2.25 Intellectuals The Intellectual
Solar geoengineering may buy time for fundamental measures such as the transformation of energy systems, but there are also many risks and uncertainties | Image source[2]
Introduction
In January 2022, a violent eruption of submarine volcanoes on Aha Apay Island in Tongahun touched the public's mood, and such large-scale volcanic eruptions could have a significant impact on the Earth's climate. At the same time, these climate impacts have inspired atmospheric scientists to come up with an extremely bold complement to the climate crisis: solar geoengineering. Such ancillary measures may buy time for fundamental measures such as the transformation of energy systems, but there are also many risks and uncertainties. This article will focus on stratospheric aerosol propagation to explore the uses, controversies, risks, uncertainties, and global scientific status of solar geoengineering.
Written by | Li Yaowei Dai Zhen
Editor-in-charge | Feng Hao
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Imagine a day a few decades from now, when humanity is overwhelmed by the tragic effects of climate change: glaciers in the Arctic and Antarctic are melting on a large scale, sea levels are rising, island nations and coastal cities are in jeopardy, extreme weather such as droughts, torrential rains, typhoons occur frequently in different regions, and burning mountain fires are raging more and more... The Earth system is on the verge of collapse.
To avoid such a possible tragic scenario, scientists are exploring the use of a temporary "analgesic" for the planet to buy more time for a fundamental "therapeutic" – greenhouse gas emission reductions.
This "analgesic" can reduce the amount of sunlight reaching the Earth's surface: it consists of millions of tiny particles artificially spread in the stratospheric region (about 20 kilometers above the ground); these particles that envelop the Earth reflect a portion of the sunlight back into space, lowering the Earth's temperature.
At an altitude of 20 kilometers above the ground, simulating the scene after a volcanic eruption? Although this sounds like science fiction, it is an engineering solution that scientists from China, the United States, britain, Germany, Japan and other countries are exploring. Collectively referred to as "solar geoengineering," these proposals are similar to analgesics and may help humanity weather the painful transition to net zero greenhouse gas emissions (carbon neutrality), but they also have side effects and a significant risk of abuse.
1
Stratospheric perturbation experiments shelved due to controversy
To study the effects of this "Earth analgesic" and its side effects, researchers at Harvard University began in 2017 with a scientific project: the Stratospheric Controlled Perturbation Experiment (SCoPEx) [1]. SCoPEx hopes to measure stratospheric particulate matter associated with solar geoengineering in the field, providing key experimental parameters for computer simulation studies.
Computer simulation studies are the primary means of assessing the risks and benefits of solar geoengineering; but due to the very limited current understanding of the microphysical and atmospheric chemistry of the relevant stratospheric particulate matter, there is a great deal of uncertainty in the results of these computer simulations. The ScoPEx project hopes to change this with field measurements — exploring precisely how the tiny amounts of anthropogenically sown particulate matter interact with stratospheric background air, solar radiation, and surface infrared radiation.
That is to say, SCoPEx is not a test of solar geoengineering itself, but a discussion of the feasibility and potential hazards inherent in solar geoengineering (more precisely, stratospheric aerosol propagation in it).
Fig. 1 Stratospheric controlled perturbation experiment SCoPEx concept diagram | source[2]
At the heart of the SCoPEx experiment is a high-altitude balloon that carries a pod equipped with propellers and a range of scientific instruments. The propeller has two functions: first, the wake generated by the propeller movement will form a well-mixed area (about 1 km long and about 100 meters in diameter) in the stratosphere experimental area, and other instruments can spread the relevant particles in this area; second, the propeller can transport the pod to different locations in the area to measure the characteristics of the sown particles.
The pod can reach a speed of several meters per second when in motion, and it takes about ten minutes to fully measure a 1 km long disturbing air mass. Therefore, the advantage of the high air balloon is that it can artificially create a small amount of controlled stratospheric air and observe the evolution of disturbed air masses multiple times over a 24-hour period. (If an airplane had been used instead of a balloon, the experiment would not have been able to achieve such a small perturbation volume or observe its evolution for a long time.) ）
After the balloon successfully reaches the stratosphere about 20 km above the ground, along with the movement of the propeller, the instrument will release a very small amount of particle matter (less than 2 kg, non-toxic minerals such as calcium carbonate particles, which have been considered as a preliminary test) [1]. These particles are extremely tiny and are also known as aerosols. Subsequently, other instruments on the pod will measure changes in the disturbing air masses, including aerosol density, optical properties of the aerosol, and the chemical processes of the aerosol on the stratosphere.
Figure 2 Schematic diagram of SCoPEx high-altitude balloon operation| Source[3] The author of this article has modified it
The project's researchers state that the tests do not pose a hazard to people or the environment: the test emits far fewer particulate matter than aircraft, rockets, etc., and the tests are limited to a small area of the stratosphere (several square kilometers) [1].
SCoPEx was originally scheduled to be tested in Sweden in June 2021 and will be one of the first field tests related to solar geoengineering. But four months before the first test flight, under pressure from Swedish residents and environmental groups, SCoPEx announced the project postponement, and there was no clear news on when it would be restarted. The main reason for the opposition of environmental groups is that solar geoengineering "may cause irreversible changes to the socio-economic system" [4], reducing people's enthusiasm for emission reduction, thus becoming a stumbling block to emission reduction policies.
Although scientists believe that the immediate risks of this study are negligible [5], because the experiments were sown with less than 2 kilograms of non-toxic substances such as calcium carbonate, which is less than the material emitted per minute by a regular airliner, and the experimental area is only a few square kilometers. Opponents still argue that SCoPEx could create a lot of other problems, and that the first field experiment in solar geoengineering would be like opening a Pandora's box: studying solar geoengineering may lead to a growing tendency to implement solar geoengineering.
2
Why proposed solar geoengineering?
The controversy over the SCoPEx project leads us to wonder: Why are scientists around the world trying to explore solar geoengineering?
Today, climate change caused by rising concentrations of greenhouse gases (mainly carbon dioxide) threatens human production and may even change the course of human civilization. Response measures fall into two main categories – emission reduction and adaptation. Emission reductions are necessary and fundamental measures, with major international climate agreements and climate negotiations such as the 1997 Kyoto Agreement, the 2009 Copenhagen Agreement and the 2016 Paris Agreement all centered on emission reductions; adaptation aims to reduce vulnerability to climate change systemic risks.
However, scientific research at this stage and actual reduction actions by countries show that we cannot reduce carbon dioxide emissions to safe limits in a short period of time to prevent the dangerous consequences of climate change, nor can we fully withstand the likely climate crisis through adaptation measures alone [6,7].
Against this backdrop, are there new, bolder ways to cool the planet faster and buy us more time to get rid of fossil fuels? Geoengineering has entered the academic field as such a temporary analgesic.
There are two main geoengineering ideas: one, removing carbon dioxide from the atmosphere to reduce the heat preservation capacity of the atmosphere; and second, reflecting more sunlight back into space to reduce the heat absorbed by the earth.
The first method is called "carbon geoengineering." Some methods in carbon geoengineering, such as carbon removal, because carbon dioxide in the atmosphere can be directly reduced, have been widely recognized by scholars as a response to climate change [8]. However, this approach remains challenging to implement, with "carbon capture and storage" technology as an example being costly and requiring a significant amount of land [9].
The second approach, the "solar geoengineering", while unlikely to completely reverse climate change caused by rising greenhouse gas concentrations in large temperatures, may be able to mitigate changes in several key climate variables, such as temperature and precipitation, in a relatively short period of time, and therefore may reduce climate risk [10].
Among them, stratospheric aerosol propagation is an extensively studied solar geoengineering method, and according to the IPCC Sixth Assessment Report, this technical route has higher cooling efficacy and technical feasibility than other solar geoengineering methods (such as cumulus lighting over the ocean and thinning of high-level cirrus clouds) [8,11], as well as relatively low economic costs [12].
In global climate governance over the past few decades, the scientific basis is not the only guiding criterion, and socio-economic considerations often affect the strength and effectiveness of carbon dioxide emission reductions. A Harvard University study on the optimal path of climate governance shows that considering climate and economic models, solar geoengineering as an auxiliary means to combat climate change can effectively reduce the economic cost of climate governance. Compared with the combined path of emission reduction and carbon removal, the addition of solar geoengineering can not only significantly flatten the curve, reduce the cost peak to about 1/6 of the original, but also greatly reduce the overall economic cost [13].
Figure 32 The economic cost of different climate governance paths under the temperature control target of 32 degrees Celsius | Source[13] Translated by the author of this article
William Nordhaus, a 2018 Nobel Laureate in Economics and a professor in the Department of Economics at Yale University, also mentioned in his book Climate Casino: The Risks, Uncertainty and Economics of Global Warming that in the face of climate change, we undoubtedly need to focus on carbon emission reduction, but both adaptation and geoengineering measures are indispensable [14].
3
Is stratospheric aerosol propagation a fantasy?
As mentioned earlier, the SCoPEx project plans to sow only very small amounts of particulate matter in the stratosphere, and simulations from climate models suggest that the actual stratospheric aerosol propagation project may require millions of tons of particulate matter per year [10].
Regardless of how long we continue to transport such a large amount of particulate matter or gas to the stratosphere, how can such a large amount of particulate matter be suspended at a high altitude and not quickly fall to the ground?
In fact, nature has shown the power of stratospheric aerosol propagation many times, most recently the tonga eruption earlier this year, which estimates now show that the tonga eruption injected about 400,000 tons of sulfur dioxide into the stratosphere [15].
These sulfur dioxide chemically converted into sulfate particles in the stratosphere. These particles are particularly tiny, most of them are in diameter from a few hundred nanometers to a few microns, less than 1/20 of the thickness of a human hair, so they can be suspended in the air. In addition, as the name suggests, atmospheric motion in the stratosphere region is mainly horizontal flow, and the movement in the vertical direction is very weak, so suspended particulate matter can stay in the stratosphere for a longer period of time (months to years).
Another more notable example is the eruption of Mount Pinatubo in the Philippines in 1991. During the eruption, Pinatubo put about 20 million tons of sulfur dioxide into the stratosphere and spread across the globe with atmospheric motion for about a year. Over the next three years, the global average temperature dropped by about 0.5 degrees Celsius despite the effects of el Niño ( a phenomenon in which the waters of the eastern Pacific ocean heat up abnormally every few years ] [ 16,17 ]
The inspiration for anthropogenic stratospheric aerosol propagation is volcanic eruptions. Paul Krutzen was awarded the Nobel Prize in Chemistry in 1995 for his research on the mechanisms of ozone layer destruction, and in a 2006 article he compared the effects of anthropogenic stratospheric aerosol propagation with the 1991 eruption of Mount Pinatubo .[18] It was this paper that led the scientific community to seriously consider the concept of stratospheric aerosol propagation.
At present, the most studied in the scientific community is still the sulfate aerosol (volcanic eruption product). In addition, scholars are also considering a series of materials such as calcium carbonate, metal aluminum, alumina and barium titanate. Some scholars have even proposed the use of diamond particles as a sowing material, because diamonds have a strong ability to reflect sunlight and the surface of diamonds is not prone to chemical reactions, which may have less possible effect on the ozone layer [19].
Now that Earth has shown us that spreading aerosols into the stratosphere can reduce the Temperature of the Earth, and that aerosols can be suspended in the stratosphere for a longer period of time, another technical question is, how to operate?
At present, scientists have proposed a number of stratospheric aerosol propagation methods, of which there are three main types of existing technologies that can be achieved [20]:
1 Use of high-altitude aircraft that can cruise the stratosphere to spread aerosols on a large scale. At present, there are many aircraft in the world that can fly at altitudes of more than 20 kilometers, including drones such as RQ-4 Global Hawk; 2 A large number of stratospheric balloons are used to spread aerosol materials into the stratosphere. Similar weather balloons are currently launched to altitudes of about 30 km around the world every day; 3 aerosols are injected into the stratosphere using artillery shells. This method is best chosen at high altitudes to make it less difficult for shells to enter the stratosphere, but at the same time the technical cost is also very high.
Currently, a new generation of high-altitude aircraft is one of the most likely choices for large-scale applications, compared to the other two methods, it is both low cost and stable operation.
Figure 4 Some of the stratospheric aerosol propagation methods proposed by scientists so far | source[20] Translated by the author of this article
4
Risks and Disputes
Although solar geoengineering, represented by stratospheric aerosol propagation, can reduce global temperatures, it is precisely because of this global climate effect that it has many natural science risks and uncertainties, and it is also accompanied by controversies in the socio-economic field.
From the most basic changes in surface temperatures, to the effects on glaciers; from local climate changes to ocean circulation reactions; from sulfur deposition to the risks of a project suddenly being forced to abort — the risks and uncertainties of solar geoengineering touch all aspects of climate change science. There is currently no unified assessment standard for these complex risks and uncertainties in academia. The lack of such unified standards may focus people's attention on factors that do not have much actual impact, and it is difficult to form a holistic understanding of solar geoengineering.
In his book A Case for Climate Engineering, David Keith, a Harvard professor who led the ScoPEx project, notes that one of the most frequently mentioned points in the media coverage of solar geoengineering is that it has the potential to reduce monsoon intensity and summer precipitation in East Asia, thus affecting local agricultural harvests [21]. In fact, on a rapidly warming Earth, the summer monsoon is likely to become too intense, causing hurricanes and torrential rains that can damage agricultural harvests. In this case, the weakening of the monsoon by solar geoengineering may actually be more conducive to the development of local agriculture.
So, how to assess the risks of solar geoengineering so that we can most constructively guide scientific research? An article published in Nature Review: Earth and the Environment argues that in order to sort out these complex risks, we can first list the uncertainties of solar geoengineering one by one and evaluate each entry according to two criteria [22]. The two criteria are: first, the possibility that solar geoengineering will occur, or that we currently lack a understanding of it; and second, the magnitude of the negative impact. Based on the comprehensive evaluation of these two criteria, we can identify the items with higher two indicators as the focus of the next step of research.
From this perspective, the most important risks and uncertainties of solar geoengineering are:
Aerosol microscopic physics and sub-grid scale aerosol mixing processes
So far, simulations of the microscopic physics of solar geoengineering aerosols have relied heavily on studies of past post-volcanic aerosol processes. There is a big difference between the artificial, controlled aerosol release and the uncontrolled volcanic eruption process: when the aerosol is artificially released, humans can choose the rate of release, location, and the total amount of aerosol, while the volcanic eruption will generally only be in a short period of time, in the same location, sharply put a large amount of aerosol into the stratosphere. At the same time, climate model simulations of aerosol processes are currently largely limited to the grid scale (i.e., the smallest unit of area of the climate model), and the size of these grids (at least a few thousand square kilometers) determines that they cannot simulate aerosol-scale physical processes. However, these processes directly affect the reflection efficiency of aerosols in solar geoengineering and indirectly downstream after delivery.
Effects of stratospheric aerosol heat absorption on surface climate
When the aerosol is discharged into the stratosphere, in addition to reflecting short-wave solar radiation and causing the surface temperature to decrease, it also absorbs long waves and heats the stratosphere. Such temperature changes can have a large impact on the surface climate, for example, changes in stratospheric temperature may change the vertical circulation of the atmosphere, thereby affecting the concentration of the stratospheric ozone layer, and even changing important atmospheric chemical processes on the surface. At the moment we know very little about these effects.
Ecosystem response
The impact of solar geoengineering on ecosystems is complex. For example, reducing sunlight on the surface may reduce the photosynthesis of plants, but because stratospheric aerosol geoengineering mainly reduces direct light and increases diffracted light, the photosynthetic efficiency of plants may increase as a result. In addition, changes in the Earth's temperature will also affect animals, and unknown questions include whether drastic changes in the Earth's temperature will prevent animals from surviving in the current living space and forced to migrate, or how animals will react to sudden loss of sunlight. At present, scientific research in these areas is very limited.
Compared with risks and uncertainties in the natural sciences, the policy and ethical dimensions are more relevant.
Due to the fluidity of the atmosphere, aerosols will eventually reach the globe wherever they are stocked. Therefore, the implementation of solar geoengineering must be agreed by all countries. However, due to the regional nature of the climate, it is likely that different countries or regions will be affected differently. A 2018 study noted that under the same solar geoengineering scenario, the precipitation cycle in the Amazon cannot be controlled in a natural (pre-industrial) state if the heat wave in Europe is effectively suppressed [23]. These regional influences, whether positive or negative, can become a point of contention in international negotiations.
The bumpy experience of the SCoPEx project is a microcosm of the difficulty of this regulation. As mentioned earlier, while SCoPEx has little physical impact on the environment, the project is being shelved due to public opposition to solar geoengineering.
On the other hand, the general concern of academics and politicians is whether the implementation of solar geoengineering will become a stumbling block to emission reduction policies, and whether the technology of solar geoengineering will be transformed into weapons of war.
In response, Nordhaus argues that any measure to ensure social stability, such as a police force or a snow-capped rescue force, increases the chances of people taking risks, but most people may prefer to live in a society with a police and rescue system .[24] He argues that perhaps solar geoengineering will reduce the impetus for emissions reduction to some extent, but that shouldn't stop us from studying it more deeply in case it's needed. In addition, some studies have pointed out that the application of solar geoengineering and emission reduction policies are not a simple trade-off or competitive relationship. Their strengths and weaknesses should be weighed together during the decision-making process [25,26]. In this way, policymakers can develop climate policies that are cost-effective.
As for concerns about its use as a weapon of war, some scholars believe that this possibility is very small [27]. This is because weapons should generally be able to be effectively and precisely controlled by the user, and have an immediate effect, while the scope of the impact of solar geoengineering on the climate is difficult to control, and its onset is very slow relative to traditional weapons.
It is conceivable that country A wants to use solar geoengineering to cause climate impact on country B in order to complete the attack. Country A, then, needs to set up equipment in the right areas to spread millions of tons of aerosols, but the observable climate impact of these aerosols on Country B can take years; and not only country B, but also most countries and regions in the world are likely to suffer. The cost and scale of this strike are unlikely to be carried out in secret, the risk of being stopped halfway is enormous, the international impact is terrible, and the benefits are difficult to determine. In modern warfare, it is hard to imagine solar geoengineering as a reasonable weapon choice.
5
More scientific research is needed
Solar geoengineering is currently a relatively small field of scientific research. As of 2018, global research funding in this area has reached a maximum of about $8 million per year [28], which accounts for less than 0.01% of research funding in the field of climate change. Although the Chinese government has not made a clear statement on the implementation of solar geoengineering, China is one of the few countries with government funding to support solar geoengineering research projects.
China's largest solar geoengineering project is the "Research on Basic Theory and Impact Assessment of Geoengineering" under the Ministry of Science and Technology's 973 Program, led by British scientist John Moore and attended by scholars from Beijing Normal University, the Chinese Academy of Social Sciences, and Zhejiang University .[29] Research in this project includes topics in the natural and social sciences, but does not include laboratory or outdoor experiments.
Compared to the size of solar geoengineering research projects supported by the U.S. government (US$100 million to US$200 million), [30], China's research projects are not large. But Chinese climate scientists are no strangers to solar geoengineering, and the overall attitude is not fundamentally different from that of American scholars. A recent study found that climate scientists in both China and the United States believe that funding for solar earth research should account for about 5% of research funding for climate science, and that they do not support the implementation of any recent specific engineering projects [31].
In addition to China and the United States, the United Kingdom, Germany, Australia, and Japan also have large-scale solar geoengineering research projects [28], which have trained a group of scholars. While these scientists can't make a decision about whether solar geoengineering is implemented or not, they may be key to advancing international collaborations on solar geoengineering and ultimately whether solar geoengineering can be implemented.
As Frank Keutsch, chief scientist at SCoPEx and a professor at Harvard University, puts it: "As a scientist, I can't sway the final decisions of society [for solar geoengineering], but I can provide a scientific basis for those who need to make decisions." （As a scientist, I have no say on the decisions that society ultimately takes. But I can help provide facts for those who do.）” [32]。
Acknowledgements: Thank you to Professor Cao Long of Zhejiang University for providing academic guidance for this article.
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About the Author
Yaowei Li, Ph.D. student in the School of Applied Science and Engineering, Harvard University, is supervised by Professor Frank Keutsch mentioned in the article; Dai Zhen, who graduated from the School of Applied Sciences and Engineering of Harvard University, is supervised by Professor David Keith mentioned in the article. ”
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Plate editor| Ginger Duck