Context - Plan B - Geo-Engineering - Overview

Introduction
Geo-Engineering – The 2 Types
Specific Geo-Engineering Possibilities – Benefits and Risks
References
Recent Updates and News Articles

(Article Note – Direct quotations are indicated in italics with emphasis highlighted in green. All references used in the Article are listed at the end of the Article.)

1. Introduction

As discussed in the Context Article on Carbon Budgets, the total amount of carbon emissions which can be further emitted into the atmosphere but still keeping the world within a ‘safe’ heating threshold of 1.5 degrees or even 2.0 degrees Celsius is very small – this is the ‘Carbon Budget’. At current rates the Carbon Budget will be exhausted within a few years for the more stringent target and the in a couple of decades for 2 degrees warming . It is effectively an acknowledgement that our current mitigation efforts are failing in the sense that they are not reducing emissions quickly enough. If this is correct, as presently looks likely, other strategies will become necessary to avoid dangerous levels of warming. It is in this context that the concept of Geoengineering takes its place.

Geoengineering (also referred to as Climate Engineering) is defined in the first major report on the topic from the Royal Society (‘Geoengineering the climate science, governance and uncertainty’, 2009) as ‘the deliberate large scales manipulation of the planetary environment to counteract anthropogenic climate change’.

It encompasses a range of technologies and strategies aimed at either:a) removing carbon dioxide (CO₂) from the atmosphere or b) reducing the amount of solar energy reaching the earth. Both approaches represent a profound shift in how humanity might confront climate change—not just by mitigating emissions, but by actively and consciously managing the very climate system itself.

However in a world reaching dangerous levels of warming, there are a range of powerful arguments that can be made both in support of and in opposition to Geo-Engineering. Solar Engineering may cool the planet without addressing ocean acidification or the underlying causes of warming, and CDR faces significant scalability, energy, and cost barriers. Both carry substantial environmental hazards; moreover, both approaches raise complex ethical and governance challenges. Who decides whether and how to deploy such technologies? How do we weigh potential risks against the harms of inaction? And what unintended consequences might arise from intervening in planetary systems at scale?

The main features of CDR and SRM will be looked at in the next section while some of the specific possibilities within each branch will be outlined and their benefits and risks examined. At a later date two further Context Articles will examine in further detail what appear to be the key representative technologies in each category; Direct Air Capture (DAC) for CDR and Sulphur Atmospheric Injection (SAI) in the case of SRM. The broader governance, risk management and ethical and indeed fundamental philosophical questions will be considered in these contexts where humans consciously and directly take on the role of managing key climate aspects and essentially ‘playing God’. The risks are real and significant in making such drastic interventions at the scale required to make a difference but as outlined in several other Context Articles as indeed most of recent climate literature, so are the risks of unchecked global heating where we are not able or simply are not modifying our emissions, social and economic structures fast enough – It seems like a true Faustian decision, but one we may shortly have no option but to make.

2. Climate Engineering – The Two Types

Geoengineering as a whole originally included the two main branches, Carbon Dioxide Removal (CDR) and Solar Radiation Modification (SRM). Whilst both of these are indeed intended as large scale manipulation of the climate, they operate in very distinct manners which has led to the IPCC largely abandoning the term ‘Geoengineering’ in favour of these two separate terms, although it continues in the popular usage.

Carbon Dioxide Removal (CDR)

CDR is a type of negative emissions technology (NET) which involves the physical removal of CO2 from the atmosphere and storage in secure natural reservoirs such in rock formation. The removal can be either i) nature based, which essentially enhances existing natural processes to extract carbon such as forestation (either new forests ‘afforestation’ or the restoration of degraded ones ‘reforestation’) or ii) technological engineering processes such as direct air capture which filters air and utilises chemical processes to extract carbon dioxide.

There has been a lot of focus on biological based CDR such as reforestation and afforestation but also engineering based approaches are also being developed but which are mostly in the early stages of deployment and relatively tiny in scale relative to what would likely be required.

IPCC in its IPCC’s 6^th Assessment Report, Working Group 3 – Climate Change 2022: Mitigation of Climate Change (6^thAR-WG3)- Technical summary (Box TS. 10, p114) provides 3 main roles for CDR:

lowering of CO2 emissions in the near term,
in the medium term, counterbalancing ‘residual emissions’ from ‘hard to abate’ industrial sectors such as steel and cement production once other sectors have effectively decarbonised, and
by the second half of this century, achieving net negative emissions where in effect more carbon is extracted from the atmosphere than is emitted by humans.

All IPCC carbon emission scenarios (high, medium and low) and, with only a few exceptions, all climate models that provide a 1.5 degrees and indeed 2.0 degrees warming limit outcome by the year 2100 involve an ‘overshoot’ of carbon dioxide emissions followed by negative emissions which remove carbon to bring the cumulative atmospheric carbon dioxide to levels that result in a reduction in global average temperatures to no more than the target climatic heating. This is because socially, economically and politically; it is not considered feasible to cut carbon dioxide at the rates required to actually stay within these heating targets in the first place and thus keep the planet within ‘safe’ heating limits as indicated in the Paris Agreement. Instead under this rationale, it is accepted that temperatures will overshoot and we will deploy CDR, both biological and technological, at massive scale to bring the temperatures back down, so the theory goes which incidentally is now relied upon by practically all carbon models.

However the removal challenge is simply enormous and all negative emission capabilities would need to be considered and probably deployed. It would also be necessary to take account of the potentially significant negative side effects of the enormous scales at which such carbon removal strategies would need to work at– this will be looked at later in this article. The IPCC’s reference rate, provided in the IPCC AR6 Synthesis report at para 3.3.4 (page 87), indicates that for every 0.1 degree Celsius reduction of global temperature, removal of 220 Gigatonnes (Gt = billion tonnes) of CO2 would be required to be removed (that is over 4 years total CO2 emitted at todays rates). It is more likely several multiples of this volume of carbon would need to be removed judging from the most recent UNEP Gap report which puts the world on a current trajectory of 2.7 degrees of warming, a staggering volumes of carbon being removed and a big question mark hanging over its true feasibility.

Solar Radiation Management/Modification (SRM)

SRM, also referred to as solar geoengineering, is the grouping of technologies and approaches that limit global warming not by reducing greenhouse gases climate forcing but by limiting global warming by reducing the amount of sunlight that actually reaches the Earth. This would be done by reflecting solar radiation away from the earth and back into space. In effect they deal with the symptoms of global warming by reducing the heating rather than reducing greenhouse gases which are the root cause.

However SRM is usually considered as complementary technological approach to CDR, mainly due to the long period of time that would be required for CDR to scale sufficiently to effectively achieve global cooling through enormous negative emissions as discussed in the above section. SRM would effectively operate as a ‘stop gap’ to buy time whist CDR scaled up sufficiently by preventing the global heating from reaching dangerous levels with potentials for climate tipping points and feedbacks putting a safer planet otherwise out of reach. IPCC’s 6th Assessment Report – WG1 Report actually indicates (section 4.6.3.3) that SRM ‘is only considered as a potential supplement to deep mitigation in overshoot scenarios.

As Carbon Brief indicates SRM technologies do not directly reduce the amount of greenhouse gases in the atmosphere or prevent carbon build up and problems such as ocean acidification would not be addressed because CO2 continues to accumulate in atmosphere and oceans regardless of its deployment. However certain critical feedbacks reaching tipping points, (see Tipping Point Introduction Context Article)could be prevented or slowed by SRM, thus preventing further se enormous amounts of further carbon dioxide and methane being released in nightmare run away scenarios as happened in the geological past (see Paleo-Climatology Context Article)

The IPCC 6AR WG1 Report is broadly optimistic about several SRM options and concludes (with high confidence) from its assessment of the climate response to SRM that ‘if practicable, [SRM] could substantially offset global temperature rise and partially offset other impacts of global warming’ although the precise amounts cannot be confidently indicated. The qualification ‘if practicable’ acknowledges that while there is substantial promise in the technology there are also potentially serious consequences for the planet which is attendant on any attempt to engineer the climate system at a global scale with something so fundamental as the amount of solar energy reaching the earth. In particular, reduction of sunlight could impact rain patterns including the monsoon at global as well as regional scales and also affect plant respiration and photosynthesis.

The Wikipedia article on the SRM highlights the risk of unintended disruptions of natural systems. This potential global scale dangers would need to considered alongside the potential for reducing global heating – the stakes are high in either direction, principally because warnings now about the burning of fossil fuels are not being taken sufficiently seriously. It raises the question whether the negative consequences might be worse than the problem it is set to resolve.

It is in response to such concerns that the Parties to the Convention on Biological Diversity in 2010 placed a ‘de facto moratorium’ on further climate geo-engineering activity until they are scientifically justified, the risks are understood and appropriate governance is in place. This position has been restated by the European Union’s scientific advisors in December 2024.

3. Specific Geo-Engineering Possibilities – Benefits and Risks

Following is an outline of the range of geo or climate engineering types including details on their development stage, their potential to reduce carbon and the associated costs and impacts. The method descriptions are taken from a combination of the Royal Society Geoengineering report, Princeton University website article and Carbon Brief sources. The following Table related to CDR also indicates the technological readiness levels (TLR Scale) from TLR 1 ‘initial concept’ to TLR 9 ‘commercial operation’; the costs and, mitigation potentials are taken from the IPCC AR6 WG3 Report and the benefits and risks from AR, Working Group 1 – Climate Change 2021: the Physical Science Basis (6^thAR-WG1), all referenced/linked at the end of this article. The comment and ‘RAG’ appraisal is an general assessment of the information by the Context article writer. Similarly with respect to the SAR analysis which follows with different criteria and layout presented.

A. Carbon Dioxide Removal (CDR) – Approach and Technology Types:

1.Afforestation: biological– growing trees to absorb and store CO2 in wood.

2.Biochar: biological – burning biomass in low oxygen environment to store carbon in soil.

3. Bioenergy with Carbon Capture and Storage (BECCS): Involves growing biomass for energy production, capturing the CO2 emitted during processing, and storing it. As of 2022, BECCS was capturing approximately 2 million tonnes of CO₂ annually.

4.Ocean Fertilization: Entails adding nutrients like iron to oceans to stimulate phytoplankton growth, which absorbs CO2. Iorn is essential for photosynthesis and is often the limiting nutrient for phytoplankton growth. Algal blooms can be created by supplying iorn to deficient waters which can nourish other organisms. However, this method’s ecological impacts are not fully understood.

5. Ocean Alkaline Enhancement: Adding lime or crushed rocks to oceans to absorb CO₂.

6. Enhanced Weathering: Spreading crushed minerals (e.g., olivine) that react with CO₂ to form stable compounds

7. Direct Air Capture (DAC): Utilizes chemical processes to capture atmospheric CO₂, which is then stored underground or utilized industrially. The first commercial DAC facility in the U.S. commenced operations in November 2023.

Method	Dev Status – TRL* (1-9)	Estimated cost range- US Dollars per 1 tonne CO2	Mitigation Potential- Billion Tonnes of CO2 per year (at max deployment)	Additional Benefits	Risks and Impacts/ Level of public acceptance –	Comment – (RED/AMBER /GREEN highlighting tech readiness
Afforestation/ Reforestation	8-9	0-240	0.5-10	Enhanced employment and local livelihoods. Improved biodiversity. Improved renewable wood provision. Improved soil carbon and nutrient cycling. Possibly less pressure on primary forest.	Land Use competition. Slow impact. Biodiversity risks for large monocultures. Potential loss of agricultural land and natural forests. Competition for biomass resource.	GREEN– Popular measure but abuses of schemes have created some scepticism. Increasing concerns on durability due to fire, drought, pests.
Biochar	6-7	10-345	0.3-6.6	Increased crop yields and reduced GHG emissions from soil. Resilience to drought.	Particulate and GHG emissions from production. Biodiversity and carbon loss from unsustainable biomass harvesting.	GREEN– Not well known. Lower impact measure so relatively popular. High cost range creates uncertainty.
Bioenergy with carbon capture and storage (BECCS)	5-6	15-400	0.5-11	Good use for waste crop residues. Employment.	Inappropriate deployment at very large scale leads to additional land and water use to grow biomass feedstock. Biodiversity and carbon stock loss if from unsustainable biomass harvest.	AMBER– Main controversy relates to large scale diversion of arable land and forests. Medium high costs.
Ocean Fertilisation with iron	1-2	50-500	1-3	Increased productivity and fisheries. Reduced upper-ocean acidification.	Nutrient redistribution. Restructuring of the ecosystem. Enhanced oxygen consumption and acidification in deeper waters. Potential return to atmosphere of captured carbon. Risks of unintended side effects. Potential for more acidification, deoxygenation;, fundamental alteration of food webs, biodiversity.	RED– High costs, low potential. The possible side effects and unintended consequences make this controversial combined with the uncertainty of duration of the carbon captured. Low tech dev status. Attempt in Alaska to use in local area came with major controversy.
Ocean Alkalinity enhancement with lime (OAE)	1-2	40-260	1-100	Limiting ocean acidification.	Increases seawater pH and saturation states and may impact marine biota. Possible release of nutritive or toxic elements and compounds. Mining impacts. Potentially increased emissions of CO2	RED– High potential but low readiness levels. Medium costs. Unknown ocean ecosystem side-effects. Potential for CO2 release reduces effectiveness. Durability issues
Enhanced Weathering	3-4	50-200	2-4/ 1-95	Enhanced plant growth, reduced erosion, enhanced soil carbon, reduced soil acidity,	Very large energy requirement to crush rock. Large particulate risks from large scale operations Heavy transportation requirement for bulky product.	AMBER– Medium readiness. Large cost uncertainty. Uncertainty of potentials. Large mining operation to scale means slow process. Potential food production make this relatively attractive option.
Direct Air Capture (DAC)	6	100-300	5-40		Very large energy requirement Increased water use Land siting of huge volume of extractor units.	GREEN – Medium-high readiness, Higher costs but with potential to reduce. Scalable for negative emissions. Very large scaling and deployment required for impact. Several Million Tonne capture sites planned. Small plants currently financed by tech firms offset schemes

B. Solar Radiation Management (SRM)

Following is a visual display summary of SRM options from Carbon Brief followed by a brief analysis of the main options.

1. Surface Albedo Enhancement (red/amber/green highlighting readiness on the comments)

Description – Strategies like painting roofs white to reflect more sunlight have been explored to reduce urban heat.. Similar effect can be obtained by increasing albedo effect of agricultural land by for example no-till farming.
Scale – local
Development – Already used in many cities (Los Angeles, New York)
Costs – Low.
Cooling Potential – Localised, moderate
Upside – Effective in reducing ‘urban heat island’ effect, Can improve energy efficiency.
Downside – Can inadvertently cause higher temperatures in adjacent areas. This occurs due to enhanced atmospheric convection at the boundaries of high-albedo regions, leading to localised changes in cloud cover and precipitation patterns
Comment : Readiness Level – Green. The unintended effects highlight the complexity of attempts at managing the climate even at a small scale.

2. Stratospheric Aerosol Injection (SAI):

Description – Involves dispersing reflective particles, such as sulfur compounds, into the stratosphere to mimic the cooling effects of volcanic eruptions.
Scale – Global -, may be directed in one region (eg Poles) but limited control due to atmospheric mixing. Requires global coordination and governance which is does not exist and strong resistance to establishment.
Development –Technically feasible, know affects from volcanic activity analogues (esp Mount Pinatubo in 1992). Extensively modelled but very limited field research largely due to opposition and lack of experimentation governance framework.
Costs – Costs of deployment relative to impact are very low – tens of billions of dollars annually.
Cooling Potential 1-8 Wm2, dispersal across atmosphere
Upside – Very effective in reducing warming. Very fast acting (within 1 to 2 years. Mean temperature can be modulated at an average global level (with with significant variances)
Downside – Impact on the hydrological cycle with potential reduced precipitation and increases. Stopping SAI where atmospheric carbon concentrations have not been reduced would result in rapid rebound affects where temperatures increase once the filtering effect is removed which could significantly impact on all other climate and ecological systems.
Comment : Readiness Level – Amber . Technologically, could be deployed without too much difficulty and highly effective at temperature reduction but highly controversial because of major attendant risks. Strong opposition from environmental groups and policy makers. Complex governance issues due to the variable impacts on different regions of the world. Fears over unintended consequences and fact that once started could not stop without risk of increased climatic impacts. Only rationale is as a potential plan B where the impacts of climate change outweighted the risks of deployment in for example clear risk of dangerous climate tipping points being out weighed.

3. Marine Cloud Brightening:

Description– Seeks to increase the reflectivity of clouds by spraying fine sea water droplets into the atmosphere, enhancing cloud albedo. Regional in scale.
Scale – local to regional
Development – limited research. Large uncertainties with cloud microphysics and interactions. Needs
Costs – moderate at localised level
Cooling Potential – 1-5 Wm2 (at local regional scale)
Upside – Use in targeted locations such as for Coral reefs and hurricane formation regions
Downside – Upsetting local biology and weather patterns and currents. Would require continuous spraying activity. Difficult to modulate/control
Comment : Readiness Level –Amber – . some scepticism due to environmental risks but may have targeted benefits. Smaller scale potential subject to further testing

4. Space Based Reflectors:

Scale – Placing mirrors or sunshades in space to block solar radiation.
Development – Very early research stages – requires advanced space engineering and launch capabilities to put very large number/size of screens into space. Long research lead time of several decades
Costs – Likely extremely high – hundreds of billions to trillions
Cooling Potential – Could significantly reduce global temperatures by blocking defined fraction of sunlight.
Upside – Offers potential for very precise control over cooling but effectiveness depends on deployment scale
Downside – Cost an time to develop as must overcome major space engineering hurdles.
Comment : Readiness Level – RED Very early development stage and highly complex – not a viable near terms solution.

4. Conclusion

As climate deadlines loom, Solar Radiation Management (SRM) and Carbon Dioxide Removal (CDR) are no longer fringe concepts—they are part of the emerging conversation about how to navigate an increasingly volatile world. As we have seen many of the technologies for SRM and CDR are in early stages of development but bring the potential to offset the harm caused by our continuing emissions and in so doing ‘buy time’ while humanity begins to ‘get its act together’, presumably. Unfortunately, whilst these two Geo-Engineering categories may buy time in different but related ways, neither are silver bullets which can solve our basic problem of largely fossil fuel carbon emissions.

For CDR, serious issues arise relating to its scalability, at least to the gigantic levels that would be required to remove enough carbon dioxide to be able to reduce temperatures in any meaningful way. This would involve a significant percentage of total global energy (probably in the form of electricity) in the case of Direct Air Capture (DAC). For ‘Nature Based’ or biological forms of removal, including BECCS, again it is the issue of the scale that would be required that is controversial; diverting a sizable portion of Earth’s land and/or waters for purposes of sequestering carbon, diverting that land away from food production or other social purposes and with the real risk of distorting or diminishing the already fragile ecosystems with large monoculture plantations which has proven so controversial, as well as real issues of the permanence of carbon sequestration.

For SRM, potentially the most effective and affordable technological solution of Stratospheric Aerosol Injection (SAI) is also the most controversial for the same reason – a true Faustian choice. It is indeed its potential to reduce temperatures at a global scale that also raises the problem of what side-effects, unintended consequences or winner and loser scenarios that this may entail at global levels that makes this technology so complex and difficult an issue. The simple question of ‘who decides’ (how, majority voting or other, rights of veto, when is decision made etc) raises enormous, complex and unique governance questions which if even possible to offer answers could simply be opposed by others who have more to lose by its deployment.

The relatively low cost could mean that some countries could unilaterally decide to deploy SAI technology ( the problem of ‘free drivers’) in very possible future scenarios of a fast deteriorating climate, potentially inviting retaliatory action from others and global conflict. Then even if the governance issue could somehow be resolved; the underlying public opposition as witnessed in the determined opposition to even small scale experimentation reflects the general unease about ‘playing God’. But as Andrew Morton in his book; The Planet Remade – How Geoengineering Could Change the World has indicated; we have already geo-engineered our planet by large scale carbon emissions, played a destructive God and already severely damaged the Earth’s climate, not intentionally of course but intention here is non consequential. Solar Engineering and indeed Direct Air Capture by this framing is no more than the prevention or reduction of the harm caused by our actions. But we come back to the dangers, risks and impacts that accompany such large scale interventions – as with the intended consequences, the unintended consequences may also be large scale.

What is clear is that over the next few short decades, based on our current emissions trajectory, these issues will have to be faced whether we wish to or not. Further consideration on these issues as well as the underlying technologies for both SAI and BECCs will be covered in later Context articles. This article serving for the purpose of introducing the technologies and briefly here some of the complex issues that arise.