Climate engineering (or geoengineering) is an umbrella term for both
carbon dioxide removal and
solar radiation modification, when applied at a planetary scale.[1]: 6–11 However, these two processes have very different characteristics. For this reason, the
Intergovernmental Panel on Climate Change no longer uses this overarching term.[1]: 6–11 [2] Carbon dioxide removal approaches are part of
climate change mitigation. Solar radiation modification is reflecting some
sunlight (solar radiation) back to space.[3] All forms of climate engineering cannot be standalone solutions to climate change, but need to be coupled with other forms of climate change mitigation.[4] Some publications place
passive radiative cooling into the climate engineering category. This technology increases the Earth's thermal emittance.[5][6][7] The media tends to use climate engineering also for other technologies such as glacier stabilization, ocean liming, and
iron fertilization of oceans. The latter would modify
carbon sequestration processes that take place in oceans.
Some types of climate engineering are highly controversial due to the large uncertainties around effectiveness,
side effects and
unforeseen consequences.[8] However, the risks of such interventions must be seen in the context of the trajectory of climate change without them.[9][10]
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".[8]: 14
Terminology
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.[1]: 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.[1]: 6–11 [2] This decision was communicated in around 2018, see for example the "
Special Report on Global Warming of 1.5 °C".[11]: 550 Specific technologies that fall into the "climate engineering" umbrella term include:[12]: 30
Mirrors in space (MIS) - satellites that are designed to change the amount of solar radiation that impacts the Earth as a form of climate engineering. Since the conception of the idea in 1923, 1929, 1957 and 1978 (Hermann Oberth) and also in the 1980s, space mirrors have mainly been theorized as a way to deflect sunlight to counter
global warming and were seriously considered in the 2000s.[17][18][19][20][21][22]
The following methods are not termed climate engineering in the
latest IPCC assessment report in 2022[1]: 6–11 but are included under this umbrella term by other publications on this topic:[24][8]
Ground-level albedo modification - a process of increasing Earth's albedo through the means of altering things on the Earth's surface. Examples include
planting light-colored plants to help with reflecting sunlight back into space.[25]
Carbon dioxide removal (CDR) is a process in which carbon dioxide (CO2) is removed from the atmosphere by deliberate human activities and durably stored in geological, terrestrial, or ocean reservoirs, or in products.[33]: 2221 This process is also known as carbon removal, greenhouse gas removal or negative emissions. CDR is more and more often integrated into
climate policy, as an element of
climate change mitigation strategies.[34][35] Achieving
net zero emissions will require first and foremost deep and sustained cuts in emissions, and then—in addition—the use of CDR ("CDR is what puts the net into net zero emissions"[36]). In the future, CDR may be able to counterbalance emissions that are technically difficult to eliminate, such as some agricultural and industrial emissions.[37]: 114
Studies using
climate models have generally shown that SRM could reduce many adverse
effects of climate change. Specifically, controlled stratospheric aerosol injection appears able to greatly moderate most environmental impacts—especially warming—and consequently most ecological, economic, and other impacts of climate change across most regions. However, because warming from greenhouse gases and cooling from SRM would operate differently across
latitudes and
seasons, a world where global warming would be offset by SRM would have a different climate from the world where this warming did not occur in the first place, mainly as the result of an altered
hydrological cycle. Furthermore, confidence in the current projections of how SRM would affect regional climate and
ecosystems is low.[40]
Passive daytime radiative cooling
Enhancing the
thermal emissivity of Earth through passive daytime radiative cooling has been proposed as an alternative or "third approach" to climate engineering[5][41] that is "less intrusive" and more predictable or reversible than stratospheric aerosol injection.[42]
PDRC surfaces are designed to be high in
solar reflectance (to minimize heat gain) and strong in
longwave infrared (LWIR)
thermal radiationheat transfer through the atmosphere's
infrared window (8–13 µm) to cool temperatures even during the daytime.[56][57][58] It is also referred to as passive radiative cooling, daytime passive radiative cooling, radiative sky cooling, photonic radiative cooling, and terrestrial radiative cooling.[57][58][54][59] PDRC differs from
solar radiation management because it increases radiative heat emission rather than merely reflecting the absorption of solar radiation.[60]
Ocean geoengineering involves adding material such as lime or iron to the ocean to affect its ability to support marine life and/or sequester CO 2. 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.[30]
Enriching seawater with calcium hydroxide (
lime) has been reported to lower
ocean acidity, which reduces pressure on
marine life such as
oysters and absorb CO 2. The added lime raised the water's
pH, capturing CO 2 in the form of
calcium bicarbonate or as
carbonate deposited in
mollusk shells. Lime is produced in volume for the cement industry.[30] This was assessed in 2022 in an experiment in
Apalachicola, Florida in an attempt to halt declining oyster populations. pH levels increased modestly, as CO 2 was reduced by 70 ppm.[30]
However, producing alkaline materials typically releases large amounts of CO 2, 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.[30]
Another 2022 experiment attempted to sequester carbon using
giant kelp planted off the
Namibian coast.[30] 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.[61][62] 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.[63] Further, some researchers dissented, arguing that this proposal could be ineffective, or even accelerate sea level rise.[64] The original authors have suggested attempting this intervention on smaller sites, like the
Jakobshavn Glacier in
Greenland, as a test run,[63][62] 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.[63]
In 2023, a modified proposal was tabled: it was proposed that an installation of underwater "curtains", made out of a flexible material and anchored to
Amundsen Sea floor would be able to interrupt warm water flow while reducing costs and increasing their longevity (conservatively estimated at 25 years for curtain elements and up to 100 years for the foundations) relative to more rigid structures. With them in place, Thwaites Ice Shelf and Pine Island Ice Shelf would presumably be able to regrow to a state they last had a century ago, thus stabilizing these glaciers.[65][66][62] To achieve this, the curtains would have to be placed at a depth of around 600 metres (0.37 miles) (to avoid damage from
icebergs which would be regularly drifting above) and be 80 km (50 mi) long. The authors acknowledged that while work on this scale would be unprecedented and face many challenges in the Antarctic (including
polar night and the currently insufficient numbers of specialized polar ships and underwater vessels), it would also not require any new technology and there is already experience of laying down
pipelines at such depths.[65][66]
Problems
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.[9]
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".[8]: 14
Ethical aspects
Climate engineering may reduce the urgency of reducing carbon emissions, a form of
moral hazard.[67] Also, most efforts have only temporary effects, which implies rapid rebound if they are not sustained.[68] 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.[69] However, several public opinion surveys and focus groups reported either a desire to increase emission cuts in the presence of climate engineering, or no effect.[70][71][72] Other modelling work suggests that the prospect of climate engineering may in fact increase the likelihood of emissions reduction.[73][74][75][76]
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.[77]
Society and culture
Public perception
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.[12] 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 radiation modification group) were largely negative and frequently associated with attributes like 'risky', 'artificial' and 'unknown effects'. Carbon dioxide removal methods were preferred over solar radiation modification. Public perceptions were remarkably stable with only minor differences between the different countries in the surveys.[12][78]
Some environmental organizations (such as
Friends of the Earth and
Greenpeace) have been reluctant to endorse or oppose solar radiation modification, but are often more supportive of nature-based carbon dioxide removal projects, such as
afforestation and
peatland restoration.[67][79]
History
Several organizations have investigated climate engineering with a view to evaluating its potential, including the
US Congress,[80] the US National Academy of Sciences, Engineering, and Medicine,[81] the
Royal Society,[82] the
UK Parliament,[83] the
Institution of Mechanical Engineers,[84] 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.[84]
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".[85] 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".[85]
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".[86] 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".[86]
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."[87][88][89] The resulting study titled Climate Intervention was released in February 2015 and consists of two volumes: Reflecting Sunlight to Cool Earth[90] and Carbon Dioxide Removal and Reliable Sequestration.[91]
In June 2023 the US government released a report that recommended conducting research on stratospheric aerosol injection and marine cloud brightening.[92]
As of 2024 the Coastal Atmospheric Aerosol Research and Engagement (CAARE) project was launching
sea salt into the marine sky in an effort to increase cloud "brightness" (reflective capacity). The sea salt is launched from the
USS Hornet Sea, Air & Space Museum (based on the project's regulatory filings).[93]
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S2CID201590290. Further, radiative cooling cannot be a complete, standalone solution, but rather is part of a more comprehensive approach that must include CO2 reduction. Otherwise, the radiative balance will not last long, and the potential financial benefits of mitigation will not fully be realized because of continued ocean acidification, air pollution, and redistribution of biomass.
^Committee on Geoengineering Climate: Technical Evaluation and Discussion of Impacts; Board on Atmospheric Sciences and Climate; Ocean Studies Board; Division on Earth and Life Studies; National Research Council (2015).
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