Atmospheric manipulation (also known as “solar radiation management” or SRM) involves reflecting sunlight to reduce global warming. The reflectivity of a surface is known as albedo. A highly reflective surface has a high albedo and reflects a lot of solar radiation back into the atmosphere, while a surface with a low albedo absorbs little of the sun’s radiation, thus absorbing it. A good example of the reflection versus absorption concept is the choice of clothes to wear. Light colors reflect more of the sun’s rays, while dark clothing absorbs more. This is why people tend to avoid dark colors in very hot weather. Ice has a high albedo. It reflects most solar radiation back into the atmosphere. This helps to keep ice cold. As surface temperatures increase due to rising water temperatures, though, icebergs in the sea are melting. Fewer iceberg mean more of the ocean’s surface is exposed to sunlight. Since liquid water has a lower albedo than ice, more sunlight is absorbed, thus raising the temperature of the water even more. This, in turn, causes more ice loss in a cycle of global warming. In this article, we will consider SRM projects designed to reflect sunlight to reduce global warming and assess the feasibility and potential dangers of these methods.
In scientific terms, SRM is a deliberate intervention in the natural evolution of physical processes in the atmosphere and other components of the earth’s climate system to achieve the desired results. The increased attention on SRM in the last few years has stemmed from the 2015 Paris Agreement. That agreement included a resolution by signing nations to limit the increase in average global temperature below 2°C above pre-industrial levels, with a goal of limiting warming to 1.5°C. Unfortunately, global emissions have increased 1.7% annually since 1992, the year in which the first climate agreement was reached. Experts argue that it would take 25 – 45 years to reach the goals of the 2015 Agreement if member nations started reducing emissions linearly now.
Whether the Agreement’s goals are achievable on its timetable or not, climatologists and other experts are discussing variations of SRM including, increasing sulfate aerosols in the stratosphere to increase the Earth’s albedo, as well as engineering projects designed to protect natural heat reflectors like sea ice, snow, and glaciers. One method would involve the injection of an aerosol into the stratosphere where it would remain for several months due to the lack of precipitation. An aerosol of sulfur is a likely candidate because it would mimic the cooling effect of large volcanic eruptions. Such eruptions cause residual atmospheric ash particles and sulphuric acid droplet scatter that would reflect some incoming sunlight. The estimated annual cost of this approach is estimated at $2B - $10B. However, some scientists warn that this tactic would cause dramatic climate changes and weather events that would cost $2 trillion annual to address, rendering the “cure worse than the disease.”
Another solar geoengineering method, Marine Cloud Brightening (“MCB”), would introduce fine mists of seawater aimed at the stratocumulus clouds, which are dark. These clouds could be lightened by introducing more cloud condensation nuclei into the lower atmosphere. After the seawater evaporates, some salt particles would stay suspended. Although MCB may be able to compensate for approximately 1°C of global warming for its projected $10B annual cost, the results likely will vary geographically because the deployment of seawater would vary.
Lastly, the geoengineering method, Cirrus Cloud Thinning (“CCT”), could work by increasing outgoing longwave radiation to disrupt Earth’s radiative balance. Cirrus clouds have a net warming effect from trapping some outgoing longwave radiation and reflecting some incoming shortwave radiation. While disbursement of high-altitude cirrus clouds could reduce global warming, and could be done inexpensively by injecting ice nuclei, such as bismuth triiodide, into the areas where cirrus clouds are likely to form, CCT may have a net warming effect.
The fact is that the way climate would respond to SRM isn’t known completely. The impact of aggressive SRM would become clear within months but could be irreversible in that timeframe. Clearly abatement measures to reverse the effect of greenhouse gas emissions will take a long time to have noticeable improvement. Given the risks of SRM, the best approach seems to be “proceed with caution.” Solar geoengineering seems best for managing short-term climate change risks from existing emissions, and not long-term abatement until more research is done.