As the United States and other countries continue to pledge to reduce emissions – with the ultimate and lofty goal of reaching net-zero by 2050 – a growing number of researchers are studying courses of action to ward off the worst effects of warming, should mitigation and adaptation prove insufficient.
Solar geoengineering – an umbrella term for different approaches to increase the Earth’s reflectivity to offset the effects of accumulated gases in the atmosphere – was an obscure field 20 years ago. Now, it is a subject of growing interest and study among scientists and governments around the world.
Thinking that solar geoengineering could be part of a portfolio for addressing a host of unknowns in a changing climate, the National Academies of Science Engineering and Medicine has recommended that the U.S. government spend up to $200 million over the next five years to further research the subject.
In a March 25 report entitled “Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance,” the National Academies stated that, “given the urgency of the risks posed by a rapidly warming planet,” the federal government should pursue research of solar geoengineering in coordination with other nations, subject to governance, and alongside a robust portfolio of climate mitigation and adaptation policies.
Solar geoengineering is currently being researched in the United States, Australia, India, China, Sweden, Norway and Germany – with Australia now conducting physical experiments in the lower atmosphere and Harvard University preparing for physical experiments of its own.
While mitigation and adaptation currently serve as the only widely acceptable options to address the effects of warming, “we might need a Plan B,” said AAPG Member James Conca, a 33-year Earth and environmental scientist and consultant for federal and state environmental and energy agencies as well as industry. “Plan A – which is decarbonization – is not working. By 2100 we might be able to get a handle on this. But we might have to put off warming until then.”
A Plea for Research
The most researched form of solar geoengineering is stratospheric aerosol injection, which essentially reproduces the effects of a volcanic eruption by releasing reflective particles into the atmosphere that are known to temporarily cool the planet. A method known as marine cloud brightening is being studied and tested and involves releasing sea water droplets in the lower atmosphere over the ocean to stimulate the thickening of clouds to increase their reflective properties.
At this time, opinions on solar geoengineering with regard to its scientific, geopolitical and ethical ramifications run all over the map. But one strong and common voice has emerged from those who support its study: Solar geoengineering should never be a substitute for reducing emissions or for trying to adapt to a warmer environment. Rather, it is a tool that – if supported by ample research – could become an option for cooling the atmosphere, should other efforts prove insufficient.
“It’s very plausible that in 10 to 15 years, we could see climate impacts that are more severe. I would rather be in a position at that point to support informed decisions about our options,” said Doug MacMartin, a senior research associate and senior lecturer in mechanical and aerospace engineering at Cornell University. “Those decisions need to be informed by the best science we have.”
Currently, the majority of research on solar geoengineering in the United States is funded by philanthropic donors.
Making a case for further study, David Keith, a professor at Harvard University’s John A. Paulson School of Engineering and Applied Sciences and at the Harvard Kennedy School, explained that reaching net-zero emissions in 2050 or 2070 will not stop the effects of warming – it will simply stop the problem from getting worse.
Speaking at a 2019 conference of the Institute of International and European Affairs that was published on YouTube, Keith noted that the global agreements in place to reduce emissions are not working. According to the U.S. Energy Information Administration, emissions are expected to increase globally by at least 0.6 percent each year through 2050.
“At this point I am not advocating for doing solar geoengineering,” Keith said. “I’m advocating for having a much larger, international, serious and open-access research program.”
Had successful efforts to curb emissions begun 10 or 15 years ago, solar geoengineering would likely not have moved to the forefront of discussions, MacMartin said.
“It’s pretty obvious that climate impacts are going to get worse than they are now. We are already seeing it with forest fires, hurricanes and droughts,” he said. “That’s what’s causing people to say, ‘Let’s take these ideas about solar geoengineering more seriously.’”
The Volcano Effect
Scientists have long known that when a volcano erupts, it releases large amounts of sulfur dioxide into the stratosphere. The resulting sulfate aerosols can remain for several years, creating a reflective global veil. When Mount Pinatubo erupted in 1991, the Earth cooled 1.3 degrees Fahrenheit, according to the U.S. Geological Survey.
If humans were to reproduce that scenario, aerosols the size of a micron would be released into the stratosphere where they would remain for roughly a year, Keith explained in his presentation.
“The stratosphere naturally has aerosols in it, including sulfate aerosol, so the most obvious idea is to increase the concentration of those sulfuric acid aerosols to reflect away a little more sunlight,” he said.
Of the roughly 500 papers written on solar geoengineering in the United States, none show that uniform solar geoengineering, if done at a moderate level, and if done in combination with emissions cuts as a supplement, could have “bad results”, Keith said.
Yet, a host of questions remain: Can aerosols be evenly dispersed? How greatly would they affect the ozone layer, contribute to air pollution or alter rainfall patterns?
“Some of the obvious questions about side effects have at least been looked at. It doesn’t mean we know all the answers,” Keith said. “It certainly doesn’t mean we can make informed decisions about deployment today, in my opinion.”
Keith believes that solar geoengineering is rapidly growing in relevance because of a steady accumulation of knowledge over the years and, most importantly, as scientists take in the results of climate modeling experiments, “these ideas look better than they did before – in the sense that as we looked, we found less risks than we thought – and the technologies look more effective in climate models than we thought,” he said.
Ready to take their research to the next level, Keith and his team have been working on launching Harvard’s SCoPEx – a Stratospheric Controlled Perturbation Experiment – to begin small-scale, controlled atmospheric experiments.
The first step is a balloon flight to test the SCoPEx platform without the release of particles, as reported on Harvard’s School of Engineering and Applied Sciences website. If approved by an independent advisory committee, the team will look to later release calcium carbonate, a common mineral dust, or other materials such as sulfates in response to evolving scientific interests. They expect “no significant hazard” to people or the environment.
The goal is to “improve the fidelity” of simulations of stratospheric aerosol injection by providing modelers with experimental results. SCoPEx will make quantitative measurements of aspects of the aerosol microphysics and atmospheric chemistry that are currently highly uncertain in the simulations.
A propelled balloon will allow scientists to create a small, controlled volume of stratospheric air and observe its evolution for an anticipated 24 hours.
The Harvard team formally asked its independent advisory committee to review its plans for a proposed platform test in collaboration with the Swedish Space Corporation in June 2021. Yet in March 2021, the advisory committee recommended that the platform test be suspended until a more thorough societal engagement process can be conducted to address issues related to solar geoengineering research in Sweden.
Over the Ocean
In an attempt to protect and restore Australia’s Great Barrier Reef, scientists at Southern Cross University, the Sydney Institute of Marine Science, the University of Sydney and Queensland University of Technology are conducting experiments to test the efficacy of marine cloud brightening.
While not as researched as stratospheric aerosol injection, marine cloud brightening is based on the fact that clouds located approximately 800 meters over the naturally dark ocean can be highly reflective.
The ocean absorbs approximately 95 percent of radiation, said Tom Ackerman, former director of the Cooperative Institute for Climate, Ocean and Ecosystem Studies at the University of Washington. When microscopic sea water droplets are sprayed into the air, they evaporate and leave behind nano-sized sea salt crystals, which can act as seeds for cloud droplets, brightening existing clouds and deflecting solar energy.
At present, scientists have “indirect evidence” that marine cloud brightening is effective, based partly on observations of smokestacks from ships that vent particles into clouds, making them brighter, Ackerman said.
“We know this works, but the real question – this is the $64-trillion question – is, can you actually do this in a controlled manner?” he said. “We have more evidence about the effects of aerosols than we do about marine cloud brightening.”
At least for now.
In March 2020, just before the pandemic, Australian scientists tested prototype equipment for marine cloud brightening off the coast of Townsville in Far North Queensland.
The team worked from two large vessels that carried a prototype cloud brightening machine and air sampling equipment that normally never leave the laboratory, stated an April 17, 2020 news release from Southern Cross. Instruments weighing more than 80 kilograms were loaded by crane through the vessel roof.
The team deployed a sprayer and observed the enhanced production of aerosol particles, though not yet in marine clouds themselves.
“We tested the hypothesis at one-tenth of the scale we’re aiming for, using a drone in the atmosphere and a sampling vessel 5 kilometers away on the sea surface and showed how we can successfully create hundreds of trillions of these sea salt crystals per second, which float up into the atmosphere to bolster the reflectivity of the existing clouds,” said Daniel Harrison of Southern Cross, who led the government-funded effort.
“Cloud-brightening could potentially protect the entire Great Barrier Reef from coral bleaching in a relatively cost-effective way, buying precious time for longer-term climate change mitigation to lower the stress on this irreplaceable ecosystem,” he said.
The team plans to triple the size of the experiment this year and scale it up to ten-fold in 2022, which researchers say should be able to brighten clouds across a 20-by-20-kilometer area.
Unlike stratospheric aerosol injection, scientists believe that marine cloud brightening has fewer unforeseen consequences. The particles added to marine clouds remain in the lower boundary layer of the atmosphere for just a few days to a week, Ackerman said.
Still, some scientists seem to favor stratospheric aerosol injection because they better understand the physics, and it is known that aerosols can cool the planet.
“We know in principle how to do it,” MacMartin said.
More physical uncertainty exists with marine cloud brightening. “I can imagine doing marine cloud brightening over smaller regions,” he added, “but stratospheric aerosol injection has more of a global effect.”
Diving into the Deep
If research on solar geoengineering continues to grow and prove effective without unwanted consequences, some say nailing down the science will be the easiest part toward possible deployment.
Geopolitical and ethical considerations will likely be much harder to solve. Getting all countries on board would no doubt be a major challenge, not to mention strategies on how to govern and monitor the process.
In terms of ethics, all scientists must abide by the No. 1 rule, which is to not do harm, Ackerman reminded.
“Maybe it is unintended consequences, but we are already doing something by letting CO₂ increase,” he said. “What’s ethically allowed here may not be allowed in other places. Is this the beginning of a slippery slope?”
Adding to the list of concerns is possibly facing a “moral hazard,” MacMartin said. “Some countries might say, ‘Great! Now we don’t need to cut emissions because we can solve climate change this way instead.’”
If countries deploy various forms of solar geoengineering and slow their efforts to reduce emissions, a dependency is immediately created to keep atmospheric temperatures from soaring, he said.
Furthermore, solar geoengineering will not solve all climate-related issues, namely ocean acidification, MacMartin added. Ocean acidification can only be solved by lowering emissions.
As more countries dig deeper into these subjects, it will likely become obvious that both stratospheric aerosol injection and marine cloud brightening could be deployed unilaterally – without global consensus.
In his presentation, Keith noted that stratospheric aerosol injection would require roughly 50 aircraft based in the Tropics flying approximately 20 kilometers high to disperse aerosols – making it possible for many countries to act alone.
Ackerman said that countries also could argue that performing marine cloud brightening in their own waters is outside the jurisdiction of others.
A murky situation at best, much must still be discussed on a myriad of levels before solar geoengineering can be considered a viable option, should it ever be needed. But nevertheless, many believe the research must move forward.
“If the government does nothing to research this, and at some point conditions become unbearable and climate engineering becomes necessary, what will we do? We have no controlled scientific experiments. We don’t want 50 different groups trying different experiments in the atmosphere, not knowing their effects and possibly making things worse,” Ackerman said. “We need to start doing research yesterday.”