Global dimming may mitigate warming, but could hurt crop yields

Shading Earth by adding a veil of particles to the upper atmosphere may help to offset global warming — but at a cost.

Crop yields could decline, as they did following two colossal volcanic eruptions that shot sunlight-blocking sulfur particles high above the cloud layer and into the planet’s stratosphere, researchers report online August 8 in Nature. The study is the first to use real-world data to evaluate the potential consequences of such “stratospheric veil” geoengineering.

Adding tiny particles called aerosols to the stratosphere, an approach known as solar radiation management, has been proposed as a way of reducing incoming sunlight to cool the planet and mitigate climate change. Some researchers have suggested that this cooling, as well as the scattering of light by the aerosols, could be beneficial to plants and improve crop yields.

But by looking at harvests of maize, soy, rice and wheat following volcanic eruptions in 1982 and 1991, scientists determined that these sulfates decreased total incoming solar radiation enough to hurt crop yields in the aftermaths of those eruptions.

The team also made projections for how solar radiation management might affect crops in 2050–2069. Those results suggest that any benefits to crops from cooling would be completely offset by decreased food production as a result of reduced sunlight.

Many scientists have been leery of proposed geoengineering schemes to alter incoming sunlight because the potential side effects are largely unknown (SN: 3/7/15, p. 14). Stratospheric aerosols reduce how much direct sunlight reaches the surface of Earth, but increase diffuse light reaching the surface. Previous studies have found that the boost in diffuse light may increase plant growth by allowing more light to penetrate tree canopies and reach more leaves. But the potential effect of this dimming on edible yields from crops hadn’t been quantified.

Dimming particles

The powerful volcanic eruptions of Mexico's El Chichón in 1982 and the Philippines' Mount Pinatubo in 1991 shot particles into the stratosphere, briefly increasing the atmospheric layer's aerosol optical depth, a measure of how much light is scattered (top). In the wake of the eruptions, crops suffered (bottom, shown here as percent changes in yield on a logarithmic scale). Globally, the average yield of maize (orange) dropped by more than 5 percent after each eruption, while the average yields of wheat (gray), soy (blue) and rice (green) also temporarily decreased.

How two large volcanic eruptions affected the atmosphere and crops

Powerful volcanic eruptions offer the only possible real-world data on the effects of large concentrations of stratospheric aerosols. In recent decades, there have been just two eruptions strong enough to shoot particles up into Earth’s stratosphere — the 1982 eruption of Mexico’s El Chichón and the 1991 eruption of the Philippines’ Mount Pinatubo. “We can use them as a natural experiment,” says study coauthor Jon Proctor, an agricultural economist at the University of California, Berkeley.

Sulfur dioxide ejected from the volcanoes was transformed into sulfate aerosols in the stratosphere and remained there for two years. Proctor and colleagues synthesized data collected from 1979 to 2009 on temperature, rainfall, cloud cover, stratospheric aerosols and satellite-based measurements of incoming solar radiation to assess the overall effect on food production.

During the two years following the 1991 Pinatubo eruption, which injected some 20 megatons of sulfur dioxide into the atmosphere, the planet dimmed, with about 2.5 percent less sunlight reaching Earth’s surface. The combination of beneficial cooling and detrimental darkening overall hurt crops. Average global yields of maize crops dropped by 9.3 percent, while those of soy, rice and wheat crops each dropped by about 4.8 percent, the researchers found. Following the El Chichón eruption, the effect was about half as harmful.

“The damages from reduced total light washed out benefits of redistribution of that light,” Proctor says. “The scattering was not worth it.”

Lowering a veil

The June 1991 eruption of Mount Pinatubo injected a burst of sulfur particles into the stratosphere. The particles soon turned into reflective sulfate aerosols (blue in this animation) and spread from the equatorial region to the poles. The animation shows monthly simulations of how the particles traveled around the atmosphere from June 1991 through September 1993. "Holes" at the poles reflect missing data due to a lack of satellite observations.

The findings don’t mean that solar radiation management should be off the table as a way to mitigate global warming, the team says. For one thing, it’s possible that farmers could switch to different sorts of crops that are more tolerant of reduced sunlight — although farmers have already been resistant to switching to more heat-tolerant crops despite rising temperatures, said study coauthor Solomon Hsiang, an economist at the University of California, Berkeley, during a news conference on August 7.

But the study marks an important first step in using real-world data to understand the potential side effects of this geoengineering strategy, Hsiang said. Future studies might use eruption-based data also to examine how solar radiation management might affect other ecosystems, such as coral reefs, or even how it affects human health or labor productivity.

David Lobell, an agricultural ecologist at Stanford University, says the results are interesting. But he warns that the eruptions can’t tell us everything about what might happen in a future geoengineered climate. “There are all sorts of issues with increased CO₂ that aren’t just about the temperature going up,” Lobell adds.

Still, given the potentially severe consequences of climate change, there’s a risk-risk trade-off to consider, Proctor says. “We don’t want to be overoptimistic about humanity’s ability to solve every problem,” he says. “But we do want to explore what might be achievable.”