A popular idea for geoengineering is to inject sulfur into the lower stratosphere, because all major volcanic eruptions have cooled the earth’s surface for a few years by forming sulfate aerosols (Ward, 2009). There are two problems.
Background: Within 21 days following the eruption of Pinatubo in the Philippines in 1991, a 99% pure sulfuric acid (75%) and water (25%) aerosol formed at an elevation of 20 to 23 km in the lower stratosphere (Oman et al., 2006; Stowe et al., 1992) that increased the optical depth of the atmosphere to 0.4, reflecting and absorbing incoming solar energy, reducing globally averaged net radiation at the top of the atmosphere by about 2.5 W m-2, causing an average global cooling of the surface temperatures by ~0.5oC for ~3 years (Stowe et al., 1992; Valero and Pilewskie, 1992) and warming the tropical lower stratosphere ~3oC (Angell, 1997).
Two issues that are often not specified are the important role of erupted water in this aerosol-forming chemical process and the resulting effects on the oxidizing capacity of the atmosphere.
1. Pinatubo erupted 491 to 921 Mt (megatons) of water and only 15 to 19 Mt of SO2, 30 to 50 times more water than SO2 (Gerlach et al., 1996). Sulfate aerosols are hygroscopic; their formation depends on the availability of one molecule of OH and ~3 molecules of water for each molecule of SO2 oxidized in the stratosphere (Bekki, 1995). Since the stratosphere is normally almost completely dry (Lelieveld et al., 2007; Rosenlof, 2003), efforts to form such an aerosol must include at least 3 times as much water as sulfur dioxide and probably more to allow for proper mixing and to allow for water that might become involved in other chemical processes.
2. The sulfate aerosol after Pinatubo formed near the base of the ozone layer. The net effect of Pinatubo included reducing ozone levels in the lower stratosphere by as much as 25% for several years (Hofmann et al., 1994; Randel et al., 1995). There was a strong downward trend in OH concentration (Krol and Lelieveld, 2003; Lelieveld et al., 2006; Prinn et al., 2005; Prinn et al., 2001) and sharp increases in the growth rates of CH4 and CO concentrations in the tropics and at high southern latitudes (Dlugokencky et al., 1996; Lelieveld et al., 2006). Manning et al. (Manning et al., 2005) calculated that an OH reduction of the order of 10%, decaying over about a year, is consistent with anomalously high CH4 growth rates during this period. Thus efforts to form such aerosols can actually increase greenhouse gases by reducing the oxidizing capacity of the atmosphere. These greenhouse gases may remain in the atmosphere longer than the initial 3 years of cooling.
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Ward, P.L., 2009, Sulfur Dioxide Initiates Global Climate Change in Four Ways: Thin Solid Films, v. 517, p. 3188–3203 DOI:10.1016/j.tsf.2009.01.005.
Peter L. Ward
U.S. Geological Survey, retired
Teton Tectonics
www.tetontectonics.org