Cooling the Earth by reflecting sunlight: How it would work

• Solar geoengineering is a branch of geoengineering that focuses on reflecting sunlight back into outer space to reduce global warming.
• There are several solar geoengineering techniques being researched; the most feasible one consists of spraying reflective aerosols in the stratosphere.
• Scientists also consider brightening marine clouds to make them more reflective.

Recently, the White House’s Office of Science and Technology Policy launched a five-year research plan to investigate methods for reflecting solar radiation back to outer space in an attempt to reduce the effects of global warming.

The strategy is called solar geoengineering, albedo modification or solar radiation modification (SRM), and it is meant to cool down the Earth by limiting the amount of sunlight that reaches the surface. Although it is not a new idea, the topic has been understudied and considered essentially taboo for a long time (from 1965 to the mid-2000s), hence the relevance of the White House’s research plan.

What is solar geoengineering?

Greenhouse gas emissions accumulate in the atmosphere and keep the Earth from “cooling itself”. Due to these heat-trapping gasses, the Earth retains more solar radiation than it releases. Solar radiation turns into heat as it enters the Earth’s atmosphere, so its excessive absorption causes the average global temperatures to rise.

This is what we call global warming and it is a major cause of climate change. Global warming leads to ice melting and rising sea levels, loss of permafrost (a subsurface layer of soil that remains below its freezing point throughout the year), stronger and longer heatwaves around the world, a higher frequency of extreme weather events, reduced soil moisture and droughts, dried-out vegetation and wildfires, water shortages, alteration of ocean biome due to rising ocean temperatures, and other effects.

The exchange of solar radiation in the atmosphere (the amount of solar radiation that enters the Earth versus the amount of solar radiation that leaves it) is called radiative forcing and it is measured in units of watts per square meter.

Currently, the Earth receives a global and annual average of almost 340 watts per square meter of solar radiation. Before the industrial era, radiative forcing (heat in and heat out) was in a close balance, meaning the Earth’s average temperature was more or less stable.

Researchers have calculated a baseline for radiative forcing based on a year before the beginning of global industrialization. Compared to this baseline, radiative forcing can directly measure the ways recent human activities have changed the planet’s climate.

Solar geoengineering refers to a branch of geoengineering that studies the possibility of helping Earth release that extra solar radiation by reflecting it back into outer space through different methods.

What are the 3 types of solar geoengineering?

There are three main proposed techniques of solar geoengineering:

• Stratospheric aerosol injection. This is the most technically feasible method right now. It consists of introducing reflective aerosols into the atmosphere. Aircraft or tethered balloons would spray these reflective aerosols in the stratosphere for a quick global cooling effect. The proposed chemical compounds to do this are sulfur dioxide (naturally released by erupted volcanoes), calcite, and alumina, but more research is needed to determine which is better (and safer) for the task.

• Marine cloud brightening. This consists of making marine clouds brighter and, therefore, more reflective. One proposal is for specially designed ships to spray saltwater into the air as a fine mist. Salt particles would act as cloud 'seeds', where water vapor condenses around them, forming water droplets. Enhancing the cloud droplet number and concentration could whiten the cloud, making it more reflective.

• Enhanced marine sulfur emissions. This regional solar geoengineering method consists of 'fertilizing' the oceans with materials that boost dimethyl sulfide production. Dimethyl sulfide is the primary source of sulfate aerosols in remote ocean regions, so increasing its amounts could increase cloud reflectivity. This method is believed to be more appropriate for reducing ice melting in the Antarctic.

Additionally, there are a number of other ideas, including:

• Space-based techniques such as introducing a space mirror into orbit to reflect incoming sunlight, dispersing sunlight before it reaches Earth with diffraction gratings or lenses, etc. These options are the least feasible because of their costs.
• Ocean mirror. A fleet of sea vessels would spread lots of long-lasting microbubbles in the ocean, forming an artificial seafoam. This artificial seafoam would be whiter and, therefore, more reflective.
• Cirrus cloud thinning. Cirrus is a type of high cloud made of ice crystals that reflect sunlight but also trap heat from infrared radiation. Thus, if we were able to thin them or reduce them, that could have a cooling effect on Earth.

Who proposed solar geoengineering?

There isn’t a single person who proposed solar engineering, but the idea can be traced back to several events:

• In 1965, U.S. President Lyndon B. Johnson’s Science Advisory Committee issued a landmark report titled “Restoring the quality of our environment”. This is the first high-level statement on global warming. While it was focused on warning about the harmful effects of carbon dioxide emissions from fossil fuels, it also proposed to “raise the reflectivity of the Earth” to countervail climate change.
• In 1974, Russian climatologist Mikhail Budyko suggested that burning sulfur in the stratosphere could create a reflective haze that would throw sunlight back into space. This idea was tested in real life with the eruption of Mount Pinatubo in 1991, whose debris formed a reflective aerosol cloud that reduced solar radiation by 10% and the average global temperature by 0.9ºF for almost two years.
• In 1992, solar geoengineering options were discussed in a climate change report from the US National Academies titled “Policy implications of greenhouse warming”.
• Atmospheric chemist and Nobel laureate Paul Crutzen wrote about stratospheric sulfur injections in 2006. His paper "Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?" was very influential in the scientific community, and it is credited with breaking the “taboo” around solar geoengineering.
• In 2011, a British academic consortium called Stratospheric Particle Injection for Climate Engineering tried to test the method on a large scale but ended up not doing it due to political issues.

Can we reverse global warming?

Most scientists agree that global warming can be slowed down or even reversed if we reduce our greenhouse gas emissions. There are several proposals on how to do this: switching to clean energy, reforesting the planet, prioritizing sustainable agriculture, etc. Researchers feel that the answer lies in adapting all or most of these suggestions.

Solar geoengineering is not widely considered an alternative solution to the climate crisis, but those who defend it believe it could help mitigate its effects, while we transition to more eco-friendly energy sources, industries, and practices.