As the impacts of climate change become increasingly severe, some scientists and policymakers are turning their attention to a set of controversial technologies known as solar radiation management. These techniques, also referred to as solar geoengineering or albedo modification, aim to cool the Earth by reflecting a small fraction of incoming sunlight back into space. Unlike efforts to reduce greenhouse gas emissions, which tackle the root cause of global warming, solar radiation management is a form of climate intervention that seeks to offset the warming effects of these gases. While some see it as a potentially rapid and low-cost tool to reduce the immediate impacts of the climate crisis, others warn of unacceptable environmental, social, and geopolitical risks.
The growing interest in solar radiation management stems from the concern that current efforts to cut emissions and remove carbon dioxide from the atmosphere may not be enough to prevent catastrophic climate scenarios. Proponents argue that, in conjunction with emission reductions, these technologies could serve as a ‘safety valve’ to achieve climate stabilization targets. However, the deployment of any solar radiation management technique would have global consequences, and there is currently no international consensus or governance framework to oversee research or potential implementation. The debate over solar radiation management is therefore not just a scientific one, but also a complex ethical and political challenge that raises profound questions about who has the right to control the Earth’s thermostat.
Proposed Intervention Techniques
A variety of solar radiation management methods have been proposed, all with the goal of increasing the Earth’s reflectivity, or albedo. These techniques are at various stages of research and development, with some being more studied than others. The most prominent methods include stratospheric aerosol injection and marine cloud brightening.
Stratospheric Aerosol Injection
Stratospheric aerosol injection is the most researched solar radiation management method. It involves injecting tiny reflective particles, or aerosols, into the stratosphere, the upper layer of the atmosphere. These particles, such as sulfur dioxide, would mimic the cooling effect of large volcanic eruptions by scattering sunlight back into space. The 1991 eruption of Mount Pinatubo, for example, is estimated to have cooled the planet by about 0.5°C for over a year. Custom aircraft are the leading proposed method for delivering these aerosols, though other mechanisms are under consideration.
Marine Cloud Brightening
Marine cloud brightening is another significant area of research. This technique aims to increase the reflectivity of low-lying marine stratocumulus clouds by spraying them with sea salt or other particles. These particles act as cloud condensation nuclei, creating more, smaller cloud droplets, which makes the clouds brighter and more reflective. Unlike stratospheric aerosol injection, which would have a global impact, marine cloud brightening is seen as a way to achieve regional-scale cooling.
Other Proposed Methods
Several other solar radiation management techniques have been proposed, though they have received less attention and research. These include space-based methods, such as deploying mirrors or sunshields in orbit to block a portion of sunlight from reaching Earth. Surface-based approaches, known as albedo modification, involve increasing the reflectivity of the Earth’s surface by, for example, painting roofs white, planting more reflective crops, or even covering large areas of the ocean or ice with reflective materials. Another proposed method is cirrus cloud thinning, which would involve removing ice crystals from high-altitude cirrus clouds to allow more heat to escape from the atmosphere.
Potential to Mitigate Climate Change
The primary motivation for exploring solar radiation management is its potential to rapidly counteract the rising global temperatures. Unlike reducing emissions or removing carbon dioxide, which take time to have an effect, some solar radiation management techniques could cool the planet within months of deployment. This rapid cooling effect could help to limit some of the worst impacts of climate change, such as extreme weather events and rising sea levels, while society transitions to a low-carbon economy. Climate models have consistently shown that solar radiation management could reduce global warming and potentially prevent the crossing of certain climate tipping points.
In addition to reducing temperatures, some studies suggest that solar radiation management could have other benefits. For example, it could reduce the frequency and intensity of extreme temperature and precipitation events in many regions. Some research also indicates potential co-benefits, such as a reduction in tropospheric ozone. Proponents argue that, given the urgency of the climate crisis, it is essential to research these technologies to better understand their potential benefits and risks. They see solar radiation management not as a replacement for emissions reductions, but as a complementary tool that could be used in a comprehensive climate strategy.
Significant Risks and Uncertainties
Despite the potential benefits, solar radiation management is fraught with risks and uncertainties. A major concern is that these technologies do not address the root cause of climate change: the high concentration of greenhouse gases in the atmosphere. As a result, they would not mitigate all the impacts of these gases, such as ocean acidification. Furthermore, solar radiation management would create a different type of climate change, with its own set of distinct impacts.
One of the most significant risks is the so-called “termination shock.” If solar radiation management were to be deployed and then suddenly stopped, global temperatures would rise rapidly to the levels they would have been at without the intervention, with potentially catastrophic consequences. There are also concerns about unintended consequences for regional weather patterns. Some regions could experience cooler or wetter conditions, while others could become warmer or drier, potentially disrupting agriculture and ecosystems. Other potential risks include depletion of the ozone layer, changes in the ratio of direct to diffuse sunlight, and increased acid deposition.
Beyond the environmental risks, there are significant social and geopolitical concerns. The uneven distribution of the benefits and negative consequences of solar radiation management could lead to international conflicts. A country that deploys such technology may not be the one that suffers the harmful side effects. There is also the risk of “moral hazard,” where the prospect of a technological fix for climate change could reduce the incentive to cut greenhouse gas emissions.
The Current State of Scientific Research
The current understanding of solar radiation management is largely based on computer modeling and observations of natural analogs, such as volcanic eruptions and ship tracks. Extensive research efforts are underway to better understand the feasibility, risks, and benefits of various techniques. The two most studied methods are stratospheric aerosol injection and marine cloud brightening. Modeling studies have demonstrated the basic efficacy of these methods in cooling the planet, but they have also highlighted significant gaps in our understanding.
Many of the processes that are most important for understanding the impacts of solar radiation management, such as the formation of clouds and aerosols, are among the most uncertain components of the climate system. There is still a great deal of uncertainty about how these interventions would affect regional climates, air quality, and large-scale atmospheric circulation. The UK’s Natural Environment Research Council is investing £10 million in four research projects to address these knowledge gaps using computer modeling and historical data. However, there is a consensus among many scientists that none of the proposed technologies are ready for deployment.
The Challenge of Global Governance
The prospect of deploying technologies that could alter the global climate raises profound governance challenges. There is currently no universally accepted international framework to oversee the research, development, or potential deployment of solar radiation management. The uneven distribution of benefits and risks means that any decision to deploy these technologies would have significant geopolitical implications. The fact that a small number of countries could unilaterally deploy some forms of solar radiation management is a particular concern.
Several international organizations have begun to address the governance of solar radiation management, but there is no consensus on the path forward. Some have called for a moratorium on the use of these technologies, while others argue that such a ban could stifle necessary research. The Solar Radiation Management Governance Initiative, established in 2010, is working to expand the global dialogue on this issue, particularly in developing countries, which are likely to be most affected by both climate change and any attempts to engineer the climate. Ultimately, the governance of solar radiation management will require a transparent, inclusive, and internationally coordinated approach to ensure that any decisions made are in the best interests of humanity as a whole.
