To date, one of the big concerns about solar geoengineering has been that the effect would be uneven, with some regions of the world experiencing more climate change and adverse impacts than they would without use of the technique. To address this concern Ben Kravitz from the Pacific Northwest National Laboratory in the US and colleagues investigated the potential for regional disparities using a multi-model experiment where solar geoengineering was deployed to offset the climate forcing from a quadrupling of carbon dioxide in the atmosphere.

The team divided Earth's continental area into 22 different regions, and modelled the temperature and precipitation changes resulting from “moderate” solar geoengineering. “In this study we took moderate to mean some geoengineering but less than it would take to return global mean temperature to its pre-industrial value,” said Kravitz.

For all models and all regions the researchers found that moderate geoengineering resulted in regional temperature values that were closer to pre-industrial levels than in an un-geoengineered, high carbon-dioxide world. “There was no one region that was consistently made 'better' or 'worse' in all models,” said Kravitz. But the models didn't depict a climate utopia either. In almost every model there was at least one region for which no amount of solar reduction could restore precipitation towards its preindustrial value.

In part this is because solar geoengineering reduces the amount of energy available at the Earth's surface for evaporation. “Also, a reduction in sunlight will cause more cooling at the surface than in the atmosphere above, which makes upward motion more difficult,” said Kravitz, whose findings are published in Environmental Research Letters (ERL).

Although Kravitz and his colleagues' study was idealized and only considered two climate effects (temperature and precipitation), it is clear that any introduction of solar geoengineering will involve trade-offs. Possibly some of those trade-offs could be alleviated by using “non-uniform” solar geoengineering – turning down the Sun more in some locations than others.

Ultimately solar geoengineering cannot be a permanent solution to climate change. “If society continues to increase carbon dioxide emissions and offsets the climate change with geoengineering, we would need to do continually more geoengineering to keep up with increasing emissions,” said Kravitz. However, the new study indicates that short-term use of moderate levels of solar geoengineering may bring some relief to most regions of the world.

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