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Page 454
(U.S. Bureau of the Census, 1988). This gives a cost of slightly
more than $1 per ton-mile for freight. If a dust distribution
mission requires the equivalent of a 500-mile flight (about 1.5
hours), the delivery cost for dust is $500/t, and ignoring the
difference between English and metric tons, a cost of $0.50/kg of
dust. If 1010 kg must be delivered
each 83 days, (provided dust falls out at the same rate as soot), 5
times more than the 1987 total ton-miles will be required. The
question of whether dedicated aircraft could fly longer distances
at the same effective rate should be investigated. However, if the
requirement is to mitigate the 1989 U.S. emissions of CO2, 500 times less dust is needed, the cost
is about $10 million per year, and implementation would require
about 1 percent of the ton-miles flown in 1987. If 10 percent of
the ton-miles flown in 1987 were used, the system could mitigate 80
Gt CO2. These costs should probably
be increased by the cost of delivered dust (say, $0.50/kg) and of
delivery systems in the aircraft, but better-than-average freight
rates could probably be arranged. Thus the costs appear to be about
$0.0025/t CO2.
Clearly, the amount of dust required could be greater by a
factor of 10, and the cost would be $0.025/t CO2. This provides a cost estimate in the
range of $0.003 to $0.03/t CO2.
Multiple Balloon Screen
A screen can be created by putting a vast number of aluminized,
hydrogen-filled balloons at a high enough altitude that they do not
interfere with air traffic. They would provide a reflection screen.
The properties of such a system are examined in Appendix Q.
The multiple balloon parasol system requiring billions of 1- to
6-m-diameter balloons would appear to cost about 20 times as much
as distributing dust in the stratosphere. The large number of
balloons, and the trash problem posed by their fall, make the
system somewhat unattractive.
Changing Cloud Abundance
A more detailed discussion of the possibility of changing cloud
abundance appears as Appendix Q.
The Approach
Independent studies estimated that an approximately 4 percent
increase in the coverage of marine stratocumulus clouds would be
sufficient to offset CO2 doubling
(Reck, 1978; Randall et al., 1984). Albrecht (1989) suggests that
the average low-cloud reflectivity could be increased if the
abundance of cloud condensation nuclei (CCN) increased due to
emissions of SO2. It
