Blocking sunlight to cool Earth won't reduce crop damage from global warming

Pinatubo, for example, injected about 20 million tons of sulfur dioxide into the atmosphere, reducing sunlight by about 2.5 percent and lowering the average global temperature by about half a degree Celsius (nearly 1 degree Fahrenheit).

The team linked maize, soy, rice and wheat production from 105 countries from 1979-2009 to global satellite observations of these aerosols to study their effect on agriculture. Pairing these results with global climate models, the team calculated that the loss of sunlight from a sulfate-based geoengineering program would cancel its intended benefits of protecting crops from damaging extreme heat.

“It’s similar to using one credit card to pay off another credit card: at the end of the day, you end up where you started without having solved the problem,” Hsiang said.

Some earlier studies suggested that aerosols might improve crop yields also by scattering sunlight and allowing more of the sun’s energy to reach interior leaves typically shaded by upper canopy leaves. This benefit of scattering appears to be weaker than previously thought.

“We are the first to use actual experimental and observational evidence to get at the total impacts that sulfate-based geoengineering might have on yields,” Proctor said. “Before I started the study, I thought the net impact of changes in sunlight would be positive, so I was quite surprised by the finding that scattering light decreases yields.”

Messing with sunlight
Despite the study’s conclusions, Proctor said, “I don’t think we should necessarily write off solar geoengineering. For agriculture, it might not work that well, but there are other sectors of the economy that could potentially benefit substantially.”

Proctor and Hsiang noted that their methods could be used to investigate the impact of geoengineering on other segments of the economy, human health and the functioning of natural ecosystems.

They did not address other types of geoengineering, such as capture and storage of carbon dioxide, or issues surrounding geoengineering, such as its impact on Earth’s protective ozone layer and who gets to set Earth’s thermostat.

“Society needs to be objective about geoengineering technologies and develop a clear understanding of the potential benefits, costs and risks,” Proctor said. “At present, uncertainty about these factors dwarfs what we understand.”

The authors emphasize the need for more research into the human and ecological consequences of geoengineering, both good and bad.

“The most certain way to reduce damages to crops and, in turn, people’s livelihood and well-being, is reducing carbon emissions,” Proctor said.

“Perhaps what is most important is that we have respect for the potential scale, power and risks of geoengineering technologies,” Hsiang said. “Sunlight powers everything on the planet, so we must understand the possible outcomes if we are going to try to manage it.”

— Read more in Jonathan Proctor et al., “Estimating global agricultural effects of geoengineering using volcanic eruptions,” Nature (8 August 2018)