Cooling Earth by injecting tiny particles high into the atmosphere, just as Mount Pinatubo’s 1991 eruption (above) did, probably wouldn’t help crop yields in a warmer world.

Jonathan Proctor and Solomon Hsiang

A plan to shoot tiny droplets into the sky would cool the planet—but wouldn’t help crops

As the climate heats up, some scientists wish to use a novel technique to cool things back down: geoengineering. One method would spew tiny particles high into the atmosphere to scatter sunlight. That would cool Earth’s surface and, in theory, protect crops like corn, which are expected to become less productive as temperatures rise. But a new study suggests any boost in crop yield due to lower temperatures would be largely counteracted by dimmer sunlight.

Scientists have long known that airborne sulfur dioxide gas—like that spewed from major volcanic eruptions—can briefly cool the planet when it reacts with water vapor to form light-scattering droplets. After the 1991 eruption of Mount Pinatubo in the Philippines, there was also evidence that it increased the uptake of carbon dioxide (CO2) in plants around the globe. That’s because light-scattering aerosols can diffuse direct sunlight, whose harsh rays typically hit only the tops of trees and other tall plants. But diffuse sunlight can reach all layers of foliage, including those below the canopy. Many researchers presumed the same effect would work to boost agricultural yields, says Jonathan Proctor, an agricultural economist at the University of California (UC), Berkeley.

To estimate what geoengineering might do to a wide variety of crops, Proctor and his colleagues analyzed agricultural records worldwide in the wake of two volcanic events: the eruption of Mount Pinatubo and the 1982 eruption of Mexico’s El Chichón. They used them to guide a model of the future in which people injected enough light-scattering aerosols into the stratosphere to counteract all human CO2 emissions after the year 2020.

During the period from 2050 through 2069, geoengineering cooled the crop-growing regions of the planet by about 0.88°C on average. That cooling boosted corn harvests about 6.3%. But the sunlight that accompanied the scattering, in addition to being more diffuse, was also slightly dimmer. And the researchers found that it had the opposite effect, decreasing yields by about 5.3%. On top of that, yields dropped by another 0.2%, thanks to subtle changes in cloudiness and precipitation patterns. Overall, geoengineering had no discernable effect on crop yields for corn, soy, rice, and wheat, the researchers report online today in Nature.

Maybe it shouldn’t be surprising that diffuse sunlight, as opposed to direct, didn’t seem to have much effect on crop yields, says Dennis Baldocchi, a bioenvironmental engineer at UC Berkeley who was not involved in the study. Unlike long-lived trees, which can be tightly packed in a forest and overshadow lower layers of foliage, crop plants stand mostly unshaded by their neighbors for much of their life cycle.

“This is an important and impressive study, hats off to the researchers,” says Gernot Wagner, a climate economist at Harvard University who was not involved in the study. The findings “are an important reminder that neither climate change nor solar geoengineering act on temperatures alone.” Other effects of the technique, such as how it might affect ocean acidification because of uptake of CO2, deserve significant study, he adds.

Although the team’s findings are disappointing in one sense, says Proctor, the approach could be easily adapted to assess the costs and benefits of geoengineering on other economic sectors, including the construction industry and health care. “Geoengineering may be ineffective at mitigating damage to agriculture, but it might still be worth doing,” he notes. “At this point, we just don’t know.”