A new proposal for climate intervention is gaining traction in the scientific community. The idea involves injecting sunlight-reflecting aerosols into the Earth's stratosphere to delay or reduce the transmission of heat from the sun into our atmosphere. But as climate interventions go, geoengineering isn't easy.
"Teragrams of material, such as sulfur dioxide (S02), will need to be prepared annually, brought to altitudes of 20 km (12.4 miles) or higher, and dispersed efficiently in the stratosphere," Karen Rosenlof and Ru-Shan Gao of the NOAA Chemical Sciences Laboratory told Interesting Engineering in an email. "Technological development is needed for all these steps."
This raises the question: how can we lift enough aerosols into the atmosphere in an efficient, sustainable way? Rosenlof, Gao, and their colleague Pengfei Yu, a research assistant at the University of Colorado, suggest mixing some black or brown carbon particles into the aerosols, which would naturally "loft" them into the ideal altitude upon heating from the sun, according to a new study published in the journal Science Advances.
This solar-powered method could significantly lower the necessary delivery altitude for Stratospheric Aerosol Injection (SAI), to heights achievable even with modern-day aircraft — bringing us substantially closer to a viable means to combat climate change.
No, there's no proof this will lead to a 'runaway freeze' of the global climate
Mixing brown or black carbon particles into the material used to create aerosols could lower the injection altitude, since the composite material would absorb solar radiation, become heated, and "loft" into the stratosphere. "Our study assumes use of black carbon (BC), which is a pure carbon aerosol," explained Rosenlof and Gao in the email to IE. "[W]e did not identify a specific type of brown carbon (BrC); these particles are less well understood. They are made of various organic molecules that absorb visible light, but less strongly than BC."
Of course, to make it happen, policymakers will need a lot of sulfur. Annually, "somewhere between 1 to 10 teragrams" — but the amount will vary depending on the desired surface temperature of the planet. In January, Microsoft Founder, Billionaire, and recent bachelor Bill Gates endorsed a similar proposal from Harvard University to test methods of blocking direct sunlight, in a geoengineering project called Stratospheric Controlled Perturbation Experiment (SCoPEx).
But the new study extends this idea, focusing on "a technique that would allow injecting the climate intervention material at a lower altitude achievable by current aircraft," said Gao and Rosenlof.
However, the new study's proposal for efficient, black carbon-assisted delivery is not without drawbacks. There remains the need to grasp which scenarios will be most efficient, in addition to a need to understand what side-effects on surface weather and climate will be, according to Rosenlof and Gao. It's important to remember that as a form of geoengineering, injecting aerosols into the air represents a level of planetary manipulation never seen before. And it won't be localized to just the United States. "The implications are awe-inspiring, and the consequences have the potential to be severe," read a blog on the Geoengineering Model Intercomparison Project.
"Global modeling studies have looked at the impacts on the stratospheric ozone layer and surface weather patterns," said Gao and Rosenlof. "There also may be changes in high altitude clouds, or the sky may appear whiter, while sunsets may be more colorful." Some of you may immediately feel reminded of popular films of the last several decades like "The Matrix" and "Snowpiercer", which depicted runaway freezing effects in the aftermath of overzealous geoengineering. But as of writing, the researchers "are not aware of any studies that show any runaway effects due to SAI."
Injecting sulfur into the stratosphere is a temporary form of geoengineering
The research team's novel proposal was inspired by the 2017 wildfires of the U.S. Pacific Northwest, when massive amounts of black carbon particles were lifted high into the Earth's stratosphere — where they remained for months. Looking at this, the researchers noted that similar black or brown particles might absorb sufficient solar energy to lift the surrounding aerosol particles high enough to reflect sunlight effectively. And this effect will remain in place long after the absorbing carbon dissipates due to oxidation.
This is a paradoxical proposal to consider: on one hand, it could serve as a viable way of delaying or deferring the effects of climate change — preserving a biosphere more friendly to life as we know it. But while there remain many unknown variables, it's perhaps comforting to know that even this level of geoengineering isn't permanent. "Previous large volcanic eruptions have demonstrated that material injected into the stratosphere has a finite lifetime," said Gao and Rosenlof. "Some Mount Pinatubo aerosol (from the large eruption in 1991) was present in the stratosphere for approximately 3 years," with a global impact extending a mere two years. The Earth's natural circulation of air "flushes injected material out of the stratosphere within a few years." If or when we "blot out" the sky, we can at least enjoy the novelty of a slightly whiter sky, with striking red sunsets and captivating sunrises.