Stratospheric aerosol modification (SAM)

The following text is a short version of a Science Perspective [1]:

Research in this area gained traction after Crutzen’s call for investigating the effects of continuous stratospheric injections of sulfur into the atmosphere (Crutzen, 2007), also called sulfur aerosol modification (SAM), as one method to deliberately mitigate anthropogenic global warming, analogue to observed lowering of temperatures after large volcanic eruptions. SAM could be seen as a last resort option to reduce the impact of climate change to society, e.g. increasing heat waves, floods, droughts, and sea-level rise.

SAM technologies are presently not developed, and scientists are just starting to grasp the potential risks and benefits of these kind of interventions (5). Various model studies have helped to improve the understanding of sulfur aerosol microphysical processes and transport of aerosols, for example in realistically reproducing aerosol distributions after recent volcanic eruptions. A feature of aerosol microphysical processes related to SAM is the reduced cooling efficiency (cooling per injected unit sulfur) with increasing injection rate (7).  In short, the more SAM is done, the less effective further injections are at reducing temperatures (Fig 1a).  But, the amount of injection required for a given level of cooling is uncertain, varying from 0.1 to 0.4 Wm-2(Teragram(Sulfur)  year-1) between different models (8), and also depends on injection location, -height, and -area.

Additionally, the resulting aerosol distribution patterns are uncertain and depend on the injection location and rate, and are also dependent on transport and chemistry in the models. Stratospheric sulfate alters the aerosol transport by warming the stratosphere due to absorption of terrestrial radiation. This warming impacts stratospheric dynamical processes, e.g.   higher wind speed in the stratosphere at the equator decreases the poleward transport of aerosols (Niemeier and Schmidt, 2017). This has consequences not only for sulfate but for the transport of all stratospheric constituents. Changes in stratospheric chemistry have been shown to impact stratospheric ozone concentrations and cause a delay of the Antarctic ozone recovery by several decades (10).

 

 

1. Niemeier Ulrike and Simone Tilmes, Sulfur injections for a cooler planet, Science, Vol. 357, Issue 6348, pp. 246-248, DOI: 10.1126/science.aan3317, 2017. (Invited paper)

3. P.J. Crutzen, Clim Change 77, 211 (2006).

5. A. Robock, Earth’s Future 4, 644 (2016).

6. U. Niemeier, H. Schmidt, K. Alterskjær, J. E. Kristjánsson, JGR 118, 11905 (2013).

7. U. Niemeier, C. Timmreck, Atmospheric Chemistry and Physics 15, 9129 (2015).

8. R. Moriyama, et al., Mitigation and Adaptation Strategies for Global Change pp. 1–22 (2016).

10. S. Tilmes, R. Müller, R. Salawitch, Science 320(5880), 1201 (2008).