Page 830

420 × 106 oz/yr × 28.35 g/oz × 1 t/106 g

= 11.9 × 103 t/yr of Ag, or

= 25.5 × 103 t/yr AgI.

Clearly there is not enough silver or AgI to consider this experiment.

For H2SO4, with a density of 1.841 g/cm3, the total weight to be added per day

= 1.841/5.7 × 1.5 × 105 t/day

= 48 × 103 t/day H2SO4

= 31 × 103 t/day SO2, if all the SO2 is converted to H2SO4 CCN. To put this number in perspective, a medium-sized coal-fired U.S. power plant emits about this much SO2 in a year; the equivalent emissions of 365 U.S. coal-burning power plants (50 percent of present U.S. SO2 emissions) would produce sufficient CCN. To estimate the value of the sulfur directly, the total weight of SO2 to be added per day is 32 × 103 t or about 16 × 103 t of sulfur, which is equivalent to about 6 megatons (Mt; 1 Mt = 1 million tons) of sulfur per year. Given the average market price of sulfur for 1983–1987 (f.o.b. mine or plant)—$96.90 (U.S. Bureau of the Census, 1988)—the minimum yearly cost would be at least $580 × 106/yr. Equating this yearly cost to the 300 parts per million by volume (ppmv) of CO2 necessary for full compensation gives $580 × 106/(2840 Mt C/ppmv CO2 × 300 ppmv CO2), or about a fraction of a cent per ton of CO2. To obtain an equivalence to conserved carbon, known emissions of carbon in 1978, 1979, and 1980 have been compared with the total measured increase of CO2 to obtain the equivalence: 3890 Mt C 1 ppmv CO2. A 4 percent increase in cloudiness was then equated to a 300-ppmv CO2 decrease, which translates into a reduction of 1200 gigatons (Gt; 1 Gt = 1 billion tons) of carbon, or 4400 Gt of CO2.

The primary cost of this process involves the mechanism for distributing SO2 in the atmosphere at the correct location. Assume a fleet of ships each carrying sulfur and a suitable incinerator. The ships are dedicated to roaming the subtropical Pacific and Atlantic oceans far upwind of land while they burn sulfur. They are vectored on paths to cloud-covered areas by a control center that uses weather satellite data to plan the campaign. In addition to choosing areas that contain clouds, it is important to distribute the ships and their burning pattern so as not to create major regional changes, or the kind of change with a time or space pattern likely to force unwanted wave patterns. These restrictions (which we may not know how to define) could be a difficult problem for such a system to solve.

From the above, 16 × 103 t/day, or 6 Mt/yr of sulfur must be burned. If 102 t per ship per day are allocated, and a ship stays out 300 days each year, roughly 200 ships of 10,000-ton capacity are needed (one reprovisioning stop every 150 days). At a cost of $100 × 106 per ship (surely generous),