More reason to be highly dubious of climate change geoengineering

It's taken a long time for someone to think of this issue:  what effect would long term spraying of sulphate aerosols into the atmosphere as a climate moderating geoengineering attempt have on crops?

Looking at the examples of two volcanos, some scientists say that the decrease in sunlight hurts crop yields significantly, and in fact likely off setting the temperature moderating effects:  

Specifically, the research team examined what happened to maize, soy, rice, and wheat yields in the wake of the Mount Pinatubo and El Chichón eruptions in the years following their eruptions until the volcanic aerosols dissipated. They chose these plants because they are highly sensitive to light and temperature, so eruptions can have a big impact on their yields. The crops are also staples for billions of people — important if you want to estimate the potential societal consequences of geoengineering.

They found that the eruptions reduced the amount of direct sunlight hitting the earth but increased the amount of diffuse light. This led to a decline in edible yields from the crops they studied. Global average maize yields declined by 9.3 percent and the harvests of soy, rice, and wheat fell by 4.8 percent after the Mount Pinatubo eruption. Making a similar estimate of the consequences of the El Chichón eruption proved to be more difficult because the data wasn’t as robust. 

The researchers then simulated what would happen to crops if humanity tried to mimic the sulfur injection into the atmosphere by Mount Pinatubo. They looked at a moderate climate change scenario that projects global emissions will peak around 2040 and then decline.

The results showed that geoengineering using sulfate aerosols to manage sunlight would indeed increase crop yields by mitigating some of the losses due to rising temperatures. However, the changes in sunlight exposure would offset these gains, as less light means the plants produce less food.

The research team did not expect that the gains and losses from geoengineering would almost completely offset each other. And so “we were surprised by our own results in what seems like the simplest of relationships,” said co-author Solomon Hsiang, who leads the Global Policy Laboratory at UC Berkeley.

The overall effect is that solar radiation management would do little to reduce crop losses stemming from climate change.

In summary:

Though geoengineering can sometimes seem like an easy, tempting solution to a complicated problem, the results show that it could introduce its own complexities into the climate system. And creating an intervention that influences the whole planet is still a difficult and expensive proposition. 

Getting the requisite 20 million tons of sulfur compounds into the atmosphere would require a vast logistical network to send dozens of aircraft flights across the sky to spray these aerosols. The researchers estimated that keeping global average temperatures from rising more than 2°C via solar radiation management would require the equivalent of a Mount Pinatubo eruption every year....

The study’s authors say there could still be other benefits from geoengineering — for instance, saving lives from extreme heat — that could outweigh the costs. But that requires further investigation. 

“We want to make it very clear and explicit that we’re not pro- or anti-geoengineering in any way,” Hsiang said. “We think that geoengineering in this case highlighted a potential side effect.”