Geoengineering, Part II

August 4, 2021

V. less promising geoengineering approaches

In Part I of this post, we described the proposed geoengineering approaches of stratospheric aerosol injection (SAI) and marine cloud brightening (MCB) in some detail. Other methods that have been suggested for climate intervention, and that are included in Fig. II.1 for completeness, have been pursued less vigorously than SAI or MCB. Nonetheless, it is useful to discuss them briefly in order to gain more insight into the challenges of geoengineering.

The conceptually simplest, but technologically most demanding, method for reducing sunlight reaching Earth by 1% involves installing a space mirror or shield to reflect 1% of the sunlight emitted toward Earth. Such a mirror would have to lie along the line between Earth and the Sun, and would have to remain along that line throughout the Earth’s annual orbit. In order for its relative position to remain fixed, the mirror would have to be placed at such a distance that the gravitational pull it feels from the Sun exceeds the opposite pull from Earth by just the right amount to provide the centripetal force needed to keep the shield orbiting around the Sun with the same orbital period as Earth. That would mean that as the Earth rotates daily about its axis, the mirror always partially shields the part of Earth exposed to sunlight each day. A straightforward calculation of the needed distance puts it at 1% of the distance from Earth to Sun, or 1.5 million kilometers from the center of Earth.

In order to shield 1% of the Sun’s cross-sectional area, the radius of the shield would have to be 0.1% of the Sun’s radius or about 700 km. In practice, allowing for less than 100% efficiency in reflecting the Sun’s rays, a more likely radius is 1000-2000 km, requiring a shield (or combination of smaller shields, see Fig. V.1) encompassing an area exceeding that of the country of Egypt. Obviously, launching such an enormous shield into space, or constructing it in space, represents an enormous and costly technical challenge.

Figure V.1. Artist’s conception of a huge array of solar reflectors between the Sun and Earth, and locked in with Earth’s orbit.

Furthermore, there are some basic challenges with the space mirror approach. One is that it would do nothing to counteract the accumulation of carbon dioxide in Earth’s oceans, which leads to ocean acidification, the bleaching of coral reefs, the extinction of fish species, etc. Another is that the suggested mirror location does not provide a stable equilibrium orbit. Namely, if any space debris were to collide with the shield and give it a radial kick, either toward the Sun or toward the Earth, upsetting the gravitational balance, the shield would then accelerate away from the preferred location in the direction of the kick. These shortcomings and the enormous technical challenges and cost of such a project have led to sparse detailed consideration of the space mirror option.

The next simplest technological approach to conceptualize is increasing the amount of incident sunlight reflected from Earth’s surface back out into space. The current albedo of Earth’s surface is dominated by ice cover in the Arctic and Antarctic regions and worldwide glaciers. Unfortunately the ongoing global warming is already leading to rapid melting of the polar ice caps, the Greenland and Antarctic ice sheets, sea ice near the poles, and of worldwide glaciers, so that the highly reflective coverage area is fading fast. This is part of the positive feedback loop that enhances global warming: the heat retained in the oceans leads to ice melt, reduced albedo, more net input solar power, and hence, more heating.  Even if we could produce a reflective coating to cover all the remaining ice pack, the ice would still be melting because of increasing heat in the oceans beneath them.

One proposal for surface albedo modification is to blow artificial snow, or add some other highly reflective coating, across the surface of glaciers and mountains. This is done on a small scale by Alpine ski companies, which use reflective polypropylene blanket covers. The cost of doing this is roughly $10M U.S. per square mile.  Currently, glacierized areas cover about 5.8 million square miles, so the cost to cover all of that surface would be about $60 trillion U.S. So this is not a cheap option.

Similarly, programs like the CoolRoofs initiative in New York City to paint rooftops white (see Fig. V.2) do not really deliver much bang for the buck. Such programs do increase albedo in populated areas, and reduce energy consumption in households in summer. But they increase energy consumption for winter heating. Furthermore, the reflected sunlight in cities is often absorbed by particles of soot and other dark air pollutants that are prevalent in urban areas, thereby heating the low atmosphere. It has been pointed out that a more effective, and probably even more cost-effective, “way to use your roof in the fight against climate change [is to] cover it with solar panels. The panels intercept sunlight before it hits the roof, so your house doesn’t heat up so much. They don’t bounce the light back into the atmosphere where it can heat up soot particles. And they generate at least some electricity without emitting greenhouse gases.”

Figure V.2. A team puts the finishing touches on painting one of New York City’s roofs white. The goal of the CoolRoofs initiative is to coat one million square feet of city rooftops. Credit: Ben Huff.

Other suggested methods for surface albedo modification also present significant problems. It has been suggested to clear high-latitude snow-covered areas of their boreal forests to increase albedo, but that also would significantly reduce carbon dioxide absorption by the trees. Agricultural crops could, in principle, be genetically engineered to produce more highly reflective leaves, but concerns about the unintended consequences of such GMOs would greatly complicate acceptance of such proposals. Adding reflective covers over polar ice or Earth’s deserts would endanger species living in those areas. The bottom line is that surface albedo modification proposals are neither cost-effective nor environmentally friendly.

The last of the techniques represented in Fig. II.1 is different from the other four techniques, in that its aim is not to reduce the solar irradiance of Earth, but rather to increase the probability that infrared radiation emitted by Earth escapes into space. Of course, the most effective way of accomplishing that aim is to reduce the concentration of greenhouse gases in the atmosphere. But a secondary effect is caused by the high-altitude, wispy cirrus clouds, which consist of small ice crystals that absorb in the infrared (see Fig. IV.3). These clouds form near the top of the troposphere, and the absorbed radiation adds to tropospheric heating. The proposal known as Cirrus Cloud Thinning is to inject particles such as sulfuric or nitric acid into the regions where cirrus clouds form, with the hope that these particles will serve as nuclei for the formation of larger, shorter-lived ice crystals that absorb less of Earth’s infrared radiation. This approach suffers from the same concerns expressed above for stratospheric aerosol injection – i.e., perturbing the atmosphere in ways whose net result is hard to predict – but also has the added drawback that it relies on the most suspect feature of climate models, namely the interaction of aerosols with clouds. It might turn out to increase, rather than decrease, infrared absorption.

VI. scientific reviews on geoengineering research and governance

In the present political climate it is likely that geoengineering will generate a more highly polarized discussion as time goes on, with both the promise and the perils overstated, as has unfortunately been the case for all topics related to climate change mitigation. This was seen already in the cancellation of the first test balloon flight for the SCoPEx experiment in June 2021, even though that flight had no intention to disperse any matter in the atmosphere. But three general statements can be made about the more promising approaches described in Sections III and IV above: (1) it is very premature to attempt any global test of any geoengineering technique; (2) it is quite likely that governments around the world will be looking for ways to mitigate serious climate change impacts by mid-century, and stratospheric aerosol injection and marine cloud brightening are approaches worthy of consideration; (3) small-scale research efforts on SAI and MCB are needed in the short term in order to have a sensible basis for discussing their effectiveness and side-effects by mid-century, but even small-scale experiments have the potential for impacts beyond national borders, and hence require some sensible international agreements on governance.

These issues have been taken up in two reports generated by the U.S. National Academies of Sciences (NAS), and also discussed by the Conference of the Parties to the United Nations-sponsored Convention on Biological Diversity (CBD). The 2015 NAS report Climate Intervention: Reflecting Sunlight to Cool Earth presented a detailed assessment of the technological and governance risks associated with the geoengineering approaches considered above, leading to a set of clear and reasonable recommendations, some of which are reproduced below:

  • Efforts to address climate change should continue to focus most heavily on mitigating greenhouse gas emissions in combination with adapting to the impacts of climate change because these approaches do not present poorly defined and poorly quantified risks and are at a greater state of technological readiness.
  • Albedo modification at scales sufficient to alter climate should not be deployed at this time.
    • There is significant potential for unanticipated, unmanageable, and regrettable consequences in multiple human dimensions from albedo modification at climate-altering scales, including political, social, legal, economic, and ethical dimensions.
    • Deployment at climate-altering amplitudes should only be contemplated armed with a quantitative and accurate understanding of the processes that participate in albedo modification. This understanding should be demonstrated at smaller scales after intended and unintended impacts to the Earth system have been explicitly documented, both of which are lacking.
    • If research and development on albedo modification were to be done at climate-altering scales, it should be carried out only as part of coordinated national or international planning, proceeding from smaller, less risky to larger, more risky projects; more risky projects should be undertaken only as information is collected to quantify the risks at each stage.
  • The committee recommends an albedo modification research program be developed and implemented that emphasizes multiple-benefit research that also furthers basic understanding of the climate system and its human dimensions.
    • Small-scale field experiments with controlled emissions may for some situations with some forms of intervention be helpful in reducing model uncertainties, validating theory, and verifying model simulations in different conditions. Experiments that involve release of gases or particles into the atmosphere (or other controlled perturbations) should be well-enough understood to be benign to the larger environment, should be conducted at the smallest practical scales, should be designed so as to pose no significant risk, and should be planned subject to the deliberative process outlined in [the next] Recommendation.
  • The committee recommends the initiation of a serious deliberative process to examine (a) what types of research governance, beyond those that already exist, may be needed for albedo modification research, and (b) the types of research that would require such governance, potentially based on the magnitude of their expected impact on radiative forcing, their potential for detrimental direct and indirect effects, and other considerations.
    • The United States should help lead the development of best practices or specific norms that could serve as a model for researchers and funding agencies in other countries and could lower the risks associated with albedo modification research.

The Conference of the Parties to the CBD made similar comments for international consumption, while noting their narrower focus: “science based global, transparent and effective control and regulatory mechanisms…for climate-related geo-engineering… may not be best placed under the Convention on Biological Diversity.” Nonetheless, they announced a decision stating:

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 that would be conducted in a controlled setting in accordance with Article 3 of the Convention, and only if they are justified by the need to gather specific scientific data and are subject to a thorough prior assessment of the potential impacts on the environment.”

It should be noted that the SCoPEx team has set up its own independent Advisory Committee specifically to provide advice on the governance of the experiment, in the hopes of providing a template for future experimental research on solar radiation management approaches. The members of that Advisory Committee have put out a statement promising to:

commit to working with Harvard University to develop and implement a credible and sound governance framework for this research project…Each of us is committed to the work of this Committee because of the possibility that governments or others may turn to solar geoengineering as an option for mitigating climate change as the impacts become more severe. While the SCoPEx research team designed the experiment to answer a set of scientific and engineering questions, we recognize that this experiment has implications far beyond these questions. Moreover, current research practices are not well adapted to the ethical, moral, or even technical issues associated with geoengineering research. The SCoPEx project presents an opportunity to pilot comprehensive and inclusive approaches to research governance that are commensurate with the myriad, interconnected, and complex challenges presented by geoengineering research. We modestly hope that the processes we develop and employ to evaluate SCoPEx can both responsibly guide this particular experiment and serve as a model for other geoengineering research. We also hope that this work will provide a foundation and test-case for broadening participation in guiding research and making sound decisions about scientific experiments.”

The cancellation of the planned first SCoPEx balloon launch in Sweden in June 2021 provides an indication already that broader participation in guiding research will be critical to get projects such as SCoPEx off the ground (literally). And that will require a robust education plan to combat both legitimate public concerns and the inevitable spread of misinformation about the scope and goals of each proposed experiment.

In order to address the many unsettled issues with regard to solar radiation management, the NAS undertook in 2021 a follow-up study – Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governanceto update the 2015 assessment of the state of understanding and to provide recommendations for how to establish a research program, what to encompass in the research agenda, and what mechanisms to employ for governing this research.” This report made many recommendations regarding goals and governance of an effective geoengineering research program, emphasizing the need for careful deliberation, with international and societal involvement, at every step along the way. Some of the most important recommendations are reproduced below:

  • The United States should implement a robust portfolio of climate mitigation and adaptation. In addition, given the urgency of climate change concerns and the need for a full understanding of possible response options, the U.S. federal government should establish—in coordination with other countries—a transdisciplinary, solar geoengineering [SG] research program. This program should be a minor part of the overall U.S. research program related to responding to climate change. The program should focus on developing policy-relevant knowledge, rather than advancing a path for deployment, and should be subject to robust governance. The program should
    • advance knowledge relevant to decision making, including design of future research efforts;
    • ensure transparency, disciplinary balance, and public and stakeholder engagement;
    • coordinate research across federal agencies and with research outside the U.S. federal government; and
    • limit research on technology with direct applicability for deployment to early-phase, fundamental research.
  • The program should establish robust mechanisms for inputs from civil society and other key stakeholders in the design of the research program, as well as promote their engagement in relevant program components. Key stakeholders include climate-vulnerable communities and underrepresented groups, including from indigenous populations and the Global South.
  • The program and its outcomes should be regularly reviewed and assessed by a diverse, inclusive panel of experts and stakeholders (including consultation with international counterparts) to determine whether continued research is justified and, if so, how goals and priorities should be updated.
  • “Exit ramps” (i.e., criteria and protocols for terminating research programs or areas) should be an explicit part of the program, with mechanisms to terminate a research activity, for example, if it is deemed to pose unacceptable physical, social, geopolitical, or environmental risks or if research indicates clearly that a particular SG technique is not likely to work.
  • A U.S. national solar geoengineering research program should operate under robust research governance and support the development or designation of an international governance mechanism. Important elements of research governance include a research code of conduct, a public registry for research, regular program assessment and review processes, permitting systems for outdoor experiments, guidance on intellectual property, inclusive public and stakeholder engagement processes, mechanisms for advancing international information sharing and collaboration (within research teams and among national scientific agencies), and establishment of an expert committee to advance discussions about international governance needs and strategies.
Figure VI.1. The three interconnected broad categories and thirteen research topic clusters in the solar geoengineering research agenda recommended in the 2021 NAS Report.
  • The agenda for the SG research program should encompass three broad, interconnected areas [summarized in Fig. VI.1] that address: (i) the context and goals for SG research, (ii) the impacts and technical dimensions, and (iii) the social dimensions. Under these broad categories, the following are recommended as key research clusters to pursue:
  • Context and Goals for SG Research
    • Program Development Pathways. Designing an SG research program to maximize the prospects for broadly beneficial outcomes.
    • Future Conditions. Exploring the range of future conditions under which SG-related decisions will be made.
    • Integrated Decision Analysis. Understanding implications of, and strategies to address, persistent uncertainties that affect decision making related to SG.
    • Capacity Building. Developing the capacities needed for all countries to engage meaningfully with SG research and research governance activities.
  • Impacts and Technical Dimensions
    • Atmospheric Processes. Understanding chemical and physical mechanisms that determine how addition of materials to the atmosphere alters the reflection and transmission of atmospheric radiation.
    • Climate Response. Assessing how different SG approaches would affect key climate outcomes.
    • Other Impacts. Assessing the potential environmental and societal impacts of SG intervention strategies.
    • Monitoring and Attribution. Designing an observational system (and understanding its limitations) for detection, monitoring, and attribution of SG deployment and impacts.
    • Technology Development and Assessment. Addressing the science and engineering issues related to hardware, materials, and infrastructure underlying SG research.
  • Social Dimensions
    • Public Perceptions and Engagement. Understanding public perceptions of SG and strategies for inclusive, meaningful societal engagement, and how to incorporate these insights into a broader research program.
    • Political and Economic Dynamics. Exploring the implications of SG for national and international relations and related incentive structures.
    • Governance. Developing effective, adaptive processes and institutions to govern SG activities.
    • Ethics. Incorporating ethics and justice considerations for current and future generations into SG research and research governance.
  • Deliberate outdoor experiments that involve releasing substances into the atmosphere should be considered only when they can provide critical observations not already available and not likely to become available through laboratory studies, modeling, and experiments of opportunity (e.g., observing volcanic eruptions, rocket plumes, or ship tracks). All outdoor experiments involving the release of substances into the atmosphere should be subject to the governance established pursuant to the [above] recommendations…
    • In addition, any outdoor substance releases should be limited to a quantity of material at least two orders of magnitude smaller than the quantity that could cause detectable changes in global mean temperature or adverse environmental effects…These limitations should apply for at least the next 5 years and then be revisited and revised if needed, based on program review guidance from a diverse and inclusive panel of experts and stakeholders, as discussed [above].

The many detailed recommendations made by the 2021 NAS panel represent a robust, well-reasoned set of guidelines for how to proceed cautiously on research delineating the promise and perils of Earth albedo modification. But it is also quite reasonable, in light of recent experience, to remain skeptical that partisan governments will pay much heed to these guidelines.

VII. conclusions

The first rule for finding a way out of a deep hole is to stop digging. In that spirit, the first and most important step in dealing with human-induced climate change is to stop as much as possible of the fossil fuel burning that has caused the problem in the first place. But even if we reach carbon neutrality globally, that is the equivalent of stopping digging; at that point, we’re still in a deep hole, it’s just no longer getting deeper. Carbon dioxide remains in the atmosphere for many decades, so even when we emit no more than is absorbed on Earth, global mean temperatures will remain at a new post-industrial high. Adapting to the resultant altered climate will be painful, involving serious modifications of agricultural procedures, many more species extinctions, frequent climate-related disasters and deaths, mass human migrations, and perhaps even warfare. Technology to remove some of the carbon dioxide from the air will be available, but will be very costly if it is to have a significant effect.

The possibility of counteracting at least a significant portion of the human-caused temperature increase by reflecting more of the sunlight incident on Earth is certainly worth considering on a mid-century timeline when we have hopefully stopped “digging” so aggressively. Both stratospheric aerosol injection and marine cloud brightening have substantial promise, not as “silver-bullet solutions” to climate change, but rather as cost-effective mitigation measures to temper some climate change impacts by increasing Earth’s albedo. But both have profound uncertainties regarding potential negative climate and ecological impacts of further human disturbance of the balance between Earth’s atmosphere, clouds and oceans.

A sensible long-term approach to decisions regarding the wisdom of enhancing albedo on a global scale requires a carefully designed, monitored and governed short-term research program, featuring small-scale, localized experiments that can test crucial aspects of the relevant climate models. The recommendations outlined in Section VI from the recent National Academies of Sciences reviews provide wise science-based suggestions to guide such a research program.

Unfortunately, there is no reason to expect the politics surrounding any such geo-engineering research program to be less pitched than it has been for any other aspect of climate change. We will, however, see new coalitions on each side of public debates. As we have already seen in the events leading to the Swedish cancellation of the first SCoPEx test flight, those opposing any geoengineering research are likely to include both climate mitigation activists and members of the public who are wary of new technologies and who fear impacts on their way of life. The mitigation activists fear, with justification, that any attention paid to geoengineering comes at the expense of the necessary, urgent focus on attaining carbon neutrality. For example, Al Gore has called geoengineering proposals “insane, utterly mad, and delusional in the extreme.” The eco-justice ETC Group (Action Group on Erosion, Technology and Concentration) “opposes geoengineering and other false solutions to climate change (e.g., proprietary, genetically-engineered ‘climate-ready’ crops)” and has called for a moratorium on geoengineering research.

Likely supporters of geoengineering research will include some erstwhile climate change skeptics, including many in the fossil fuel industries and the politicians (e.g., former Representative Lamar Smith of Texas) and think tanks they support with significant donations. These groups, motivated by the desire to maintain fossil fuel profits, will join with some serious scientists and some politicians who want to understand whether solar radiation management can produce an effective way to mitigate some of the disastrous effects of climate change.

In an interesting recent (June 2020) development, Democrats on the U.S. House of Representatives’ Select Committee on the Climate Crisis have issued a nearly 550-page report entitled Solving the Climate Crisis: The Congressional Action Plan for a Clean Energy Economy and a Healthy, Resilient and Just America. The bulk of the report represents a detailed, coherent plan for decarbonizing the U.S. economy and supporting measures to aid adaptation to climate change. But among many “building blocks” regarding specific legislation to be considered by appropriate House committees, the report includes one entitled Invest in Research on the Risks and Governance for Atmospheric Climate Intervention (ACI), the name they use at NAS’ suggestion to replace “geoengineering.” The building block refers to a bill introduced by Rep. Jerry McNerney, Democrat of California, “which would establish a program within NOAA to study potential approaches to ACI and to provide reporting oversight for climate intervention experiments.”

The House Democrats’ report includes the explicit recommendation: “Congress should draw upon the findings of the forthcoming National Academies of Sciences, Engineering, and Medicine study to establish a research program to investigate potential ACI approaches, their risks, and governance frameworks.” This recommendation involves, as it should, only a tiny fraction of the appropriations the report would propose overall, but it represents a credible step to approach the issue scientifically. Although this particular issue might attract some bipartisan support, it remains a long shot to survive the fraught Congressional legislative processes in this highly partisan era.


J. Temple, What is Geoengineering — and Why Should You Care? MIT Technology Review, Aug. 9, 2019,

What is Solar Geoengineering?,

National Research Council, Climate Intervention: Reflecting Sunlight to Cool Earth (The National Academies Press, 2015),

National Academies of Sciences, Engineering, and Medicine, Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance (The National Academies Press, 2021),

1965 President’s Science Advisory Committee Report on Atmospheric Carbon Dioxide,

GOP Embraces Geoengineering … Which Terrifies Geoengineering Researchers, MIT Technology Review, Nov. 8, 2017,

K. Ellison, Why Climate Change Skeptics Are Backing Geoengineering, Wired, March 28, 2018,

K. Huff, The Effects of Volcanic Sulfur Dioxide on the Ozone Layer,

P.J. Crutzen, Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?, Climatic Change 77, 211 (2006),

S. Tilmes, R. Müller and R. Salawitch, The Sensitivity of Polar Ozone Depletion to Proposed Geoengineering Schemes, Science 320, 1201 (2008),

A. Thompson, Two Evils Compete: Global Warming vs. Ozone Hole,

Simulated Geoengineering Evaluation: Cooler Planet, but With Side-Effects,

 S. Tilmes, et al., Effects of Different Stratospheric SO2 Injection Altitudes on Stratospheric Chemistry and Dynamics, Journal of Geophysical Research: Atmospheres, March 30, 2018,

H. Fountain and C. Flavelle, Test Flight for Sunlight-Blocking Research is Canceled, New York Times, April 2, 2021,

Climate-Cooling Trial Balloon Gets Canceled, May 18, 2012,

D. Keith, The Peril and Promise of Solar Geoengineering,

K. Brent, J. McGee, J. McDonald and M. Simon, Putting the Great Barrier Reef Marine Cloud Brightening Experiment Into Context, May 13, 2020,

National Academies of Sciences, Engineering and Medicine, Critical Issues in Weather Modification Research (National Academies Press, 2003),

R. Wood, Lessons Learned from Field Campaigns on Shallow Clouds, April 8, 2015,

H. Hickey, Could Spraying Particles Into Marine Clouds Help Cool the Planet?, UW News, July 25, 2017,

J.  Latham, Control of Global Warming?, Nature 347, 339 (1990),

J. Latham, et al., Marine Cloud Brightening, Philosophical Transactions of the Royal Society A 370, 4217 (2012),

IPCC Report AR5, Working Group 1, Chapter 7, Clouds and Aerosols,

S. Salter, G. Sortino and J. Latham, Sea-Going Hardware for the Cloud Albedo Method of Reversing Global Warming, Philosophical Transactions of the Royal Society A 366, 3989 (2008),

P.V. Hobbs, et al., Emissions from Ships with Respect to Their Effects on Clouds, Journal of the Atmospheric Sciences 57, 2570 (2000),

D. Keith, O. Morton, Y. Shyur, P. Worden and R. Wordsworth, Reflections on a Meeting About Space-Based Solar Geoengineering,

Surface Albedo Modification,

The Easy Fix That Isn’t: White Roofs May Increase Global Warming,

Cirrus Cloud Thinning,

Conference of the Parties to the Convention on Biological Diversity, Climate-Related Geoengineering and Biodiversity,

B. Geman, Al Gore: Geoengineering to Counter Warming is ‘Utterly Mad’, The Atlantic, Jan. 15, 2014,

House Select Committee on the Climate Crisis Majority Staff Report, Solving the Climate Crisis: The Congressional Action Plan for a Clean Energy Economy and a Healthy, Resilient, and Just America,