Reflecting Sunlight to Cool Earth: The NAS Weighs Controversial Measures in New Report

, director of science & policy | February 10, 2015, 2:41 pm EDT
Bookmark and Share

The president’s science advisor John P. Holdren has often observed that humanity has three basic options for dealing with climate change: Mitigation (reducing heat-trapping emissions), adaptation (coping with unavoidable impacts of climate change), and suffering.  The more swiftly we both mitigate and adapt, the less suffering we endure and impose on future generations.

Suppose, however, that we falter and temperatures continue to rise to dangerous levels. In a climate emergency, facing high risks of major and otherwise unavoidable impacts, should the U.S. or other governments consider forced cooling of Earth by injecting reflecting aerosol particles into the stratosphere?

Today, the U.S. National Academy of Sciences (NAS) weighs in with a pair of major reports examining the scientific basis for considering this and other possible “climate interventions” — deliberate, potentially large-scale actions to reflect sunlight away from Earth or remove carbon dioxide from the atmosphere should mitigation and adaptation prove insufficient to limit the risks of dangerous climate warming.

Kudos to the National Research Council (NRC) panel, chaired by Marcia McNutt, editor-in-chief of the journal Science and former director of the U.S. Geological Survey, for tackling this set of challenging and controversial issues. It is one of a growing number of scientific and related policy assessments on a suite of potential and problematic climate responses most commonly referred to as “geoengineering.”

Reflecting sunlight to cool Earth

Here’s a synopsis of key findings from the NRC report on Climate Intervention: Reflecting Sunlight to Cool Earth. (In a related post, my colleague Brenda Ekwurzel looks at their report on Climate Intervention: Carbon Dioxide Removal and Reliable Sequestration):


“Technologies that prevent sunlight from reaching Earth’s surface could reduce average global temperatures within a few years, similar to the effects of large volcanic eruptions. While many albedo-modification [i.e. solar energy reflecting] techniques have been proposed… two strategies that could potentially have a significant impact are injection of aerosols into the stratosphere and marine cloud brightening. [T]hese methods would not require major technological innovation to be implemented and are relatively inexpensive…

However, albedo modification would only temporarily mask the warming effect of greenhouse gases and would not address atmospheric concentrations of CO2 or related impacts such as ocean acidification. In the absence of CO2 reductions, albedo-modification activities would need to be sustained indefinitely and at increasingly large scales to offset warming, with severe negative consequences if they were to be terminated. In addition, albedo modification introduces secondary effects on the ozone layer, precipitation patterns, terrestrial and marine ecosystems, and human health, with unknown social, political, and economic outcomes.

Many of the processes most relevant to albedo modification — such as those that control the formation of clouds and aerosols — are among the most difficult components of the climate system to model and monitor. Present-day observational capabilities lack sufficient capacity to monitor the environmental effects of an albedo-modification deployment. Improvements in the capacity to monitor direct and indirect changes on weather, climate, or larger Earth systems and to detect unilateral or uncoordinated deployment could help further understanding of albedo modification and climate science generally.

 [I]t would be “irrational and irresponsible” to implement sustained albedo modification without also pursuing emissions mitigation, carbon dioxide removal, or both. [The Committee] oppose[s] deployment of albedo-modification techniques, but recommend[s] further research, particularly “multiple-benefit” research that simultaneously advances basic understanding of the climate system and quantifies the technologies’ potential costs, intended and unintended consequences, and risks.

 Albedo-modification research will have legal, ethical, social, political, and economic ramifications. The committee recommend[s] the initiation of a serious deliberative process to examine what international research governance structures may be needed beyond those that already exist, and what types of research would require such governance. The degree and nature of governance should vary by activity and the associated risks, and should involve civil society in decision-making through a transparent and open process.”

No substitute for dramatic reductions in heat-trapping emissions

In other words: Proposed strategies to alter the amount of sunlight hitting the Earth’s surface by (for example) deliberately injecting millions of tons of sulfate aerosols into the stratosphere pose enormous risks and uncertainties and don ‘t address the underlying causes of global warming or other major risks from rising concentrations of carbon dioxide, such as ocean acidification. They should not be deployed today and we should do everything possible to avoid their being deployed in the future. As the NRC report emphasizes, there is no substitute for dramatic reductions in heat-trapping emissions. Preventative medicine is far more attractive than getting treated in the emergency room.

But the Committee also recommends the U.S. government invest in an “albedo-modification research program” focused on improving understanding of the intended and unintended impacts of these technologies on climate, people, and ecosystems. They consider — and firmly reject — the “moral hazard” argument that such research would somehow distract from efforts to reduce emissions, concluding that “as a society we have reached a point where the severity of the potential risks from climate change…outweigh[s] the potential [moral hazard] risks associated with a suitably designed and governed research program.”

I strongly agree. We need to better understand these technologies and their risks, even if we are determined to never deploy them. They are relatively low-cost, and if deployed unilaterally by others, would have global consequences. In the U.S. and internationally, societal debate over their use would be well served by better understanding their risks and consequences. A fuller understanding of their risks, informed by science, might well reinforce our collective determination to never use them and motivate greater commitment to mitigation and adaptation. And, should we falter in that effort, we would be well-served to better understand the impacts of such emergency-room measures.

Needed: a transparent, participatory process to guide research on impacts and risks

That said, the question of who decides what research is appropriate is tricky. To date, studies have largely been confined to computer modeling. The NRC notes that “small-scale field experiments with controlled emissions [e.g. releasing reflecting aerosols into the atmosphere] may….be helpful.” Some scientists are eager to initiate field research. In my view, the NRC Committee has it exactly right when they call for any planning of such research to be subject to a “serious deliberative process” to weigh options for its governance. Such a process, they argue, should be fully transparent and informed by the active participation of civil society.

That process should begin now and subsequent guidance on the governance of albedo-modification research established before the U.S. supports any scale-up of albedo-modification research.

Posted in: Global Warming Tags: ,

Support from UCS members make work like this possible. Will you join us? Help UCS advance independent science for a healthy environment and a safer world.

Show Comments

Comment Policy

UCS welcomes comments that foster civil conversation and debate. To help maintain a healthy, respectful discussion, please focus comments on the issues, topics, and facts at hand, and refrain from personal attacks. Posts that are commercial, self-promotional, obscene, rude, or disruptive will be removed.

Please note that comments are open for two weeks following each blog post. UCS respects your privacy and will not display, lend, or sell your email address for any reason.

  • The simple proof that CO2 change does not cause climate change has been hiding in plain sight and here it is:

    CO2 has been considered to be a forcing with units Joules/sec. Energy change, which is revealed by temperature change, has units Joules. Average forcing times
    duration produces energy change. Equivalently, a scale factor times the
    time-integral of the CO2 level produces the temperature change.

    During previous glaciations and interglacials (as so dramatically displayed in An Inconvenient Truth) CO2 and temperature went up and down nearly together. This is impossible if CO2 is a significant forcing (scale factor not zero) so this actually proves CO2 CHANGE DOES NOT CAUSE SIGNIFICANT AVERAGE GLOBAL TEMPERATURE CHANGE.

    Application of this analysis methodology to CO2 levels for the entire Phanerozoic
    eon (about 542 million years) (Berner, 2001) proves that CO2 levels up to at least 6 times the present will have no significant effect on average global temperature.

    See more on this and discover the two factors that do cause climate change (95% correlation since before 1900) in a peer reviewed paper published in Energy and Environment, vol. 25, No. 8, 1455-1471 or search “agwunveiled”.

  • Hi Peter –

    Thanks for the very helpful comments. In my view, the committee got all the
    important things right, roughly as follows:

    1. Never forget that emission cutting and adaptation are the first and most important
    responses to climate change.

    2. But even the best response on these would leave us facing serious remaining risks, which call for investigating climate engineering. (Aside: There’s a lot of useless argument over what to call this, and what to call the two major approaches within it – actively messing with the global carbon cycle, or actively messing with the
    Earth’s radiation balance. For here, I’ll call them “carbon stuff” and “sunlight stuff”.)

    3. These two approaches are so different in their potential benefits and risks that they have to be considered separately. Carbon stuff looks like emission-cutting in reverse, but with some disadvantages (it costs more, and you need to worry about the long-term stability of where you put the carbon) and some advantages (It’s flexible in where, when, and at what scale you do it; and it holds the unique prospect of driving net human emissions negative, so we can move beyond slowing or stopping harm, but actually undo some of the harm we’ve done.) But carbon stuff can only work slowly. Sunlight stuff can undo some (not all) climate-change risks fast, but it’s only a partial solution and carries a bunch of novel risks – some direct, but many related to its potential for misuse.

    4. Expanded research on both approaches should proceed – faster, less encumbered, and more aimed at operational capability for carbon stuff, and with more reservations, a limit to tiny scales of active perturbation with tiny direct risks, and (maybe) additional regulatory encumberances for sunlight stuff.

    None of these points is new or surprising, but the committee’s exploration of them is measured, cogent, and helpful.

    My biggest problem with the report is that they mostly ducked the hardest and most contentious questions, particularly about governance of research. In particular, they are quite ambiguous on the sharply contested question of whether a new system of regulatory control needs to be in place before the first, smallest outdoor climate intervention experiments take place. My guess is that they will be read, on
    balance, as saying go ahead and take the next (small) step – outdoor field experiments, including small-scale active aerosol perturbations (both stratospheric and marine cloud brightening). But plenty of people will parse their text to claim they said the opposite, and the text is ambiguous enough to support both readings.

    With all the brain-power and resources they had, I wish they had not just issued yet another call for a “serious deliberative process” to consider these questions, but actually taken them forward a step or two.

    If I have to guess about the impact of this report, I think it has a good chance of moving the debate forward, mainly by getting research program managers and their bosses to start discussing concretely what specific research to fund, by what criteria, and under what (if any) additional oversight and control to address the related risks. On balance I think this would be a good outcome, but I worry about the possibility that these conversations take place exclusively in the United States, rather than immediately engaging foreign counterparts – which I think is an essential step toward managing the longer-term risks posed by these technologies, most of which are political, not technical.

    • Peter Frumhoff

      Thanks for these good comments, Ted. Perhaps the ambiguity of the report on the difficult question of research governance reflects the wildly differing views of report authors (as they have published elsewhere) – for example, Granger Morgan’s endorsement of “governance as you go”, moving forward with small-scale experiments with limited oversight ( and Ray Pierrehumbert’s argument that a deliberate debate about governance of “sunlight stuff” research is both essential and unlikely (

      A committee charged with recommendations on research governance would surely benefit from participation far broader than scientists alone.

  • lycophidion

    “But the Committee also recommends the U.S. government invest in an
    “albedo-modification research program” focused on improving
    understanding of the intended and unintended impacts of these
    technologies on climate, people, and ecosystems.” Strongly disagree. The hazard isn’t moral. Research in “albedo modification” (what Orwellian bullshit!) would of necessity involve “albedo modification,” unless you are talking about modeling, which, you note, would currently be meaningless: “Many of the processes most relevant to albedo modification — such as
    those that control the formation of clouds and aerosols — are among the
    most difficult components of the climate system to model and monitor.” That means such a research program, of necessity would involve introducing aerosols on a large scale, which is unacceptable, or on a local scale, which is both unacceptable and meaningless. Beyond that, development of such magic bullet solutions, which inevitably entail unforeseen and detrimental consequences, DO set the stage for their deployment. Their deployment is a political and economic decision, not a scientific one (or we would not have a problem with antibiotic resistant bacteria, neonicotinoids, etc.), will be on the desk for policy-makers, and WILL happen, under one or another administration, precisely because the bipartisan consensus is to place market concerns ABOVE climate change, which means that the type of strategy needed to halt fossil fuel emissions and global warming will not be implemented. In short, we need system change, not technofixes.

    • Peter Frumhoff

      The concerns you raise are a good example of why any such research beyond computer modeling should be subject to serious public discussion of risks and trade-offs – it may be that “unacceptable under any conditions and at any scale” would be the outcome. But as these are low cost technologies that can be deployed today by virtually any government (or, for that matter, wealthy individuals), that’s a necessary dialogue to have, in my view. It would be unwise to avoid it. And – perhaps – it will help jolt us into the system change that I agree we need.

  • kfree33

    I don’t know if this question makes sense, but I’m asking it. Do solar panels reduce heat-trapping emissions in any way?

    • Peter Frumhoff

      Yes, by enabling us to replace fossil fuels (principally, coal) with an emissions-free means of generating electricity.

  • Richard Solomon

    A word of warning: politicians, motivated by their wish/need to please their corporate contributors whose bottom line is profits and not societal welfare, will probably ignore the “serious deliberative process” in evaluating these options. Instead, they will go for ‘the quick fix’ of these kinds of geo-engineering efforts without the need to really change our lifestyles amid promises that more technology can solve the challenges we face from global warming. The law of unintended consequences can be very, very harsh in situations like these!

  • Spennypenny

    This is very interesting. Not sure why we as a society are still pretending that we are not spraying Aerosol into the atmosphere. Here in Portland we have had increasing use of Aerosols over the past 5-10 years and we are at a point where we have significantly more days without direct sun. I think the recommendations for research, careful consideration and open and transparent dialog are critical here. At this point we are conducting experiments with weather modification and solar reflection without any open research into the long term impacts on plants, animals, and people, not to mention the Aerosols that come back down to earth as rain and eventually back into the water system.

    • Expl.geo

      What weather modification and solar reflection experiments are there being conducted? Can someone direct me to peer reviewed papers on these experiments? I understand that we are conducting climate altering experiments in an uncontrolled manner with our emissions of greenhouse gasses. We are also emitting cooling aerosols in the form of other industrial wastes; 50 million tons of sulfur per year I have read; but again, this is not a scientific experiment, rather in support of our consumer society.

  • lassoatrain

    I have some news for you . they have already begun the aerosol spraying of our atmosphere here in California quite some time ago. It has been well documented that Barium is the main ingredient in the Aerosol being released in to the atmosphere. my concern with the use of Barium is that according to a 1931 U.S. Bureau of standards publication volume. 6 titled an investigation in to the measuring of extreme solar Ultraviolet radiation. the published report by well respected professional scholars claim that they were able to measure the ultraviolet light reaching the surface of the Earth by the use of Barium flint glass which blocked all wavelengths of light except that in the range of ultraviolet which passed through the filter, and also the fact that barium refracts light down towards the planet where I believe it releases its energy as heat into the atmosphere, I mean really what is the difference if there is a hole in the ozone or if they are warming the planet by refracting light down upon us? (not reflecting)
    ether way we are being radiated or is that eradicated? and when they speak of 200 species a day going extinct where is the list? or do they really mean blood lines? I hope someone will gain some benefit from this my own breaking news report.

    • Expl.geo

      Can anyone tell me if this is true and direct me to peer reviewed papers in scientific publications explaining the science behind it? The last time I was concerned about barium, it was being used as an additive to drilling muds to make them heavier. I think there may be some difficulty in keeping this metal in the air.

  • We can not only reflect solar energy, but also convect it through the troposphere by the use of the atmospheric vortex engine.