Geoengineering, also called climate engineering or climate intervention, refers to planned, large-scale actions taken to fight climate change caused by humans. This field 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 works by reflecting some sunlight away from Earth back into space to lower global temperatures. Though these methods were once grouped together, they differ in how they work, how much time they take, and the risks they might cause. They are now usually discussed separately. Other large-scale ideas, such as slowing the melting of ice in polar and alpine regions, are sometimes included in the category of geoengineering.
Some geoengineering methods raise political, social, and ethical concerns. A common worry is that relying on these technologies might make it harder to reduce greenhouse gas emissions. Many experts agree that strong rules and international cooperation are needed to manage these efforts.
Major scientific groups have studied the possibilities, risks, and rules for managing geoengineering. These include the U.S. National Academies of Sciences, Engineering, and Medicine, the Royal Society, the United Nations Educational, Scientific and Cultural Organization (UNESCO), and the World Climate Research Programme.
Methods
Carbon dioxide removal (CDR) is a process where humans intentionally take carbon dioxide (CO₂) from Earth’s atmosphere and store it in places like underground areas, forests, oceans, or in products. This process is also called carbon removal, greenhouse gas removal, or negative emissions. CDR is increasingly being used in climate policies as part of plans to reduce climate change. To reach net zero emissions, the most important step is to greatly and continuously reduce emissions. After that, CDR may be used to balance out emissions that are hard to eliminate, such as some emissions from farming and industry.
CDR methods can be done on land or in water. On land, these include planting trees, improving farming practices to store carbon in soil, using bioenergy with carbon capture and storage (BECCS), and capturing CO₂ directly from the air and storing it. In water, methods include adding nutrients to oceans to help them absorb more CO₂, making ocean water more alkaline to store CO₂, restoring wetlands, and using natural coastal areas to store carbon. To determine how much CO₂ a CDR method removes, scientists study the entire process, including how it affects the environment over time and how it is monitored, reported, and checked for accuracy. 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 to reduce global warming by reflecting more sunlight back into space. It is not meant to replace efforts to cut greenhouse gas emissions but could help limit warming as an extra tool. SRM is a type of geoengineering.
The most studied SRM method is stratospheric aerosol injection (SAI), which involves putting small reflective particles into the upper atmosphere to reflect sunlight. Other methods include making ocean clouds more reflective (marine cloud brightening) or building space structures to block sunlight.
Glacial geoengineering is a set of proposed methods to slow the melting of glaciers, ice sheets, and sea ice in polar and mountain areas. These ideas aim to address concerns that melting ice could worsen climate change through effects like less sunlight being reflected, faster ice movement, and methane release from frozen ground.
Proposed glacial geoengineering methods include using solar radiation management in specific areas, thinning clouds to let more heat escape, and using engineering tools to support ice. Ideas being studied include using SAI in polar regions, making ocean clouds more reflective, covering ice with reflective materials, managing water under ice, and protecting ice shelves with underwater barriers.
Glacial geoengineering is still in early research stages, and many ideas face challenges related to technology, the environment, and how decisions are made. Some people believe these methods could help slow ice loss, reduce rising sea levels, and prevent dangerous climate changes. However, scientists warn that these methods are not well understood and might cause unexpected problems. Glacial geoengineering is generally seen as a possible addition 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 checks before starting, and be monitored by proper authorities. In 2016, they encouraged more research that combines different fields of study 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 by Dumping of Wastes and the London Protocol have discussed "marine geoengineering." In 2013, the London Protocol added a rule that requires certain activities, such as ocean fertilization, to be reviewed and approved before happening. This rule is not yet active because not enough countries have agreed to it. In 2022, the parties to both agreements noted increasing interest in marine geoengineering, listed four techniques for further study, and urged careful evaluation of projects using current guidelines while considering possible new rules. In 2023, they warned that these techniques might cause serious harm to the environment, pointed out scientific uncertainty about their effects, and called for strict use of review processes and stronger international teamwork. Their work is supported by the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection from the International Maritime Organization.