Congratulations. You’ve done everything humanly possible to cut carbon dioxide—to zero. But what if even that won’t be enough?
It’s one of the most uncomfortable realizations in climate research. Inertia in the climate system implies that even if emissions stopped, temperatures and especially sea levels would continue to rise for a long time. The logical conclusion leads almost immediately to the specter of solar geoengineering, an attempt to use technology to reflect a portion of sunlight back into space. The principle behind solar geoengineering is simple enough. With less sunshine coming through the atmosphere, the planet would invariably cool—and fast. At least temporarily. There’s even a natural analogue: the eruption of Mount Pinatubo in the Philippines. In June 1992—ironically, the same time as the pivotal Rio de Janeiro Earth Summit—global average temperatures were about 0.5C cooler than they would have been without all the ash and sulfur dioxide, SO₂, catapulted into the lower stratosphere by the volcano a year prior.
Alas, the millions of tons of gunk from Mount Pinatubo soon fell out of the stratosphere, temperatures shot back up—and they’ve been increasing since.
That leads to another thought experiment. What if some entity, be it an international body or a lone nation, decided to use large-scale tech to re-create the cooling effects of a volcanic eruption? The engineering would be straightforward: release SO₂ near the equator about 20 kilometers (12.4 miles) up into the stratosphere. The SO₂ would turn into tiny reflective sulfate particles that would spread around the globe within weeks and linger for months. A bit of sunlight would be reflected away, and everything down below would be cooled.
This is the premise of solar geoengineering via stratospheric aerosols. It’s fast. Unlike cutting CO₂, adding SO₂ cools the Earth within weeks, not decades. It’s powerful. Millions of tons of SO₂ could help offset the global warming effects of hundreds of billions of tons of CO₂. It’s also highly imperfect and risky. It’s akin to adding one type of pollution (SO₂) to help counter the effects of another pollutant (CO₂). Think of it as an experimental drug taken in a pandemic. It might show promise, but watch out for unknown side effects.