Climate engineering (also called geoengineering) is a term used for both carbon dioxide removal and solar radiation management, also called solar geoengineering, when applied at a planetary scale.: 6-11 However, they have very different geophysical characteristics which is why the Intergovernmental Panel on Climate Change no longer uses this overarching term.: 6-11 Carbon dioxide removal approaches are part of climate change mitigation. Solar geoengineering involves reflecting some sunlight (solar radiation) back to space. All forms of geoengineering are not a standalone solution to climate change, but need to be coupled with other forms of climate change mitigation. Another approach to geoengineering is to increase the Earth's thermal emittance through passive radiative cooling.
Carbon dioxide removal is defined as "Anthropogenic activities removing carbon dioxide (CO2) from the atmosphere and durably storing it in geological, terrestrial, or ocean reservoirs, or in products. It includes existing and potential anthropogenic enhancement of biological or geochemical CO2 sinks and direct air carbon dioxide capture and storage, but excludes natural CO2 uptake not directly caused by human activities."
Some types of climate engineering are highly controversial due to the large uncertainties around effectiveness, side effects and unforeseen consequences. However, the risks of such interventions must be seen in the context of the trajectory of climate change without them.
Climate engineering (or geoengineering) has been used as an umbrella term for both carbon dioxide removal and solar radiation management (or solar geoengineering), when applied at a planetary scale.: 6-11 However, these two methods have very different geophysical characteristics, which is why the Intergovernmental Panel on Climate Change no longer uses this term.: 6-11 This decision was communicated in around 2018, see for example the "Special Report on Global Warming of 1.5 °C".: 550
Some authors, for example in the mainstream media, also include passive daytime radiative cooling, "ocean geoengineering" and others in the term of climate engineering.
Specific technologies that fall into the "climate engineering" umbrella term include:: 30
The following methods are not termed "climate engineering" in the latest IPCC assessment report in 2022: 6-11 but are nevertheless included in other publications on this topic:
Carbon dioxide removal (CDR), also known as carbon removal, greenhouse gas removal (GGR) or negative emissions, is a process in which carbon dioxide gas (CO2) is removed from the atmosphere by deliberate human activities and durably stored in geological, terrestrial, or ocean reservoirs, or in products.: 2221 In the context of net zero greenhouse gas emissions targets, CDR is increasingly integrated into climate policy, as an element of climate change mitigation strategies. Achieving net zero emissions will require both deep cuts in emissions and the use of CDR. CDR can counterbalance emissions that are technically difficult to eliminate, such as some agricultural and industrial emissions.: 114
Solar geoengineering, or solar radiation modification (SRM), is a type of climate engineering in which sunlight (solar radiation) would be reflected back to outer space to limit or offset human-caused climate change. There are multiple potential approaches, with stratospheric aerosol injection (SAI) being the most-studied method, followed by marine cloud brightening (MCB). Other methods have been proposed, including a variety of space-based approaches, but they are generally considered less viable, and are not taken seriously by the Intergovernmental Panel on Climate Change. SRM methods could have a rapid cooling effect on atmospheric temperature, but if the intervention were to suddenly stop for any reason, the cooling would soon stop as well. It is estimated that the cooling impact from SAI would cease 1-3 years after the last aerosol injection, while the impact from marine cloud brightening would disappear in just 10 days. Contrastingly, once any carbon dioxide is added to the atmosphere and not removed, its warming impact does not decrease for a century, and some of it will persist for hundreds to thousands of years. As such, solar geoengineering is not a substitute for reducing greenhouse gas emissions but would act as a temporary measure to limit warming while emissions of greenhouse gases are reduced and carbon dioxide is removed.
Enhancing the thermal emissivity of Earth through passive daytime radiative cooling has been proposed as an alternative or "third approach" to geoengineering that is "less intrusive" and more predictable or reversible than stratospheric aerosol injection.
Passive daytime radiative cooling (PDRC) is a zero-energy building cooling method proposed as a solution to reduce air conditioning, lower urban heat island effect, cool human body temperatures in extreme heat, move toward carbon neutrality and control global warming by enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zero energy consumption or pollution. Application of PDRCs may also increase the efficiency of systems benefiting of a better cooling, such like photovoltaic systems, dew collection techniques, and thermoelectric generators.
Ocean geoengineering involves adding material such as lime or iron to the ocean to affect its ability to support marine life and/or sequester CO2. In 2021 the US National Academies of Sciences, Engineering, and Medicine (NASEM) requested $2.5 billion funds for research in the following decade, specifically including field tests.
Enriching seawater with calcium hydroxide (lime) has been reported to lower ocean acidity, which reduces pressure on marine life such as oysters and absorb CO2. The added lime raised the water's pH, capturing CO2 in the form of calcium bicarbonate or as carbonate deposited in mollusk shells. Lime is produced in volume for the cement industry. This was assessed in 2022 in an experiment in Apalachicola, Florida in an attempt to halt declining oyster populations. pH levels increased modestly, as CO2 was reduced by 70 ppm.
A 2014 experiment added sodium hydroxide (lye) to part of Australia's Great Barrier Reef. It raised pH levels to nearly preindustrial levels.
However, producing alkaline materials typically releases large amounts of CO2, partially offsetting the sequestration. Alkaline additives become diluted and dispersed in one month, without durable effects, such that if necessary, the program could be ended without leaving long-term effects.
Another 2022 experiment attempted to sequester carbon using giant kelp planted off the Namibian coast. Whilst this approach has been called "ocean geoengineering" by the researchers it is just another form of carbon dioxide removal via sequestration. Another term that is used to describe this process is blue carbon management and also marine geoengineering.
Some engineering interventions have been proposed for Thwaites Glacier and the nearby Pine Island Glacier to stabilize its ice physically, or to preserve it by blocking the flow of warm ocean water, which currently renders the collapse of these two glaciers practically inevitable even without further warming. A proposal from 2018 included building sills at the Thwaites' grounding line to either physically reinforce it, or to block some fraction of warm water flow. The former would be the simplest intervention, yet still equivalent to "the largest civil engineering projects that humanity has ever attempted": it is also only 30% likely to work. Constructions blocking even 50% of the warm water flow are expected to be far more effective, yet far more difficult as well. Further, some researchers dissented, arguing that this proposal could be ineffective, or even accelerate sea level rise. The original authors have suggested attempting this intervention on smaller sites, like the Jakobshavn Glacier in Greenland, as a test run, as well as acknowledging that this intervention cannot prevent sea level rise from the increased ocean heat content, and would be ineffective in the long run without greenhouse gas emission reductions.
According to climate economist Gernot Wagner the term "geoengineering" is "largely an artefact and a result of the terms frequent use in popular discourse" and "so vague and all-encompassing as to have lost much meaning".: 14
Interventions at large scale run a greater risk of unintended disruptions of natural systems, resulting in a dilemma that such disruptions might be more damaging than the climate damage that they offset.
Climate engineering may reduce the urgency of reducing carbon emissions, a form of moral hazard. Also, most efforts have only temporary effects, which implies rapid rebound if they are not sustained. The Union of Concerned Scientists points to the danger that the technology will become an excuse not to address the root causes of climate change, slow our emissions reductions and start moving toward a low-carbon economy. However, several public opinion surveys and focus groups reported either desire to increase emission cuts in the presence of climate engineering, or of no effect. Other modelling work suggests that the prospect of climate engineering may in fact increase the likelihood of emissions reduction.
If climate engineering can alter the climate, then this raises questions whether humans have the right to deliberately change the climate, and under what conditions. For example, using climate engineering to stabilize temperatures is not the same as doing so to optimize the climate for some other purpose. Some religious traditions express views on the relationship between humans and their surroundings that encourage (to conduct responsible stewardship) or discourage (to avoid hubris) explicit actions to affect climate.
A large 2018 study used an online survey to investigate public perceptions of six climate engineering methods in the United States, United Kingdom, Australia, and New Zealand. Public awareness of climate engineering was low; less than a fifth of respondents reported prior knowledge. Perceptions of the six climate engineering methods proposed (three from the carbon dioxide removal group and three from the solar geoengineering group) were largely negative and frequently associated with attributes like 'risky', 'artificial' and 'unknown effects'. Carbon dioxide removal methods were preferred over solar geoengineering. Public perceptions were remarkably stable with only minor differences between the different countries in the surveys.
Some environmental organizations (such as Friends of the Earth and Greenpeace) have been reluctant to endorse or oppose solar geoengineering, but are often more supportive of nature-based carbon dioxide removal projects, such as afforestation and peatland restoration.
Several organizations have investigated climate engineering with a view to evaluating its potential, including the US Congress, the US National Academy of Sciences, Engineering, and Medicine, the Royal Society, the UK Parliament, the Institution of Mechanical Engineers, and the Intergovernmental Panel on Climate Change. The IMechE report examined a small subset of proposed methods (air capture, urban albedo and algal-based CO2 capture techniques), and its main conclusions were that climate engineering should be researched and trialed at the small scale alongside a wider decarbonization of the economy.
The Royal Society review examined a wide range of proposed climate engineering methods and evaluated them in terms of effectiveness, affordability, timeliness, and safety (assigning qualitative estimates in each assessment). The key recommendations reports were that "Parties to the UNFCCC should make increased efforts towards mitigating and adapting to climate change, and in particular to agreeing to global emissions reductions", and that "[nothing] now known about geoengineering options gives any reason to diminish these efforts". Nonetheless, the report also recommended that "research and development of climate engineering options should be undertaken to investigate whether low-risk methods can be made available if it becomes necessary to reduce the rate of warming this century".
In 2009, a review examined the scientific plausibility of proposed methods rather than the practical considerations such as engineering feasibility or economic cost. The authors found that "[air] capture and storage shows the greatest potential, combined with afforestation, reforestation and bio-char production", and noted that "other suggestions that have received considerable media attention, in particular, "ocean pipes" appear to be ineffective". They concluded that "[climate] geoengineering is best considered as a potential complement to the mitigation of CO2 emissions, rather than as an alternative to it".
In 2015, the US National Academy of Sciences, Engineering, and Medicine concluded a 21-month project to study the potential impacts, benefits, and costs of climate engineering. The differences between these two classes of climate engineering "led the committee to evaluate the two types of approaches separately in companion reports, a distinction it hopes carries over to future scientific and policy discussions." The resulting study titled Climate Intervention was released in February 2015 and consists of two volumes: Reflecting Sunlight to Cool Earth and Carbon Dioxide Removal and Reliable Sequestration. According to their brief about the study:
Climate intervention is no substitute for reductions in carbon dioxide emissions and adaptation efforts aimed at reducing the negative consequences of climate change. However, as our planet enters a period of changing climate never before experienced in recorded human history, interest is growing in the potential for deliberate intervention in the climate system to counter climate change... Carbon dioxide removal strategies address a key driver of climate change, but research is needed to fully assess if any of these technologies could be appropriate for large-scale deployment. Albedo modification strategies could rapidly cool the planet's surface but pose environmental and other risks that are not well understood and therefore should not be deployed at climate-altering scales; more research is needed to determine if albedo modification approaches could be viable in the future.
In June 2023 the US government released a report that recommended conducting research on stratospheric aerosol injection and marine cloud brightening.