Could humans cool the planet by copying what a volcano does?

Probably, at least in part, but nobody knows yet how well or how safely. The idea is called stratospheric aerosol injection: deliberately putting tiny particles, aerosols, high into the stratosphere to cool the Earth through the same dimming and extra reflectivity a big volcanic eruption causes. The IPCC calls it the most-researched method of its kind, with high agreement that it could limit warming to below 1.5 °C. But like every solar geoengineering approach, it would work imperfectly, and other effects are possible, especially if it were used badly.

The model is real. In 1991, Mount Pinatubo erupted and cooled the whole planet by about half a degree Celsius for up to three years. Volcanic ash falls out of the sky quickly, but sulfur dioxide rises into the stratosphere, reacts with water vapor and forms sulfate aerosols that reflect sunlight, sometimes for a year or more. The climatologist Mikhail Budyko is believed to have been the first, in 1974, to suggest doing it on purpose if global warming ever became pressing.

We have even been doing a version by accident: aerosols from burning fossil fuels and biomass masked global warming, cooling the average surface temperature by about 0.66 °C between 1850 and 2014, mostly thanks to sulfates, while black carbon soot does the opposite and warms the air.

Supporters claim that at a moderate intensity it could counter most changes in temperature and rainfall, act fast, cost little to deploy and be reversible in its direct effects. The open questions are big ones. Its effect on rainfall patterns is one of the most-discussed uncertainties, and modelling supports concerns that sulfur aerosols could damage the ozone layer. As of 2021 there was no leading candidate material, with alumina, calcite and salt also under consideration, and the favorite delivery method is custom aircraft.