August 4, 2021
Human civilization has advanced and flourished over ten millennia, in large part as a result of an unusually stable Earth climate. During this so-called Holocene period, global mean Earth temperatures, as reconstructed from ice cores (see Fig. I.1) and other proxies, remained constant within about ±0.2°C, facilitating the development of agriculture and human population centers. But humans have largely taken that climate stability for granted and failed to appreciate the delicate balance of atmospheric and environmental factors required to maintain it. The climate consequences of global warming caused by human emissions of greenhouse gases are becoming clearer by the day, with striking increases in the frequency of severe storms, droughts, floods, fires and heat waves around the planet. Having disturbed that delicate balance by emissions into Earth’s atmosphere, humans are now contemplating counteracting global warming by adding new emissions into the atmosphere to introduce a cooling effect. Those contemplated actions go under the general name of geoengineering or solar radiation management.
Geoengineering is a very delicate game, with enormous possibilities for unintended consequences. The preferred approach to address ongoing global warming is by weaning human societies off of the fossil fuel burning that causes the warming, just as the successful efforts to counteract stratospheric ozone depletion resulted from international agreements to ban future production of the chlorofluorocarbons (CFCs) that caused the problem in the first place. Injecting other substances into the atmosphere to address such problems is a risky proposition. It would be akin to injecting sodium into the stratosphere to bind with the chlorine released from CFCs, to form ordinary salt in the hope that the chlorine would then never be freed again and the added sodium would not cause its own chemical perturbations to the atmosphere.
Unfortunately, the lobbying power of the fossil fuel industry and the politically motivated spread of misinformation about climate change have introduced dangerous delays in efforts to seriously curtail fossil fuel burning. Technology to remove carbon dioxide from the air is promising, but it would be enormously costly and technically unfeasible to do this on a magnitude and time scale needed to be more than an incremental aid in combating global climate change. These delays might well lead to long-lasting global temperature increases and substantial human suffering, prompting future political calls, from at least some politicians in some countries, for “emergency quick-fix” measures to cool the Earth on a short time scale and at achievable costs. Such measures could only be accomplished via geoengineering approaches, making it necessary to consider them seriously now.
Thus, in many ways the issues surrounding geoengineering are similar to those we discussed for the new technology of gene drives in a previous post. Is it worth pursuing scientific research on a new technology that has some promise of mitigating serious threats to the well-being of Earth’s populations, but also has a suite of potentially disastrous consequences that are not yet well understood? Does such research place one on a slippery slope toward the eventual deployment of these technologies, whether or not the consequences are clear, and with politicians, rather than scientists, inevitably making the decisions about deployment? Is it morally defensible to simply deny funding for such research, when the technology could potentially save millions of lives? Or do hopes for the outcome of such research relieve political pressure to pursue alternative, less uncertain but possibly more costly, avenues for mitigation of the threats? If government funding is denied, will private venture capital fund the research, and maybe even eventual deployment, anyway? Might some countries decide to go ahead with research and deployment, leading to global consequences, without international agreements governing the deployment? These are controversial issues in both the gene drives and geoengineering cases.
In this post we will review the science, the questions, and preferred guidelines for research on various technical approaches to geoengineering.
II. Summary of considered climate intervention methods
In our earlier post on climate change problem-solving, we considered many options to reduce greenhouse gas concentrations in the atmosphere, by weaning humanity off of fossil fuel burning and using forest growth or technology to remove carbon dioxide from the air. Geoengineering options take a different approach, noting that Earth temperatures reflect the balance between power input from the Sun and power output from the heated Earth. Reducing greenhouse gas concentrations in the atmosphere is meant to reduce the trapping of infrared (long wavelength) radiation emitted from the Earth, thereby increasing the power output to space and lowering Earth temperatures. Most geoengineering approaches aim instead to reduce power input from the Sun by increasing the fraction of incident (short wavelength) sunlight reflected back out to space, i.e., by increasing Earth’s albedo. This is not a new idea; it was first suggested to President Lyndon Johnson in a 1965 report from his Presidential Science Advisory Committee, as an approach worth considering to counteract the global warming they anticipated from greenhouse gas emissions. Four such albedo enhancement approaches indicated schematically in Fig. II.1 are often labeled as Solar Radiation Management (SRM) techniques.
A fifth proposed geoengineering approach, also represented in Fig. II.1, is aimed at reducing atmospheric absorption of the infrared radiation from Earth without reducing greenhouse gas concentrations. All five suggested approaches in Fig. II.1 have the potential to cool the Earth, but they would not have direct impact on other ongoing effects of increased greenhouse gases, such as increasing ocean acidification, with serious consequences for coral reefs and fish species.
The methods considered to increase albedo involve making Earth land masses or low-lying marine clouds brighter, or injecting aerosols into the stratosphere, or installing enormous mirrors out in space. The suggested atmospheric engineering approach to increase infrared transmission involves thinning out high-altitude cirrus clouds that absorb in the infrared. Considerations of the science, the technical challenges, and the potential cost-effectiveness of these proposals has constrained current focus to two of the five: stratospheric aerosol injection (SAI) and marine cloud brightening (MCB). We will discuss these two approaches, respectively, in some detail in Sections III and IV below, and then we will briefly summarize the problems faced by the other approaches in Section V. A thorough review of the technical aspects of SAI and MCB approaches was carried out by the U.S. National Academies of Sciences in 2015.
Figure II.2 shows a schematic illustration of how SAI and MCB might potentially be realized. The fundamental problem that arises in both cases is that Earth’s atmosphere and Earth’s oceans form an enormously complex, dynamic system of coupled fluids, where physical and chemical impacts of composition changes are sometimes hard to predict. We have seen that already in the ozone depletion saga, where the emission of CFCs from Earth dangerously depleted the stratospheric ozone layer, exposing the Earth’s surface to dangerous short-wavelength ultraviolet radiation. This was realized only after the fact, though still in time to come to international agreement to ban further production of CFCs. Because of the dangers of significant changes to the chemical makeup of the atmosphere, one cannot really carry out experiments to study the effects of geoengineering on a global scale.
Thus, one has to rely on modeling and computer simulations, which many people already doubt in the case of greenhouse gas impacts, though in that case there is a long history of Earth temperature and greenhouse gas measurements to which models can be fitted. Furthermore, climate modeling most often deals with the daunting issues coupling the troposphere (lower portion of the atmosphere) to Earth’s surface, while SAI modeling necessarily adds in uncertainties associated with the chemistry and physics of the stratosphere and the ozone layer. Paradoxically, but not too surprisingly, some of the same politicians and institutes that complain about the science of climate change because it is based on suspect models seem eager to grasp the even more suspect models of geoengineering to provide a theoretical solution to global warming that will not jeopardize their fossil fuel industry donors’ bottom lines. In addition to reliance on the models, one can contemplate small, localized experiments to test features of the geoengineering models. Some of these small proposed experiments will be discussed in the following sections, along with suggestions for the governance, funding and desired restrictions on such research.
III. injecting aerosols into the stratosphere
The most widely discussed approach for solar radiation management takes its cue from nature. The cues are seen in Fig. III.1 from the occasional few-year cooling “spikes” associated with known worldwide volcanic eruptions, superimposed on a smoother rise in global mean temperatures correlated with the growth in atmospheric carbon dioxide concentrations. This volcanic cooling is understood to arise from the emission during eruptions of large quantities of sulfur dioxide (SO2), some of which rises to the stratosphere, where “after two months, most SO2 is converted to sulfuric acid by reaction with hydroxyl radicals (OH). This condenses into aerosols in the atmosphere.” The aerosol droplets typically survive for a couple of years in the stratosphere and have sizes comparable to the wavelengths of incident sunlight, allowing them efficiently to reflect some of that sunlight back out into space. For example, the 1991 eruption at Mt. Pinatubo in the Philippines released 15-30 megatons of sulfur dioxide and led to a sharp few-year drop in global mean temperatures by about 0.4°C.
Of course, volcanic eruptions also cause a number of harmful environmental impacts. A fair fraction of the emitted sulfates are returned to surface air layers in acid rain, where they introduce pollution that leads to premature human deaths and substantial ecological damage. Furthermore, analysis of the chemical reactions occurring in the stratosphere indicates that the eruptions indirectly deplete the stratospheric ozone (O3) layer (see Fig. III.2), which is essential for absorbing harmful deep ultraviolet solar radiation before it reaches Earth’s surface. The major ozone depletion seen during the late 20th century was attributed to the catalytic role played by chlorine (Cl), supplied via CFCs from earthbound refrigerants and aerosols, in reactions that break up ozone molecules. However, the sulfuric acid aerosols resulting from the Mt. Pinatubo eruption in 1991 appear partially to blame for the lowest recorded ozone concentrations in 1993-4. The mechanism is believed to result from the interactions of the sulfuric acid aerosols with nitrogen oxide molecules, which normally mitigate ozone depletion by reacting with Cl and ClO molecules to form less ozone-depleting compounds. However, this mitigating process is disrupted because the nitrogen oxides get removed from the stratosphere when they “react with the surface of the [sulfuric acid] aerosols to form nitric acid (HNO3).”
The challenge for stratospheric aerosol injection (SAI) is thus to capture the cooling effect of the aerosols while lessening the negative environmental impacts of volcanic eruptions. The artificial injection of sulfate aerosols was first suggested as a climate change mitigation in 1974 by the Russian climatologist Mikhail Budyko. The idea received a substantial boost in 2006 when Paul Crutzen, who had received the 1995 Nobel Prize in Chemistry for his work studying the atmospheric chemistry of ozone, suggested Albedo Enhancement by Stratospheric Sulfur Injections as a contribution worth considering to resolve the public policy dilemmas that had delayed reductions in fossil fuel burning. But as indicated by the complex chemical reaction cycles involving sulfate aerosols referenced above, and their potential impact on the stratospheric ozone layer, detailed and credible modeling is needed to assess the possible negative impacts.
Such modeling has pointed out several drawbacks of sulfur injection. Simone Tilmes of the National Center for Atmospheric Research and her collaborators considered the effect of injected sulfate aerosols on stratospheric ozone levels in polar winters. At very cold temperatures the injected sulfur helps the formation of polar stratospheric clouds containing ice crystals, on the surface of which chlorine-induced ozone destruction is greatly enhanced. Their analysis indicates that “Over the next few decades, these hypothetical injections would likely destroy between about one-fourth and three-fourths of the ozone layer above the Arctic. Because atmospheric circulation patterns over the Arctic tend to ‘wobble,’ this Arctic ozone hole even could sweep over populated areas… Ozone wouldn’t suffer the same depletion over Antarctica, ‘because it’s already gone,’ Tilmes told LiveScience. But the sulfates would delay the expected recovery of the ozone hole [from the Montreal Protocol ban on CFCs] by about 30 to 70 years, the model found.”
More recent and more detailed simulations reported by the National Oceanic and Atmospheric Administration (NOAA) also point toward significant effects of injected sulfate aerosols on hemispheric circulation and precipitation patterns at Earth’s surface: “…the additional sulfate aerosols cause a strengthening of the Northern Hemisphere stratospheric polar vortex, a band of strong westerly winds that forms between about 10 and 30 miles above the North Pole every winter. A stronger polar vortex in turn shifts the North Atlantic Oscillation, or NAO, which influences the location of storm tracks across the North Atlantic, to a more positive phase, resulting in a stronger Atlantic jet stream and a northward shift of the storm track. During a positive NAO, northern Europe sees warmer-than-average temperatures that are associated with the air masses that arrive from lower latitudes, along with increased precipitation. At the same time, southern Europe sees less precipitation. The model simulations show that these trends reverse during summer when the stratospheric polar vortex is not present. “
Another significant risk arises because sulfur dioxide molecules absorb ultraviolet radiation from the Sun, so their injection would contribute to stratospheric heating. Recent simulations suggest a stratospheric temperature increase by as much as 10-15°C, leading also to a substantial increase in the water vapor stored in the stratosphere. The impacts of such heating and water retention on the dynamics of interactions between the stratosphere and the troposphere, and on their complex coupling to Earth’s climate, are only beginning to be modeled.
The risks and uncertainties associated with sulfate aerosol injection have led a Harvard team to consider injection of alternative aerosol materials. “Harvard researchers have analyzed a range of alternative materials, including diamond [Lucy in the Sky With…?], and have found that calcium carbonate could be promising. Early research suggests that it has near-ideal optical properties, meaning that for a given amount of reflected sunlight it would absorb far less radiation than sulfate aerosols, causing significantly less stratospheric heating; and it has the potential to greatly reduce the activation of ozone-depleting halogen species compared to sulfate aerosol, meaning that it could reduce ozone loss. Yet, calcium carbonate does not exist naturally in the stratosphere even though it is non-toxic and earth abundant [as limestone]. Therefore, though we can almost certainly expect that calcium carbonate will not have the stratospheric reactivity of sulfate, the actual stratospheric reactivity is not known, which means laboratory and outdoor studies are needed.”
The Harvard team has therefore proposed a balloon flight experiment to test some aspects of the interactions of injected calcium carbonate particles with each other, with the ambient stratospheric air, and with solar and infrared radiation. The experiment is called SCoPEx (Stratospheric Controlled Perturbation Experiment). It is intended not as a comprehensive test of solar geoengineering, but rather to “make quantitative measurements of aspects of the aerosol microphysics and atmospheric chemistry that are currently highly uncertain in the simulations.” In other words, it is designed to provide experimental results to constrain the simulation models for calcium carbonate aerosols, in particular.
As indicated in Fig. III.3, “At the heart of SCoPEx is a scientific balloon, fitted with propellers.
• This high-altitude balloon would lift an instrument package approximately 20 kilometers (12 miles) into the atmosphere.
• Once in place, a very small amount (100 grams to 2 kilograms) of material would be released into an air mass mixed by the propellers, roughly one kilometer long and one hundred meters in diameter.
• The same balloon would then measure resulting changes in the perturbed air mass, including changes in aerosol density, atmospheric chemistry, and light scattering.
• Instrumentation carried on the balloon would also measure wind, which could allow inference of atmospheric turbulence for testing hypotheses about how stratospheric turbulence is generated.“
SCoPEx planned a first equipment test flight to take off in June 2021 from the Esrange Space Center in Kiruna, Sweden. This first flight was intended not to release any material into the stratosphere, but rather “to review the gondola’s horizontal and vertical control using the winch system and propellers as well as the power, data, navigation, and communication systems.” However, that first test flight has been canceled by the Swedish Space Corporation “after an outcry from environmentalists and others.” Niclas Hällström, a member of the What Next environmental research group headquartered in Uppsala, Sweden, said of the outcry: “The mobilization against this project in Sweden has been remarkable, uniting scientists, civil society and the Saami people, against the danger of a slippery slope toward normalization of a technology that is too dangerous to ever be deployed.”
An earlier project called SPICE, aiming to test technology for stratospheric injections, also had its first, environmentally benign, scheduled equipment test canceled in Britain in 2012, partly as a result of analogous concerns about a “slippery slope” and partly due to patent disputes. These cancellations highlight the considerable challenges faced today by any experiment that aims to constrain modeling of SAI. We will discuss the establishment of governance restrictions that might allow such tests in Section VI of this post.
How, theoretically, would SAI actually be achieved if there is ever a decision that the benefits outweigh the risks? An excellent source of information on the technology is available in the YouTube video of a Harvard lecture by applied physicist David Keith, who is a member of the SCoPEx team. The magnitude of the injections that might be needed is set by the anticipated change in Earth’s power balance that would accompany a future doubling of the atmospheric concentration of carbon dioxide, from a pre-industrial level of 280 parts per million (ppm) to 560 ppm — a level that would be reached well before the end of the 21st century along humans’ present fossil fuel burning trajectory. That change in the power balance, called the radiative forcing of a CO2 doubling, is estimated at 3.7 watts per square meter (W/m2) of surface area. Keith estimates the cost and amount of sulfur (if that were the material eventually used) that would need to be injected annually to counteract half of the CO2-doubling increase, or to effect a radiative forcing reduction by about 2 W/m2. The mean solar irradiance at Earth’s surface, averaged over the globe and over a full year, is about 230 W/m2. Thus, the needed effect corresponds to increasing Earth’s albedo by about 1%, i.e., modifying the stratosphere to reflect 1% more of the incident sunlight than the Earth does presently.
Increasing the albedo by 1% would require releasing about 1.5 million tons of sulfur into the stratosphere per year, or an annual addition equivalent to about 20% of what the Mt. Pinatubo volcanic eruption added (via SO2 molecules) in 1991. For comparison, current global emissions of sulfur as low-lying atmospheric pollution are about 50 million tons per year, but it must be noted that such pollution causes millions of incremental human deaths and illnesses per year. Those 1.5 million tons of injected sulfur would offset about half of the warming potential of the more than 50 billion tons of carbon dioxide to be released annually by mid-century global fossil fuel burning, in the absence of significant changes in our fossil fuel usage. One requires much less sulfur than carbon dioxide because the sulfur will form aerosol droplets of size comparable to the wavelength of light, making them highly efficient for reflecting light.
Releasing the 1.5 million tons of sulfur would require a new airplane fleet of about 100 craft, cumulatively making about 120,000 flights per year (or about 0.3% of current commercial flights per year) at an altitude of order 20 km in the tropical stratosphere. The choice of the tropics is because natural airflow patterns in the tropical stratosphere go upward in the tropics and follow long arcs leading downward toward the poles. So injection in the tropics could lead to spread more or less evenly around the globe. The estimated annual cost for such an operation is about $5 billion U.S., compared to the estimated annual cost of $20 trillion U.S. incurred by unmitigated climate change impacts by 2100. So the financial cost of injecting the aerosols is relatively modest, but the cost of handling risky side-effects is unknown. Model calculations to date of those unintended consequences are somewhat encouraging, but they are insufficiently constrained by relevant data.
A likely future climate change scenario is that nations do finally get their act together to reduce greenhouse gas emissions, but too late and too slowly to keep global mean temperature increases below the IPCC-suggested guidelines of 1.5°C or 2.0°C above pre-industrial levels. Even if humans were to reach carbon neutrality the cumulative additions to atmospheric greenhouse gas concentrations up to that point would remain in the atmosphere for many decades, producing long-lasting global temperature increases and climate change impacts beyond what could be accommodated comfortably by human adaptation.
Thus, as one approached carbon neutrality, many nations might then consider using some level of mid-century solar geoengineering to mitigate at least part of the global temperature increase. David Keith then argues for an eventual gradual introduction of aerosols into the stratosphere, so that we can monitor the impacts of a small amount, before we decide to increase the injections in a staged manner. This reasonable approach is then, of course, not a “quick fix,” but rather a helpful mitigation. But if stratospheric aerosol injection is to provide mitigation on a mid-century timescale, the need for relevant research to constrain modeling of the impacts is critical within the next decade.
IV. marine cloud brightening
The alternative approach to albedo enhancement known as marine cloud brightening (MCB) is currently being pursued at a localized level by Australia, in an attempt to counteract the ongoing bleaching of the Great Barrier Reef. “MCB involves spraying [nano-sized] sea salt particles into clouds above the ocean to enhance the number of water droplets and hence the amount of sunlight reflected by the cloud,” just as the nano-sized aerosol droplets injected into the stratosphere would also act to increase reflection of incident sunlight (see Fig. IV.1). “Australian scientists conducted the first outdoor marine cloud brightening (MCB) experiment in late March . The purpose of this experiment was to test a delivery mechanism comprised of 100 high-pressure nozzles [see Fig. IV.2] that can spray nano-sized sea-salt particles into the air above the reef… MCB was initially proposed as a means of cooling temperatures at a global scale to offset rising temperatures associated with climate change. However, MCB also has potential to be used at a local or regional scale for cooling effects that might protect high value but vulnerable natural assets from climate change impacts, such as polar regions and coral reefs.”
Concepts of cloud seeding have been around for more than a century. There have been multiple attempts to increase precipitation by injecting clouds with chemicals such as silver iodide, potassium iodide, dry ice, or even table salt. The belief has been that such chemicals can form ice crystals within the cloud that grow to larger size than the surrounding water droplets, thereby stimulating precipitation. However, scientific evidence demonstrating the actual effectiveness of these techniques is largely lacking. A 2003 report from the U.S. National Academies of Sciences stated that “…science is unable to say with assurance which, if any, seeding techniques produce positive effects. In the 55 years following the first cloud-seeding demonstrations, substantial progress has been made in understanding the natural processes that account for our daily weather. Yet scientifically acceptable proof for significant seeding effects has not been achieved.”
The idea to use sea salt injections into marine clouds to combat global warming was first proposed by John Latham in a 1990 letter to the journal Nature. It is expected that such injections would have greater effect on low-lying clouds over oceans than over land, because the marine clouds generally have “a deficit of cloud condensation nuclei due to lower levels of dust and pollution at sea.” While many groups, including Latham’s, have worked to embellish the concept, modeling evidence for its effectiveness on a global scale remains limited. As shown by the extensive worldwide efforts to improve global climate modeling summarized in the most recent IPCC report, “Clouds and aerosols continue to contribute the largest uncertainty to estimates and interpretations of the Earth’s changing energy budget.”
Part of the complication is indicated in Fig. IV.3, which shows that thick, low-lying clouds tend to reflect some solar radiation back out into space, producing a cooling effect, but also some infrared radiation back to Earth’s surface, producing a warming effect. Models tend to show that the cooling effect dominates over the heating, but the error bars on that conclusion are rather large and the difference clearly depends on the size of the droplets formed inside the clouds. In addition, modifying clouds can cause complex changes to weather and precipitation patterns at Earth’s surface, leading again to a wide variety of potential unintended consequences. A reduction in sea surface temperatures might change ocean circulation patterns, and therefore also weather over land, and adversely affect marine ecosystems; increased cloud cover could negatively affect photosynthesis, plant growth and agriculture. A research coalition called the Marine Cloud Brightening Project, formed in 2009 and now headquartered at the University of Washington, encompasses modeling, field experiments, technology development and policy research to study cloud-aerosol effects and MCB.
In a 2008 paper, atmospheric scientist John Latham worked with engineer Stephen Salter to consider the size and design of a fleet of ships that would be needed to counteract a doubling of atmospheric CO2 via MCB. They proposed a worldwide fleet of around 1500 unmanned, wind-driven ships, like that illustrated in Fig. IV.2, that would spray mist generated from sea water into the air at sites that feature a high fraction of low-level marine stratocumulus clouds. Each such ship would have to spray about 50 cubic meters per second of carefully filtered sea water that passed through “micro-nozzles with piezoelectric excitation to vary drop diameter.”
A 2015 National Academies study suggested that the cost for such a program would run a few billion U.S. dollars per year, comparable to the relatively modest estimated costs for stratospheric aerosol injection. For comparison, the estimated operating cost for the more technically developed removal of carbon dioxide directly from the air is about $100 per ton removed, or about $4 trillion U.S. per year if one asked such plants to keep up with the 40 billion tons of CO2 currently being added annually to the atmosphere from worldwide fossil fuel burning.
While there are clearly many modeling uncertainties, MCB has two significant advantages over SAI. First, it is subject to localized testing, as in the Great Barrier Reef project; in fact, small-scale MCB already occurs unintentionally due to the aerosols in ships’ exhaust. Second, as summarized by Wikipedia, “The climatic impacts of marine cloud brightening would be rapidly responsive and reversible. If the brightening activity were to change in intensity, or stop altogether, then the clouds’ brightness would respond within a few days to weeks, as the cloud condensation nuclei particles precipitate naturally.” Thus, it is certainly worthy of a well-regulated research program on much less than global scales, as is also the case for stratospheric aerosol injection.
— Continued in Part II —