Volcanic Eruptions as Analogs for Stratospheric Geoengineering Impacts

In response to the global warming problem, there has been a recent renewed interest in geoengineering “solutions” involving “solar radiation management” by injecting particles into the stratosphere, brightening clouds or the surface, or blocking sunlight with satellites between the Sun and Earth.  No systems to conduct geoengineering now exist, but a comparison of different proposed stratospheric injection schemes, using airplanes, balloons, and artillery, shows that using airplanes to put sulfur gases into the stratosphere would not be expensive.  Nevertheless, it would be very difficult to create stratospheric sulfate particles with a desirable size distribution.  While volcanic eruptions have been suggested as innocuous examples of stratospheric aerosols cooling the planet, the volcano analog also argues against geoengineering because of ozone depletion, regional hydrologic responses, and other negative consequences.  Volcanic eruptions are an imperfect analog, since solar radiation management proposals involve the production of a permanent stratospheric aerosol layer, while volcanic layers are episodic.  Nonetheless, we can learn much from the volcanic example about the microphysics of stratospheric sulfate aerosol particles; changes in atmospheric circulation, producing regional climate responses, such as changes to the summer monsoon; atmospheric chemistry; changes of the partitioning of direct and diffuse insolation; effects on satellite remote sensing and terrestrial-based astronomy; and impacts on the carbon cycle.  There are 26 reasons why geoengineering may be a bad idea, and five reasons why it might be a good idea.  Some of these can be evaluated with climate modeling, and some using the volcanic analog.  Observations of the next large volcanic eruption will help to understand the evolution in stratospheric sulfate aerosol size distribution over the first few months after the eruption.  Much more research is needed before we can quantify each of these, so that policymakers in the future can make informed decisions about whether to ever implement stratospheric geoengineering.  Given what we know today, global efforts to reduce anthropogenic emissions and to adapt to climate change are a much better way to address anthropogenic global warming.