by Colleen Golja
figures by Brad Wierbowski
Articles with dystopian titles like “Is it OK to Tinker With the Environment to Combat Climate Change?” and “To Curb Global Warming Science Fiction May Become Fact” have begun to surface regularly in prominent news sources like The New York Times, The Guardian, NPR, The Economist, and many others. Just this past October, a cinematic portrayal of a climate-modified future, Geostorm, was released. It seems as if the concept of an artificially altered planet has begun to capture the public’s attention. But what exactly is the technology that these sources are talking about? Can we really control our climate?
Many forms of this technology—called “geoengineering”—exist. Geoengineering is broadly defined as the deliberate altering of environmental processes to induce changes in the earth’s climate. One type attempts to combat the temperature rise associated with climate change by altering the Earth’s albedo, which describes the amount of sunlight reflected by the Earth’s surface back into space (Figure 1). Light colors are more reflective, so snow-covered or desert regions tend to exhibit a higher albedo than dark regions like oceans or forests which absorb light. By absorbing less sunlight, the surface in high albedo areas maintains a lower temperature (all else being equal).
Solar geoengineering is a specific form of albedo modification in which highly reflective particles are introduced into the atmosphere to increase Earth’s albedo. This would reduce incoming light (radiation) from the sun, and thereby decrease the amount of energy (heat) reaching Earth’s surface. Scientists know that certain types of clouds and small particles in the atmosphere called aerosols cause this exact effect. Solar geoengineering focuses on the use of aerosols to increase Earth’s albedo.
An important caveat of solar geoengineering is its inability to attack the root cause of changing temperatures: increased atmospheric carbon dioxide (CO2). Therefore, solar geoengineering alone is not being proposed as a solution to climate change. Many researchers believe that solar geoengineering coupled with emissions reductions could reduce risks associated with climate change (Figure 2). Due to the slow nature of instituting new energy policies, however, some researchers suggest that solar geoengineering could reduce some of the warming from rising greenhouse gases during the period of transition to greener energy platforms and be phased out once emissions have stabilized.
But really, what are aerosols and what are they doing in the atmosphere?
You may immediately think of an aerosol spray can, used for products such as spray paint. These products use a fluid under pressure, called a propellant, to push something out of an orifice. For example, when you push the trigger on a can of spray paint, the orifice is opened, allowing the pressurized propellant fluid to escape. If the can was well-shaken before use, this propellant will be mixed with tiny paint particles. This mixture is a colloidal suspension of liquid particles in propellant, meaning that the particles are evenly distributed and small enough that they will not settle out of the solution over time. More technically, an aerosol is generally defined as “a colloidal suspension of particles dispersed in air or gas.” (Figure 3)
Small solid and liquid particles are not just in spray paint—they are also in our atmosphere, ranging in size from nanometers to micrometers. For reference, the average diameter of human hair is 100 micrometers, while one nanometer is 1,000 times smaller than one micrometer. Thus, even though they’re tiny, aerosols can be a wide range of sizes. These particles can be naturally occurring, like dust, sand, fog (water vapor), and smoke (ash), or derived from man-made sources, such as the burning of fossil fuels (combustion). They enter the atmosphere when sand is blown off deserts, volcanoes erupt, forests burn, and when power plants are firing, among various other processes (Figure 4). These particles both scatter and absorb incoming sunlight. Together, scattering and absorption are referred to as the aerosol “direct effects.” Shape, size, and chemical composition dictate the magnitude of these effects. As you might imagine, increasing the number of reflective particles in the atmosphere increases Earth’s albedo.
Scientists seek to understand how aerosols’ direct effects can be altered—in particular, the absorption and scattering of light. Absorption describes a particle’s ability to absorb light (energy), which is ultimately converted to heat. Materials being evaluated for geoengineering exhibit low absorption to minimize impacts on the atmosphere around them. Scattering, meanwhile, describes the way in which light is displaced by a particle. Aerosols exhibit both towards-Earth and towards-space scattering properties. Aerosols that are desirable in geoengineering exhibit high scattering towards space, or reflectivity. The shape and chemical composition of the particle has a great influence on the magnitude of these effects.
However, these effects are all about location, location, location. The lower part of the atmosphere in which we live, the troposphere, holds particles for a relatively short period of time before processes like precipitation quickly remove them via a phenomenon called deposition. However, aerosols are also found in a much higher layer of the atmosphere known as the stratosphere. Particles naturally arrive in this region either through deep convection—or thermally driven movement—from the troposphere or as a result of strong volcanic eruptions.
Aerosols present in the stratosphere are not heavily impacted by deposition processes because the stratosphere sits above the height at which cloud formation and rain would typically scavenge and remove particles. Aerosols in this region, therefore, remain in circulation for about a year before they are removed by natural sedimentation and stratospheric circulation into the troposphere. With such a long lifetime, particles can be efficiently transported all over the globe by stratospheric winds. Solar geoengineering recommends the introduction of aerosols into the stratosphere to leverage their long lifetime, global distribution, and increased reflectivity due to lack of competition with tropospheric cloud systems. A variety of methods for introducing aerosols into the stratosphere have been recommended for geoengineering, ranging from the use of stratospheric balloons to high-altitude jets.
But what makes researchers think this could actually work?
The year 1816 was called “The Year Without a Summer.” In fact, abnormally low summer temperatures and rainy conditions in Europe are even given credit for Mary Shelley’s creation of the famous novel “Frankenstein,” which was (partially) a result of boredom and being trapped indoors. The global changes in temperature and precipitation have since been attributed to the eruption of the Indonesian volcano, Mt. Tambora, on April 15, 1815, when a large volume of volcanic ash and other aerosols were introduced into the stratosphere. This caused an average temperature decrease of 0.4 to 0.7 °C (0.7 to 1.3 °F) worldwide as sunlight reaching earth was blocked by reflective aerosols from the volcano. While many volcanos have erupted in the past century, not all have been strong enough to inject material into the stratosphere.
This natural event illustrates the same general impacts predicted by climate models that incorporate various geoengineering scenarios. All of the models agree that solar geoengineering can help cool the planet on average, and they indicate that increases in planetary albedo will alter precipitation patterns, leading to changes in the location and intensity of rainfall. But even assuming these precipitation changes, model studies have noted that fewer “extreme” weather events occur under solar geoengineering compared to climate change scenarios. This means that models predict that solar geoengineering would lead to fewer occurrences of high intensity rainfall and drought periods. However, different models suggest differences in the exact location of these changes, making this a highly active area of research. Researchers are comparing multiple models to more fully understand the impact of aerosols on the global climate. It is likely that changing the chemical and radiative properties of the stratosphere will have unforeseen, far-reaching consequences that must be studied further to gain a more realistic idea of solar geoengineering’s impact on the planet.
The big picture
Curbing emissions is a challenge that various nations are approaching differently. The United States is currently changing a significant source of its energy from coal to natural gas because vast natural gas resources allow for an enormous reduction in cost. While this switch will decrease emissions for some time, it must be paired with the adoption of renewable sources of energy like wind and solar. Current challenges to be overcome include managing the intermittency of sunlight and wind; while energy storage technology is developing quickly, the scale required to manage long periods of low performance has yet to be largely applied. It is possible that advances in infrastructure will emerge in the next decade, but many scientific and political challenges must be addressed before this technology is adopted both in the United States and all over the world. Solar geoengineering is not a long-term replacement for these types of systemic reforms, but it can buy us some valuable time.
Colleen Golja is a Ph.D. student at Harvard University’s John A. Paulson School of Engineering and Applied Sciences in Environmental Science and Engineering.
For more information:
- Ethics of Solar Geoengineering
- Economics of Solar Geoengineering
- Recent articles from The Guardian, Smithsonian Magazine, The New York Times, NPR, The New York Times Magazine
- Journal articles from Journal of Geophysical Research: Atmospheres, Geoscientific Model Development, Atmospheric Chemistry and Physics, Quaternary Research, and Atmospheric Aerosols