According to the new paper, solar geo-engineering’s consequences will override its benefits. Proctor and his colleagues find that spraying volcanic gas into the air will reduce global temperatures, aiding crops. But those same gases will also scatter sunlight and reduce the amount of radiation that reaches the surface, hampering photosynthesis.

The net effect is roughly nil: Global yields will not emerge from a geo-engineered world any more bountiful than they were before. Solar geo-engineering cannot reverse the worldwide agricultural damage wrought by climate change.

The results were published in this week’s edition of the scientific journal Nature. Proctor compared them to a disappointing clinical trial.

“If you think of solar geo-engineering as an experimental surgery, then the side effects of the treatment—the changes in sunlight—are kind of just as bad as the original disease,” he told me.

But he didn’t want to throw out the idea just yet. “Just because the first test of an experimental surgery had bad side effects for a particular part of the body, it doesn’t mean the procedure should be automatically abandoned,” he said.

Proctor and his colleagues were only able to run the test because something like solar geo-engineering has happened before—in fact, it occurred in living memory.

In 1991, the Philippine volcano Mount Pinatubo erupted, injecting 20 megatons of sulfur dioxide into the high atmosphere.* Within a few weeks, the sulfate gas—which can reflect away enormous amounts of visible light—enveloped the planet. Eventually, this new layer of sulfate scattered so much sunlight that the planet cooled by roughly half a degree Celsius (or 1 degree Fahrenheit). The sulfates fell out of the atmosphere a couple years later, and climate change continued its steady march.

Plants seemed to prosper in Pinatubo’s dimmer, chillier world. Forests, especially, flourished: One study found that photosynthesis in a tract of Massachusetts woods increased by 23 percent on sunny days in 1992, as compared with before Pinatubo’s eruption. Biologists speculated that the volcanic gas scattered the sun’s rays in every direction, allowing more of them to dodge the dense leaves in a forest’s canopy and reach its understory.

So Proctor and his colleagues expected solar geo-engineering to boost global agriculture as well. After all, solar geo-engineering is nothing more than an endless Pinatubo: If humanity commits to solar geo-engineering the planet, then it will have to regularly spray sulfate gas into the sky in order to keep a lid on Earth’s temperature.

But when the team looked at how agriculture reacted to Pinatubo—and to the eruption of Mexico’s El Chichón volcano in 1982—it found that global crop yields fell, even after controlling for temperature. Global corn yields decreased by 9.3 percent in Pinatubo’s aftermath; wheat, soy, and rice yields fell by 4.8 percent.