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Page 817
Appendix Q
Geoengineering Options
This appendix is divided into four sections: (1) naval rifle
system, (2)- balloon system, (3) multiple balloon system, (4)
changing cloud abundance. Each section either describes the system
or indicates how the costs were computed.
Naval Rifle System
The current cost of a naval projectile weighing 1900 pounds (lb)
is $7000 to $8000. The cost of propellant alone (if the shell is
furnished) is $900. It seems that a reasonable estimate for a 1-t
shell, dust (commercial aluminum oxide can be obtained for
$0.25/lb), and a propellant for each shot is $10,000. An efficiency
of one-half is assumed: one-half of the shell is dust, and the
other half consists of the packaging, dispersal mechanisms, and so
on, necessary to make the shell function. Thus the cost of the
ammunition for 40 years will be
The number of shots required in the 40 years is
If a single rifle can fire 5 shots per hour (naval rifles can
fire faster than this, but cooling intervals between shots can
lengthen the barrel life) and the rifle operates 250 working days
per year, then a rifle can fire 5 shots/hour × 24 hours/day
× 250 days/yr = 3 × 104 shots/yr per rifle.
Page 818
Thus
are required. Therefore, operating inventory of 4 ×
102 riflescan be assumedat any
time.
A gun barrel will have to be replaced approximately every 1500
shots; thus over the 40 years,
will be needed. A gun barrel probably would cost (in continuous
production
several hundred thousand dollars—say a million dollars. The
total cost of rifle barrels is thus3 × 105 barrels × 106 $/barrel = $3 ×1011 for barrels.
If the rifles are organized into 10-barrel stations, on land or
at sea, and a billion dollars is allocated for the capital cost of
each station, one might expect to buy 40 10-barrel stations to keep
350 barrels operating at a time, thus giving a cost for stations of
40 stations × 109 $/station
= $4 × 1010.
This should probably be doubled, at least; to allow for
overhead, power, maintenance, replacement, and so on. Multiplying
by 5 gives $2 × 1011 for
stations.
Finally, people are needed to operate the system. Although the
system would probably be highly automated, assume that it will work
like current operations. Then allocate 10 people/barrel × 4
× 102 barrels × 3
shifts × $105/person/yr
× 40 years = $48 × 109
$5 × 1010,
which can be doubled to include indirect personnel, overhead, and
so on, giving $1011 for operators.
Therefore, 24,000 people are assumed to be involved at any
time.
To sum up,
Ammunition
$4 × 1012 =
4.0 × 1012
Rifle barrels
$3 × 1011 =
0.3 × 1012
Stations
$2 × 1011 =
0.2 × 1012
People
$1 × 1011 =
0.1 × 1012
TOTAL
$4.6 × 1012
$5 × 1012 for
40 years,
giving an annual undiscounted cost of $50/40 × 1011 = $100billion.
Page 819
Clearly, the cost of the project is dominated by ammunition, and
the number of stations and rifles is reasonable, as is the amount
of activity, considered on a large industrial scale. The rifles
could be deployed at sea or in empty areas (e.g., military
reservations) where the noise of the shots and the fallback of
expended shells could be managed.
Balloon System
Consider a hydrogen balloon floating at 20 km, using the
Archimedes principle and noting that the density of hydrogen-gas is
one-fourteenth that of air:
md(isplaced) =
mg(as inside balloon) +
mb(alloon) + mp(ayload)
4/3pr3ro=4/3pr31/14ro + 4pr2Drrs(kin)
+ mp
mp=4/3pr3ro13/14-4pr2Drrs
=4pr3[13/(3x14)ro-(Dr/)/rs]
If
r = 100 m (radius of balloon)
ro =
88 g/m3 = 8.8 × 10-2 kg/m3 (density of air at 20 km)
Dr = 1 mm = 10-3 m (thickness of balloon skin)
ro =
1.15 g/cm3 (nylon) × 10-3 kg/g × [102 cm/m]3 = 1.15 × 103 kg/m3.
Then
mp = 1.26 × 107 (2.7 × 10-2 - 1.15 × 10-2)
= 1.26 × 105 (1.55)
= 1.95 × 105 2 × 105
kg.
The mass of the balloon for a 1-mm thickness is
4pr2Drr = 12.6 × 104 × 10-3 × 1.15 × 103
= 1.26 × 1.15 × 105
× 10-3 × 103 kg
= 1.5 × 105 kg.
If the balloon is 2/3-mm-thick (assumed for convenience), its
mass from the previous computation is 1.5 × 105 kg and the mass of dust lifted, if a
50 percent efficiency factor is used to account for instruments,
dust dispenser, container, and so on (this is conservative), is
105 kg. Nylon of the appropriate
gauge for weaving into a 2/3-mm-thick fabric (1050 denier is about
0.3 mm) costs $2/lb = $4.4/kg. If this is tripled for fabric and
balloon manufacture
Page 820
(the cost of parachute fabric is about 3 times the cost of the
yarn, based on information from a colleague at Du Pont Industrial
Fabrics), cost of controls, dust dispensing, and so on, $15/kg can
be estimated or 1.5 × 105
kg/balloon × $15/kg = 2.25 × 106 $/balloon.
Twenty lifts are necessary in 40 years:
2 × 106 balloons ×
2.25 × 106$/balloon = $4.25
× 1012.
Consider the additional costs of infrastructure and support:
there will be 2 × 106 lifts
in 40 years or
If there are 100 crews (each responsible for 2 lifts per day on
250 days a year) and each crew has 100 people,
104 people × 105 $/person/yr × 40 years = $4
× 1010
$1011 with an
overhead of 150%.
If each station is capitalized at $109, another $1011 is required, but this infrastructure
barely affects costs, as does the cost of dust even at $0.50/kg or
hydrogen at $10/kg.
Hydrogen can currently be purchased as liquid hydrogen in
1500-gallon lots (equivalent to 169,000 standard cubic feet) for
$2.5/100 ft3. For conversion, 1 kg
of hydrogen = 432.3 standard cubic feet. Thus the cost is
In quantities of 100 × 106 ft3/day, Ogden and Williams (1989) quote
costs lower than $30/GJ. This is
Each balloon has a mass of 4.2 × 106 m3
× 1/14 × 8.8 × 10-2 kg/m3 = 2.6 × 104 kg of hydrogen. At 5 × 104 balloon lifts per year, the annual
quantity is
13.2 × 108 kg 109 kg = 423
× 109 ft3 109 ft3/day = 102 × 106 ft3/day.
Page 821
The total mass of hydrogen required for 40 years is
2.6 × 104 kg/balloon
× 2 × 106 balloons =
5.2 × 1010 kg.
At $10/kg, this costs $5.2 × 1011 = $0.52 × 1012.
[Design note: The breaking strength of 1200 denier ( 0.4 mm) nylon is over 25 lb (Du Pont, 1988). The
equatorial circumference of the balloon is
2pr = 6.3 × 102 m × 103 mm/m = 6.3 × 105 mm;
therefore, the payload will be suspended from a double (actually
2.5) set of nylon strings 0.4 mm in diameter:
6.3 × 105 mm × [25 lb/(2.2 lb/kg)] × 2 = 142
× 105 kg.
Because the payload weighs 1.18 × 105 kg, the safety factor = 121
times!]
By using hydrogen at $10/kg, costs may be summarized as
Balloons
$4.25 × 1012
Infrastructure and personnel
$0.10 × 1012
Capital for launch stations
$0.10 × 1012
Hydrogen
$0.52 × 1012
TOTAL
$4.97 × 1012
$5 × 1012.
This mitigates 1012 t of carbon
or 4 × 1012 t of CO2. An undiscounted cost the same as that
for the naval rifle system is obtained:
$5/t C = $1.25/t CO2
$5/40/t C/yr = $0.125/t C/yr = $0.03/t CO2/yr.
All of the above material assumes no reuse of balloons, and no
allowance is made for the automation of launch, and so on. The
possibility of some reuse, and of automation, probably reduces the
total cost. If not controlled to land for reuse, balloons could be
"chased" and controlled to land for collection and disposal, or to
land in the deep ocean and sink promptly.
Consider hot air balloons. Again by using the Archimedes
principle,
mdisplaced = mgas + mballoon + mpayload
Vro = Vri +
mballoon + mpayload
Using the perfect gas law
piVi = miRTi
pi = RriTi
poVo = moRTo
po = RroTo
where m = mass, V = volume, po = outside pressure, pi = inside pressure,
ro = density of air outside,
and ri =
density of gas inside. At floating equilibrium,po=pri,
because the balloon is limp. Therefore,
Page 822
If r and dr are
expressed in meters, and roand rs are in
each r must be multiplied by
1/10-3 = 103:
mp = 12.6 × 103 r3 { (ro/3)[(Ti - To)/Ti] - (Dr/r)rs}
= 1.26 × 104
r3 { (ro/3)[(Ti - To)/Ti] - (Dr/r)rs}
where r (specific gravity) is
expressed in grams per cubic centimeter, r in meters, and
mp in kilograms. At 20
km,
ro =
(88 g/m3)(102 cm/m)3 = 88 × 10-6 g/cm3
for
To = -58.5°C = 217 K
(Kelvin)
Ti = 104°C = 377 K
(Kelvin)
r = 102 m
Dr = 1 mm
r s =
1.15
= 1.26 × 1010 (1.23
× 10-5 - 1.15 ×
10-5)
= 0.1 × 105 kg = 104 kg.
If 2/3-mm nylon is used, mp = 6 ×4 kg.
Thus the costs of a hot air balloon system can be expected to be
at least 4 to 10 times higher than the cost of a hydrogen balloon
system. These costs
Page 823
could be decreased by running the balloon at higher temperature,
but to get 105 kg of payload per
balloon with 1-mm nylon a temperature of 658 K (385°C) is
required, and with 2/3-mm nylon 475 K (202°C), which seems
difficult to manage. The breaking strength of nylon goes to zero
percent of its room temperature value by 250°C. While the skin
temperature of a hot air balloon is well below the core gas
temperature, the management of temperature to guarantee skin
strength with so large a differential between average and skin
temperature seems rather difficult, although the skin might be
insulated as some weight penalty. The results are sensitive to the
factors. Hot air balloons seem to be nearly competitive with
hydrogen balloons. This question would have to be explored further
before choices between hydrogen and hot air systems could finally
be made.
Multiple Balloon System
The mass of a bubble filled with hydrogen is one-fourteenth the
mass of the air displaced. The total mass of the hydrogen-filled
balloon will be (at any altitude)
At floating equilibrium, we have
1/14·4/3pr3ra=4pr2Drrs=4/3pr3ra
Drrs=13/14rra
Drrs=13/3x14rra=3x10-1
rra
If plastic with density of 1 g/cm3 and a skin thickness of
Dr = 10-1 mm = 10-4 m = 10-2 cm
(which is plausible) is used, then
At 19 km = 12 miles 62,000 feet, ra 10-4 and
r3x10-2/10-4=300cm=3.
Such a balloon has a disk area of
pr2 = 9p = 28 m2 = 3
× 10 m2. Thus,
5x1012/3x10
2x1011 = 200x109 balloons of 3 -m radius
Page 824
are required. If the balloon is 10-mm
material, a balloon of 3 × 10-1 m (30-cm) radius is obtained and
20,000 × 109 balloons are
needed.
Hydrogen will diffuse through the skin of the balloons, which
probably means that the system must be refreshed annually. The fall
of collapsed balloons might be an annoying form of trash rain.
Because the area of the material required for a balloon is 4pr2,
the material requirement is
of material for any size balloon. At $0.10/m2 (20 m2 of wrapping plastic can be bought in
the supermarket for about $2), this is $2 × 1012. Over 40 years, this amounts to $80
× 1012. It offsets 1012 t of carbon, so the cost is $80/t C
or $80/40 = $2/t C/yr or $0.50/t CO2/yr. A reasonable cost range of $0.50 to
$5/t CO2/yr can be assumed.
Changing Cloud Abundance
A study was undertaken to consider the various factors that
would be required to increase the albedo effect of global cloud
cover sufficiently to balance the temperature increase that is
projected to occur with a doubling of CO2. Toward this end, the temperature
sensitivity to different (high, middle, and low) cloud layer
properties was calculated by using a radiative-convective
atmospheric model. In addition, cost estimates have been made.
These amelioration processes are reversible and inexpensive. If
they were determined to be deleterious or if cost-competitive
programs were developed, these measures could be discontinued
immediately.
At the outset it cannot be emphasized too strongly that there
are tremendous uncertainties associated with these intellectual
exercises. As a case in point, circumstantial evidence teaches that
we have a very limited understanding of the role of cloud abundance
because a warming accompanied the measured increase in cloud
coverage over the past century. Consequently, a much better
understanding of the system is necessary before any large-scale
operations could reasonably be proposed.
The Climatic Effect of Clouds
Earlier, Reck (1978) studied the effect of increases in cloud
cover and, using a radiative-convective atmospheric model, found
that a 4 to 5 percent increase in low-level cloud cover would be
sufficient to offset the warming predicted from a doubling of
preindustrial CO2. This value is in
reasonable agreement with Randall et al. (1984), who estimated that
a 4 percent increase was required in the amount of marine
stratocumulus, which comprises the bulk of the low clouds on a
global basis. Unfortunately, many
Page 825
assumptions are contained in these estimates, and to understand
those assumptions and the role that clouds could play, cloud
sensitivity calculations have been made to illustrate the range of
surface temperature for various assumptions of cloud
properties.
In these calculations, the Mitigation Panel used the assumed
abundances and optical properties shown in Table Q.1 and a global
surface albedo of 15.4 percent. The model has three layers of
clouds under global average conditions. It is assumed that clouds,
once formed, will have the same effects over their entire lifetimes
and that they will have optical properties identical to those of
current low-level clouds, which are assumed to be unchanging during
the seeding process. Unfortunately, these assumptions contain many
uncertainties. These sensitivity calculations show that the effects
of clouds depend not only on the fraction of a given cloud type,
but also on the surface albedo beneath the clouds. The special role
of the low-level cloud and its varying effect as the surface albedo
changes add considerable complication because the surface albedo
varies from about 4 to 20 percent over some water to as high as 90
percent over pure snow or ice (Hummel and Reck, 1979). This means
that once a cloud is formed it may start with a cooling effect and
end up in an area where it could produce either greater or lesser
cooling, with the slight possibility of even a heating effect.
Albrecht (1989) (see also Twomey and Wojciechowski, 1969)
suggests that the average low-cloud reflectivity would increase if
the abundance of cloud condensation nuclei (CCN) were to increase
through emission of SO2.
TABLE Q.1 Assumed Properties of Average Global Clouds
Cloud Type
High
Middle
Low
Cloud Abundances
Fraction of shortwave cloud cover
0.181
0.079
0.302
Fraction of longwave cloud cover
0.181
0.079
0.302
Cloud Optical Properties
Solar albedo of cloud cover
0.21
0.48
0.69
Solar absorptivity of cloud cover
0.005
0.02
0.035
Infrared absorptivity of cloud cover
0.50
1.00
1.00
Page 826
To test for the sensitivity to this part of the problem, the
surface temperature changes with varying optical properties were
calculated and are shown in Table Q.2. For comparison purposes the
sensitivity of high and middle clouds was also included. Clearly,
the estimate depends strongly on the value of assumed low-cloud
solar reflectivity. For example, a change of 4 percent in the
reflectivity value (low-cloud abundance—see Table Q.2) would
be sufficient to cause the calculated surface temperature to change
by 3°C. With a sensitivity of this magnitude, clearly a large
potential exists for forced changes provided they could be
controlled, and provided large regional anomalies and uncontrolled
long-distance effects are not created.
There is also a height dependence in the radiation field that
varies greatly with latitude and altitude (Ramanathan et al.,
1987). The cloud fraction variation with latitude is shown in Table
Q.3. In the present environment, there is a greater probability of
having clouds over water than over land, with more clouds over land
in the afternoon and more clouds over water in the morning. This
occurs because cloud height and optical properties are intimately
related to humidity and physical conditions. For example, the role
of a cloud at a given latitude is controlled by the zenith angle of
the sun. If the cloud were to move to a more northern latitude, its
cooling effect would be expected to diminish in proportion to the
change in the cosine of the sun's zenith angle. As can be noted
from the cosines listed in Table Q.3, a cloud at 5° latitude
could have about twice as large a contribution as the same cloud at
65° latitude. Many less predictable features are also crucial
(such as the degree of evaporation).
Reck (1978, 1979), using a model based on that of Manabe and
Wetherald (1967), has also illustrated cloud height effect. These
calculations show heating from high-level clouds and cooling from
middle- and lower-level
TABLE Q.2 Calculated Surface Temperature Sensitivity to
Changes in Cloud Properties
Cloud Type
High
Middle
Low
Sensitivity (°C) per percent change in cloud
abundance
0.36
-0.35
-0.66
Sensitivity (°C) per percent change in cloud
albedo
-0.16
-0.06
-0.35
Sensitivity (°C) per percent change in cloud
absorptivity
-0.062
0.048
0.045
Page 827
TABLE Q.3 Latitudinal Variation of Assumed Annual Cloud
Cover
Fraction of Cloud Cover
Latitude (degrees)
Cosine of Zenith Angle
Upper Cloud
Middle Cloud
Lower Cloud
5
0.61
0.225
0.075
0.317
15
0.593
0.181
0.064
0.264
25
0.560
0.160
0.063
0.248
35
0.512
0.181
0.079
0.302
45
0.450
0.210
0.110
0.388
55
0.381
0.242
0.131
0.438
65
0.309
0.254
0.119
0.444
75
0.259
0.252
0.111
0.424
85
0.243
0.205
0.092
0.375
ones. One possible error in the estimates presented here is the
assumptionof either a fixed cloud altitude or a fixed cloud
temperature. Reck (1979)has shown a greater model sensitivity to a
fixed cloud temperature. Mixedbehavior might be observed in the
real atmosphere. Clearly with all thepossible heating or cooling
effects, the presence of naturally occuring cloudscould complicate
the analysis of data obtained to test the role of
humanintervention. See, for example, the cloud experiments
suggested below.
With all the above assumptions in mind, it is proposed both that
CCN emissions should be done over the oceans at an altitude that
will produce an increase in the stratocumulus cloud albedo only,
and that the clouds will remain at the same latitudes over the
ocean where the surface albedo is relatively constant and low. As
noted in Figure Q.1, an increase in surface albedo, should the
cloud float over land, would only enhance its cooling effect. This
is true provided the latitude of the cloud does not change, as
discussed previously.
How Cloud Condensation Nuclei Can
Change Climate
Despite the lack of knowledge about cloud processes, the
possibility of altering clouds has been considered for a long time.
The idea of cloud seeding for agricultural purposes became popular
in the 1950s and 1960s, but because of the lack of precision and
the litigation that resulted, it has not been very succesful (see,
for example, Todd and Howell, 1985; and Kerr, 1982). Changes in
cloudiness on a regional scale were also proposed some time ago by
Russian scientist, who considered decreasing the cloudiness
Page 828
FIGURE Q.1 Calculated surface temperature
variation with changes in low-cloud cover and surface
albedo.
in the arctic region to promote ice melting and improved growing
conditions in Siberia. Before the more recent satellite
measurements, most of what was known about cloud processes and how
they contribute to the global radiative balance came from climate
modeling, and in climate models, most of the details of the cloud
processes were not included. Certainly, no individual clouds were
included on the grid scale of the general circulation models (GCM);
thus specific details of the microphysics, as it might involve
seeding or CCN, could not be studied within the concept of
GCMs.
Proposed Change in Low-Cloud Albedo
Through Emissions of Cloud Condensation Nuclei
In a recent paper, Albrecht (1989), following a hypothesis of
Twomey and Wojciechowski (1969), grossly estimated the additional
CCN that would be necessary to increase the fractional cloudiness
or albedo of marine stratocumulus clouds by 4 percent. He estimates
that this increase in low-level fractional cloudiness would be
equivalent to that attributed to a 30 percent increase in CCN. As
noted from Table Q.3, this 4 percent increase, if it were strictly
in lower-level cloud abundance at global average conditions
(35° latitude), would be more or less equivalent to the
cloudiness at 4° latitude further north. Albrecht's idealized
stratocumulus cloud, which he argues is typical, has a thickness of
375 m, a drizzle rate of 1 mm per day,
Page 829
and a mean droplet radius of 100 mm;
he also assumes that each droplet is formed by the coalescence of
1000 smaller droplets. The rate at which CCN are depleted by this
model is 1000/cm3 per day.
Consequently, about 300/cm3 per
day (30 percent of 1000) of CCN would be needed to be discharged at
the base of the cloud to maintain a 4 percent increase in
cloudiness. This assumes that the perturbed atmosphere would remain
sufficiently close to saturation in the vicinity of the CCN that
additional cloud cover would be formed every time the number of CCN
increased.
Now an extrapolation will be made to the entire globe, while
keeping in mind Albrecht's assumption that cloudiness in a typical
ocean region is limited by the small number of CCN. On the average,
31.2 percent of the globe is covered by marine stratiform clouds
(Charlson et al., 1987). If no high-level clouds are present, the
number of CCN that must be added per day is
= 4p × (radius of earth)2 × (cloud-layer thickness)
× 31.2 percent × CCN/volume
= 4p × (6.37 × 108
cm)2 × (3.75 × 104 cm) × 0.312 × 300/cm3/d
= 1.8 × 1025 CCN per
day.
The three materials that have been used for cloud seeding are
silver iodide (AgI), lead iodide (PbI), and dry ice. Dry ice is not
applicable to this situation because it does not create CCN. It is
used because of its precipitation-enhancing properties. Lead iodide
will not be considered because it was used before full awareness of
the environmental problems associated with lead. Although adverse
environmental consequences will probably also be associated with
AgI, a calculation will be made anyway. Calculations will also be
performed using sulfuric acid (H2SO4),
because most of the CCN that occur naturally over the oceans are
believed to be H2SO4 CCN arising from the oxidation of
dimethyl sulfide (DMS) produced by planktonic algae in the seawater
(Charlson et al., 1987).
The mass of a CCN is (4/3
pr3 ×
density), and it is assumed that the mean radius r = 0.07
× 10-4 (Charlson et al.,
1987). Because the density of AgI is 5.7 g/cm3, the CCN mass is
= 4/3p × (0.07 × 10-4 cm)3
× 5.7 g/cm3
= 8.2 × 10-15 g.
The total weight of AgI to be added per day is
= (total number to be added) × (weight of average CCN)
= 1.8 × 1025/day ×
8.2 × 10-15 g
= 1.5 × 1011 g/day or
about 1.5 × 105 t per
day.
Worldwide silver production in 1985 was 420 × 106 ounces (U.S. Bureau of the Census,
1987). This is converted to metric tons:
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),
Page 831
the capital cost of the fleet is $2 × 1010. Amortized over 20 years, an annual
capital cost of $1 × 109 may
be used. The sulfur will cost another $0.6 × 109 per year, and $2 × 106 per ship per year may be allocated for
operating costs ($10,000 per operating day), to give a total cost
of $2 × 109 annually. Over
40 years (until 2030) this means $8 × 1010, or $1011. This continuously mitigates
˜103 Gt = 1012 t for a cost of $0.10/t of CO2. Of course, this continues to be a
yearly cost of $1 × 109/yr.
The SO2 could also be emitted
from power plants. These plants could be built in the Pacific Ocean
near the equator (hopefully on small deserted islands) and would
serve to furnish power for nearby locations (e.g., South America).
Transmission or use of the power in the form of refined materials
could be considered, or possibly the use of superconducting power
transmission systems. It is estimated that eight large power plants
using spiked coal would be required (with 4 times the normal amount
of sulfur) at a cost of $2 to $2.5 × 106 per plant. Most of the cost would be
borne by those buying the power, so the cost might be at most 10
percent per year (the interest on the investment), or a total of $2
× 109 per year (with the
above conversion, $2 × 109/3890 × 106
$0.0005/t CO2).
Comparison of the Cloudiness and
Proposed Cloud Condensation Nuclei Emissions with Current Estimates
in the Real Atmospher
Total U.S. SO2 emissions are 65.7
× 103 t per day, which is
roughly 2 times the amount calculated in the previous paragraph.
Consequently, there should already be some cloud-enhancing effects
evident in the northern hemisphere if Twomey and Wojciechowski's
hypothesis, as implemented by Albrecht, is correct. An examination
of available CCN data shows that the mean CCN concentration at
oceanic locations in the northern Atlantic is about 5 times higher
than at remote locations in the southern Pacific (see Schwartz
(1988), who, however, concludes that there is no discernible
contribution of anthropogenic SO2
emissions to the global cloud cover effect on planetary albedo or
temperature). Furthermore, several studies have examined trends in
cloudiness in the northern hemisphere and have all come to the same
conclusion: The total cloud amount has been increasing in the
northern hemisphere (study areas include United States, North
America, the North Atlantic, and Europe) since the early 1900s
(Henderson-Sellers, 1986, 1989; Changnon, 1981; Angell et al.,
1984; Warren et al., 1988). The largest increases in cloudiness in
the United States occurred from the 1930s to about 1950 and from
the mid-1960s to about 1980. The first period corresponds to a
period of rapid growth of U.S. SO2
emissions after the Depression and extends to the end of World War
II; the second period corresponds to the proliferation of tall
stacks. From 1965 to 1980 the mean effective stack height (physical
height of stack plus plume rise) of SO2
Page 832
emissions doubled from about 300 to 600 m. This, of course,
increased the lifetime of discharged emissions in the atmosphere
and transformed the SO2 pollution
problem from primarily a local issue in many localities to a
long-range transport issue.
Between 1900 and 1980 the mean cloud cover over the conterminous
United States has increased about 10 percent (Henderson-Sellers,
1989), which should be more than sufficient to compensate for an
equivalent doubling of CO2. Because
CO2 increased only about 12 percent
during the same period, the net effect should have been a cooling.
However, analyses of temperature data in the northern hemisphere
over the same periods consistently indicate that the mean
temperature has risen about 0.5° to 0.7°C overall, but no
trend was evident in the conterminous United States (Jones et al.,
1986; Hansen and Lebedeff, 1987; Hanson et al., 1989). This
suggests either that the effects of clouds are not understood, or
that other factors, such as the very poor data reliability for
cloudiness and the effect of cloud height, need to be
considered.
Wigley (1989) presents some crude calculations suggesting that
SO2/CCN-derived forcing could be
large enough to have offset any temperature increase due to CO2 in the northern hemisphere. Schneider
(1972) points out that SO2 emissions
are regionally heterogeneous, which would lead to long-wave forcing
anomalies that in turn could lead to long-wave anomalies plus
teleconnections. In any event, all of this is quite speculative and
underscores the fact that much is yet to be understood about the
causes of climate variations during the last century.
Impacts of Enhanced Acid
Deposition
One must now consider whether the injection of this much
additional SO2 into the atmosphere
will cause an acid deposition problem. It should be kept in mind
that the principal component of naturally occurring CCN is sulfate
formed from DMS emission from marine algae. Schwartz (1988) quotes
estimates of 16 to 40 × 1012
g/yr or perhaps about 25 × 109 kg/yr emitted from this source. The
addition about 6 × 109 kg/yr
is being considered, approximately 25 percent of the natural
amount, although locally much more than 30 percent may be added to
the amount naturally present. The oceans have an enormous buffering
capacity (Stumm and Morgan, 1970), so the additional rainout of
sulfate (especially after dilution through cloud dispersal and
droplet coalescence) seems unlikely to have any effect, even
locally, although there is clear disagreement on this point. The
principal concern is to avoid additional sulfate deposition over
land. With a 30 percent rainout per day, this could be ensured to a
90 percent level by operating about a week upwind of land. Such a
constraint would have to be added to the others already stated.
Page 833
Another possible way of dealing with the acid rain concern would
be to introduce sulfate in the form of ammonium sulfate or
bisulfate, each of which is a neutral salt. This would avoid the
acid question from the start. Both salts are used frequently as
fertilizers and in the dilutions to be seen here would have a mild
fertilizing effect locally. These salts can be made by reacting
ammonia with sulfuric acid. The price of ammonia is about $100/t,
so the cost of the CCN might double, and there would be an
additional cost for equipment to run the reaction at sea. These
additional costs might increase the total by as much as 50 percent
to $0.15/t of carbon mitigated per year or $0.04/t CO2.
Necessary Cloud Condensation Nuclei
Experiments
If global-scale CCN emissions were to be considered in a serious
way a number of fundamental studies would need to be performed.
Among these would be the following:
• Exploratory studies of the effectiveness of CCN for
enhancing stratocumulus cloud cover, with a full statistical
analysis of covariates, and so on.
• Determination of CCN properties: (1) lifetimes of CCN at
various altitudes; (2) effectiveness in cloud enhancement; and (3)
effect of their precipitation on oceans.
• Determination of the fraction of SO2 emissions converted to CCN and the
resulting particle size distribution.
• Extension of the idea of CCN enhancement from local and
regional to global dimensions: a careful study of the scale
dependence of the effectiveness of cloud enhancement processes and
the interaction of clouds with the radiation field.
• Full confirmatory analysis of the effectiveness of CCN on
fractional cloudiness with carefully selected test statistics. A
multiplicity of analysis would have to take into account all
variables such as the humidity profile, convective processes, and
CCN count, along with methods for the study of precipitation
processes.
Note
1. Throughout this report, tons (t) are metric; 1 Mt = 1 million
tons; and 1 Gt = 1 billion tons.
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