Geoengineering for Climate Stabilization

Abstract

Engineering the climate by means of carbon dioxide removal (CDR), Earth radiation management (ERM), and/or solar radiation management (SRM) approaches has recaptured the attention of scientists, policy makers, and the public. Climate engineering is being assessed as a set of tools to deliberately, and on a large scale, moderate or retard global warming. There are several concepts available, like injecting aerosol-forming SO₂ into the stratosphere or placing huge objects in orbit to partly shade Earth from incoming radiation or fertilizing the ocean with iron for increased algae growth and creation of carbon sinks. Such concepts are highly speculative, and irrespective of whether they would work, they bear huge risks, from adversely affecting the complex climate system on a regional or global scale to potentially triggering droughts, famine, or wars. More research is needed to better understand promising concepts and to work them out in depth, so that options are made available in case they should become necessary in the future, when climate change mitigation and adaptation measures do not suffice and fast action becomes imperative. Apart from the technological hurdles, which are anyhow mostly far beyond today’s engineering capabilities, huge social, moral, and political issues would have to be overcome. The purpose of this chapter is to highlight a few common concepts of CDR, ERM, and SRM for climate engineering to mitigate climate change.

Introduction

Climate engineering (also dubbed geo-engineering , geoengineering) is defined as “the deliberate large-scale intervention in the Earth’s climate system, in order to moderate global warming” (Shepherd 2009). Another, more positive term found in the literature is “climate remediation” or “climate intervention.”

It can be considered a variant of macroengineering (the implementation of extremely large-scale design projects such as the Panama Canal) and similar in type to terraforming (planetary engineering, i.e., altering the environment of an extraterrestrial world). The expression is not to be confused with geological engineering (likewise termed geoengineering or geotechnical engineering, which is concerned with the design and construction of earthworks, including excavations, hydraulic fracturing (fracking), drilling, and underground infrastructure).

Climate engineering can be seen as the most desperate, bizarre climate change mitigation measure. Yet, due to slow progress with conventional and incremental measures, it has recaptured widespread attention among scientists, politicians, and the public. Climate engineering or “hacking the planet” (Kintisch 2010) is hyped as “quick fix” and “only solution” on the one hand and bedeviled and rejected as wacky idea, simply gambling, being impossible, and very dangerous on the other hand. Some see it as a metaphoric “Faustian bargain” or man’s attempt to “play God.” Finally, one needs to acknowledge that climate engineering concepts mostly “treat the symptoms rather than cure the illness” of climate change.

It is not so easy to find one’s position toward climate engineering, and according to Heyward and Rayner (2013), some “scientists involved in geoengineering discourse convey mixed messages about the need for technocratic management of the anthropocene at the same time as expressing strong commitments to the importance of public participation in decision making about geoengineering.” The Intergovernmental Panel on Climate Change (IPCC) states that every option has to be considered, yet it expresses a critical attitude toward climate engineering due to the inherent, unknown risks and assesses it in its 2007 report as “largely speculative and unproven and with the risk of unknown side-effects.” It was around the year 2008 (Ming et al. 2014) to 2009 that a critical discourse of geoengineering started to emerge, mainly in American magazines (Biello 2009; Kunzig 2008) and German newspapers (Anshelm and Hansson 2014). Kennedy et al. (2013) write that “No study of coping with climate change is complete without considering geoengineering.”

Social science teaches that transformation dynamics evolve from hope-inspired alternative choices rather than fear-driven technical constraints (Stirling 2014). With a lot of disappointment from commitment and implementation of climate change mitigation measures over the last years, and continued GHG emissions, many scientists feel certain despair, giving an inclination toward options provided by climate engineering.

Climate engineering can be considered a complementary approach to conventional measures: Preserving the climate (quick fix) while CO₂ is gradually brought under control by natural and/or artificial processes. In this scenario, climate engineering would “buy time” for mankind and the globe.

The major issue, even with reversible actions of climate engineering, is that the climate system is very complex. Identifying unintended consequences is not a trivial – if at all possible – task. Such consequences could be most severe and irreversible, like droughts or wars. In this context, it is worthwhile to think about the theory of chaos, which is rooted in the pioneering work of MIT meteorologist and mathematician Edward N. Lorenz (1963). Moreover, a slight drifting of the continents or a minor shifting of ocean currents may bring ice to one land and desert sands to another; see Lorenz (1972).

Safe Limits

The concept of Earth as a self-regulatory system was developed in the late 1960s by J. E. Lovelock and became popular under the name “Gaia hypothesis” and “Daisyworld” model (it is a parable on the biological homeostasis of the global environment. “Daisyworld” contains white and black flowers. When temperatures rise, more white daisies grow, increasing reflection. Sinking temperatures are counteracted by a growth of black daisies: They absorb more sunlight. Hence the balance of white to black daisies controls the temperature and stabilizes it. The simplistic Daisyworld model intuitively describes the coupling between climate and the biosphere). Lovelock’s concept is being discussed controversially (Weaver and Dyke 2012; Boston 2008). For sure nature can buffer anthropogenic impact to some extent, but not endlessly, and climate change is testimony for this finite buffering capacity.

In their seminal paper “A safe operating space for humanity” (compare The Limits to Growth work by the Club of Rome in 1972), Rockström et al. write that “Although Earth has undergone many periods of significant environmental change, the planet’s environment has been unusually stable for the past 10,000 years. This period of stability — known to geologists as the Holocene — has seen human civilizations arise, develop and thrive” (Rockström et al. 2009). They define nine interlinked planetary boundaries, three of which have already been overstepped. For instance, the estimated safe threshold identified for atmospheric CO₂ is 350 ppm or a total increased warming of 1 W/m² (current warming is approx. 1.9 W/m² radiative forcing from 400 ppm of CO₂ (Butler and Montzka 2013), not considering the additional radiative forcing by other greenhouse gases such as CH₄).

Climate Engineering Approaches

Climate engineering is still in its infancy, at a theoretical stage, where ideas are being generated, discussed, and elaborated. The Secretariat of the Convention on Biological Diversity concluded that “There is no single geoengineering approach that currently meets all three basic criteria for effectiveness, safety and affordability. Different techniques are at different stages of development, mostly theoretical, and many are of doubtful effectiveness” (Secretariat of the Convention on Biological Diversity 2012).

Global climate engineering is untested and mostly untestable (MacMynowski et al. 2011). Its roots go back to 1965, when advisors to US President Lyndon B. Johnson suggested spreading reflective particles over 13 million km² of ocean in order to reflect an extra 1 % of sunlight away from Earth (Kintisch 2010). This was one of the first high-level acknowledgements of climate change. Interestingly, no suggestions to cut down CO₂ emissions were reported to have been made. The president did not follow these early geoengineering suggestions.

Even prior to that, in 1955, John von Neumann foresaw “forms of climatic warfare as yet unimagined” in Fortune magazine (von Neumann 1955). In 1974, the Russian researcher Mikhail Budyko suggested that cooling down the planet could be achieved by burning sulfur in the stratosphere, which would create a haze from the resulting aerosols (higher albedo) (Teller et al. 1997). This and other concepts will be touched upon below. Space-based geoengineering concepts build upon Tsiolkovsky’s and Tsander’s 1920s idea of utilizing mirrors for space propulsion (Kennedy et al. 2013). As these examples show, ideas to engineer the climate came up quite early.

Small-scale weather modification can already be achieved today, e.g., by cloud seeding to induce rainfall. The historical Project “Stormfury” (1962–1983) attempted to weaken tropical cyclones with silver iodide (Willoughby et al. 1985). For a brief review on “rainmaking attempts” and “weather warfare,” which is outside the scope of this chapter, see Chossudovsky (2007) and Climate Modification Schemes, American Institute of Physics (AIP) (2011). Weather modification action has been limited by the international community, e.g., during war by the 1977 UN Environmental Modification Convention. Another regulation in this respect is the London Convention (1972) and its 1996 Protocol, which are global agreements regulating dumping of wastes at sea. Article 6 prohibits exports of wastes for dumping in the marine environment, which includes, e.g., CO₂ in CCS (carbon capture and storage) schemes (Dixon et al. 2014).

Examples where man has modified local climate (impacts) include artificial snow in skiing resorts or irrigation for crop yield amelioration. Previous environmental interventions by man have sometimes brought about unwanted – and unexpected – effects, also in the near past, e.g., streamlining riverbeds leading to local floods or the creation of urban heat islands.

Joe Romm, founding editor of the blog Climate Progress, has linked “geo-engineering to a dangerous course of chemotherapy and radiation to treat a condition curable through diet and exercise — or, in this case, emissions reduction” (McGrath 2014). Al Gore, former vice president of the USA, was quoted on climate engineering to be “utterly mad and delusional in the extreme.” He said that searches for an instant solution were born out of desperation, were misguided, and could lead to an even bigger catastrophe (Goldenberg 2014). “The idea that we can put a different form of pollution into the atmosphere to cancel out the effects of global warming pollution is utterly insane” (Goldenberg 2014).

In fact, the idea of “engineering” the Earth’s climate is a shocking one. There is yet little information available, and “technically feasible” concepts are totally vague on costs, effectiveness, reversibility, risks, and side effects.

However, serious scientists have started to investigate options for climate engineering more deeply, since swift remedial action might be needed once the Earth’s climate system reaches a “tipping point” (positive feedback, thermal runaway, e.g., thawing of permafrost releases CH₄, which further increases temperatures). It seems necessary to study climate engineering, to be prepared. There is also the threat of unilateral action by another country (Dean 2011), should a local benefit from such action be expected. Tipping point rhetoric is challenged in Heyward and Rayner (2013).

Climate engineering ideas and concepts fall into two broad groups: carbon dioxide removal (CDR) and solar radiation management (SRM). Several researchers discern Earth radiation management (ERM) from SRM, where ERM techniques focus on atmospheric convection enhancement (building of thermal bridges) and increasing outgoing IR heat radiation (i.e., long wavelength). The focus of SRM is on (short wavelength) incoming radiation. The term ERM was introduced by David L. Mitchell et al. (2011). He includes CRD and cirrus cloud reduction into SRM (Mitchell and Finnegan 2009). CRD techniques are remediation, whereas SRM are intervention.

CDR techniques are generally not considered that controversial, and they do not seem to introduce global risks, as they work on the local scale. Costs and technical feasibility have been limiting CDR deployment, e.g., reforestation or CCS. CRD attacks the root cause of climate change. However, the effects work slowly to bring down temperatures again.

SRM targets an increase in the amount of solar energy radiated back into space, effectively dimming the Sun. The necessary albedo enhancement is envisioned for deserts, oceans, mountains, clouds, and also manmade objects like roofs or roads. Prominent concept examples include deployment of giant orbiting sunshields in space, emission of huge amounts of SO₂ (Crutzen 2006) and particles into the stratosphere to mimic the action of volcanoes, increase of the Earth’s albedo by “painting” deserts white, spraying sea water into the atmosphere to produce and whiten clouds, redirecting ocean streams and changing their salinity (Could a massive dam 2010), or pumping seawater into pole regions and creating ice. Such techniques bear the risk of upsetting the Earth’s natural rhythms. SRM approaches act quickly. However, they do not remove the root cause of climate change, mainly CO₂ levels in the atmosphere, so other aspects like ocean acidification are not tackled.

Raymond Pierrehumbert, professor in Geophysical Sciences at University of Chicago, said “The term ‘solar radiation management’ is positively Orwellian. It’s a way to increase comfort levels with this crazy idea” (Rotman 2013).

According to Shepherd (2009), CDR methods should be regarded as preferable to SRM methods. SRM methods are expected to be cheaper, though.

The Royal Society wrote in a 2009 report (Shepherd 2009): “Solar Radiation Management methods could be used to augment conventional mitigation. However, the large-scale adoption of Solar Radiation Management methods would create an artificial, approximate, and potentially delicate balance between increased greenhouse gas concentrations and reduced solar radiation, which would have to be maintained, potentially for many centuries. It is doubtful that such a balance would really be sustainable for such long periods of time, particularly if emissions of greenhouse gases were allowed to continue or even increase.”

Although technological hurdles exist, it is expected that devising working technologies (i.e., installations that cool the atmosphere) are easier than understanding their effects or how governance (Shepherd 2009) should be applied.

The focus of this chapter lies on SRM, which directly modify the Earth’s radiation balance; compare Fig. 1. It also covers CDR, which influences the global carbon cycle (see Fig. 2), and ERM, as well as touching upon governance and other related aspects of climate engineering.

Fig. 1

Schematic showing the global average energy budget of the Earth’s atmosphere. Yellow indicates solar radiation; red indicates heat radiation; and green indicates transfer of heat by evaporation/condensation of water vapor and other surface processes. The width of the arrow indicates the magnitude of the flux of radiation and the numbers indicate annual average values. At the top of the atmosphere, the net absorbed solar radiation is balanced by the heat emitted to space (Source: Shepherd 2009)

Fig. 2

Simplified representation of the global carbon cycle. The values inside the boxes are standing stocks (in Pg C); the arrows represent annual fluxes (Pg C/y). The black arrows and numbers show the preindustrial values of standing stocks and fluxes; the red arrows and numbers indicate the changes due to anthropogenic activity (Source: Cole 2013)

Radiation Balance

Energy on Earth mainly comes from the Sun. The solar constant is approx. 1,361 W/m², which translates into a power of 1.730 × 10¹⁷ W for the entire Earth. The average incoming solar radiation is approx. ¼ of the solar constant (342 W/m²). The radiation balance of the Earth is shown in Fig. 1 in a simplified version.

Climate engineering aims at modifying this radiation balance to achieve a lower net heating effect. In climate science, radiative forcing or climate forcing is defined as the difference of insolation (sunlight) absorbed by the Earth and energy radiated back to space. Currently, it is 2.916 W/m², which corresponds to 479 CO_2-eq. 1.88 W/m² thereof is due to CO₂ and 0.51 W/m² due to CH₄ (Butler and Montzka 2013).

Global Carbon Cycle

Figure 2 shows the simplified global carbon cycle in Gt of carbon per year (1 Gt = 1 Pg = 10¹⁵ g). One can see that the ocean is the largest sink.

The various carbon sinks present opportunities for geoengineering. Subsets of special techniques are biogeoengineering and Arctic geoengineering. In biogeoengineering, plants or other living organisms are used or modified to beneficially influence the climate on Earth, e.g., by creating carbon sinks. An example is iron fertilization of the oceans. Iron is a growth-limiting factor, so fertilization would be expected to produce more algae, taking up CO₂, like land-based biomass.

“Global dimming” is an aspect that could be exploited for climate engineering. Monoterpenes from boreal forests (Rinnan et al. 2011; Aaltonen et al. 2011) were found to contribute to global dimming (cooling), apart from being a CO₂ sink, so tree planting would be a working biogeoengineering approach. Global dimming, generally, is caused by an increase in particulates such as sulfate aerosols in the atmosphere due to human action. The effect of anthropogenic global dimming has interfered with the hydrological cycle by reducing evaporation and so may have reduced rainfall in some areas. Global dimming also creates a cooling effect that may have partially counteracted the effect of greenhouse gases on global warming. With sulfur levels in fuels being further reduced, e.g., for ships, the global warming contribution of combustion emissions will increase in the future.

Arctic geoengineering focuses geographically on the Arctic, which plays a key role in maintaining current climate due to its albedo and stored methane. The Arctic ice is disappearing quickly, though, and concepts have been envisioned to support ice buildup.

Impacts of Climate Engineering

The targeted impact of climate engineering is to bring down global air and surface temperatures. Undesired side effects might also occur, though, particularly in SRM schemes. Several researchers have run computer models to investigate the effect of blocking part of the solar radiation. Shading the Sun would, according to the models, reduce the global temperatures, but also lead to profound changes to precipitation patterns including disrupting the Indian Monsoon (Shepherd 2009). Anthropogenic SO₂ in the stratosphere at a level necessary to counteract the radiative forcing of human CO₂ and CH₄ could cut rainfall in the tropics by 30 % (Ferraro et al. 2014). Also, it would lead to acid rain. There is further concern that SO₂ in the troposphere can harm the ozone layer; see also section “Stratospheric Sulfate Aerosols.”

Evidence that such action would in fact result in a net cooling was provided by the eruption of the volcano Mt. Pinatubo in the Philippines in June 1991. It resulted in a −0.5 °C variation in the Earth surface temperature, due to the effect of sulfate aerosol-induced albedo enhancement. However, already by the year 1995, the effect had vanished, and the temperature returned to the former value (Gomes and de Araújo 2011). Note: Another volcanic event with transient, global impact on the climate was the 1815 eruption of Mount Tambora in Indonesia, which led to a “year without summer” and famine due to reduced crop yields (Stilgoe et al. 2013a).

Sticking with this geoengineering example, potential side effects of SO₂ injected into the stratosphere by, e.g., balloons, artillery, or jet planes, are:

• CO₂ emissions from the missions

• Litter, e.g., from returning balloon shells

• Noise, e.g., from the artillery

• Depletion of ozone

• Regional droughts, e.g., in Africa and Asia from weaker monsoon activity

• Impact on cloud formation, particularly cirrus clouds, with unpredicted effects

• Acidic rain, leading to further ocean acidification, and other effects on the ecosystem

• Whitening of the sky due to aerosols, more diffuse radiation

• Less yield from solar energy collectors, impacting renewable energy production

• Temperature changes in the stratosphere, influencing atmospheric circulations in the troposphere with unknown effects

The Geoengineering Model Intercomparison Project (GeoMIP) around Ben Kravitz assesses the projected impacts of geoengineering by different climate models, focusing on SRM (http://climate.envsci.rutgers.edu/GeoMIP/publications.html). In 2013, 12 climate models simulating quadrupled atmospheric carbon dioxide levels and a corresponding reduction in solar radiation were compared (Kravitz 2013). In Fig. 3, an overview by the Convention on Biological Diversity (http://www.cbd.int/convention/) shows which intended and unintended effects might result from geoengineering.

Fig. 3

Conceptual overview of how greenhouse gas emission reductions and the two main groups of geoengineering techniques may affect the climate system, ocean acidification, biodiversity, ecosystem services, and human well-being (Numbers refer to the chapters in the cited source, from which reproduction with permission was made (Secretariat of the Convention on Biological Diversity 2012))

It is expected that both SRM and SDR would affect biodiversity and ecosystems, which finally have a significant impact on human well-being. As stated above, quantification of intended and also identification of unintended consequences of SRM and to a lesser extent ERM and CDR techniques are difficult to achieve.

On the benefits, risks, and costs of stratospheric geoengineering, see e.g., Robock et al. (2009).

Legal, Moral, and Social Issues

“Whose hand will be on the planetary thermostat?”(Robock 2014). Action by one nation would impact climate globally, but who is entitled to enact and control climate engineering? Would the target of climate engineering be to reduce future global warming, i.e., to maintain current temperatures; to limit global warming to, e.g., 2 K; or to bring back temperatures to preindustrial levels? Who would set the target? These questions cannot be answered at this point in time, as outlined in this section of this chapter.

Legal Issues

Signed by 150 government leaders at the 1992 Rio Earth Summit, the Convention on Biological Diversity is dedicated to promoting sustainable development. Conceived as a practical tool for translating the principles of Agenda 21 (a voluntarily implemented action plan of the United Nations with regard to sustainable development), it states “that no climate-related geo-engineering activities that may affect biodiversity take place, until there is an adequate scientific basis on which to justify such activities and appropriate consideration of the associated risks for the environment and biodiversity and associated social, economic and cultural impacts, with the exception of small-scale scientific research studies” (http://www.cbd.int/convention/). Thereby, private or public experimentation and adventurism are avoided, yet research is possible. R&D in climate engineering is justified so that man understands his options once a said environmental tipping point has been surpassed (contingency planning to have “something on the shelves” when needed). Research priorities in this respect are worked out in Shepherd (2009).

Moral and Social Issues

While anthropogenic greenhouse gas emissions are an unwanted side effect, climate engineering constitutes a large-scale, intentional effort to alter the climate.

Responsibilities and global political governance are not clear. It is conceivable that different governments have different targets for global temperatures. Some areas of the world show higher crop yield in an elevated temperature scenario, for instance. So actions by one country to alter the climate, motivated by expected local benefits, might result in war. Multilateral commitments and agreements over time periods of several 100 years would be necessary, as this is the time that, e.g., SO₂ from climate engineering would have to remain in the stratosphere in a delicate balance with anthropogenic CO₂ emissions it is offsetting, so there would also have to be imperative controls over CO₂ levels at the same time.

The governance of emerging science and innovation is discussed in Stilgoe et al. (2013b), citing canceling the geoengineering project “SPICE” (see below) as an example. For public perception of geoengineering, see Corner et al. (2013) and Sikka (2012). Governance principles concerning climate engineering were also elaborated in the 2010 Asilomar International Conference on Climate Intervention Technologies (http://climate.org/resources/climate-archives/conferences/asilomar/report.html).

Preliminary Climate Engineering Field Experiments

Climate engineering has a global scale, and documented field trials to date are very limited. Some concepts can hardly be tested at all.

One of the largest experiments, known as LOHAFEX, was an Indo-German fertilization experiment in 2009, in which six tonnes of iron as iron sulfate solution was spread over an area of 300 km² (Ebersbach et al. 2014) in the South Atlantic. It was expected to trigger an algal bloom, resulting in CO₂ update and some of the algae ending up in the ocean bed as carbon sink.

A much disputed, similar experiment was carried out in July 2012 by entrepreneur Russ George, who put approx. 100 t of iron sulfate into the Pacific Ocean several hundred miles west of the islands of Haida Gwaii/Canada. The intention was to increase the production rate of phytoplankton for salmon fishing (Sweeney 2014).

In 2005, a pilot project in Switzerland to cover a glacier with a reflective foil was carried out. On the Gurschen glacier, it was found that the blanket reduced the melding by 80 % (Pacella 2007). More trials on an area of more than 28,000 m² were done on the Vorab glacier (Pacella 2007).

Painting the Andes: In 2009, the World Bank has awarded a seed grant to 26 innovative climate adaptation projects, selected from 1,700 proposals (World Bank). Among them was one idea from Peruvian inventor Eduardo Gold to whiten the Chalon Sombrero peak in the Andes (Collyns 2010). This pilot project (see Fig. 4) has received positive media attention.

Fig. 4

Whitening the mountain Chalon Sombrero in Peru in a geoengineering pilot project (Source: Collyns 2010)

In the UK SPICE project (Stratospheric Particle Injection for Climate Engineering, 2015), a trial balloon flight was planned; see Fig. 5.

Fig. 5

Concept of the SPICE experiment (Source: Vidal 2011)

The idea was to send a balloon 1 km into the sky and to eject water droplets. These droplets should create clouds, increasing the albedo. The experiment had to be canceled due to opposition from environmental groups (Shukman 2014; Zhang et al. 2015).

Tree planting (reforestation, afforestation) (Zomer et al. 2008; Schirmer and Bull 2014; Trabucco et al. 2008) and peatland restoration (Bonn et al. 2014) activities are being considered in several parts of the world. According to the IPCC, reforestation refers to establishment of forest on land that had recent tree cover, whereas afforestation refers to land that has been without forest for longer time periods (IPCC 2015).

Cool roof experiments: In cities, the temperature is typically 1–3 °C higher than in the surrounding countryside, due to, e.g., heat-absorbing infrastructure such as dark asphalt parking lots and dark roofs (Oke 1997). By increasing the reflectivity, more radiation is sent back into space, and energy costs (air conditioning) can be reduced. Pilot projects are, e.g., the “White Roof Project” (http://www.whiteroofproject.org/) and New York’s “NYC °CoolRoofs” (http://www.nyc.gov/html/coolroofs/html/home/home.shtml).

Keeping groundwater level and salinity low. In Australia, rising levels of salty groundwater pose a problem for farmers. By pumping that groundwater into shallow evaporation ponds, crops are protected, with a positive side benefit of increased albedo (Edmonds and Smith 2011); see Fig. 6 (note that “geoengineering” is a side effect here).

Fig. 6

Image of a 700 × 900 m² wheat field in Western Australia in which a 66 m diameter evaporation pond was created (Source: Edmonds and Smith 2011)

Edmonds and Smith (2011) also describe reflective covers on water bodies to prevent evaporation losses. According to Ming et al. (2014), 40–50 % of the water stored in small farm dams of “hot” countries may be lost due to evaporation. Such covers, as a side effect, increase the albedo and thereby contribute to climate change mitigation; compare Fig. 7.

Fig. 7

Reflective evaporation covers on a mine reservoir at Parkes in Australia (Edmonds and Smith 2011)

Proposed Strategies for Climate Engineering

Potential approaches are surface based (e.g., albedo modification of land or ocean), troposphere based (e.g., cloud whitening), stratosphere based (e.g., injection of SO₂ or Al₂O₃), and space based (e.g., gigantic space-based mirrors, lenses, or sunshades). Below, several selected concepts are briefly introduced.

Carbon Dioxide Removal (CDR)

As mentioned above, the first set of concepts can be summarized as CO₂ removal schemes (CDR) as visually summarized in Fig. 8.

Fig. 8

Depiction of some popular CDR concepts. See text for details (Source: Climate 2010)

Carbon capture and storage (CSS) and carbon sequestration projects are out of the scope of this chapter; see elsewhere in this handbook and in the DOE/NETL CO₂ capture and storage roadmap (2010). Other CDR concepts include (Shepherd 2009):

• Use of biomass as carbon sink.

• Protection of and (re)creation of terrestrial carbon sinks such as grasslands.

• Enhanced weathering to remove CO₂ from the atmosphere.

• Direct capturing of CO₂ from the ambient air (concepts to wash CO₂ out of the atmosphere include “artificial trees” and scrubbing towers), known as industrial air scrubbing (IAS) or direct air capture (DAC) (de_Richter et al. 2013). Costs are expected to be prohibitively high (House et al. 2011).

• Enhancement of oceanic uptake of CO₂, for example, by fertilization of the oceans with naturally scarce nutrients such as iron or by changing ocean currents.

• Biochar (when biomass is pyrolyzed, char (biochar) remains. It can be mixed with soil to create terra preta, a carbon sink (Hyland and Sarmah 2014)).

There are numerous other concepts, such as removing (dark) vegetation from the mountain tops or changing the composition of ship and aircraft exhaust. The interested reader will find a collection of ideas in various internet sources such as Wikipedia. Out of the concepts presented from (Shepherd 2009) above, two are described briefly as an example.

Enhanced Weathering

In enhanced weathering, inorganic matter is used to take up CO₂, a process that occurs in nature, but slowly. For instance, if carbonates are formed, CO₂ is stored long term. This chemical approach to geoengineering involves land- or ocean-based techniques. Examples of land-based enhanced weathering techniques are in situ carbonation of silicates such as ultramafic rocks (ultrabasic rocks, which are igneous and metaigneous rocks with a very low silica content and a high magnesium and iron content). Ocean-based techniques involve alkalinity enhancement of the sea, e.g., by grinding, dispersing, and dissolving olivine, limestone, silicates, or calcium hydroxide against ocean acidification and for CO₂ sequestration. Enhanced weathering is considered as one of the most cost-effective options. CarbFix (2015) is a feasibility project of enhanced weathering in Iceland. For details on mineral carbonation/mineral sequestration, see, e.g., Herzog (2002) and Goldberg et al. (1998).

Bioenergy with Carbon Sequestration (BECS), Biochar, and Wood Burning

BECS is a hybrid approach in which bioenergy crops are grown and used as fuel, and the CO₂ emissions are captured and stored (see CCS elsewhere in this handbook). Biochar and BECS could together contribute a carbon sink of 14 GtC/year by 2100 (Edenhofer et al. 2012). The concept of burying wood in anoxic environments (e.g., deep in the soil) is that decomposition would be much slower, providing a long-term carbon sink; compare Fig. 9. According to Zeng (2008), the long-term carbon sequestration potential for wood burial is 10 ± 5 GtC per year, and currently about 65 GtC is available on the world’s forest floors in the form of coarse woody debris suitable for burial. The cost for wood burial is estimated to be lower than the typical cost for power plant CCS. Approx. 100 tC are bound as coarse wood carbon from a typical mid-latitude forest area of 1 km² in 1 year (Zeng 2008). However, there is the potential for counterproductive emissions of methane from anaerobic decomposition of the buried wood.

Fig. 9

Schematic diagram of forest wood burial and storage (Source: Zeng 2008)

It is estimated that, by storing carbon in deep sediments, deep ocean sequestration can capture up to 15 % of the current global CO₂ annual increase. It was hence suggested to dump crop residues in the deep ocean (Strand and Benford 2009).

Solar Radiation Management (SRM)

The second set of techniques for climate engineering is the SRM category. SRM stands for “solar radiation management” or “sunlight reflection methods;” compare Fig. 10.

Fig. 10

Depiction of solar radiation management (Source: Climate 2010)

Four of such SRM concepts are explained below.

Cloud Reflectivity Modification

This approach considers altering the reflectivity of clouds in two ways: thinning of cirrus clouds and brightening (low) marine clouds. High, cold cirrus clouds let sunlight penetrate but capture infrared radiation. Hence, thinning or removing cirrus would have a net cooling effect on Earth. By contrast, low, warm clouds (stratocumulus, which cover approx. 1/3 of the ocean’s surface) reflect sunlight efficiently. This “cloud whitening” or “marine cloud brightening” could be achieved with cloud condensation nuclei (CCN) such as fine seawater droplets. The effect is considered to be more pronounced on the sea than on the land, as clouds over the landmass have more (natural and anthropogenic) CCN available. Proposed schemes include seawater sprays produced by unmanned ship, ocean foams (Evans et al. 2010) from air bubble bursting, ultrasonic excitation (Barreras et al. 2002), and electrostatic atomization.

Stratospheric Sulfate Aerosols

SO₂ is known to cause global dimming, as it leads to aerosol formation, and the aerosols reflect sunlight. The mechanism is that SO₂ is oxidized to sulfuric acid, which is hygroscopic, has a low vapor pressure, and hence forms aerosols (Robock 2014). It was suggested to inject sulfur into the stratosphere as SO₂, sulfuric acid, or hydrogen sulfide by artillery, aircraft, and balloons (Rasch et al. 2008). According to estimates by the Council on Foreign Relations, “one kilogram of well placed sulfur in the stratosphere would roughly offset the warming effect of several hundred thousand kilograms of carbon dioxide” (Victor et al. 2009).

This approach was estimated to be over 100 times cheaper than producing the same temperature change by reducing CO₂ emissions (Keith et al. 2010). The SO₂ injection would have to be maintained, as tropospheric sulfur aerosols have a comparatively short atmospheric lifetime. Also, other particles have been considered, e.g., Al₂O₃.

Space Lenses, Space Mirrors, and “Dyson Dots”

Space-based concepts aim at transforming the solar constant into a controlled solar variable (Kennedy et al. 2013). They envision large space-based objects, which might be manufactured on the moon, mining local materials, or using material from asteroids. Concepts of giant lenses (Early 1989), dust rings (Bewick et al. 2013), and sunshades (Kosugi 2010) to block part of the Sun’s incoming radiation using the effects of reflection, absorption, and diffraction were worked out. A convex lens with 1,000 km in diameter is considered sufficient, and in a Fresnel embodiment, it would only be a few millimeters thick (Early 1989). Shading the Sun by approx. 55,000 orbiting mirrors with 100 km² size, made from wire mesh, or by trillions of smaller mirrors (comparable to a DVD), was suggested (Ming et al. 2014); however, such concepts are widely viewed as unrealistic. Current engineering capabilities are far from being able to realize such science-fiction-like concepts, not speaking about the costs, which are estimated at a century worth of global domestic product of all nations combined (Ming et al. 2014).

The “mirrors and smoke in space” concept was refined and coined “Dyson dots” (Kennedy et al. 2013). The concept is to place one or more large lightsail(s) in a radiation-levitated position sunward of the Lagrange point 1 (L1, SEL1). In this point, the gravitational forces on an object exerted by Earth and the Sun are equal. L1 is approx. 1.5 million km from Earth.

A 700,000 km² parasol in L1 would reduce insolation on Earth by at least 0.25 %. A photovoltaic power station on the sunny side of the parasol could “beam” energy to Earth via a maser (microwave laser) on the order of global demand, hence essentially funding the entire project. The “Dyson dot” concept is shown in Fig. 11.

Fig. 11

Dyson dot concept with “self-funding” master energy delivery to the Earth (Reproduced with permission from Kennedy et al. 2013)

The expression “Dyson dot” is based on the concept of a “Dyson sphere,” a hypothetical megastructure imagined by Freeman Dyson in 1960, who speculated in a science article entitled “Search for Artificial Stellar Sources of Infrared Radiation” that advanced extraterrestrial civilizations could have housed in their star with a megastructure, maximizing energy capturing.

A 0.25 % reduction in the Sun’s energy output was observed in the period of mid-sixteenth to mid-seventeenth century dubbed “sunspot cycle shutdown time,” “Maunder Minimum,” or “Little Ice Age,” so this order of magnitude is what space geoengineers are aiming at.

Dust Clouds

Clouds of extraterrestrial dust placed in the vicinity of the L1 point are an alternative concept to thin-film reflectors, aiming at significantly reducing the manufacturing efforts. The material should be mined from captured asteroids, being moved by solar collectors or mass drivers (Bewick et al. 2012); see Fig. 12.

Fig. 12

Impression of an L1 positioned dust cloud for space-based geoengineering (Source: Bewick et al. 2012)

For details on such a dust concept, see, e.g., Bewick et al. (2012). Dust for sunlight blocking might also be mined on the moon.

Other Greenhouse Gas Remediation Ideas

There are many other geoengineering concepts than those introduced above, some of which are mentioned here:

CFC Destruction by Lasers

Chlorofluorocarbons (CFC) are persistent in the atmosphere, having huge GWP, yet they are accessible via their photochemistry (Stix 1993). Extremely powerful lasers might be used to break up tropospheric CFC.

Ocean Heat Transport

Ocean heat transport (downwelling of ocean currents) is outlined in Zhou and Flynn (2005). This concept aims at changing oceanic currents to shovel heat energy to deeper regions of the ocean. Also, solar-driven heat pumps might be used to this end.

Methane Remediation

Since methane is also a GHG of big concern, other geoengineering concepts target reducing CH₄ emission, e.g., by soil oxidation into CO₂ (Tate 2015).

ERM and Energy Production

Earth radiation management (ERM) aims at increasing the long wavelength radiation sent into space, which today is being trapped by GHG. ERM can be combined with energy production in so-called meteorological reactors (Ming et al. 2014). The term “meteorological reactor” stands for a climate engineering installation that fulfills two purposes: reduction of radiative forcing and energy production. Possible embodiments are:

• Solar updraft tower

• Solar downdraft energy tower

• Atmospheric vortex engine

• Heat pipes

• Radiative cooling, emissive energy harvesters (EEH)

Figure 13 shows an overview of such ERM schemes.

Fig. 13

Principal longwave radiation targets of meteorological reactors (Source: Ming et al. 2014)

The “chimney effect” is used to create air motion, which can drive a generator. The hot air is moved into higher layers of the atmosphere, where it can radiate off heat energy. In Fig. 14, emissive energy harvesters (EEH) designs are depicted.

Fig. 14

Two possible EEH designs. (a) In a thermal EEH, a heat engine operates between the ambient temperature and a radiatively cooled plate. (b) In an infrared rectenna EEH, the whole panel is at ambient temperature, but the circuit’s electrical noise is coupled to the cold radiation field via antennas (Source: Byrnes et al. 2014)

For details on “meteorological reactors” in ERM mode, see Ming et al. (2014) and http://www.solar-tower.org.uk/meteorological-reactors.php.

Climate Engineering in the Context of Climate Change Mitigation and Adaptation

Figure 15 is an illustration of the conceptual relationship between SRM and CDR with climate change adaptation and mitigation, in the context of the interdependent human and climatic systems.

Fig. 15

Illustration of mitigation, adaptation, solar radiation management (SRM), and carbon dioxide removal (CDR) methods in relation to the interconnected human, socioeconomic, and climatic systems and with respect to mitigation and adaptation. The top part of the figure represents the Kaya identity. REDD stands for Reducing Emissions from Deforestation and Forest Degradation (Source: Edenhofer et al. 2012)

The Kaya identity (O’Mahony 2013) mentioned in the caption of Fig. 15 is based on Japanese scientist Kaya and can mathematically be expressed as F = pop * (GDP/pop) * (E/GDP) * (F/E), with F being global anthropogenic CO₂ emissions, pop being global population growth, G the world GDP, and E the global energy consumption. Carbon emissions F can be estimated as the product of growth (pop), economic expansion (GDP/pop), energy intensity (E/GDP), and carbon efficiency (F/E).

Is It Geoengineering or Not?

The term geoengineering expresses, as stated initially, deliberate large-scale intervention in the Earth’s climate system. CDR methods with a local to regional and/or low global impact are hence not real geoengineering approaches. The delineation is not exactly clear-cut. An attempt was made by the 2011 IPCC Expert Meeting on Geoengineering; see Fig. 16.

Fig. 16

Scale and impact are important determinants of whether a particular CDR method and specific application should be considered as geoengineering or not. Note that the specific positioning of the different methods is only illustrative and does not constitute a consensus view of the experts participating in the 2011 IPCC Expert Meeting on Geoengineering that produced this chart (Source: Edenhofer et al. 2012)

As Fig. 16 shows, ocean fertilization and ocean alkalinization are seen as geoengineering-type projects, as can be large afforestation/reforestation.

Discussion

Having presented some geoengineering concepts, a discussion about their targeted effectiveness and commercial viability has to be carried out. Geoengineering appraisals in their context frames were studied in Bellamy et al. (2012), where “climate emergency,” “insufficient mitigation,” and “climate change impacts” were cited most often. The appraisals were found to be mostly expert analytic, involving calculations/computer modeling, expert reviews and opinions, economic assessments, and MCA (multi-criteria analysis) (Bellamy et al. 2012). This study also investigated the frequency of different geoengineering proposals; see Fig. 17.

Fig. 17

Relative abundance of geoengineering concepts in the scientific literature. “Others” were cited no more than once (Source: Bellamy et al. 2012)

Stratospheric aerosols and space reflectors were investigated most often. There was a balance between solar- and carbon-based concepts.

A qualitative ranking of storage potentials and local vs. global impact is shown in Fig. 18.

Fig. 18

The relative estimated total storage potential for emission reduction and sink creation projects at different scales (Source: Edenhofer et al. 2012)

As Fig. 18 shows, concepts with a large estimated global potential are carbon sinks, with the ocean being particularly important. For these, transboundary issues arise.

Blue carbon is the carbon captured by the world’s oceans and coastal ecosystems (Blue Carbon Initiative 2015).

An overall evaluation in terms of affordability and effectiveness, reproduced from Shepherd (2009), is shown in Fig. 19.

Fig. 19

Preliminary overall evaluation of the geoengineering technique (Shepherd 2009)

The color of the bullets in Fig. 19 indicates the level of system safety (red = low; yellow = medium; green = high), whereas the size of the bullets relates to the timeliness of the techniques (large = quick; small = slow). One can see from Fig. 19 that urban surface albedo enhancements like “white roofs” are safe, but lack effectiveness technically and financially. Afforestation, also a safe technique, is affordable, but has a lower effectiveness potential than stratospheric aerosols, which are more risky, are more costly, and take more time. Such comparison charts can help define research priorities.

Results from another, similar study are depicted in Figs. 20 and 21.

Fig. 20

Schematic overview of the climate geoengineering proposals considered. Black arrowheads indicate shortwave radiation; white arrowheads indicate enhancement of natural flows of carbon; gray downward arrow indicates engineered flow of carbon; gray upward arrow indicates engineered flow of water; dotted vertical arrows illustrate sources of cloud condensation nuclei; and dashed boxes indicate carbon stores (Source: Lenton and Vaughan 2009)

Fig. 21

Summary of estimates of the radiative forcing potential of different climate geoengineering options from Lenton and Vaughan (2009). The potential of longwave (CO₂ removal) options is given on three different time horizons, assuming a baseline strong mitigation scenario. The rightward pointing arrows, which refer to mirrors in space, stratospheric aerosols, and air capture and storage on the year 3,000 timescale, indicate that their potential could be greater than suggested by the diamonds (which in these cases represent a target radiative forcing to be counteracted: 3.71 W/m² due to 2 × CO₂ = 556 ppm for the shortwave options and 1.43 W/m² due to 363 ppm CO₂ in the year 3000 under a strong mitigation scenario) (Source: Lenton and Vaughan 2009)

Lenton and Vaughan (2009) concludes “only stratospheric aerosol injections, albedo enhancement of marine stratocumulus clouds, or sunshades in space have the potential to cool the climate back toward its pre-industrial state. Strong mitigation, combined with global-scale air capture and storage, afforestation, and bio-char production, i.e., enhanced CO₂ sinks, might be able to bring CO₂ back to its pre-industrial level by 2100, thus removing the need for other geoengineering.”

A third study (Goes et al. 2010) which is being presented here has compared four scenarios: BAU (business as usual), CO₂ abatement, intermediate geoengineering (next 50 years), and continuous geoengineering from the present until 2150; see Figs. 22 and 23. The two geoengineering scenarios deploy stratospheric aerosol injection.

Fig. 22

Radiative forcing (panel a), global mean atmospheric CO₂ (panel b), global mean surface temperature change (panel c), and the rate of global mean surface temperature change (panel d) for BAU (circles), abatement (dashed line), intermittent geoengineering (crosses), and continuous geoengineering (solid line). Note that these results neglect potential economic damages due to aerosol geoengineering forcing. BAU business as usual, GWP gross world product (Source: Goes et al. 2010)

Fig. 23

Economic damage of climate change (panel a), total costs (i.e., CO₂ abatement costs and climate change damages cost), abatement, (panel b), fraction of CO₂ abatement (panel c), and per capita consumption (panel d) for BAU (circles), optimal abatement (black dashed line), intermittent geoengineering (crosses), and continuous geoengineering (solid line). Note that these results neglect potential economic damages due to aerosol geoengineering forcing. BAU business as usual, GWP gross world product (Source: Goes et al. 2010)

CO₂ emissions are assumed to be equally increasing in all scenarios except the abatement one. Two key observations from this study (Goes et al. 2010) are:

• Radiative forcing in the “intermediate geoengineering” scenario would reach the same levels as that in the BAU scenario soon after the geoengineering was stopped.

• Compared to the BAU scenario, a temperature rise of up to 1.5 K per decade, as opposed to less than 0.5 K per decade, would result. Such a strong change might finally be even worse for flora and fauna – and humans than a steady increase.

Figure 23 gives projections on the costs of the four scenarios.

As one can deduct from Fig. 23, damage and total costs of the BAU and intermediate geoengineering scenarios are highest, whereas the continuous geoengineering scenario presents itself as the economically most favorable one. As the authors conclude, aerosol geoengineering for CO₂ abatement can be an economically ineffective strategy. Failure to sustain the aerosol forcing can lead to huge and abrupt changes to the climate: “Substituting aerosol geoengineering for greenhouse gas emissions abatements constitutes a conscious risk transfer to future generations, in violation of principles of intergenerational justice which demands that present generations should not create benefits for themselves in exchange for burdens on future generations” (Goes et al. 2010).

Conclusions

As this brief, introductory chapter to geoengineering has shown, several concepts that at first sight look tempting to “quickly fix global warming” have been developed. Ideas range from more tree planting to huge constructions in space, they include techniques to substantially alter the albedo of manmade objects, deserts, or mountains, and they consider injecting vast amounts of chemicals into the ocean and/or the stratosphere. At the present time, the consequences of such measures, and even the magnitude of their very effect, are hard if not impossible to predict, possibly generating huge risks from irreversibly messing up the complex climate system of our Earth for centuries, altering rainfall patterns, and provoking severe military activities, to name but a few possible side effects. Yet, climate engineering poses an option to deal with the impending aggravation of climate change, and once scientists know more about the various options, one or the other of them might in fact become a viable support in global climate change mitigation and adaptation measures to bring the anthropogenic impacts back under control. On the question of geoengineering ethics, Alan Robock concludes that “in light of continuing global warming and dangerous impacts on humanity, indoor geoengineering research is ethical and is needed to provide information to policymakers and society so that we can make informed decisions in the future to deal with climate change. This research needs to be not just on the technical aspects, such as climate change and impacts on agriculture and water resources, but also on historical precedents, governance, and equity issues. Outdoor geoengineering research, however, is not ethical unless subject to governance that protects society from potential environmental dangers…Perhaps, in the future the benefits of geoengineering will outweigh the risks, considering the risks of doing nothing. Only with geoengineering research will we be able to make those judgments” (Robock 2012).

So to conclude, one can say that climate engineering is an interesting topic of research, and CDR techniques that are less risky than SRM techniques might complement conventional climate change mitigation actions. For approaches with global impact, clear governance rules need to be established and enforced.

Outlook

Research of geoengineering should be enhanced, as recommended, e.g., by the UK Royal Society, the American Meteorological Society, the American Geophysical Union, the US Government Accountability Office, and prominent scientists (Robock 2014). Unrealistic and potentially dangerous concepts will be abandoned, and new, innovative ones emerge, possibly providing new options for climate change mitigation and adaptation.