Geoengineering the Climate System
Can Control Points in the Climate System Be Manipulated Safely? (Modified 17 October)
(Modified 17 October 2021)
This Substack site addresses what we know about weather and climate, and how we know it. Developing these essays has led to an appreciation of the amazing technologies that have been deployed, and the international networks of researchers using them, that let us “see” the dynamics of the climate system in real time and envision our climate future. I have purposely avoided dealing with economics, politics and policy, having no expertise in those areas, and knowing how many other voices there are out there on those topics. Hopefully, focusing on the science helps to establish a solid technical foundation on which policy discussions can be built.
But the topic of this essay may bring us to a grey area where social concerns direct what kind of science is allowed to occur. Social and ethical limitations on research have a long history in many fields, so the idea of balancing risk and benefit is well-established, if exceedingly complex.
What about climate research? My previous essays have focused on recent, sophisticated research using advanced technologies to measure the net radiant energy balance of the Earth, and the complex patterns of global ocean currents. I’ve also reached back into the 1800s and the early calculations by Svante Arrhenius on the impact of carbon dioxide on global temperature.
All of these have been built on observations of the system as it currently operates. None addressed proposed research that involved active intervention into the climate system.
A recent editorial by David Keith prompted a renewed personal interest in the idea of geoengineering a solution to our climate change crisis, and generated the topic for this essay. In that article, he emphasizes the urgent need to reduce the rate of global warming, and while supporting efforts to reduce greenhouse emissions, or enhance removal, suggests that the long-term, slow-response process of remaking the energy economy might take too long. The alternative he offers is geoengineering – specifically using tiny sulfur-based particles injected into the upper atmosphere to reflect sunlight. The advantages he mentions include that the effect would be both immediate and temporary, buying time for the required but longer-term reductions in greenhouse gas emissions.
The concept of injecting materials into the atmosphere to modify weather is not a new one, and can be traced all the way back to the centuries-old efforts to “seed” clouds to increase local rainfall. While making for some good movie scenes, there is little evidence that local rain or snow can be increased significantly in this way, but research continues and applications are attempted.
In the global climate context, geoengineering generates heated debates about both the potential to slow global warming through this technology, and the potential environmental side effects. An excellent recent article by Rachel Kaufman puts the risks and rewards in perspective by reporting on conversations with a number of atmospheric scientists both affiliated with and outside of ongoing research efforts. Two reports from the National Academy of Sciences in 2015 and 2021 capture both the detailed science and the nature of the debate on what research might be done. These sources provide much of the information summarized here. Others are listed at the end of this essay.
With the goal of exploring the science and not the policy, let’s ask these 4 questions about geoengineering increased reflectance of sunlight to reduce global temperatures: How does it work? Can it be Effective? How could it be implemented? What are the environmental impacts? We will ask the same questions about a proposed methods of bioengineering – fertilizing the oceans with iron – in the next essay.
First, some context about the Earth system and why subtle perturbations to that system might affect major change.
A goal in the study of any complex system, be it climate, forests, or the global economy, is to identify control points. Where are the key interactions that control, or at least have the biggest impact on, the direction of the whole? Which knobs do you turn if you want to manage that system. Think of the ways that interest rates or money supply are used to speed up, slow down, or even rescue the economy.
The shopworn but useful example of a control point in a complex system is your home heating/air conditioning system. That system is connected to an energy source, be it electric, gas or oil, provided by a very complex system of energy production and delivery. Even if you are off-grid with solar or geothermal energy, the supply chain that made it possible to construct that system is still complex. Once the system is activated, there is another fairly complex system of pipes, ducts, fans, radiators, or whatever that uses the energy delivered to your home to warm or cool.
And what determines how that system affects the temperature in your home? It is that little box on the wall, the thermostat, that signals when the system should turn on and off, and maybe even in which mode it should operate – heating or cooling. A simple twist of the dial controls the actual operation and effectiveness of that entire complex system.
Geoengineering is based on the idea that we can identify one of these “knobs” and define a minor intervention into the climate system that will allow us to control the direction or intensity of climate change without serious side effects or unsupportable expense. But there is a crucial difference to keep in mind between geoengineering and your thermostat.
You know precisely how the thermostat works and what changing it means. As a human construct its responses are clearly defined. While we know enough about the existing climate system to predict the direction and rate of change, introducing a new process or element into a natural system can produce unexpected results. And as the Earth is an integrated and complex system, how you turn those knobs can have ramifications for biology and chemistry as well as for the physics of the atmosphere and oceans. Our fourth question on environmental impacts will try to address some of those unintended consequences.
The most common proposal for geoengineering global climate is to distribute into the upper atmosphere tiny sulfur-containing particles, called aerosols, that have the demonstrated effect of reflecting sunlight.
So lets ask our four questions.
How does it work?
The temperature of the atmosphere and the surface of our planet is determined by the balance between incoming solar radiation (the top yellow arrow) and the combination of loss of infrared or heat radiation to space (top red arrow) and reflectance of the atmosphere and surface (the curved yellow arrow). An amazing set of technological innovations, covered in a previous essay, allows the direct measurement of this balance, and substantiates that the net balance is positive (energy is accumulating, temperature is increasing).
Reflectance (or albedo to use the technical term) is determined by many factors. The reflectance of the land surface of the Earth changes with conversion of forests (low reflectance) to bare ground (high reflectance) or from ice (very high reflectance) to water (very low reflectance). Proposals for modifying surface reflectance are many and varied (white roofs, land management, etc.). Concern runs high over the continuing loss of ice cover in a warming world, especially in the Arctic Ocean, and how that could decrease reflectance, accelerate temperature increases, enhance melting, and accelerate the rate of warming of the Earth.
In the diagram, the goal is to increase the size of that curved yellow arrow above the current value of 100 by injecting sulfate aerosols into the upper atmosphere. Some models suggest that increasing reflectance by 2% (102 versus 100) could counteract much of the effect of greenhouse gas warming.
Aerosols are tiny particles that can remain suspended in the air for relatively long periods of time. They have different properties depending on the physical and chemical characteristic of the particle. The sulfur-containing particles that are proposed for use are known to have high reflectance. Delivering these aerosols to the upper atmosphere can increase reflection of incoming sunlight and cool the planet.
Can it be effective?
There is very good evidence that sulfur aerosols can increase reflectance and reduce temperatures. The best global-scale examples come from the largest volcanic eruptions that inject millions of tons of sulfur (and many other compounds) into the upper atmosphere. This table summarizes the estimated (often approximate) impacts of four of the largest eruptions over the last 240 years, including the amount of sulfur dioxide emitted (SO2, the oxidized form of the element also emitted by coal-burning power plants), and the estimated impact on global temperatures (see Sources at the end for references).
In my home region of New England, the winter of 1816, following the Tambora event, was known as “eighteen hundred and froze to death.” Frost occurred in every month in some areas and crop production was affected over much of North America and Europe. The event has been immortalized in folklore and song, while more serious reports document the human suffering resulting from severe cold and loss of crop production. The acclaimed climate scientist, James Hansen, famously, and accurately, predicted that the eruption of Pinatubo in 1991 would deflect increasing global temperatures for a number of years.
The most recent reports on the current energy balance of the planet include as much as a 0.5 degree Celsius net impact of the current aerosol load in the atmosphere resulting largely from human activity. Some of this is through changes in direct reflectance and some is through alteration of cloud formation.
How could it be implemented?
Aerosols have a temporary impact on reflectance. The table above on volcanic eruptions and the detailed descriptions in the 2015 report from the National Academy of Sciences, suggest that major effects are reduced within a year or two, although some impacts appear to be detectable up to 10 years out. As denser-than-air particles, gravity eventually wins the battle against air currents and turbulence that would keep these particles afloat. Implementation of a strategy to reduce global warming using aerosols would then require a program of continuous additions to the upper atmosphere.
How much would be required? Modeling studies suggest the addition of 2.5 Million tons per year of sulfur as sulfur dioxide. Dr. Keith puts the number at about 2 million tons of sulfur per year. Methods for distribution into the atmosphere are many and varied, but it seems that the technology of distribution is not a major impediment. There is much discussion about the optimal timing and location of release.
What are the potential environmental impacts?
So if it seems that the Earth could be cooled by this intervention, and if the effect is known to be temporary, why not try it? That is the argument in David Keith’s editorial which addresses many topics beyond the scope of this essay, including impacts on economic productivity and the distribution of wealth. He favors testing this idea and is involved in research to that end.
Questions on the chemical and climate impacts of the proposed intervention abound, as reported in the article by Rachel Kaufman and the National Academy documents.
The industrial world has worked long and hard to reduce sulfur emissions and the deposition of the resulting sulfuric acid to land and water. Since 1980, major changes in energy systems have been made to reduce emissions by as much as 80% in Europe and North America. Would injecting sulfur into the upper atmosphere reverse these gains?
For perspective, current human emissions of sulfur to the atmosphere are about 45 million tons, down from about 75 million tons in 1980 (these are values for the amount of sulfur – data reported as sulfur dioxide [SO2 ] would be twice these values). Current estimates are that these human emissions are about 4 times greater than natural sources.
An additional 2 million tons would represent about a 4-5% increase in global deposition. The impacts of acid rain were most severe in regions immediately downwind from major sources. The sulfur in aerosols added to the upper atmosphere could be expected to be deposited at lower concentrations over much wider areas.
While these numbers suggest minimal direct environmental impacts from the added sulfur, both of the National Academy reports describe a host of additional consequences. These include changes in the amount and distribution of precipitation, impacts on the ozone layer, reductions in solar energy delivered to solar energy collectors and native ecosystems. All of these rely on existing models as the only global-scale observations available are from the volcanic eruptions listed above and other smaller events.
One interesting figure in the 2015 report models the impact on climate should this geoengineering approach be enacted and then suddenly terminated. Essentially, the Earth would return to the rate of warming being driven by greenhouse gases within a short period of time. The annual rate of change following cessation of aerosol additions would be greatly accelerated.
So this intervention would be temporary, perhaps buying time to solve greenhouse-gas-driven climate change, but not offering a permanent solution. It is treating the symptom, but not the cause.
There is a difference in focus between the two National Academy reports. The 2015 document describes the science of both atmospheric reflectance and unintended consequences in great technical detail. The 2021 report places more emphasis on the need for international collaboration, regulation and very strict controls on any proposed research.
Context and Current Status of Research
We have been geoengineering the planet for centuries. The byproducts of industry and agriculture have altered air and water resources in uncounted ways. The emission of greenhouse gases has geoengineered our current climate crisis. Through decades of research and practice, good science applied to critical issues has allowed us to reverse-geoengineer the impacts of serious pollutants. This has led to progress on some major issues, including acid rain and the ozone hole. In those cases, a focus on the detailed science of the processes affecting strong acids in precipitation and soils, and the formation and breakdown of ozone, have driven the ultimate policy decisions and protocols. Constructive international collaboration was also essential to realizing those outcomes.
The long-term solution to the increase in greenhouse gases in the atmosphere is a fundamental reorganization of the energy and agricultural systems of the planet. Is geoengineering in the climate context a potential temporary fix? Could it work, and be used without deflecting efforts to reduce greenhouse gas concentrations?
A recently planned, small-scale test of a balloon system that might be used to distribute aerosols was halted, even though no distribution of aerosol was to occur, essentially because the research was part of a program to eventually test geoengineering of atmospheric reflectance. Perhaps an opportunity to learn something about wind currents and balloon technologies was lost by this action. Perhaps, by ruling out an entire category of research, we are also missing an opportunity to learn something about the energetics and dynamics of the atmosphere.
It seems that there is a question of scale here. Should we disallow small-scale research because the application of the results on a global scale might be more detrimental than helpful? Current summaries suggest there are lots of unknowns in geoengineering the energy balance of the planet. Experimentation under controlled conditions is one way to reduce those uncertainties.
Some sources have suggested that reflectance might be useful or needed in a “climate emergency” where the climate system reaches an unpredicted tipping point and the rate of change accelerates beyond expectations. Will there be a point where the immediate risks of climate change might argue for direct, short-term intervention to delay warming while the longer-term and essential restructuring of the global economy occurs? If that should occur, would it not be best to know as much as we can about the technology before it is applied?
Actual application of reflectance to geoengineer climate would require international agreements and logistics, topics far beyond the scope of this site. On the science side, it might be useful to learn more about the process before contemplating its use.
Sources
The editorial by David Keith is here:
https://www.nytimes.com/2021/10/01/opinion/climate-change-geoengineering.html?searchResultPosition=1
The article by Rachel Kaufman is here:
The National Academy of Sciences reports are here:
https://www.nap.edu/catalog/18988/climate-intervention-reflecting-sunlight-to-cool-earth
https://www.nap.edu/download/25762
One visualization from NASA of the disappearance of artic sea ice is here:
Some background on cloud seeding can be found here:
https://en.wikipedia.org/wiki/Cloud_seeding
and results of a recent experiment are here:
Background information on geoengineering with sulfur aerosols is here:
https://en.wikipedia.org/wiki/Stratospheric_aerosol_injection
https://en.wikipedia.org/wiki/Stratospheric_sulfur_aerosols
https://en.wikipedia.org/wiki/Solar_geoengineering#Stratospheric_aerosol_injection
Information on the four major eruptions listed in the table can be found here:
https://www.usgs.gov/natural-hazards/volcano-hazards/volcanoes-can-affect-climate
https://en.wikipedia.org/wiki/1991_eruption_of_Mount_Pinatubo
https://en.wikipedia.org/wiki/1883_eruption_of_Krakatoa
https://en.wikipedia.org/wiki/Year_Without_a_Summer
https://en.wikipedia.org/wiki/Laki
https://volcano.oregonstate.edu/laki-iceland-1783
https://historydaily.org/eighteen-hundred-and-froze-to-death-the-year-without-summer
The image of changes in temperature in Europe following Tambora is here:
https://upload.wikimedia.org/wikipedia/commons/d/d8/1816_summer.png
One estimate of net global warming due to aerosols can be found here:
https://www.epa.gov/climate-indicators/climate-change-indicators-climate-forcing
Data on total sulfur dioxide emissions globally and by country are here:
https://ourworldindata.org/grapher/so-emissions-by-world-region-in-million-tonnes
The estimate that 80% of sulfur dioxide emissions to the atmosphere is by human activity, and 20% from natural process such as volcanic eruptions, is here:
Estimates for Ecosystem Light Deprivation and Inhibition of Solar Technologies are here:
https://en.wikipedia.org/wiki/Stratospheric_sulfur_aerosols#Climate_engineering
The two articles describing the cancellation of a test balloon flight related to aerosol distribution are here:
https://www.nytimes.com/2021/04/02/climate/solar-geoengineering-block-sunlight.html