Albedo Enhancement by Stratospheric Sulfur Injections: A Contribution to Resolve a Policy Dilemma?

ALBEDO ENHANCEMENT BY STRATOSPHERIC SULFUR

INJECTIONS: A CONTRIBUTION TO RESOLVE A POLICYDILEMMA?An Editorial EssayFossil fuel burning releases about 25 Pg of CO2 per year into the atmosphere, which

leads to global warming (Prentice et al., 2001). However, it also emits 55 Tg S as

SO2 per year (Stern, 2005), about half of which is converted to sub-micrometer size

sulfate particles, the remainder being dry deposited. Recent research has shown that

the warming of earth by the increasing concentrations of CO2 and other greenhouse

gases is partially countered by some backscattering to space of solar radiation by

the sulfate particles, which act as cloud condensation nuclei and thereby influence the micro-physical and optical properties of clouds, affecting regional precipitation patterns, and increasing cloud albedo (e.g., Rosenfeld, 2000; Ramanathan

et al., 2001; Ramaswamy et al., 2001). Anthropogenically enhanced sulfate particle

concentrations thus cool the planet, offsetting an uncertain fraction of the anthropogenic increase in greenhouse gas warming. However, this fortunate coincidence

is bought at a substantial price. According to the World Health Organization, the

pollution particles affect health and lead to more than 500,000 premature deaths

per year worldwide (Nel, 2005). Through acid precipitation and deposition, SO2

and sulfates also cause various kinds of ecological damage. This creates a dilemma

for environmental policy makers, because the required emission reductions of SO2,

and also anthropogenic organics (except black carbon), as dictated by health and

ecological considerations, add to global warming and associated negative consequences, such as sea level rise, caused by the greenhouse gases. In fact, after earlier

rises, global SO2 emissions and thus sulfate loading have been declining at the

rate of 2.7% per year, potentially explaining the observed reverse from dimming

to brightening in surface solar radiation at many stations worldwide (Wild et al.,

2005). The corresponding increase in solar radiation by 0.10% per year from 1983

to 2001 (Pinker et al., 2005) contributed to the observed climate warming during the

past decade. According to model calculations by Brasseur and Roeckner (2005),

complete improvement in air quality could lead to a decadal global average surface air temperature increase by 0.8 K on most continents and 4 K in the Arctic.

Further studies by Andreae et al. (2005) and Stainforth et al. (2005) indicate that

global average...