Solar Geoengineering and Climate Change

Solar Geoengineering and Climate Change

May 9, 2023

Congressional Research Service

https://crsreports.congress.gov

R47551

SUMMARY

Solar Geoengineering and Climate Change

Solar geoengineering (SG) refers to a set of methods aimed at cooling the Earth in order to

counteract the warming effects of increases in greenhouse gases (GHGs). Interest in SG has

developed due to concerns that current strategies of climate change mitigation, primarily through

GHG emissions reductions, may not be sufficient to stabilize global temperatures at levels

quickly enough to avoid adverse climate impacts resulting from global warming.

R47551

May 9, 2023

Jonathan D. Haskett

Analyst in Environmental

Policy

The Earth’s energy budget is the ratio of energy entering the Earth’s climate system in the form

of shortwave solar radiation and the energy leaving the Earth’s climate system either as reflected

shortwave radiation energy or reradiated longwave heat radiation energy. SG methods seek to offset global warming by either

reducing the amount of energy coming into the system from the sun, or increasing the amount of energy lost from the system

to space.

Three SG methods are discussed in this report: stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and

cirrus cloud thinning (CCT).

SAI seeks to reproduce the cooling effect of volcanic eruptions by injecting sulfates into the stratosphere, to form a reflective

aerosol. There is uncertainty about the method’s degree of cooling, the potential effect on the Earth’s climate, and concern

about damage to the Earth’s protective stratospheric ozone layer.

MCB seeks to increase the reflectivity of low-altitude oceanic clouds, reducing incoming solar energy and having a cooling

effect, by spraying a mist of seawater into such clouds. Some studies indicate that MCB could offset the warming of a

doubling of atmospheric CO2 (a common metric of climate sensitivity) and others indicating that it could not. Some climate

modeling studies suggest that MCB could affect global precipitation, with other studies yielding mixed regional effects.

CCT differs from SAI and MCB in that rather than decreasing the amount of incoming energy, CCT attempts to increase the

amount of outgoing energy. High-altitude cirrus clouds prevent some longwave outgoing heat energy from reaching space.

CCT adds an aerosol to the clouds to thin them, allowing more longwave heat energy to escape. Research on CCT

effectiveness has produced mixed results, with some researchers finding a cooling effect and others not.

Although some international agreements include sections on geoengineering, there is no international agreement exclusively

governing SG research and implementation. Governance of SG is an area of active discussion for researchers and

policymakers.

As of the publication of this report, Congress has not passed legislation that exclusively regulates or governs SG research or

implementation; however, there are aspects of some U.S. statutes that may be relevant. For FY2022, Congress appropriated

funds for the Office of Science and Technology Policy (OSTP) to form an interagency task force in cooperation with other

federal agencies, whose mandate includes the establishment of a research governance framework for publicly funded SG

research. The OSTP has solicited public comments on rapid climate intervention research, including research on SG.

Some scientists have stated that the scientific understanding of SG is not currently sufficient to consider implementation.

Some commentators have raised the concern that the perceived availability of SG may be seen as an alternative to mitigation

strategies, such as emissions reductions, and delay or reduce efforts at such reductions.

Congressional deliberations on SG policy may include an evaluation of trade-offs between benefits and risks. On the one

hand, SG may provide a cooling effect to offset global warming, reducing the risk of adverse climate change effects. On the

other hand, risks associated with SG include possible damage to stratospheric ozone, reductions in precipitation, and

reduction of ocean primary productivity. There is uncertainty about the climate response to SG, including the possibility of

adverse impacts at global and regional levels. There may also be a risk of unilateral action on SG by countries or nonstate

actors in the absence of U.S. law or an international agreement specifically addressing SG.

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Solar Geoengineering and Climate Change

Contents

Introduction ..................................................................................................................................... 1

The Earth’s Energy Budget ............................................................................................................. 3

Solar Geoengineering: Selected Methods........................................................................................ 4

Stratospheric Aerosol Injection (SAI) ....................................................................................... 5

Potential Cooling by SAI .................................................................................................... 5

Potential Concerns/Drawbacks of SAI ............................................................................... 6

Marine Cloud Brightening (MCB) ............................................................................................ 7

Potential Cooling Effects of MCB ...................................................................................... 8

Potential Concerns/Drawbacks of MCB ............................................................................. 8

Cirrus Cloud Thinning (CCT) ................................................................................................... 9

Potential Effects of CCT on Global Temperatures .............................................................. 9

Potential Concerns/Drawbacks of CCT ............................................................................ 10

Concerns About SG ....................................................................................................................... 10

Federal Law and SG ....................................................................................................................... 11

International Agreements and SG ................................................................................................... 11

Congressional Action and Considerations ..................................................................................... 12

Figures

Figure 1. The Earth’s Energy Budget .............................................................................................. 4

Figure 2. Ship Tracks off the Iberian Peninsula .............................................................................. 7

Contacts

Author Information........................................................................................................................ 13

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Solar Geoengineering and Climate Change

Introduction

This report presents information on solar geoengineering (SG), a type of climate intervention for

mitigating climate change.1 SG is a set of proposed methods for limiting global warming by

reducing the amount of solar energy entering the Earth’s climate system or increasing the amount

of energy leaving the Earth’s climate system, and by doing so preventing global temperatures

from increasing over time.2

Interest in the concept and methods of SG as a way of offsetting global warming has developed in

part due to concerns that current strategies of climate change mitigation, through emissions

reductions and reducing the atmospheric carbon dioxide (CO2) concentration through carbon

dioxide removal (CDR), may not be sufficient to stabilize global temperatures quickly enough to

avoid adverse climate impacts.3 In 2017, the U.S. Global Change Research Program (USGCRP)

published its Fourth National Climate Assessment that stated the following:

Limiting the global mean temperature increase through emissions reductions or adapting

to the impacts of a greater-than-3.6°F (2°C) warmer world have been acknowledged as

severely challenging tasks by the international science and policy communities.

Consequently, there is increased interest by some scientists and policy makers in exploring

additional measures designed to reduce net radiative forcing through other, as yet untested

actions, which are often referred to as geoengineering or climate intervention (CI) actions. 4

SG is distinct from the other principal methods of mitigating climate change—reducing emissions

of greenhouse gases, and removing CO2 from the atmosphere—that seek to influence global

temperatures through changes in the concentrations of greenhouse gases (GHGs) in the

atmosphere. SG, by contrast, seeks to influence global temperatures by managing the balance of

incoming and outgoing energy. To do this, SG includes methods for increasing the reflectivity of

the Earth and thereby reducing the absorption of incoming solar energy or by reducing the

capacity of high-altitude clouds to retain outgoing energy in the form of heat. Either approach, if

successful, would create a cooling effect, offsetting, to some extent, the global warming effect of

increases in GHGs.

In a study published in 2021, the National Academies of Science, Engineering, and Medicine

(NASEM) examined three SG methods, selected on the following basis:

This particular study focuses specifically on atmospheric-based interventions—both

because these strategies are a source of growing research interest and because they pose

1 In addition to solar geoengineering, there are several terms in use for this group of methods, including solar radiation

management, solar radiation modification, albedo modification, and sunlight reflection. See also Harvard’s Solar

Geoengineering Research Program, “Geoengineering,” https://geoengineering.environment.harvard.edu/

geoengineering.

2 National Academies of Science, Engineering, and Medicine, Reflecting Sunlight: Recommendations for Solar

Geoengineering Research and Research Governance (Washington, DC: The National Academies Press, 2021)

(hereinafter NASEM 2021) provides the following description: “solar geoengineering (SG), which refers to attempts to

moderate warming by increasing the amount of sunlight that the atmosphere reflects back to space or by reducing the

trapping of outgoing thermal radiation” (p. 1).

3 The White House Office of Science and Technology Policy: Legal, “Request for Input to a Five-Year Plan for

Research on Climate Intervention,” https://www.whitehouse.gov/ostp/legal/. See also Scientific American, “Solar

Geoengineering Should Be Regulated, U.N. Report Says,” February 28, 2023, https://www.scientificamerican.com/

article/solar-geoengineering-should-be-regulated-u-n-report-says/.

4 U.S. Global Change Research Program (USGRP), Climate Science Special Report: Fourth National Climate

Assessment, Volume I, 2017, p. 401 (hereinafter USGCRP NCA4 2017).

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particularly large governance challenges, given the inherently transboundary, global nature

of such interventions.5

The methods selected for examination by NASEM are presented in this report: stratospheric

aerosol injection (SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT). The

report includes an overview of the techniques and a discussion of their potential for affecting

global temperatures, as well as the potential risks and uncertainties associated with each

technique.

Other methods of SG have been proposed, including space-based methods, and modification of

the reflectance of land surfaces.6 In addition to the NASEM selection criteria, the analysis in this

report was limited to atmospheric methods due to the likely high costs and technical challenges of

space-based methods relative to atmospheric methods, and concerns raised by researchers

regarding the effectiveness of surface reflectivity modification.7

Although some international agreements, such as the Convention on Biodiversity and the London

Protocol, include sections on geoengineering, there is no international agreement exclusively

governing SG research and implementation.8 Governance of SG is an area of active discussion by

researchers and policymakers.

As of May 2023, Congress has not introduced or passed legislation that exclusively regulates or

governs SG research or implementation. There are, however, aspects of some U.S. statutes that

may be relevant to SG. For FY2022, Congress appropriated funds for the Office of Science and

Technology Policy (OSTP) to form an interagency task force in cooperation with other federal

agencies whose mandate includes the establishment of a research governance framework for

publicly funded SG research.9 On August 19, 2022, the OSTP posted a notice in the Federal

Register soliciting public comments on rapid climate intervention research, including research on

SG.10

5 NASEM 2021, p. 1

6 D. G. MacMartin et al., “Solar Geoengineering as Part of an Overall Strategy for Meeting the 1.5°C Paris Target,”

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 376, no.

2119 (2018), 20160454 (hereinafter MacMartin 2018). See also J. Lee et al., “Future Global Climate: Scenario-Based

Projections and NearTerm Information” in Climate Change 2021: The Physical Science Basis. Contribution of Working

Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change, ed. Masson-Delmotte et

al., (Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA), 2021, p. 553 (hereinafter

IPCC AR6 WGI Chapter 4). See also Peter J. Irvine et al., “Climatic Effects of Surface Albedo Geoengineering,”

Journal of Geophysical Research: Atmospheres, vol. 116, no. D24 (2011).

7 Roger Angel, “Feasibility of Cooling the Earth with a Cloud of Small Spacecraft near the Inner Lagrange Point (L1),”

Proceedings of the National Academy of Sciences, vol. 103, no. 46 (2006), p. 17184. See also Peter J. Irvine et al.,

“Climatic Effects of Surface Albedo Geoengineering,” Journal of Geophysical Research: Atmospheres, vol. 116, no.

D24 (2011).

8 United Nations Convention on Biological Diversity, May 22, 1992, 31 I.L.M. 818. See also 1996 Protocol to the

Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter, November 7, 1996, 36

I.L.M. 7.

9 U.S. Congress, House Committee on Appropriations, Consolidated Appropriations Act, 2022 (H.R. 2471; P.L. 117103): Provisions Applying to All Divisions of the Consolidated Appropriations Act, committee print, 117th Cong.

10 U.S. Global Change Research Program, “Request for Input to a Five-Year Plan for Research on Climate

Intervention,” https://www.globalchange.gov/content/request-input-five-year-climate-intervention-research-plan.

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The Earth’s Energy Budget

The methods of SG described in this report are intended to influence some of the energy

components of the Earth’s climate system, known as the energy budget. The Earth’s energy

budget is the balance between the energy coming into the Earth’s climate system and the energy

leaving the Earth’s climate system (Figure 1).11 This balance determines the Earth’s climate, and

when the gain and loss of energy are in balance, the Earth’s climate is stable. When there is a

change in the balance of the energy budget, the Earth’s average temperature, and therefore

climate, will change.12

Energy comes from the sun in the form of shortwave radiation, which includes visible light, and

when it reaches the Earth’s atmosphere a portion is reflected back into space, a portion is

absorbed by the atmosphere itself, and a portion is transmitted through the atmosphere to the

Earth’s surface (Figure 1A). At the Earth’s surface, a portion of the solar energy that has been

transmitted through the atmosphere is reflected back into space and the remainder is absorbed.

Some of the energy absorbed by the Earth’s surface is reemitted back to the atmosphere as

longwave (infrared or heat) radiation. Greenhouse gases (e.g., water vapor, CO2, methane [CH4])

do not absorb the incoming shortwave solar radiation but can absorb the longwave radiation

emitted from the Earth’s surface. Some of this heat energy is reemitted back toward the Earth’s

surface (Figure 1B). This absorption and reemission of longwave radiation or heat energy by the

atmosphere keeps some energy from being lost directly into space and keeps the Earth’s surface

warmer than it would be without the atmosphere. This keeps the Earth’s temperature within a

range that can support human life.

11 See glossary in Intergovernmental Panel on Climate Change, The Physical Science Basis: Contribution of Working

Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge, United

Kingdom: Cambridge University Press, 2013): “The climate system is the highly complex system consisting of five

major components: the atmosphere, the hydrosphere, the cryosphere, the lithosphere and the biosphere, and the

interactions between them.”

12 When the Earth’s temperature is referred to in this report, the phrase refers to the average temperature of the Earth.

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Figure 1. The Earth’s Energy Budget

Source: Adapted from images presented on the National Aeronautics and Space Administration (NASA)

website, “The Earth’s Radiation Budget,” https://science.nasa.gov/ems/13_radiationbudget.

Note: The specifics of the Earth’s energy budget are described in the text of the report.

When GHGs are added to the atmosphere through human-influenced (anthropogenic) emissions,

the increased levels of GHGs in the atmosphere increase the amount of heat energy that can be

absorbed and reemitted by the atmosphere. This leads to more heat energy trapped in the

atmosphere and less of this energy transmitted back into space. In this case, incoming and

outgoing energy are not in balance, the Earth warms, and the temperature of the Earth increases.

An imbalance of this kind between incoming and outgoing energy can be expressed

mathematically and is termed radiative forcing.13 When the incoming solar energy retained in the

Earth’s climate system is greater than the outgoing energy reemitted back to space, the radiative

forcing is positive, and this has a warming effect for Earth. When outgoing energy is greater than

the incoming solar energy retained in the Earth’s climate system, then radiative forcing has a

negative value, and this has a cooling effect for the Earth.

Solar Geoengineering: Selected Methods

There are several methods of SG, referred to as atmospheric-based interventions, identified by

the NASEM as the focus of ongoing research. These methods—stratospheric aerosol injection

(SAI), marine cloud brightening (MCB), and cirrus cloud thinning (CCT)—all involve the impact

13 Radiative forcing has units of watts per square meter (W/m2) and is a measure of the movement of energy.

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of aerosols in the atmosphere and are discussed in more detail below.14 Research on these SG

methods has primarily involved modeling studies.15 However, the SG research has also included

examining natural events like volcanic eruptions, and anthropogenic features like cloud aerosol

pollution from ships, as real-world examples of the effects of adding aerosols to the atmosphere.16

The effects of the natural and anthropogenic increases in organic aerosols in the atmosphere on

cirrus cloud formation have also been studied.17

Stratospheric Aerosol Injection (SAI)

The SAI method is based on the observation that past volcanic eruptions, like the 1991 eruption

of Mt. Pinatubo in the Philippines,18 that emitted large quantities of sulfates (such as sulfur

dioxide) into the stratosphere, led to a reduction in the amount of incoming solar energy, resulting

in a short-term global cooling effect. The sulfate gases released during an eruption are converted

into a dispersed aerosol of sulfuric acid. The aerosol reflects and thereby reduces the amount of

incoming solar energy, resulting in short-term global cooling. In the case of the 1991 eruption of

Mt. Pinatubo, this process resulted in a decrease of global temperatures by approximately 0.3oC

for about three years.19

SAI as an SG method would require the dispersion of sulfates in the stratosphere to create

manmade aerosol particles in order to make the atmosphere more reflective to incoming solar

radiation energy and mimic the cooling effect observed after a large volcanic eruption.20A variety

of methods have been proposed to add sulfates to the atmosphere, including “aircraft, rockets,

artillery, and pipes elevated to high altitudes carrying aerosol precursors.”21

Potential Cooling by SAI

Estimates of the degree of cooling that SAI could provide, and the quantity of sulfates needed for

this cooling, have varied. A review of modeling study results for equivalent amounts of sulfate

injected into the atmosphere have differed in the amount of cooling estimated between studies, by

14 The National Institute for Occupational Safety and Health (NIOSH) defines an aerosol as “... a suspension of tiny

particles or droplets in the air, such as dusts, mists, or fumes.” See also NIOSH, “Aerosols,” https://www.cdc.gov/

niosh/topics/aerosols/default.html. See also NASEM 2021.

15 NASEM 2021.

16 D. Visioni et al., “Sulfate Geoengineering: A Review of the Factors Controlling the Needed Injection of Sulfur

Dioxide,” Atmospheric Chemistry and Physics, vol. 17, no. 6 (2017), p. 3879. See also M. S. Diamond et al.,

“Substantial Cloud Brightening From Shipping in Subtropical Low Clouds,” AGU Advances, vol. 1, no. 1 (2020).

17 Martin Wolf et al., “A Biogenic Secondary Organic Aerosol Source of Cirrus Ice Nucleating Particles,” Nature

Communications, vol. 11 (2020), p. 4834. See also K. Ignatius et al., “Heterogeneous Ice Nucleation of Viscous

Secondary Organic Aerosol Produced from Ozonolysis of α-Pinene,” Atmospheric Chemistry and Physics, vol. 16, no.

10 (2016), p. 6495.

18 The eruption of Mount Pinatubo in the Philippines, in 1991, was the second-largest volcanic eruption of the 20th

century. See also National Oceanographic and Atmospheric Administration (NOAA), “Mt. Pinatubo, Philippines:

Facts,” https://www.ngdc.noaa.gov/hazard/stratoguide/pinfeat.html.

19 NASEM 2021.

20 NOAA, “Layers of the Atmosphere,” https://climate.nasa.gov/news/2919/Earths-atmosphere-a-multi-layered-cake/,

includes the following definition of the stratosphere: “The stratosphere extends from 4 -12 miles (6-20 km) above the

Earth’s surface to around 31 miles (50 km). This layer holds 19 percent of the atmosphere’s gases but very little water

vapor.” See also earlier studies: David Keith and Hadi Dowlatabadi, “A Serious Look at Geoengineering,” Eos,

Transactions American Geophysical Union, vol. 73, no. 27 (1992), p. 289; and M. I. Budyko, “Climatic Changes,”

American Geophysical Union, Waverly Press, 1977.

21 NASEM 2021, p. 77.

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a factor of three.22 Based on observations of volcanic eruptions and some modeling studies, in

order to maintain any potential cooling effects longer than a few years, the quantity of sulfate

aerosol in the stratosphere would need to be replenished regularly.23

Potential Concerns/Drawbacks of SAI

Ninety percent of the atmosphere’s ozone (O3) is in the stratosphere, and this ozone provides

protection from the harmful ultraviolet (UV) radiation from the sun.24 Some research indicates

that SAI using sulfates could cause depletion of the protective stratospheric ozone layer.25 The

1991 eruption of Mt. Pinatubo provided evidence for this effect.26 Other modeling studies on the

effects of SAI on the ozone layer produced mixed results, with increases of ozone in some

regions and decreases in others.27

Using sulfate aerosols may also increase water vapor content, and depending on where in the

stratosphere the aerosols are introduced, could affect cloud formation.28 Cloud formation could

lead to warming of the stratosphere and could reduce the degree of cooling achieved by sulfatebased SAI.29

These concerns about sulfate-based SAI have led to research into alternative materials that could

be aerosolized and used instead of sulfates, such as alumina and diamond.30 There is uncertainty

about the behavior of nonsulfate aerosols because they do not occur naturally in the

stratosphere.31

Estimates of the degree of cooling that SAI could provide have varied. If SAI were implemented,

there is some research that indicates an abrupt termination of SAI would lead to rapid warming of

the planet, to the temperature level that it would have had if SAI had not been implemented and

with the risk of adverse climate effects. These findings imply that SAI would need to be

continued indefinitely at levels approximating an annual volcanic eruption like that of Mount

22 D. Visioni et al., “Sulfate Geoengineering: A Review of the Factors Controlling the Needed Injection of Sulfur

Dioxide,” Atmospheric Chemistry and Physics, vol. 17, no. 6 (2017), p. 3879.

23 NASEM 2021, p. 81. See also S. Kremser et al., “Stratospheric Aerosol—Observations, Processes, and Impact on

Climate,” Reviews of Geophysics, vol. 54, no. 2 (2016), p. 278.

24 NASA, Goddard Space Flight Center, “NASA Ozone Watch,” https://ozonewatch.gsfc.nasa.gov/facts/SH.html

(accessed April 2023).

25 S. Tilmes et al., “The Sensitivity of Polar Ozone Depletion to Proposed Geoengineering Schemes,” Science, vol. 320,

no. 5880 (2008), p. 1201 (hereinafter Tilmes 2008).

26 Tilmes 2008.

27 G. Pitari et al., “Stratospheric Ozone Response to Sulfate Geoengineering: Results from the Geoengineering Model

Intercomparison Project (GeoMIP),” Journal of Geophysical Research: Atmospheres, vol. 119, no. 5 (2014), p. 2629.

28 K. S. Krishnamohan et al., “The Climatic Effects of Hygroscopic Growth of Sulfate Aerosols in the Stratosphere,”

Earth’s Future, vol. 8, no. 2 (2020).

29 D. W. Keith et al., “Stratospheric Solar Geoengineering without Ozone Loss,” Proceedings of the National Academy

of Sciences of the United States of America, vol. 113, no. 52 (2016), p. 14910 (hereinafter Keith 2016).

30 Keith 2016; see also J. A. Dykema et al., “Improved Aerosol Radiative Properties as a Foundation for Solar

Geoengineering Risk Assessment,” Geophysical Research Letters, vol. 43, no. 14 (2016), p. 7758.

31 J. A. Dykema, D. W. Keith, and F. N. Keutsch, “Improved Aerosol Radiative Properties as a Foundation for Solar

Geoengineering Risk Assessment,” Geophysical Research Letters, vol. 43, no. 14 (2016), p. 7758.

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Pinatubo.32 Some researchers have questioned the likelihood of such an abrupt halt to SAI.33

Other researchers have suggested that SG methods such as SAI might be combined, on a

temporary basis, with other climate mitigation strategies until atmospheric CO2 concentrations

reach levels consistent with global temperature stabilization targets.34

Marine Cloud Brightening (MCB)

MCB is a proposed method of cooling the Earth’s climate by increasing the reflectivity or

brightness of low-altitude marine clouds.35 MCB is based on the observation that, in a cloud,

smaller water droplets are more reflective than larger droplets and that by increasing the number

of small droplets, a cloud can be made more reflective. This phenomena has been observed with

“ship tracks,” which are lighter lines within cloudy ocean areas produced by ships’ emissions of

aerosols. These emissions cause an increase in water droplets in cloudy areas the ships pass

through (Figure 2).

Figure 2. Ship Tracks off the Iberian Peninsula

Source: National Oceanographic and Atmospheric Administration (NOAA), National Environmental Satellite

Data and Information Service, “Ship Tracks off the Coast of the Iberian Peninsula,” https://www.nesdis.noaa.gov/

news/ship-tracks-the-coast-of-the-iberian-peninsula.

32 K. E. McCusker et al., “Rapid and Extensive Warming Following Cessation of Solar Radiation Management,”

Environmental Research Letters, vol. 9, no. 2 (2014), 024005 (hereinafter McCusker 2014). See also NASEM 2021,

p. 81.

33 Andy Parker and Peter J. Irvine, “The Risk of Termination Shock From Solar Geoengineering,” Earth’s Future,

vol. 6, no. 3 (2018), p. 456.

34 D. G. MacMartin et al., “Solar Geoengineering as Part of an Overall Strategy for Meeting the 1.5°C Paris Target,”

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 376, no.

2119 (2018), 20160454.

35 J. Latham et al., “Marine Cloud Brightening,” Philosophical Transactions. Series A, Mathematical, Physical, and

Engineering Sciences, vol. 370 (2012), p. 4217 (hereinafter Latham 2012).

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Note: Image acquired January 15, 2018, by the VIIRS instrument on board the Suomi NPP Satellite, showing ship

tracks over the ocean to the west of the Iberian Peninsula.

The MCB technique would involve spraying a mist of submicrometer seawater particles into

marine stratocumulus clouds.36 The spray particles would form small droplets within the cloud;

these droplets would increase the cloud’s reflectivity, increasing the amount of incoming solar

energy reflected back out into space. The method can be implemented only where appropriate

marine cloud types occur, and the most extensive oceanic occurrence of these clouds is near the

western coasts of North and South America and Africa.37

Potential Cooling Effects of MCB

Using climate models to estimate the potential effectiveness of MCB, some researchers have

found that MCB could provide a cooling effect that would mitigate some of the warming effect

caused by the increase in concentrations of atmospheric CO2 from human activities.38

Some research indicates that MCB would be able to offset the warming effect of a doubling of

atmospheric CO2 as compared to levels before the industrial revolution; however, other studies

indicate that while there would be some cooling effect, it would not be sufficient to offset this

level of warming.39

Potential Concerns/Drawbacks of MCB

Some modeling studies of MCB have suggested it could alter precipitation patterns at global and

regional levels, although studies have differed in their results.40 For example, one modeling study

indicated that MCB caused a 50% decrease in Amazonian precipitation.41 Other modeling studies,

however, found that precipitation in the Amazon was not sensitive to MCB, increased, or

decreased.42 Other modeling studies that examined the potential impacts of MCB on Arctic and

36 Latham 2012. A micrometer is one-millionth of a meter, or 0.001 millimeters.

37 J. Latham et al., “Marine Cloud Brightening: Regional Applications,” Philosophical Transactions of the Royal

Society A: Mathematical, Physical and Engineering Sciences, vol. 372, no. 2031 (2014), 20140053 (hereinafter Latham

2014).

38 Latham 2012. The experimental design of the study was described in the journal article cited as follows:

The three simulations used in our current studies are (1) a control (CON) with carbon dioxide levels

held at a 2020 projected level of 440 ppm; (2) a climate change (CC) simulation, where the carbon

dioxide fraction increases by 1% p.a. until 2045, where it is held steady at 560ppm for the duration

of the simulation, until 2090; and (3), an MCB simulation, which is the same as the [CC]

simulation except that it includes seeding in the three stratocumulus regions defined above. All

simulations were run for 70 model years with the final 20 years (2070–2090) used for analysis.

39 Latham 2012. See also P. Forster et al., “The Earth’s Energy Budget, Climate Feedbacks, and Climate Sensitivity” in

Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of

the Intergovernmental Panel on Climate Change (Cambridge, United Kingdom: Cambridge University Press, 2021), p.

943. See also Spencer Hill and Yi Ming, “Nonlinear Climate Response to Regional Brightening of Tropical Marine

Stratocumulus,” Geophysical Research Letters, vol. 39, no. 15 (2012); and A. Jones et al., “Climate Impacts of

Geoengineering Marine Stratocumulus Clouds,” Journal of Geophysical Research: Atmospheres, vol. 114, no. D10

(2009) (hereinafter Jones 2009).

40 G. Bala et al., “Albedo Enhancement of Marine Clouds to Counteract Global Warming: Impacts on the Hydrological

Cycle,” Climate Dynamics, vol. 37, no. 5 (2011), p. 915 (hereinafter Bala 2011). As of the date of this report,

atmospheric CO2 concentration was below 430 ppm; see also NOAA, “Global Monitoring Laboratory: Trends in CO2,”

https://gml.noaa.gov/ccgg/trends/.

41 A. Jones et al., “Climate Impacts of Geoengineering Marine Stratocumulus Clouds,” Journal of Geophysical

Research: Atmospheres, vol. 114, no. D10 (2009) (hereinafter Jones 2009).

42 Bala 2011; see also Spencer Hill and Yi Ming, “Nonlinear Climate Response to Regional Brightening of Tropical

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Antarctic sea ice extent have also produced variable results.43 As different models were used in

these studies, direct comparison of results is challenging.

Researchers have identified a number of key knowledge gaps regarding the use of MCB to lower

global temperatures. These include both the basic science relevant to the method, such as the

current state of knowledge of cloud physical processes, and technological implementation

challenges, such as the development of appropriate spray technology.44

As with SAI, some research indicates that an abrupt halt of MCB could result in global

temperatures increasing to levels comparable to temperature levels occurring if MCB had not

been implemented.45

Cirrus Cloud Thinning (CCT)

Wispy, high-altitude cirrus clouds help prevent longwave heat energy from leaving the Earth.

CCT is a proposed method of reducing cirrus clouds, allowing more longwave heat energy to

escape into space, which could compensate for some of the warming caused by anthropogenic

GHG emissions.46

Cirrus clouds consist of ice crystals, and the extent to which these clouds persist in the

atmosphere is determined in part by how quickly the clouds’ ice crystals fall out of the

atmosphere. CCT would involve adding an aerosol of insoluble particles to the cloud, which

would form larger crystals than the smaller ice-only crystals. These larger crystals would fall out

of the atmosphere more quickly than the naturally forming ice-only crystals, thus thinning the

cirrus cloud and allowing more heat energy to escape.47 One proposed CCT method would be to

use specially adapted commercial aircraft to disperse nucleation, or seed, aerosols into cirrus

clouds.48

Potential Effects of CCT on Global Temperatures

There is scientific debate about whether CCT would have the desired cooling effect. A 2009 peerreviewed theoretical study indicated that CCT could provide a significant reduction in radiative

forcing, although later studies have found a range of results.49 One study, for example, found that

CCT could provide a cooling effect with optimum ice nuclei seeding concentrations, but that

higher concentrations produced a warming rather than a cooling effect.50 Other studies did not

Marine Stratocumulus,” Geophysical Research Letters, vol. 39, no. 15 (2012).

43 P. J. Rasch et al., “Geoengineering by Cloud Seeding: Influence on Sea Ice and Climate System,” Environmental

Research Letters, vol. 4, no. 4 (2009), 045112. See also B. Parkes et al., “The Effects of Marine Cloud Brightening on

Seasonal Polar Temperatures and the Meridional Heat Flux,” ISRN Geophysics, vol. 2012 (2012), p. 1.

44 Knowledge gaps have been identified in Latham 2012 and in the Marine Cloud Brightening Project, “About Us,”

https://mcbproject.org/about-us/, including cloud-aerosol interactions and cloud physical processes; climate models’

inability to adequately simulate some marine clouds; and designs for the necessary sea spray technology.

45 Jones 2009.

46 NASEM 2021.

47 David L. Mitchell and William Finnegan, “Modification of Cirrus Clouds to Reduce Global Warming,”

Environmental Research Letters, vol. 4, no. 4 (2009), 045102 (hereinafter Mitchell and Finnegan 2009).

48 Mitchell and Finnegan 2009.

49 Mitchell and Finnegan 2009.

50 T. Storelvmo and N. Herger, “Cirrus Cloud Susceptibility to the Injection of Ice Nuclei in the Upper Troposphere,”

Journal of Geophysical Research: Atmospheres, vol. 119, no. 5 (2014), p. 2375 (hereinafter Storelvmo and

Herger 2014).

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find CCT to be effective in providing a cooling effect or found that different models produced

different degrees of cooling.51 A scientific consensus on the effectiveness of CCT has not been

reached.

Potential Concerns/Drawbacks of CCT

Researchers have identified a number of knowledge gaps with respect to the potential

development of CCT, including how ice crystals in cirrus clouds nucleate and freeze, which

aerosols are effective for this process, and how the technology for implementing CCT would

work, among other challenges.52 One researcher stated that “uncertainties in both observations

and modeling of cirrus clouds place some doubt on all cirrus seeding studies.”53

Concerns About SG

Uncertainties about SG include









uncertainty about some of the underlying physical and chemical processes of SG

methods,

uncertainty regarding the technical feasibility of SG implementation,

uncertainty that the proposed SG methods will be able to provide large-scale

cooling effects even if successfully deployed, and

uncertainty about the potential regional and global effects on the Earth’s climate,

apart from the possible cooling effects.

As of May 2023, no SG field experiments at scale have been carried out, and one effort to do so

proved controversial and was canceled. Thus, most of the current understanding about SG comes

from theoretical and modeling studies.54

Scientific and academic groups are divided over SG research and implementation in the United

States and other countries.55 In January 2022, an international coalition of scientists and scholars

launched an effort advocating for a solar geoengineering non-use agreement in an “open letter.”

The open letter advocates for the adoption of an official policy of SG non-use, including a

51 J. E. Penner et al., “Can Cirrus Cloud Seeding Be Used for Geoengineering?” Geophysical Research Letters, vol. 42,

no. 20 (2015), p. 8775; these authors do not find CCT to be an effective cooling technique. See also B. Gasparini et al.,

“To What Extent Can Cirrus Cloud Seeding Counteract Global Warming?” Environmental Research Letters, vol. 15,

no. 5 (2020), 054002; these researchers found that different models produced differing results.

52 Storelvmo and Herger 2014; see also D. J. Cziczo et al., “Clarifying the Dominant Sources and Mechanisms of

Cirrus Cloud Formation,” Science, vol. 340, no. 6138 (2013), p. 1320 (hereinafter Cziczo 2013); B. Gasparini and

Ulrike Lohmann, “Why Cirrus Cloud Seeding Cannot Substantially Cool the Planet,” Journal of Geophysical

Research: Atmospheres, vol. 121, no. 9 (2016), p. 4877 (hereinafter Gasparini and Lohmann 2016); J. E. Kristjánsson

et al., “The Hydrological Cycle Response to Cirrus Cloud Thinning,” Geophysical Research Letters, vol. 42, no. 24

(2015), p. 10,807 (hereinafter Kristjánsson 2015).

53 Gasparini and Lohmann 2016; see also Storelvmo and Herger 2014; J. E. Penner et al., “Can Cirrus Cloud Seeding

Be Used for Geoengineering?” Geophysical Research Letters, vol. 42, no. 20 (2015), p. 8775; B. Gasparini et al., “To

What Extent Can Cirrus Cloud Seeding Counteract Global Warming?” Environmental Research Letters, vol. 15, no. 5

(2020), 054002.

54 Henry Fountain and Christopher Flavelle, “Test Flight for Sunlight-Blocking Research Is Canceled,” New York

Times, April 2, 2021, https://www.nytimes.com/2021/04/02/climate/solar-geoengineering-block-sunlight.html.

55 Alejandro de la Garza, “A Controversial Technology Is Creating an Unprecedented Rift Among Climate Scientists,”

Time, March 17, 2023, https://time.com/6264143/geoengineering-climate-scientists-divided/.

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commitment not to use government funds for the development of SG.56 The group is concerned

that the potential adverse impacts of SG are not well understood, that the perception of SG’s

availability may reduce the policy commitment to emission reductions, and that global

governance systems, both institutional and informal, are inherently incapable of providing

equitable control of SG.

Another group of scientists and scholars calling for “balance in research and assessment of solar

radiation modification” takes the position that the risk of adverse impacts from climate change is

so great that a well-regulated program of research should be considered to provide SG as an

option if necessary.57 In an open letter published online in 2023, the group cites the possibilities

that climate mitigation efforts such as emissions reductions and CO2 removal may prove

inadequate, by themselves, in averting catastrophic adverse climate impacts. They state that

research on SG is necessary to make informed decisions about SG, including a decision not to

implement SG if research findings indicate that it is too risky. The group advocates the formation

of a governance framework based on a set of ethical principles before any possibility of

implementation of SG.

Federal Law and SG

A number of statutes may be relevant to SG, although Congress has not considered or passed any

law with the exclusive purpose of covering SG activities. For example, the Weather Modification

Reporting Act of 1972 (15 U.S.C. §§330 et seq.) has a reporting requirement for activities

“Modifying the solar radiation exchange of the Earth or clouds, through the release of gases,

dusts, liquids or aerosols into the atmosphere.”58 Also, if sulfate aerosols used in SAI are found to

have a damaging effect on stratospheric ozone, their use in SG might be regulated under Title VI

of the Clean Air Act Amendments of 1990 (42 U.S.C. §7401).

International Agreements and SG

To date, no multilateral treaty is in force or has been proposed with the exclusive intent of

addressing the full spectrum of possible geoengineering activities, including SG. However,

principles of customary international law and existing international agreements may be relevant

to SG research or deployment projects. Customary international law refers to the general and

consistent practices by countries that are followed from a sense of legal obligation.59 Under

customary international law, countries have a duty not to cause significant transboundary harm.60

Solar Geoengineering Non-Use Agreement, “Open Letter,” https://www.solargeoeng.org/non-use-agreement/openletter/.

57 Claudia Wieners et al., “Solar Radiation Modification Is Risky, but So Is Rejecting It: A Call for Balanced

Research,” https://www.call-for-balance.com/letter.

58 C.F.R. 908.3(a)(3).

59 Restatement (Third) of Foreign Relations Law §102, (1987).

60 Ibid. at §601(1) (stating that a nation is generally obligated to take “such measures as may be necessary, to the extent

practicable under the circumstances, to ensure that activities within its jurisdiction or control ... are conducted so as not

to cause significant injury to the environment of another state.”) Countries are also obligated under international law to

take necessary measures to the extent practicable to prevent, reduce, and control pollution that is causing or threatening

to cause significant injury to the marine environment. Ibid. at §603(2).

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Because SG carries with it the likelihood of transboundary effects, this duty could be relevant to

SG research and/or deployment projects.61

The international agreements on climate change encourage their parties to implement national

policies and mitigation actions to reduce their greenhouse gas emissions.62 As SG does not

directly affect atmospheric GHG concentrations, only those aspects that address climate change

mitigation more broadly are likely to be relevant to SG.63 In addition, the Convention on

Biological Diversity (CBD), in a decision adopted on December 2016 at the 14th Congress of

Parties to the Convention, noted the need for geoengineering research.64 MCB specifically may

fall under the marine geoengineering provisions of the London Protocol, as MCB carries the risk

of deleterious effects.65 Aspects of other international agreements may subsequently be found to

be relevant to SG.

Congressional Action and Considerations

The 117th Congress appropriated funds for FY2022 for activities related to SG in the

Consolidated Appropriations Act of 2022.66 These included the preparation of a report by the

Office of Science and Technology Policy (OSTP) in conjunction with the National Oceanic and

Atmospheric Administration (NOAA) and the National Science Foundation (NSF):

NOAA is directed to support OSTP, in coordination with DOE and the National Science

Foundation (NSF), to provide a five-year plan, not later than 180 days after enactment of

this Act, with a scientific assessment of solar and other rapid climate interventions in the

context of near-term climate risks and hazards. The report shall include: (1) the definition

of goals in relevant areas of scientific research; (2) capabilities required to model, analyze,

observe, and monitor atmospheric composition; (3) climate impacts and the Earth’s

radiation budget; and (4) the coordination of Federal research and investments to deliver

this assessment to manage near-term climate risk and research in climate intervention. 67

Further, under this legislation OSTP, working with NOAA, the National Aeronautics and Space

Administration (NASA), and the Department of Energy (DOE), is tasked to form an interagency

61 CRS Report R41371, Geoengineering: Governance and Technology Policy, by Kelsi Bracmort and Richard K.

Lattanzio.

62 United Nations Framework Convention on Climate Change, May 9, 1992, 1771 U.N.T.S. 107; S. Treaty Doc No.

102-38, htttps://unfccc.int/resource/docs/convkp/conveng.pdf. See also Paris Agreement to the United Nations

Framework Convention on Climate Change, December 12, 2015, T.I.A.S. No. 16-1104, https://unfccc.int/sites/default/

files/english_paris_agreement.pdf; CRS Report R44609, Climate Change: Frequently Asked Questions About the 2015

Paris Agreement, by Jane A. Leggett and Richard K. Lattanzio.

63 Jesse L. Reynolds, “Solar Geoengineering to Reduce Climate Change: A Review of Governance Proposals,”

Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 475, no. 2229 (2019),

20190255 (hereinafter Reynolds 2019).

64 “[T]hat more transdisciplinary research and sharing of knowledge among appropriate institutions is needed in order

to better understand the impacts of climate related geoengineering on biodiversity and ecosystem functions and

services, socio economic, cultural and ethical issues and regulatory options”; see Conference of the Parties to the

Convention on Biological Diversity, December 4-17, 2016, Decision XIII/14, https://www.cbd.int/doc/decisions/cop13/cop-13-dec-14-en.pdf.

65 London Protocol, “Annex 4 Resolution LP.4(8) On the Amendment to the London Protocol to Regulate the

Placement of Matter for Ocean Fertilization and Other Marine Geoengineering Activities,” adopted October 18, 2013.

66 U.S. Congress, House Committee on Appropriations, Consolidated Appropriations Act, 2022 (H.R. 2471; P.L. 117103): Provisions Applying to All Divisions of the Consolidated Appropriations Act, committee print, 117th Cong.

67 U.S. Congress, House Committee on Appropriations, Consolidated Appropriations Act, 2022 (H.R. 2471; P.L. 117103): Provisions Applying to All Divisions of the Consolidated Appropriations Act, committee print, 117th Cong.

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working group to manage SG risk and research and to develop a governance framework for

publicly funded SG research.68

OSTP put out a public call for input on the five-year plan limited to comments regarding research

on the plan’s climate intervention components. The comment period ended on September 9,

2022.69 As of May 2023, OSTP has not published the report on solar geoengineering specified in

the statute.70 The USGCRP operates under OSTP and provides a list of climate-related

interagency working groups. As of May 2023, the interagency working group tasked to

coordinate SG research and create an SG research governance framework does not appear on the

list of interagency working groups.71

As described above, a number of U.S. statutes may be relevant to SG, and Congress may review

and amend the existing statutes such that they refer specifically to SG. Research and

implementation of SG, including unilateral action by countries or nonstate actors, could have

transboundary global effects that could affect international relations. Congress may consider these

potential effects in determining SG policy.72

Congressional actions regarding deploying SG would likely need to assess the trade-off in

choosing between the potential benefits provided by SG—atmospheric cooling to offset GHGdriven global warming—against the risks associated with SG described above. Congress also may

consider how the potential need to deploy SG might evolve if global GHG emissions increase or

decrease in the future: a rise may increase focus on SG as a means to offset global warming

associated with more GHGs in the atmosphere, whereas a decrease in GHG concentrations may

reduce the potential need for SG.

There are uncertainties regarding SG with respect to the underlying science, the technological

implementation, the effects on the Earth’s climate, and the mechanisms for the governance of SG

research and development, indicating that SG is currently not a technology that is likely to be

ready for development and implementation in the near or medium term.73

Author Information

Jonathan D. Haskett

Analyst in Environmental Policy

68 U.S. Congress, House Committee on Appropriations, Consolidated Appropriations Act, 2022 (H.R. 2471; P.L. 117-

103): Provisions Applying to All Divisions of the Consolidated Appropriations Act, committee print, 117th Cong.

69 The White House, “Request for Input to a Five-Year Plan for Research on Climate Intervention,”

https://www.whitehouse.gov/ostp/legal/.

70 P.L. 117-103 became law on March 15, 2022, specifying that the five-year plan was to be provided within 180 days

of after enactment—that is, on or before September 11, 2022.

71 U.S. Global Change Research Program, “Interagency Groups,” https://www.globalchange.gov/about/iwgs.

72 Reynolds 2019; see also Arunabha Ghosh, “Environmental Institutions, International Research Programmes, and

Lessons for Geoengineering Research,” in Geoengineering Our Climate? (New York: Routledge, 2018), pp. 199-213,

https://doi.org/10.4324/9780203485262-37.

73 NASEM 2021; see also M. Diamond et al., “To Assess Marine Cloud Brightening’s Technical Feasibility, We Need

to Know What to Study—and When to Stop,” Proceedings of the National Academy of Sciences, vol. 119, no. 4 (2022),

e2118379119; see also United Nations Environment Program, One Atmosphere: An Independent Expert Review on

Solar Radiation Modification Research and Deployment, February 28, 2023.

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Disclaimer

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under the direction of Congress. Information in a CRS Report should not be relied upon for purposes other

than public understanding of information that has been provided by CRS to Members of Congress in

connection with CRS’s institutional role. CRS Reports, as a work of the United States Government, are not

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copy or otherwise use copyrighted material.

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