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The seriousness of the risks posed by climate change demands that we examine all possible means of response, but how we do so makes all the difference.
A combination of greenhouse-gas emission cuts and solar geoengineering could keep temperatures under the 1.5-degree aspirational target of the recent Paris Agreement. It could make the world cooler at the century's end than it is today. Emissions cuts alone cannot achieve either objective with high confidence. The climate's inertia along with uncertain feedbacks such as loss of permafrost carbon mean there is a small but significant chance that the world will continue to warm for more than a century after emissions stop.
Solar geoengineering is the process by which humans might deliberately reduce the effect of heat-trapping greenhouses gases, particularly carbon dioxide, by reflecting a small fraction of sunlight back to space. The most plausible solar geoengineering technology appears to be the addition of aerosols (fine droplets or powder) to the stratosphere, where they would scatter some sunlight back to space, thus cooling the planet by reducing the amount of heat that enters the atmosphere.
Solar geoengineering could also limit global warming's predicted side effects such as sea level rise and changes in precipitation and other weather patterns. Because these changes would have their most powerful impact on the world's most vulnerable people, who lack the resources to move or adapt, one can make a strong ethical case for research to explore the technology.
Climate risks such as warming, extreme storms, and rising seas increase with cumulative emissions of carbon dioxide. Solar geoengineering may temporarily reduce such climate risks, but no matter how well it works it cannot eliminate all the risk arising from the growing burden of long-lived greenhouse gases. We can draw three important conclusions from these two facts. First, net emissions must eventually be reduced to zero to limit climate risk. Second, eliminating emissions does not eliminate climate risks, because it does nothing to address emissions already in the atmosphere. Third, the combination of solar geoengineering and emissions cuts may limit risks in ways that cannot be achieved by emissions cuts alone.
Solar geoengineering is not a substitute for cutting emissions. It is-at best-a supplement. We can't keep using the atmosphere as a free...