The impact of SO2, including its effectiveness and atmospheric residence time, varies based on its deployment location (latitude, longitude, altitude), concentration, and particle size.
For instance, the International Maritime Organization (IMO) 2020 regulations have reduced SO2 emissions from cargo ships in the troposphere, leading to decreased respiratory illnesses near ports and less acid rain. However, this reduction in SO2 has also warmed ocean shipping lanes, prompting discussions about reintroducing sulfur into the troposphere. [1]
Deploying SO2 in the stratosphere, above most of the atmospheric water vapor and where winds reach speeds of 200kph, allows it to spread globally and remain airborne longer (1 to 3 years). This higher placement necessitates less frequent applications for the desired reflective effect. The 1991 eruption of Mt. Pinatubo demonstrated this, injecting 20 million tons of SO2 into the stratosphere [2], cooling the Earth by 0.5°C and, according to some models, temporarily reversing decades of warming.
Stratospheric aerosol injection (SAI) is likened to Earth's sunscreen, [3] a temporary measure to reflect the Sun's energy and mitigate warming while we address the larger challenge of removing over a trillion tons of greenhouse gases emitted since the 1850s and transitioning away from fossil fuels.
Climate change demands immediate action, and SAI offers us the critical time needed to live in a world with fewer catastrophic climate events.
[1] https://docs.google.com/document/d/1vGEDuEn6uikvGZpR9oZplPPh...
[2] For reference, estimates suggest that global SO2 emissions were around 131 million tons in 1970 and continued to rise, peaking at approximately 150 million tons by the late 1980s in the troposphere.
[3] https://twitter.com/MakeSunsets/status/1741336520366444822