What is Geoengineering? Earth's "Climate Plan B"?

Contributor: Ava Bertolotti

Eruption of Calbuco Volcano in Chile. Getty Images

Introduction

Geoengineering, climate engineering or climate intervention, the deliberate, large-scale manipulation of climate systems to counteract anthropogenic global warming, could be Earth’s “Climate Plan B.” Criticized as a sci-fiesque corporate greenwashing scheme and political diversionary tactic meant to divert resources and scrutiny from emissions reductions policy and mitigation efforts, proponents of geoengineering have alternately promoted it as a means to buy time to decarbonize the world economy or to avert the worst-case warming scenario (e.g. the RCP8.5 IPCC emissions pathway).

Most proposed geoengineering techniques fall into one of two categories named for the climate drivers they target. While forms of greenhouse gas removal have been employed with mixed results, solar radiation management is still a largely theoretical approach.

Solar Geoengineering / Solar Radiation Management (SRM)

SRM strategies aim to counteract overly positive radiative forcing (RF) caused by the greenhouse effect by reflecting solar radiation back into space, thereby stalling atmospheric warming or even eliciting global cooling. Proposals have been advanced primarily as mitigation tools.

Albedo enhancement – increasing the reflectivity of Earth’s surface through cloud brightening or whitening of land or water surface to reduce solar radiation absorption.
It could be an effective local adaptation strategy: painting roofs white or with even a moderately reflective coating can regulate the temperature of urban heat islands, improving public health and potentially decreasing heat-related mortalities projected to increase due to climate change. This could also curb the use of air conditioning, reducing urban energy usage and cutting emissions. Some studies suggest that resultant decreases in ambient temperatures would reduce smog through reductions of photochemically-generated ozone concentrations by slowing photochemical reaction rates.
Vegetative or agricultural albedo enhancement via bioengineering – increasing the reflectance of plant cuticles, hairs, or glaucousness within photosynthetic constraints – could reduce evaporative water loss and heat stress with minimal impacts on crop productivity, even potentially increasing agricultural efficiency. A cooling effect anywhere comparable to what has been achieved through urban cool roof and reflective pavement installation would help protect field workers and mitigate temperature-related productivity losses.
Marine cloud brightening (spraying sea salt into the marine boundary layer to encourage the condensation of thicker, brighter clouds) or oceanic microbubble injection (oceanic albedo enhancement via microscopic, seafoam-esque bubbles created in the wake of ships) could increase surface albedo while limiting the heat absorption of the ocean. The deep ocean absorbs as much as 93 percent of solar radiation and is the world’s largest – although rapidly acidifying – carbon sink. The concept of cooling surface waters to slow acidification has gained traction, but how effective this will be on a relatively local scale is yet to be seen. Manipulating cloud cover or cloud reflectivity could have adverse impacts on regional weather, and fluctuating ocean temperatures or localized “cool spots” in the latter case especially could affect “crucial ocean currents” (NOAA) and emulate if not influence climate oscillations, e.g. the Pacific La Niña.

The Flisvos Project (2012) in Athens, Greece was a successful experiment in microclimate regulation by reflective pavements. The experiment used pigmented concrete blocks with a solar reflectivity of ~ 60% (~ 0.9 emissivity). The reflective pavements reduced surface temperatures by up to 12°C and lowered the peak daily ambient temperature by 1.9°C on average.
In 2010, Berkeley Lab researchers used a global land surface model to analyze the effects of installing “cool roofs” and increasing pavement reflectivity in cities with populations greater than 1 million with consideration to surface variables, such as topography, evaporation, radiation, cloud cover and temperature. For that simulated summer, cities in the northern hemisphere offset 57 gigatons of CO2 emissions – 31 Gt from roofs (0.25 average increase in reflectance) and 26 Gt from pavements (0.15 average increase in reflectance).
In May 2020, Australia conducted an open-air trial of a marine cloud brightening technique in an attempt to shade the Great Barrier Reef and reverse bleaching. The prototype turbine sprayed nano-sized salt crystals into the air, where they would induce the formation of larger constituent water droplets and brighten the low-altitude clouds over the reef to reflect sunlight away from the ocean’s surface. This form of geoengineering relies on and enhances natural processes, and is also vulnerable to climate change: the technique was less effective at higher temperatures, so it could not be used as a “substitute” for emissions reductions.

Stratospheric aerosol injection – likely the riskiest solar geoengineering proposal, reflective sulfate particulates are injected into the stratosphere to heighten the Earth’s albedo in simulation of volcanic eruptions, such as Mount Tambora (Indonesia) in 1815 and Mount Pinatubo (Philippines) in 1991, both of which caused significant global cooling (by 3°C for at least a year and 0.6°C for two years, respectively). Tropospheric sulfur dioxide is a key ingredient in smog and acid rain, where it causes millions of air pollution-related deaths a year. Using pollution to combat the radiative effects of co-pollutant-laden greenhouse gas emissions seems counterintuitive, but when juxtaposed with steep projected cuts in tropospheric sulfur dioxide emissions, SAI proponents claim that the trade-off will be negligible. Attempts have been made to quantify and analyze the public health and ecological impacts of SO2 deposition, but they hinge on uncertainties about warming pathways.
One concern is that SAI programs would have to be maintained in perpetuity to avoid the “termination shock” – a potentially catastrophic temperature fluctuation resulting from continued buildup of atmospheric GHGs with a steep decrease in albedo – that would come with stopping regular deployment. If the “radiation shield” is implemented as a temporary measure, it cannot come at the expense of emissions reductions.

Note: like MCB, regional shifts in the hydrologic cycle may disrupt seasonal weather patterns and regional climate. SAI deployment is unlikely to stop deep ocean or polar warming.

Space reflectors – another risky and very sci-fiesque solar geoengineering technique, proposals have ranged from paper-thin mirrors mounted on satellites or carried by solar-powered robots, a “giant sunshade” at the first Lagrange point (L1, a point of relative gravitational equilibrium between the Sun and Earth) that would uniformly cool the Earth, to clouds of lunar dust – each expected to reflect 2-4% of sunlight that would otherwise contribute to warming. No matter how effective in theory, current technological limitations and a prohibitively high price tag makes even the most mainstream proposals for ‘space reflectors’ practically untenable.

Related: cloud seeding

Most of these strategies are intended to be used in concert – there is no silver bullet to reverse climate change.

Carbon Geoengineering / Greenhouse Gas Removal (GGR)

Greenhouse gas removal targets the source of global warming, using negative emissions technology (NET) to extract CO2 from the atmosphere.

Carbon capture and storage technologies – CCS is a NET procedure that has received a lot of attention from heavy emitters despite having yet to be deployed on a commercial scale. Research has contested its efficacy, short and long-term viability and cost-effectiveness, with discrepancies in efficiency between models suggesting deployment at scale could be a significant financial risk in most cases.

Ambient air capture, also known as Direct Air Carbon Capture and Storage (DACCS), relies on a high concentration of CO2 in ambient air to operate at peak efficiency, otherwise hinging on the amount of sorbent the CCS facility is capable of producing; this is a heat and energy-intensive process. Liquefied CO2 would be stored in subterranean carbon reservoirs via geologic sequestration in depleted oil and gas fields or deep saline formations, where there would be potential for pipe leakage or leaching – there are regulations on the potential location of CO2 reservoirs to avoid pollution of wells or freshwater aquifers.

Bioenergy with carbon capture and storage (BECCS) was embraced in the IPCC’s 5th Assessment Report because it is viable on an experimental scale, but some deployment pathways project an almost certainly unsustainable amount of land mass being dedicated to growing biomass. Given global agricultural demand, corporatized monocrop plantations dedicated to producing just a few types of ideal biomass – e.g. rapidly-growing tree species such as eucalyptus and acacia to be planted at high density and clear-cut; possibly algae – could disincentivize forest conservation, threatening natural, high-capacity carbon sinks with deforestation. Appropriation of farmlands for biomass generation would threaten the food security and livelihoods of independent farmers and people already most affected by the climate crisis.

Steel slag (a byproduct of metal extraction from ore, usually consisting of metal oxides, silicon dioxide and elemental metals) from electric arc furnace steelmaking (one of the largest industrial emitters, along with cement), can sequester carbon dioxide emitted during steal production through CO2 mineralization and be reincorporated into construction materials, such as Portland cement. Similarly to natural weathering processes, accelerated carbonation of the slag can sequester CO2 and reduce the risk of heavy metal leaching from slag. The carbonation process is water and energy intensive, so wastewater and waste materials sourced directly from the steelmaking process would help circularize industrial steel and cement production.

The production of biochar as a byproduct of pyrolysis (a bioenergy generation process) makes simultaneous biosequestration and bioenergy generation possible. While carbon fixed in biomass feedstocks is normally released through decomposition, it can be stabilized and locked it into long-lived charcoal. Biochar enhances soil quality when repurposed as a soil amendment (à la Amazonian terra preta), and can improve nutrient retention by preventing leaching – if used in agriculture, it could scale down fertilizer usage.

Enhanced weathering (EW) / ocean alkalinity enhancement (OAE) – both proposed methods involve accelerating chemical weathering by pulverizing gigatons of minerals that react with CO2 and distributing them over a broad land surface (EW) or in the ocean (OAE). OAE could ameliorate ocean acidification by dissolving minerals into seawater to form calcium, magnesium, or sodium ions, which would bond with CO2 to increase the proportion of bicarbonate ions, thus neutralizing dissolved CO2 and carbonic acid. The efficiency of precipitation is largely dependent on the temperature-pressure reactors and mineral used (e.g. calcium and magnesium, carbonate minerals had higher dissolution and sequestration rates than silicates). However, an increase in the concentration of mineral dissolution products (Si, Ca, Mg, Fe, Ni, etc.) could have positive or negative consequences in marine ecosystems – potential ramifications involve cyanobacteria and calcifying organism proliferation, which could increase albedo by ‘whitening’ the ocean or increase productivity and sequestration potential by ‘greening’ it, to the gain of some species and the loss of others.

Proposed mechanisms for mineral distribution [Renforth et al., 2017]

Weathering efficiency also depends on the minerals used – silicate and olivine are contenders for distribution in shallow seawater or over arable land, in the vein of agricultural liming – a carbon-positive process that can be turned negative by silicate application, but one that could also reverse acidification in the watershed and act as an alkaline fertilizer.

On a global scale, the energy and labor intensiveness and environmental cost of mining gigatons of rapidly-weathering minerals could be unsustainable.

Ocean fertilization – seeding seawater with iron could stimulate marine algal blooms and phytoplankton growth, which would sequester ocean and atmospheric carbon dioxide. There is uncertainty about whether there would be enough downward transport of the CO2 sequestered into the deep ocean to prevent ocean acidification as the green biomass decomposes, releasing significant amounts of carbon dioxide at high concentrations. Like EW/OAE, it could disrupt marine ecosystems and have unforeseen impact on oceanic biochemical processes.

Afforestation / reforestation – reforestation (reforesting deforested areas) is a restoration and conservation method widely considered the most reliable means to sequester carbon dioxide emissions, and afforestation (foresting areas previously without tree cover) could convert large swaths of the planet into carbon sinks. Project Drawdown estimates of the sequestration potential of temperate and tropical reforestation is 19.42–27.85 and 54.45–85.14 gigatons from 2020-2050, respectively. Reforesting areas to restore soil quality, reestablish thriving ecosystems and restore natural capital has broad support from the scientific community.

Alternatively, afforestation would increase carbon dioxide sequestration, but it could also influence local climate and increase precipitation in traditionally dry or arid areas. Depending on the ‘reflectivity’ or the sparseness of vegetation in the biome afforested and the species of tree or other plants used, planting dense swaths of vegetation can run the risk of lowering the Earth’s albedo and fundamentally changing global weather patterns, resulting in land cover reductions in other areas due to declining precipitation. It could also evict wildlife adapted to the region and change the ways of life of people living in steppe, desert and semi-arid regions, for better or for worse.

Conclusion

While only more research and controlled trials will tell whether geoengineering is a viable approach to combatting climate change, many experts warn that so far more experimental SRM technologies (particularly SAI) should only be considered as last resort measures. In most areas, the science is promising and the cost (discounting associated risk and uncertainties) is low relative to the costs of mitigation and adaptation, but as of yet the political dimensions have gone largely unexplored. There is a de facto moratorium on most geoengineering experiments as per the UN Convention on Biological Diversity (2016). Any country unilaterally deploying geoengineering technology risks causing unpredictable weather events in other parts of the globe, with the impact likely to be more pronounced in Global South countries. Changing precipitation patterns resulting from localized SRM deployments (similar concerns have been voiced about cloud seeding to relieve drought depriving countries downstream of rain) could be a new source of geopolitical tension, and avenues for “weaponization” risk violating the 1977 Environmental Modification Convention (ENMOD). Arbitrary use of techniques like stratospheric aerosol injection by Global North countries is antithetical to the ideal of “climate justice” and incompatible with sustainable development; many argue that it undermines the obligation of countries with geoengineering resources to fulfill their common but differentiated emissions reductions responsibilities as parties to the Paris Climate Accords, and disincentivizes decarbonization.

Some technologies, such as urban or crop albedo enhancement (via cool roofs and reflective pavements, bioengineering), reforestation, biochar, local-scale coastal and agricultural enhanced weathering and ocean alkalinity enhancement, and other local-impact projects could be cost-effective and common sense measures that would serve to complement decarbonization programs. With more research, sufficient investment, equitable, just and science-informed regulation, deployment policy and oversight, these innovative climate solutions could become vital facets of the global mitigation effort.