Blocking sunlight won't reduce crop damage from warming: Study

Injecting particles into the atmosphere to cool the planet and counter the warming effects of climate change would do nothing to offset the crop damage from rising global temperatures, according to a study.

Researchers at the University of California(UC), Berkeley in the US analysed the past effects of Earth-cooling volcanic eruptions, and the response of crops to changes in sunlight.

The study, published in the journal Nature, concluded that any improvements in yield from cooler temperatures would be negated by lower productivity due to reduced sunlight.

"Shading the planet keeps things cooler, which helps crops grow better. But plants also need sunlight to grow, so blocking sunlight can affect growth.

"For agriculture, the unintended impacts of solar geoengineering are equal in magnitude to the benefits," said Jonathan Proctor from UC Berkeley.

"It is a bit like performing an experimental surgery; the side-effects of treatment appear to be as bad as the illness," Proctor said.

Some people have pointed to past episodes of global cooling caused by gases emitted during massive volcanic eruptions, such as Mt Pinatubo in the Philippines in 1991.

They argued that humans could purposely inject sulphate aerosols into the upper atmosphere to artificially cool Earth and alleviate the greenhouse warming caused by increased levels of carbon dioxide.

Aerosols - in this case, minute droplets of sulphuric acid - reflect a small percentage of sunlight back into space, reducing the temperature a few degrees.

"It is like putting an umbrella over your head when you are hot. If you put a global sunshade up, it would slow warming," Proctor said.

Pinatubo, for example, injected about 20 million tonnes of sulphur dioxide into the atmosphere, reducing sunlight by about 2.5 per cent and lowering the average global temperature by about half a degree Celsius.

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 sulphate-based geoengineering programme would cancel its intended benefits of protecting crops from damaging extreme heat.

"It is 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," said Solomon Hsiang, an associate professor at UC Berkeley.

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, researchers said.