Climate geoengineering is the most prominent type of geoengineering, and that is the focus of the current controversy. This glossary focuses exclusively on climate geoengineering approaches.
There are currently four main approaches to engineering the climate system, described below:
Specific acronyms and terms in each domain are spelt out below the general descriptions of these terms.
Solar Radiation Management describes a suite of proposed technologies that aim to reflect sunlight back into space before it warms the earth’s climate. These proposed technique would achieve this in one of a number of ways: by spraying sulphur dioxide or hydrogen sulphide into the stratosphere (the upper reaches of the atmosphere), by launching a 300 km2 barrier into heliosynchronous orbit, by making the albedo (reflectivity) of clouds, or by launching billions of reflective balloons into the stratosphere.
Stratospheric Aerosol Injection (SAI)
A solar radiation management proposal 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.
High-Albedo Crops and Snow Forest Clearance
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. 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) or Cloud Reflectivity Enhancement
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.
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.
ERM refers to a suite of proposed techniques that aim to increase the flow of warm air from the earth into space.
Cirrus Cloud Thinning
By thinning cirrus clouds (wispy, elongated at high altitudes), some researchers have proposed that more heat could be allowed to escape into space, creating an overall cooling of the climate.
Greenhouse Gas Removal (GGR) refers to an technique that removes greenhouse gases from the atmosphere through mechanical means. A more common umbrella term is Carbon Dioxide Removal (CDR), which excludes methods that remove other greenhouse gases such as methane.
Carbon Capture and Storage (CCS)
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)
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)
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)
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.
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)
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.
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.
Weather modification refers to various techniques for changing weather and precipitation patterns without intending to change overall climate patterns. These include cloud seeding and other related techniques.
Bio-geoengineering refers to efforts to cool the climate using plant life. Proposed techniques include increasing the albedo (reflectivity) of large areas through planting, and growing plants that absorb CO2. BECCS, Ocean Fertilization and efforts to bioengineer algae to convert liquid CO2 into biofuels can also be considered bio-geoengineering.
EOR – Enhanced Oil Recovery
EOR is not a geoengineering technique, but a likely destination for the vast amounts of liquified carbon gathered by CDR schemes. EOR involves pumping liquified CO2 into oil-bearing underground deposits to force an increased flow of oil or other fossil resources. Many CDR companies are looking to sell their liquified CO2 to EOR markets.
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