“Benefit number one would be, if you could do it, you would reduce global warming, and many of its risks,” Robock said. “We know that if you could get the aerosols up there, it would work because it doesn’t involve creating or affecting clouds in the troposphere, it’s just putting a shield up there to reflect sunlight.”

Researchers know that big volcanic eruptions, like the 1991 eruption of Mount Pinatubo, cooled the Earth. But these natural solar geoengineering experiments have also given them the ability to observe other pitfalls, Robock said.

“We know that there were other things that were not so good; (the eruption) destroyed ozone,” he said. “And you actually get a huge reduction of monsoon rainfall. We observed that after Mount Pinatubo.”

Volcanic eruptions only cause the Earth to cool for a year or two, because the aerosols eventually dissipate. However, if stratospheric aerosols were to be used as solar geoengineering to cool the Earth, their use could alter monsoon rainfall for a much longer period, which could result in famine, Robock said.

Some modeling has shown that solar geoengineering could in fact have less of an impact on monsoons than global warming, but nevertheless, the issue illustrates just how difficult making these predictions are.

Who decides?

Then there are issues such as insect-borne diseases, like malaria, Muri points out. How would solar geoengineering affect mosquito populations and the potential spread of malaria?

And what if a failure to cut CO₂ emissions and reduce global warming results in devastating heat waves, where thousands of people die? Is that enough to outweigh other negatives?

“There are still so many areas where we don’t know enough,” she said.

Finally, there are areas that are far outside of what climate scientists who study the physical effects of climate change can predict. The biggest question is who decides what the temperature of the planet should be?

The political decision making surrounding solar geoengineering is daunting, if you consider the difficulty the nations of the world have already had in trying to agree to curb CO₂ emissions, Muri said.

“How would one deal with geoengineering in terms of geopolitics and governance?” Muri said. “We need to develop regulations. Who sets the thermostat and how would you go about agreeing on something like that?”

In a companion piece to Muri and her colleague’s article on geoengineering and agriculture, Ben Kravitz, an assistant professor at Indiana University’s Earth and Atmospheric Sciences Department, summed it up like this.

“Agriculture is one important piece in our understanding of the effects of climate engineering,” he wrote. “Gaining a better picture of the impacts of climate engineering requires looking at numerous effects in addition to food supply, including water security, geopolitics, and environmental justice…. It is important to figure out whether climate engineering would ultimately be more or less risky than climate change (and to whom).”

What is solar geoengineering?

Researchers are studying a number of engineering approaches as possible methods for cooling the planet. The three described here have been identified by a March report by the US National Academies of Sciences, Engineering and Medicine as meriting further study. The three approaches either rely on controlling the amount of sunlight reaching the Earth, or reducing the amount of heat trapped by the atmosphere.

Stratospheric aerosol injection

This technique requires injecting aerosol particles, like sulphates or pre-cursor gases, like sulphur dioxide, into the stratosphere, which is the layer of air 10-50 km above the Earth’s surface. Most studies are looking at placing aerosols at about 20 km above the Earth, where the particles scatter and reflect solar radiation to cool the planet. This technique mimics what happens with large volcanic eruptions. When Mount Pinatubo erupted in 1991, it sprayed 15 – 20 megatons of sulphur dioxide into the atmosphere, which cooled the Earth by about 0.4°C for two years. Currently, however, there are no planes capable of flying into the stratosphere to do this.

Cirrus cloud thinning

This technique involves spraying chemicals into cirrus clouds, at about 6-13 km above the Earth’s surface, to cause them to thin or disappear. The clouds trap heat, so thinning them or reducing them cools the planet by allowing heat to escape the atmosphere. The challenge for this technique is that cirrus clouds are in the region of the atmosphere where jets fly, which could make implementing this measure difficult.

Marine cloud brightening

This approach would add particles to low laying liquid clouds over the ocean to make them thicker and more reflective, which would cool the Earth, if it did not have side effects on other clouds. This mimics what happens now under certain conditions when ships spew pollution into the atmosphere.  The effect only works for a few days, and sea salt could be sprayed up from the ocean to seed the clouds.

Original Article: Blocking the sun to control global warming

More from: Norwegian University of Science and Technology | Rutgers University