It’s widely accepted that voluntary emissions reductions alone will not be enough to reach the goal of the 2016 Paris agreement to keep this century’s global temperature increase below 2°C, leading some policy makers and scientists to argue that geoengineering techniques should also be considered to limit the impacts of global warming. One proposed technique involves injecting reflective sulfate aerosol particles into Earth’s lower stratosphere to cast a small proportion of the inbound sunlight back into space and cool the planet off.
Yet geoengineering proposals that intentionally manipulate the amount of light reflected by Earth’s surface—or albedo—are not sufficiently understood. The potential risks around sulfate aerosol solar geoengineering include alteration of regional precipitation patterns, its effects on human health, and the potential damage to Earth’s ozone layer by increased stratospheric sulfate particles.
Previous research exploring the climate’s response to aerosol injection scenarios has suggested that these methods could be tailored to reduce the potential side effects. No studies have systematically explored the extent of such control, however, in part because of the computational expense of running general circulation models with comprehensive aerosol chemistry. To provide guidance for future high-resolution simulations, Dai et al. used a computationally cheaper, two-dimensional chemical transport model to systematically estimate the effects of injecting sulfur dioxide and sulfate aerosols at a range of altitudes, latitudes, and time frames for 62 separate scenarios.
The results indicate that aerosol injections can be carefully tailored to achieve desired results, such as a minimal albedo increase near the equator, rather than a globally uniform response. For example, the simulations show that equatorial injections of sulfuric acid at high altitudes—where aerosols have a longer residence time—are the most effective at reflecting incoming radiation per unit of sulfur. They also indicate that sulfate aerosol has a higher efficacy than sulfur dioxide in all injection scenarios. In addition, the findings suggest that different injection scenarios may be combined to achieve specific climate objectives.
Although the authors caution that their results are approximations intended to guide future modeling efforts, this study provides fundamental information regarding the relative difficulty of achieving desired albedo modification effects and is an important starting point for understanding the limits of what is widely considered one of the most viable solar geoengineering techniques. (Geophysical Research Letters, https://doi.org/10.1002/2017GL076472, 2018)
—Terri Cook, Freelance Writer