According to the IPCC Fifth Assessment Report

When Mount Pinatubo erupted in 1991, it injected the stratosphere with about 15 million tons of sulfur dioxide (SO₂). The measured effect from this eruption was a 1°F (or 0.6°C) global temperature increase.

There are many aerosol interactions and characteristics that explain this global temperature increase. For one, aerosols have two major interactions with incoming solar radiation: 1) scattering and 2) absorbing. When aerosols absorb solar radiation, the environmental effect is a warming of the climate system. When aerosols scatter solar radiation, the climate experiences a cooling period.

When a large amount of aerosols are injected into the atmosphere, the next observed effect is global cooling. Numerous computer models also produce this result, but there are many unknown variables and interactions that are not accounted for in the models. Aerosols contribute to global cooling due to scattering, but the decrease in worldwide temperature is also due to the length of time that aerosols can deflect incoming solar radiation, reducing the incoming flux to the energy balance within the sun-Earth system. The energy balance is based on many layers of the Earth’s atmosphere.

Impact of Vertical and Horizontal Transport

Vertical transport from the surface of the Earth to the planetary boundary layer (PBL) takes a few days to a week, and vertical transport to the tropopause from the surface can take a month. However, vertical transport from the surface into the stratosphere by usual processes takes 5-10 years on average, and it takes another 1-2 years for exchange to occur between the stratosphere and the troposphere. This is due to the level of stability of the atmosphere. The top of the troposphere is very stable, and a strong temperature inversion exists in the stratosphere, which is the same reason for the lack of weather and clouds in the stratosphere on average.

Horizontal transport times can factor into vertical transport times. Since air is mainly transported to the stratosphere from the troposphere in the tropical latitudes, air carrying a high amount of aerosols must first be transported to tropical regions. In the case of Mount Pinatubo, the aerosols were already in the tropical region. For other sources of aerosols, both natural and anthropogenic, horizontal transport can take weeks to a year.

Therefore, only aerosols or gases with long lifetimes will make it into the upper troposphere or into the stratosphere. Gases are often subject to chemical interactions, and aerosols can experience chemical interactions as well as other physical loss sources, which are primarily wet deposition (precipitation) and dry deposition. Consequently, it is relatively difficult for aerosols to be transported to the stratosphere.

How do aerosols that make it into the stratosphere impact climate?

Once aerosols penetrate the stratosphere, however, they affect the global climate of the Earth for at least the next 1-2 years. And due to the strong stratospheric winds, aerosols are typically distributed relatively evenly around the globe. Therefore, the aerosols can act as a temporary shield around Earth, scattering incoming solar radiation from striking the Earth’s surface.

The Earth absorbs the visible shortwave radiation from the sun, and it emits in longwave infrared radiation. Of the emitted infrared radiation, only some of that energy escapes to space; the other portion is absorbed by greenhouse gases and re-emitted such that Earth actually absorbs both visible and infrared radiation. This is partially why we are experiencing a warming planet.

Although aerosols can absorb solar radiation, the net effect is observed and tested to be a cooling effect on the planet. The natural cooling phenomenon produced by volcanoes has helped to inspire the idea of geoengineering, which claims to reduce the impacts of global warming.

Geoengineering Solutions

Geoengineering includes two main fields: 1) solar radiation management (SRM) and carbon dioxide removal (CDR).

Solar radiation management attempts to reduce the amount of incoming solar radiation from the sun, which would reduce photochemical interactions in the troposphere and would reduce the available energy that the Earth absorbs and later re-emits. This method does not attempt to remove or slow down the process of carbon dioxide (CO₂) and other greenhouse gas emissions.

There are many initial drawbacks to SRM, mainly the unknown and hypothesized climate side effects, the lack of trustworthy models that accurately reflect the chaotic nature of the atmosphere, the lack of testing, the questionable feasibility, and the high costs of SRM projects.

However, SRM may be useful in the future as fewer countries emit aerosols due to cleaner technologies, which induce an overall warming effect on the planet. Additionally, SRM might delay some of the negative results from climate change, giving people more time to produce long-term, sustainable methods of reducing global warming.

The most popular types of SRM geoengineering are space-based methods, stratospheric injection, cloud brightening, thinning cirrus clouds, and surface albedo changes.

Could we just prevent solar radiation from getting to Earth?

One theoretically simple solution to reducing the incoming solar radiation is to just physically block solar radiation from entering the Earth system. Proposals to do this involve placing objects in space around Earth, and these objects could range from dust particles to refractive disks. One potential problem with this is the maintenance of keeping a single or many large refractive disks in orbit.

Satellites require an average of $10 – $400 million dollars to be launched into space, and then they require additional fuel and human monitoring to make course adjustments for the duration of their orbital lifetime.

If satellites were left to their own devices, they would all eventually fall to Earth (or burn up in the atmosphere) from the influence of gravity and friction. Additionally, there could be high capital associated with damage repairs from space debris colliding with the refractive disks. And, if the disks do fall into Earth’s orbit, they could either crash into another satellite, release chemicals into the atmosphere from reentry burnup, or cause land damages from surface impact.

The lack of models to study these features of space-based SRM prevents answers to the question of whether or not the decrease in global warming from refractive disks would be significant enough to outweigh these potential negative side effects.

Could we make clouds in the stratosphere?

The next proposed SRM method evaluates artificially produced stratospheric clouds. There is debate about the types of chemicals to use, the region of injection, and how often injection should occur.

The cooling effect would resemble the impacts of the Mount Pinatubo (and other volcanic) eruptions, so the injection would need to be replicated annually. The chemicals being considered are primarily sulfur dioxide (SO₂), sulphuric acid (H₂SO₄), and other sprayed aerosols. Some models predict that to maintain a decrease of 4 W/m^2 in solar radiation, the same amount of material would need to be annually injected into the stratosphere as the Mount Pinatubo eruption.

The feasibility and costs of essentially replicating the Mount Pinatubo eruption have not been significantly studied. One potentially large negative feedback from stratospheric injection is secondary chemistry in the stratosphere and the troposphere. Sulfur dioxide is a precursor to PM2.5, which is associated with many health issues, such as lung and respiratory disease.

Additionally, after the Mount Pinatubo eruption, there was a loss of stratospheric ozone. This is due to the chemical reduction of ozone by sulfuric acid. Ozone is lost in the stratosphere due to the Chapman mechanism and by catalytic cycles of SOx, CLOx, and NOx. Therefore, introducing sulfuric acid and sulfur dioxide can largely deplete stratospheric ozone and significantly delay the recovery of the Antarctic Ozone Hole by 30-70 years.

Since stratospheric ozone photolyzes with UV radiation, not much of the incoming UV radiation from the sun reaches the surface of the Earth. However, if the Antarctic Ozone Hole recovery is heavily delayed, then this will increase the amount of harmful UV radiation that reaches the surface.

Secondary Impacts from Stratospheric Cloud Injection

One point to consider is the scattering by stratospheric cloud aerosols. While solar radiation will be reflected back to space, there will also be more diffuse radiation across all latitudes, and this could increase terrestrial ecosystem photosynthesis.

Vegetation is a sink for carbon dioxide, so this could act to balance out greenhouse gas emissions. However, plants also emit volatile organic compounds (VOCs), which can be carcinogenic and react with other compounds to become toxic. The increase in diffuse radiation would also lower the efficiency of some green energy sources, such as solar panels, although the efficiency decrease has not been largely studied.

Could we brighten our clouds?

Although there could be many problems associated with stratospheric injection, another SRM strategy is cloud brightening to alter Earth albedo, which works much like the space-based SRM methods to decrease the overall incoming solar radiation flux that reaches the Earth’s surface. Earth’s radiation budget depends on the albedo of the planetary surface, so brighter clouds will reduce the incoming radiation.

Clouds are made brighter by having more small particles for light to reflect off of, so having a higher droplet concentration will increase albedo. In order to have a higher droplet concentration, the air needs to contain more cloud condensation nuclei (CCN), which can be chemicals or larger sea salt particles. Marine stratocumulus clouds are suggested to be the ideal cloud since they have relatively weak precipitation, and this is a benefit to increasing albedo since cloud layers with a propensity to precipitate produce a strong negative forcing from the increase in liquid water content.

Initial studies estimate that there would only be a decrease of 1 W/m^2 if 5% of the air above the ocean surface was seeded with these boundary layer clouds. A larger decrease of 4 W/m^2 is estimated if 75% of the air above the ocean surface was seeded. The current amount of cloud droplet concentrations would need to increase by a factor of five for these numbers, but these ratios of seeded cloud to ocean surface would be difficult to maintain with the chaotic nature of the atmosphere.

The cloud-aerosol interactions are not well understood, and some studies suggest that higher seeding rates would be required for even the 1 W/m^2 decrease, while other studies proclaim that cloud seeding would be more effective than initially thought. Although brightening boundary layer clouds could cool the planet, the thinning or removal of cirrus type clouds could also act as a cooling technique.

Could we remove cirrus clouds?

Cirrus clouds form around 20,000 feet and are primarily composed of ice crystals. Due to this physical structure, cirrus clouds are effective at absorbing and re-emitting longwave radiation from Earth’s surface and atmosphere, enhancing the greenhouse effect of the Earth. The estimated decrease in solar radiation is 2 W/m^2 from this method.

However, ice microphysical processes are not well understood, and some studies show that cirrus cloud removal methods could actually increase the opacity of the clouds, inducing a positive forcing that would ultimately exacerbate the greenhouse effect.

The conclusion from the studies on cloud brightening and cirrus thinning that have been conducted is that there is a large sum of ambiguous and unknown characteristics of the aerosol interactions with the atmosphere. However, there are other SRM techniques that involve changing albedo that do not deal directly with the atmosphere.

Could we genetically engineer vegetation?

Earth albedo is dependent on clouds, glaciers, sea ice, and also by sands, grasslands, and trees. Objects vary in degree of albedo, but they all contribute to the Earth’s radiation budget. One method of geoengineering involves genetically engineering croplands, grasslands, and other vegetation to have a higher albedo. These species would then replace the native species.

Although this method does not propose as many unknowns on the atmosphere as cloud brightening, it does introduce potentially very harmful feedbacks on the ecological systems of Earth, such as lower carbon uptake, lowering biodiversity, and altering photosynthetic activity. However, some models predict that a decrease as high as 5 W/m^2 could be achieved if grassland albedo was 25% higher than it currently is.

Other avenues of this albedo modification include that of urban areas, deserts, and the ocean surface. The ocean surface albedo could be increased with microbubbles layered on the top, but there have not been any studies in to the feasibility of this or what the complete effects of a sustained microbubble layer would have on ocean circulation or air-sea interactions.

Comparatively, albedo increases in urban areas would not be as difficult to maintain and would be a feasible technique to aid SRM. Increasing the albedo of roads, transforming vacant lots into gardens, and whitewashing roofs of all buildings are relatively cost effective in light of other SRM methods, and they are more manageable by cities. In terms of a low risk form of SRM, modifying urban albedo is at the top of the list. However, there is still relatively little information on the effectiveness and feedbacks of these various SRM methods.

Summary of Problems with Solar Radiation Management (SRM)

There are many avenues which make investing in solar radiation management difficult. For example, there are numerous types of models that vary in evaluation techniques and in model design, which makes comparison between studies problematic. In addition, the quantity of studies is relatively scarce, so results vary from very encouraging results from SRM techniques to ambiguous results to producing positive feedbacks on the climate system. Adding to these existing issues is the gap in understanding of atmospheric properties, such as cloud-aerosol interactions and air-ocean surface interactions, and in ecological, terrestrial, and hydrologic Earth systems. Nevertheless, there may be supportive reasons for some SRM methods.

Could we use models to better understand SRM?

Although there are many models on SRM that do not perfectly align, the Geoengineering Model Intercomparison Project (GeoMIP) is currently working on systematically evaluating a range of models to give idealized SRM scenarios. Some of the models from this project suggest a decrease of 3-4 W/m^2 in global temperature. However, some discrepancy comes from the balance of radiative forcing and CO₂ removal.

For example, radiative forcing only influences the climate system during the daytime, while CO₂ removal has an impact over the whole diurnal cycle, and this imbalance could have unknown effects on surface temperature. Also, it is not well known if strong SRM techniques could have a larger impact long-term over the climate system, or if CO₂ removal techniques will produce more effective results. A prominent argument against SRM focuses on the longevity of CO₂, which will last for the next one-thousand years in the atmosphere.

To effectively counteract the influence of increasing CO₂ emissions and their greenhouse effect on Earth, SRM techniques would also need to be in place for the same time period. Looking back on humanity’s history, this does not seem very reasonable. Current worldwide political regimes may agree to support an intense SRM project, but these same political regimes can change power and withdraw their financial support for SRM projects 4-10 years later. The United States’ relationship with the Paris Climate Agreement is evidence of this political behavior. It is quite unlikely that SRM techniques could be maintained based on the lack of global unity in the past and in the present day.

In a model run by GeoMIP, SRM projects ceased after 50 years. In this scenario temperatures associated with the projected levels of CO₂ are present again within 1-2 decades. This will amplify the rate at which temperatures increase, which will be a higher rate than if no SRM methods had been implemented at all. Ecological systems, precipitation rates, and human activity would be largely impacted by the rapid temperature increase. Some studies suggest that very aggressive CO₂ removal methods coupled with timid SRM techniques could produce a strategy with less intense positive temperature feedbacks.

Um, can I get a summary of that?

Solar radiation management techniques propose to help reduce the temperature rise due to increased CO₂ and other greenhouse gas emissions.

Some SRM models predict that temperature could be reduced by SRM countering impacts on sensitive parts of the climate, such as sea ice, global temperature, and precipitation. However, there are many unknowns in the physical interactions of the Earth system and in the sustainability of SRM techniques.

Processes like cloud-aerosol interactions or land-ocean interactions with the atmosphere are not still completely decoded, so implementing SRM methods could actually enhance the greenhouse effect and bring ecological devastation to certain regions. Additionally, SRM techniques are not well studied enough to properly evaluate costs and feasibility of projects. SRM techniques may not be very effective in reducing temperatures globally, yet plenty of time and monetary funds may be dedicated to them instead of focusing on CO₂ removal methods.

Some low risk SRM techniques, such as space reflectors or increasing urban albedo, are good stepping stones, but the physical interactions of the atmosphere and the lack of statistically sound models on SRM do not recommend implementing most of the current ideas for solar radiation management.

The risk of a strong positive feedback in response to SRM techniques currently largely outweighs the possibility of reducing temperature through SRM. In this case, it would be better to do nothing than to accidentally enhance the greenhouse effect due to lack of knowledge concerning the Earth system.

Want to know more?

• IPCC. (2013). Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 1535 pp.
• Seinfeld, J. H., & Pandis, S. N. (2016). Chapter 24 Aerosols and Climate. In Atmospheric Chemistry and physics: From air pollution to climate change, 3R. essay, John Wiley & Sons.
• Wallace, J. M., & Hobbs, P. V. (2011). Chapter 6 Cloud Microphysics. In Atmospheric science: An introductory survey. essay, Elsevier Acad. Press.