Hacking the Planet—Part 3

Climate Engineering: Solar Radiation Management

By Srivathsan Karanai Margan

We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology.

—Carl Sagan

Ever since climate change started unleashing catastrophic events and was categorized as a global risk, the global response—which includes discussions, agreements, policies, strategies, and action—has primarily focused on mitigation and adaptation. Despite the promise to drastically alleviate the impact of climate change, climate engineering remained confined to academic research and was strictly forbidden territory from a practical response perspective.

With the mitigation efforts failing to achieve the expected success, climate scientists and policymakers were compelled to look at the two climate engineering techniques: carbon dioxide removal (CDR) and solar radiation management (SRM). Of the two techniques, CDR was considered a bit more lenient than SRM, which continued to be taboo as it was considered more aggressive and controversial.

Though both the climate engineering techniques focus on reducing global warming, the paths to achieving the objective are different: CDR aims to achieve it by removing carbon dioxide (CO₂) from the atmosphere, whereas SRM intends to potentially dim the sun. SRM focuses on intentionally increasing the amount of sunlight that is reflected by the Earth and atmosphere back into space. The evolving climate desperation has driven climate scientists and policymakers to discuss SRM as an additional option to CDR, to prevent the eventuality of the global average temperature limits set by the Paris Agreement are breached.

This article discusses SRM, which is considered the last resort for saving the planet when all other climate responses fail.

Dimming the Sun

SRM is an overarching term that is used to describe a set of hypothetical and, in some cases, even speculative approaches to reducing global warming by increasing the albedo of the Earth. Albedo, which in Latin means whiteness, is the property of a planet to reflect the incoming sunlight back into space. SRM is essentially climate change happening in the reverse. SRM methods focus on reflecting a small portion of the incoming light (short-wave radiation) from the sun or enabling more thermal radiation (long-wave radiation) from Earth to escape into space to cool the Earth. Due to the cloud cover, the average albedo of the earth from the upper atmosphere, or planetary albedo, is 0.3-0.35. This means about 65–70% of the sun’s radiation is absorbed by the planet. Increasing the albedo by a meager 0.01-0.02 could be enough to offset the warming from a doubling of CO₂ emissions from the pre-industrial level.

Climate change mitigation and CDR approaches focus passively or actively on reducing or removing the cause of global warming—that is, the concentration of greenhouse gases (GHG) in the atmosphere—to facilitate the cooling of the planet on its own. This follows a longer timescale than SRM methods that directly and aggressively intervene with the warming process and could potentially start cooling the planet over a shorter timescale. It is projected that, in comparison with mitigation or CDR, SRM could be deployed cheaper and faster. Estimates suggest that the direct financial cost of implementing stratospheric aerosol injection (SAI), the most studied and controversial of the SRM methods, will be approximately 18 billion USD (in 2020 USD) per year to avoid 1 degree Celsius of warming.

Considering the complexity and sensitivity of SRM, the deployment goes through two stages: pre-deployment (research and development) and the actual deployment. (See Table 1.)

Solar Radiation Management Approaches

The albedo of the earth is determined by a combination of factors such as surface (ice, deserts, vegetation, water bodies, and urban landscape), atmosphere (clouds, aerosols, and gases), geographical and seasonal factors (latitude, season, and topography), human factors (deforestation and land use, industrial pollution), and solar factors (solar angle and solar spectrum). SRM approaches consider surface-based, space-based, and atmosphere-based albedo modification methods (See Figure 1). Each approach differs in the scale of its potential and the range of possible uses.

1. Surface-based

Surface-based, or ground-level, albedo modification is probably the simplest of the SRM approaches. The methods range from increasing roof albedo in urban areas by artificially whitening the roofs of buildings and roads to crop albedo by planting crops with high reflective properties, desert albedo by covering deserts with reflective material like vegetation, using reflective covers or sand over rocks, or snow and ice albedo by using reflective particles. Of these, roof albedo could help reduce warming in densely populated urban areas, and crop albedo could help in important agricultural regions and improve yields under certain conditions. The effects of these two albedo modification methods would be limited to regional levels and ineffective to counter global warming on a planetary scale. On the other hand, desert or snow and ice albedo modification could, in principle, result in substantial global cooling. However, implementing them would require vast resources, including material and infrastructure, and the logistics of distributing and maintaining reflective materials in remote and extreme environments.

2. Space-based

The idea of space-based methods is to deflect or block a fraction of incoming solar radiation by placing space mirrors, also known as space-reflectors, in orbit between the earth and the sun at Lagrange point L1, where the gravitational forces of the two bodies cancel each other to create a stable region in space. Space mirrors could be a giant shade, one large space mirror, or a fleet of smaller mirrors. They are expected to have the potential to block out approximately 1.8% of the solar radiation. Placing space mirrors is widely considered a hypothetical and unrealistic option due to the complexities of deploying and maintaining them at an altitude of 1.5 kilometers from the Earth. Space mirrors could be ineffective when the reflective material is in the shadow of the Earth and the position of the reflector may need to be continuously monitored and corrected. The development timescales of space mirrors are not clear, and the deployment costs are extremely prohibitive.

Placing space mirrors is widely considered a hypothetical and unrealistic option due to the complexities of deploying and maintaining them.

3. Atmosphere-based

Clouds play a significant role in influencing the distribution of solar radiation. Their properties affect how much solar radiation is absorbed, transmitted, scattered, and reflected in the atmosphere. Atmosphere- or cloud-based methods involve manipulating clouds to increase their reflectivity (albedo) and, in turn, reduce the amount of solar energy that reaches the Earth’s surface. Cloud-based methods are constrained by our limited understanding of how aerosols and clouds will interact in real time. This creates uncertainty on where and how much cloud albedo can be modified and what the actual impact would be on warming at local, national, and global levels. Cloud-based SRM methods are still largely theoretical and experimental, and their potential benefits, risks, and challenges are subjects of ongoing research and discussion.

3.1 Cirrus Cloud Thinning

Cirrus clouds are a type of high-altitude cloud that form at elevations typically above 4,000–12,000 meters and are composed of ice crystals instead of water droplets. Cirrus clouds both reflect sunlight and trap outgoing thermal radiation, but they absorb more than they reflect, resulting in a net warming effect. The idea of cirrus cloud thinning (CCT) is to modify or reduce the thickness and coverage of cirrus clouds to allow more heat to escape from the Earth’s surface and atmosphere to space, thereby reducing global warming. CCT involves injecting ice-nucleating particles, such as bismuth triiodide or aerosol particles such as sulfuric or nitric acid, into regions where cirrus clouds form. This would result in fewer but larger ice crystals, which have less optical depth and a reduced lifespan. The thin clouds would allow more infrared radiation to escape into space. However, it is predicted that in some instances CCT could be counterproductive by providing a net warming effect. The effectiveness of this method is unknown due to a limited understanding of the properties of cirrus clouds and the microphysical processes determining how they may be altered.

3.2 Marine Cloud Brightening

Marine cloud brightening (MCB), also known as marine cloud seeding, involves increasing the reflectivity of low-altitude marine stratocumulus clouds near the surface of the ocean. MCB involves seeding sea-salt aerosols into the cloud layer, where they become cloud condensation nuclei and form small cloud droplets. This would result in a short-term brightening of clouds with a higher reflectivity, thus reducing the amount of solar radiation that reaches the surface of the Earth. Research shows that the deployment of MCB is possible on a small scale, and the brightening effect would potentially last only for hours or days. Due to these limitations, MCB is not directly included in climate change models. MCB could have regional deployments, such as to slow arctic sea ice melt or protect against coral bleaching. The potential risk attributed to MCB is that a reduction in ocean temperature and available sunlight could impact the quantum of carbon absorbed by oceans.

3.3 Stratospheric Aerosol Injection

Of all the SRM approaches, strat­ospheric aerosol injection (SAI) is the most talked about and accepted as the most promising technically viable option for impacting the global temperature in a shorter period. SAI aims to replicate the volcanic eruption of Mount Pinatubo in the Philippines in 1991. When it erupted, Mount Pinatubo produced a plume of gas containing about 20 million tons of sulfate aerosols, some of which were forced into the stratosphere. Instead of falling on the Earth, the sulfate aerosols remained in the stratosphere and circulated around it for a few weeks. These aerosols remained suspended in the stratosphere for roughly one to two years, and droplets acted like tiny light-shattering mirrors, preventing the full heat of the sun from reaching the planet’s surface. This is said to have caused a reduction in the global average temperature by about 0.5^oC for a couple of years while the aerosols remained in the stratosphere.

SAI involves using a fleet of high-flying aircraft, stratospheric balloons, artillery shells, and rockets for injecting particles such as sulfur dioxide, calcium carbonate, titanium dioxide, aluminum oxide, or engineered nanoparticles into the stratosphere, which starts at 10 kilometers and ends at 50 kilometers above the surface of the earth. Most of the current research on SAI is focused more on sulfate injection than any other particle, which is often referred to as “the Pinatubo option.” To be effective, SAI must be a globally coordinated, multi-decadal exercise in which the injections are made at multiple locations around the planet. For this, it would require lifting millions of tons of the selected particles every year to altitudes of 18–25 kilometers. It is estimated that it would cost $2.25 billion per year to achieve a cooling impact of around 0.3^oC. The efficacy of SAI would depend on the composition of the aerosols used, the quantities used, the latitude and season at which they are deposited, and over what time frame.

SAI uses the Mount Pinatubo eruption as historical evidence of reducing global warming on a planetary scale. It is, however, uncertain if the effects of a sustained anthropogenic albedo modification through SAI would be like those of a brief volcanic eruption. Currently, the technologies to inject massive quantities of aerosols into the stratosphere at the required altitude do not exist, and it is predicted that the deployment technology could be developed in a decade. The potential risks from SAI could include stratospheric ozone loss, dramatic climate changes, changes to precipitation (including amount and patterns), an increase in acid rain, crop failures, and negative health effects.

Rays of Contention

As of now, all the SRM approaches are in the pre-­deployment stage, limited to either indoor research or small-scale outdoor experiments. They are yet to demonstrate the planetary-level scalability that is required to modify the albedo of the Earth to cool the planet. The uncertainties in modeling climate change, the deployment scope of the SRM approaches, and the risks of such albedo modification make it difficult to say with confidence whether successful deployments are possible, whether they will produce a tangible impact, and the benefits and consequences of such deployments.

There are several ethical, legal, equity, and justice issues related to deploying climate engineering on a planetary scale. Issues that are very specific to SRM are listed below.

1. Failing to address the actual cause of the problem: The main complaint against SRM is that it focuses only on the most obvious symptom, which is increasing global average temperature, but does nothing to address the underlying root cause of anthropogenic climate change, which is the massive increase of GHGs in the atmosphere. If the GHG concentrations are not reduced, even after successful SRM deployments, the CO₂ concentrations in the atmosphere will continue to rise, making it more toxic, and the oceans will continue to absorb it, causing rapid acidification.
2. Perpetrate inequality: The research and experiments on SRM are primarily restricted to the Global North. There is an inherent fear that this could lead to a concentration of power, and the large-scale deployments that impact every country in the world will not be made in a globally inclusive, equitable, and transparent manner where the interests of the Global South are also protected.
3. Lack of laws: Currently, there are no international laws specific to the research, development, and deployment of any SRM approach. If a country engages in deploying SRM on a global scale unilaterally or in coalition with some countries, despite opposition, there are no SRM-specific laws or governing bodies, even when such deployments are seen as a violation of national sovereignty. This raises potential gaps in how to monitor and govern large-scale deployments that may impact every other country in the world. While small-scale surface-based approaches could be deployed within a country without impacting others, large-scale surface- and atmosphere-based approaches could spill over to the atmospheric commons, which is a shared resource not owned by any individual, group, or nation.
4. Transboundary weather changes: Climate models suggest that the effects of global-scale SRM deployment could be heterogeneously distributed across the globe, impacting countries in different ways. Optimistic forecasts suggest every region of the world will reap benefits, and some regions will see more benefits than others. Pessimistic forecasts suggest that SRM deployment could have negative impacts on the monsoons in Asian and African countries that would have serious ramifications on food supplies for billions of people, thus escalating already existing social and interstate tensions.
5. Unintended weather impact: Global-scale SRM deployment could interfere with the natural modes of climate variation, causing changes to ocean and atmospheric circulation. Large-scale SRM could turn the sky hazy or milky, which could alter the quality of the light plants use for photosynthesis. SRM could disrupt the hydrological cycle, destroy the ozone layer, and cause acid rain. Besides this, ionizing the atmosphere makes it more electrically conductive, which could potentially result in dry lightning and increase forest fires.
6. Slippery slope: Any climate engineering approach should be coordinated with the existing reduction efforts. While CDR methods broadly align with this principle, SRM initiatives are radical and aggressive. Manipulating the albedo could lead to unforeseen, unintended, and harmful effects on weather patterns, precipitation, and regional climates. Once deployed, SRM might create a chain of events that cannot be stopped, especially when the outcome is undesirable or detrimental.
7. Termination shock: When SRM is deployed to reduce high-level global warming, it may need to be continued for decades or centuries by replenishing the aerosols continuously. If the SRM is stopped, all the warming that was artificially controlled will rebound, and the planet will undergo a rapid acceleration in warming. The sudden rise could be harder to manage than any gradual increase in the temperature that would have happened without the SRM intervention.

The Last Resort

The various SRM approaches depend on future technology developments. They differ in their ease of feasibility, scale, range of use, and possible benefits. Surface-based SRM options like more reflective crops or white roofs on buildings could provide some localized cooling. Even though these are low-risk and noncontroversial, deploying them on a larger scale across multiple locations could pose some unquantifiable risks. The space-based approach that is at the other extreme is currently hypothetical in nature. The real possibilities of their deployment, effectiveness, and risks are unknown. The atmosphere-based SRM approaches, especially SAI, hold the most promise to influence warming on a global scale. However, the assessment of the potential benefits and risks of the approaches still relies primarily on the modeling results and their underlying scenario assumptions. Hence, the atmosphere-based albedo modification draws strong objections from various quarters.

Climate change and global warming are accelerating. In the month of July 2023, the average global temperature increased to roughly 1.5oC above the pre-industrial average, making it the hottest month in 120,000 years. This made the secretary-general of the United Nations, Antonio Guterres, warn that “The era of global warming has ended; the era of global boiling has arrived.” In addition to climate change, this increase in temperature is influenced by another climate phenomenon called El Niño which involves the periodic warming of the sea surface. Though a temporary breach of 1.5oC is not something to panic about, the speed of the change is alarming. With insincere and tepid climate change mitigation approaches failing to provide significant relief, the implementation of failsafe adaptation roadmaps stretching into the future, and the longer timescales required for CDR to create global impact, SRM approaches may get the required levels of legal and financial patronage.

The ideal option for SRM deployment will be to implement only those SRM approaches that have a localized impact and avoid ones that have potential global risks. However, to stop a catastrophic rise in average global temperature, the geopolitically and economically fragmented global community may be tempted to exploit all the SRM, even if the approaches are not fully researched and proven safe. In such a scenario, the practical option would be to consider SRM only as a temporary option for peak-shaving the global temperature increase, to reduce the peak temperature during the overshoot period. While this would be useful in gaining time until the following key climate goals are achieved, the hope is that such borrowed time is not stretched forever:

1. Climate mitigation goals are realized.
2. The relatively benign CDR schemes are deployed at massive scales to stabilize and reduce CO2 concentrations.
3. Effective adaptation initiatives are deployed.
4. The global temperature stabilizes within the limits set out by the Paris Agreement.

Considering that SRM is the last resort in climate response strategies, the limitations, effectiveness, and risks of the different approaches must be understood before they are adopted. There is also an urgent need to draft international laws, governance rules, and development and deployment principles before any global-scale climate-altering deployments are attempted. Any large-scale deployment must be carried out only after international planning, consensus, and oversight. It must always be remembered that using SRM is an action born out of extreme desperation and panic to tackle global warming that should have been resolved much earlier with traditional emissions abatement. Hence, SRM must be treated in moderation as a supplement to mitigation or even CDR, but never as a substitute to cover our collective apathy and failure.

SRIVATHSAN KARANAI MARGAN works as an insurance domain consultant at Tata Consultancy Services Limited.