(note: this is part 3 of our series, abridged excerpts from pages 2-4 of Paul Crutzen's essay on geo-engineering plan using sulfur compounds)
ALBEDO ENHANCEMENT BY STRATOSPHERIC SULFUR INJECTIONS: A CONTRIBUTION TO RESOLVE A POLICY DILEMMA? (PDF)
We will next derive some useful metrics.
First, a loading of 1 Tg S (1 teragram/kiloliter sulfur = 1 000 000 tonne/cubic meters, source from online conversion) in the stratosphere yields a global average vertical optical depth (optical depth gives a measure of how opaque a medium is to radiation passing through it, from Scienceworld.wolfram.com) of about 0.007 in the visible [spectrum] and corresponds to a global average sulfur mixing ratio of ∼1 nmol/mole, about six times more than the natural background.
Second, to derive the radiative forcing caused by the presence of 1 Tg S in the stratosphere, we adopt a simple approach based on the experience gained from the Mount Pinatubo volcanic eruption.
For the Mount Pinatubo eruption, Hansen et al. (1992) calculated a radiative cooling of 4.5 W/m2 (W/M2 is short for: Watts per square Meter, from Answers.com) caused by 6 Tg S (6 million tons of sulfur), the amount of that remained in the stratosphere as sulfate six months after the eruption from initially 10 Tg S (10 million tons of sulfur) (Bluth et al., 1992 PDF). Linear downscaling results in a sulfate climate cooling efficiency of 0.75 W/m2 per Tg S in the stratosphere.
The estimated annual cost to put 1 Tg S in the stratosphere, based on information by the NAS (1992), at that time would have been US $25 billion (NAS, 1992; Ron Nielsen, personal communication).
Thus, in order to compensate for enhanced climate warming by the removal of anthropogenic aerosol (human induced air pollution), a stratospheric sulfate loading of 1.9 Tg S would be required, producing an optical depth of 1.3%.
This can be achieved by a continuous deployment of about 1–2 Tg S per year for a total price of US $25–50 billion, or about $25–50 per capita in the affluent world, for stratospheric residence times of 2 to 1 year, respectively.
The cost should be compared with resulting environmental and societal benefits, such as reduced rates of sea level rise.
Also, in comparison, current annual global military expenditures approach US$1000 billion, almost half in the U.S.A.
The amount of sulfur that is needed is only 2–4% of the current input of 55 Tg S/year (Stern, 2005).
Although the particle sizes of the artificial aerosols are smaller than those of the volcanic aerosol, because of greater continuity of injections in the former, the radiative forcings are rather similar for effective particle radii ranging between 0.1 and 1 μm (see Table 2.4, page 27, Lacis and Mishchenko, 1995).
However the smaller particles have a longer stratospheric residence time, so that less material needs to be injected to cool climate, compared to the volcanic emission case.
It should be mentioned that Anderson et al. (2003a,b) (2005 version) state that the radiative cooling by the aerosol could be much larger than the figure of 1.4 W/m2, derived by Crutzen and Ramanathan (2003), which is based on the assumption of constant relative humidity in the troposphere.
If Anderson et al. (2003a,b) are indeed correct, the result might be a stronger climate heating from air pollution cleanup than derived above (see also Andreae et al., 2005).
To compensate for a doubling of CO2, which causes a greenhouse warming of 4 W/m2, the required continuous stratospheric sulfate loading would be a sizeable 5.3 Tg S, producing an optical depth of about 0.04.
The Rayleigh scattering optical depth at 0.5 μm is about 0.13, so that some whitening on the sky, but also colorful sunsets and sunrises would occur.
It should be noted, however, that considerable whitening of the sky is already occurring as a result of current air pollution in the continental boundary layer.
Locally, the stratospheric albedo modification scheme, even when conducted at remote tropical island sites or from ships, would be a messy operation.
An alternative may be to release a S[ulfur]-containing gas at the earth’s surface, or better from balloons, in the tropical stratosphere.
A gas one might think of is COS (Carbonyl sulfide), which may be the main source of the stratospheric sulfate layer during low activity volcanic periods (Crutzen, 1976), although this is debated (Chin and Davis, 1993).
However, about 75% of the COS emitted will be taken up by plants, with unknown long-term ecological consequences, 22% is removed by reaction with OH, mostly in the troposphere, and only 5% reaches the stratosphere to produce SO2 and sulfate particles (Chin and Davis, 1993).
Consequently, releasing COS at the ground is not recommended. However, it may be possible to manufacture a special gas that is only processed photochemically in the stratosphere to yield sulfate.
The compound should be non-toxic, insoluble in water, non-reactive with OH (hydroxide), it should have a relatively short lifetime of less than about 10 years, and should not significantly contribute to greenhouse warming, which for instance disqualifies SF6 (Sulfur hexafluoride).
The albedo modification scheme presented here has been discussed before, however, without linking opposite climate warming and improved air quality considerations.
Instead of sulfur, it has also been proposed to launch reflecting small balloons or mirrors, or to add highly reflective nano-particles of other material than sulfur (Teller et al., 1997; Keith, 2000 PDF).
An interesting alternative could be to release soot particles to create minor “nuclear winter” conditions.
In this case earth’s albedo would actually decrease, but surface temperatures would, nevertheless, decline.
Only 1.7% of the mass of sulfur would be needed to effect similar cooling at the earth’s surface, making the operations much cheaper and less messy.
However, because soot particles absorb solar radiation very efficiently, differential solar heating
of the stratosphere could change its dynamics.
It would, however, also counteract stratospheric cooling by increasing CO2 and may even prevent the formation of polar stratospheric cloud particles, a necessary condition for ozone hole formation.