Geoengineering, also called climate engineering or climate intervention, refers to planned large actions in Earth's climate system to help reduce the effects of climate change caused by humans. This term includes two main types: large-scale carbon dioxide removal (CDR) and solar radiation modification (SRM). CDR uses methods to take carbon dioxide out of the air and is usually seen as a way to reduce the effects of climate change. SRM tries to lower global warming by bouncing some sunlight back into space instead of letting it reach Earth. These two methods work differently, take different amounts of time, and have different risks, so they are now discussed separately. Other large actions, such as slowing the melting of ice in polar and alpine areas, are sometimes also called geoengineering.
Some geoengineering ideas raise problems related to politics, society, and ethics. One concern is that using these technologies might make people less focused on reducing greenhouse gas emissions. Experts say it is important to have clear rules and international agreements to manage these efforts.
Groups like the US National Academies of Sciences, Engineering, and Medicine, the Royal Society, the UN Educational, Scientific and Cultural Organization (UNESCO), and the World Climate Research Programme have studied the possibilities, risks, and rules needed for geoengineering.
Methods
Carbon dioxide removal (CDR) is a process where carbon dioxide (CO₂) is taken from Earth's atmosphere through intentional human actions and stored permanently in places like rocks, soil, oceans, or in products. This process is also called carbon removal, greenhouse gas removal, or negative emissions. CDR is increasingly included in climate policies as part of plans to reduce climate change. To reach net zero emissions, the first step is to greatly and continuously reduce emissions. Then, CDR may be used to balance emissions that are hard to eliminate, such as some emissions from farming and industry.
CDR includes methods done on land or in water. On land, these methods include planting trees, using farming practices that trap carbon in soil, using bioenergy with carbon capture and storage (BECCS), and capturing carbon directly from the air and storing it. Methods in water include adding nutrients to oceans to help plants grow, increasing ocean alkalinity to absorb more CO₂, restoring wetlands, and using blue carbon strategies. To measure how much CO₂ a method removes, a detailed analysis is needed. This includes checking the entire process from start to finish and using monitoring, reporting, and verification (MRV). Carbon capture and storage (CCS) is not considered CDR because it does not remove CO₂ already in the atmosphere.
Solar radiation modification (SRM), also called solar geoengineering, is a group of large-scale methods that aim to reduce global warming by reflecting more sunlight back into space. SRM is not meant to replace efforts to cut greenhouse gas emissions but to help limit warming when emissions are reduced. SRM is a type of geoengineering.
The most studied SRM method is stratospheric aerosol injection (SAI), which involves putting tiny reflective particles into the upper atmosphere to reflect sunlight. Other methods include making ocean clouds more reflective (marine cloud brightening) or building space-based structures to block sunlight.
Glacial geoengineering includes proposed methods to slow the melting of glaciers, ice sheets, and sea ice in polar and alpine regions. These ideas aim to address concerns that melting ice could worsen climate change through effects like ice-albedo feedback, faster glacier movement, and methane release from permafrost.
Proposed glacial geoengineering methods include using solar radiation management in specific areas, reducing heat-trapping clouds, and using engineering structures to support ice. Some strategies being studied include injecting aerosols into the stratosphere over polar regions, brightening ocean clouds, using reflective materials on ice surfaces, draining water under glaciers, and protecting ice shelves with underwater barriers.
Glacial geoengineering is still in early research stages and faces many technical, environmental, and governance challenges. Supporters say targeted actions might help slow ice loss, reduce sea-level rise, and prevent dangerous climate tipping points. However, experts note that these methods are not well understood and could have unexpected effects. Glacial geoengineering is seen as a possible supplement to, not a replacement for, efforts to reduce greenhouse gas emissions.
Governance
Most rules and decisions about geoengineering depend on the type of method used. However, some international agreements have discussed geoengineering as a whole.
The Conference of Parties to the Convention on Biological Diversity has made several decisions about "climate-related geoengineering." In 2010, they created a set of guidelines that are not legally required but suggest that activities related to climate geoengineering should be based on scientific research, include environmental reviews, and be monitored carefully. In 2016, they encouraged more research from different scientific fields and sharing of knowledge to better understand the effects of climate-related geoengineering.
The parties to the London Convention on the Prevention of Marine Pollution and its London Protocol have focused on "marine geoengineering." In 2013, they added a rule to the London Protocol that requires certain steps, such as assessments and permits, before ocean fertilization projects can begin. This rule is not yet active because not enough countries have agreed to it. In 2022, the parties to both agreements noted increased interest in marine geoengineering, identified four techniques for further study, and advised using current guidelines to evaluate projects while considering possible new rules. In 2023, they warned that these techniques might cause serious harm to the environment, noted uncertainty about their effects, and called for strict use of evaluation rules and greater international teamwork. Their work is supported by a group of experts from the International Maritime Organization.