 is typically captured before the emissions leave the smokestack, generally with a sorbent chemical. The liquified CO2 is then pumped into underground aquifers for long term storage. CCS is not regarded as geoengineering under the UN Convention on Biodiversity’s definition.
Direct Air Capture (DAC)
Type: Carbon Dioxide Removal Location: Land Impacts: Land, Air Proposal: Suck carbon dioxide out of the air
Extracting CO2 or other greenhouse gases from the atmosphere by chemical and mechanical means, generally using a chemical sorbent and large fans to move air through a filter. The CO2 is then available as a stream of gas for CCS or EOR or other uses.
Carbon Capture Use and Storage (CCUS)
Type: Carbon Dioxide Removal Location: Land Impacts: Land, Air Proposal: Filter out carbon at the smokestack, use it for manufacturing
The idea that captured CO2 from either industry or the atmosphere can be used as a feedstock for manufacturing, resulting in CO2 stored in products. One hypothetical example involves feeding captured CO2 to algae which produce biofuels; another is reacting CO2 with calcifying minerals to produce concrete for building purposes.
Bioenergy with Carbon Capture and Storage (BECCS)
Type: Carbon Dioxide Removal Location: Land Impacts: Land, Air Proposal: Cut down trees, burn wood, capture carbon, bury carbon, plant more trees
Capturing CO2 from bioenergy applications (e.g. ethanol production or burning biomass for electricity) and subsequently sequestering that CO2 through either CCS or CCUS. The theory is that BECCS is “carbon negative” because bioenergy is theoretically “carbon neutral,” based on the idea that plants will regrow to fix the carbon that has been emitted. Many critics say this overlooks emissions from land use change and life cycle emissions.
Type: Carbon Dioxide Removal Location: Ocean Impacts: Ocean Proposal: Dump iron pellets into the ocean to stimulate plankton growth
Ocean fertilization refers to dumping iron (as powdered iron sulphate) or other nutrients (e.g. urea) into the ocean in areas with low biological productivity in order to stimulate phytoplankton growth. In theory, the resulting phytoplankton draw down atmospheric CO2 and then die, falling to the ocean bed and sequestering carbon.
Enhanced Weathering (EW)
Type: Carbon Dioxide Removal Location: Ocean, Land Impacts: Ocean Proposal: Crush millions of tonnes of silicate minerals, spread them in oceans and on land
EW techniques propose to dissolve crushed minerals (particularly silicate minerals) on land or in the sea in order to react with and fix atmospheric carbon dioxide into oceans and soils.
Type: Carbon Dioxide Removal Location: Land Impacts: Land Proposal: Millions of tonnes of silicate minerals, crushed and spread in oceans and on land, would absorb some carbon from the atmosphere
Biochar techniques propose to burn biomass and municipal waste in the absence of oxygen to create charcoal. This charcoal is then mixed into soils as a soil additive, directly burying carbon into the soil. The approach is inspired by (but very different from) Amazonian Terra Preta black soils where indigenous communities have used charcoal to improve fertility.