1 Although it should be noted that the
amount of carbon stored in the ocean and lithosphere is much larger
than that in either the atmosphere or the land, the time scales
over which they equilibrate with the atmosphere are very large.
The land area of the United States has lost close to 25 percent
of its forest cover since settlement of the North American
continent began, and forest cover continues to decline. This is
notwithstanding the planting of more and more trees over time
(Figure 28.1). The 1989 decrease in tree planting is due to a
decline in planting under the Conservation Reserve Program (CRP).
In 1989, tree planting increased on National Forest and other
federal lands, but decreased on private, state, and nonfederal
public lands. A breakdown of total planting and seeding by
ownership category in 1989 is given in Table 28.1; private sources
planted 85 percent of the trees
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FIGURE 28.1 Historical summary of U.S. forest
planting.
SOURCE: U.S.
Forest Service (1990).
TABLE 28.1 Total Planting and Seeding by Ownership
Category in FY 1989
Acres
Percent of All Planting
Federal government
National forests
307,138
10.2
Department of the Interior
52,006
1.7
Other federal agencies
9,257
0.3
TOTAL
368,401
12.2
Nonfederal public
State forests
57,133
1.9
Other state agencies
6,013
0.2
Other public agencies
13,515
0.4
TOTAL
76,661
2.5
Private
Forest industries
1,248,565
41.3
Other industry
22,225
0.8
Nonindustrial owners
1,306,096
43.2
TOTAL
2,576,886
85.3
GRAND TOTAL
3,021,948
100.0
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FIGURE 28.2 Total planting and seeding by region
in FY 1989.
SOURCE: U.S.
Forest Service (1990).
in the United States in that year. Figure 28.2 shows that the
great majority of planting (76.3 percent) is in the southern United
States (U.S. Forest Service, 1990).
Storing Carbon in Trees
Forests take up carbon fastest during their early years of rapid
growth (which may be up to 80 years for some species). As trees
age, their growth rates decline and the rate at which they
sequester carbon also declines. As a result, stands of young trees,
on a net basis, actively increase stores of carbon (per unit area)
more rapidly than mature forests, where photosynthesis is more
closely balanced by respiration and death. On the other hand, a
mature forest generally contains more stored carbon overall than a
younger forest does.
The ability of a particular type of tree to store carbon depends
on a number of factors, including its intrinsic growth rate as well
as site and stand attributes. Carbon is stored in stemwood,
branches, and roots, and in the soil around the tree. Carbon
incorporated in leaves is recycled rapidly, often on an annual
basis, and thus is less important from a carbon storage
standpoint.
Researchers have documented drastic improvements in the present
net productivity (carbon uptake) of several species of trees.
Heilman and Stettler (1985) managed short rotations of hybrid
cottonwoods on fertilized, irrigated plots in western Washington
and achieved nearly 14 t of carbon uptake per hectare (ha) per year
of total production. A study by Steinbeck and Brown (1976) of
intensely managed American sycamores on a 4-year rotation
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in Georgia yielded a carbon uptake of 6.5 t C/ha/yr. On the
average, however, Marland (1988) estimates that U.S. commercial
forests have an uptake of roughly 0.82 t C/ha/yr. If they were
fully stocked, the average forest's productivity could increase to
1.35 t C/ha/yr. In sum, both the species of tree and the management
practices are important considerations in reforestation policy, as
are the kinds of land on which the trees are planted and the
climatic zones in which planting occurs. Therefore a reforestation
strategy for sequestering carbon might theoretically involve the
use of fast-growing species with advanced silviculture techniques
on optimal sites. However, there is a trade-off between maximum
carbon storage and maximum rate of carbon uptake. For example, in
the sycamore experiment mentioned, trees were harvested every 4
years to maintain the high growth rates of young vigorous plants.
These data on short-rotation forestry demonstrate that rates of
carbon uptake can be dramatically increased by forest management
strategies. For net U.S. carbon emissions to be reduced, trees must
be either protected from oxidizing to CO2 or used to replace fossil fuel burning.
In general, more intensive management requires more energy inputs,
and these must be compensated to determine net carbon benefit.
The most comprehensive analysis of the potential for
sequestering carbon in trees in the United States is that
undertaken by Moulton and Richards (1990) of the U.S. Forest
Service. This is a detailed analysis of the land available in the
United States that could support trees, the carbon uptake that
might be expected, and actual costs for each type of land to be
managed. According to Moulton and Richards, it is possible to
sequester up to 720 Mt C on economically marginal and
environmentally sensitive pasture and croplands and nonfederal
forestlands. After analyzing the potential carbon uptake and cost
per ton in 70 region and land-type classes, Moulton and Richards
arrange these in order by cost per ton and assemble a supply curve
for carbon sequestering. The analysis concludes that up to 56.4
percent of U.S. CO2 emissions could
be sequestered in domestic trees at costs ranging from $5.80 to
$47.75/t C.
Recognizing that the Moulton and Richards analysis suggests that
56.4 percent of U.S. CO2 emissions
could perhaps be offset with a massive commitment to a
reforestation program, the Mitigation Panel takes a very
conservative approach in estimating the carbon offset that might be
envisioned. As discussed in Appendix P, the Mitigation Panel's
analysis accepts that the 10 percent objective described by Moulton
and Richards is a reasonable initial target and that reforestation
of economically marginal or environmentally sensitive pasturelands
and croplands and nonfederal forestlands to a total of 28.7 Mha
could take place at costs as described in their analysis.
Several factors in the Moulton and Richards analysis, however,
heavily influence the numeric results and are likely to elicit some
discussion as to the magnitude and cost of reforestation. Their
analysis has a 40-year time
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horizon, so it does not confront the consequences of declining
growth rates as trees approach maturity or of the long-term
possibilities for tree maintenance or harvest. In addition, land
rental rates and the ratio between carbon uptake in marketable
timber and total ecosystem carbon uptake are somewhat uncertain.
Taking these factors into account, the Mitigation Panel's analysis
suggests that 240 Mt CO2 equivalent
per year could be sequestered at costs between $3 and $10 per ton
of CO2 (average cost is $7.20/t
CO2). Demonstration projects could
verify the lower costs and higher targets for total sequestration
suggested by Moulton and Richards.
Obstacles to Implementation
There are several constraints on implementing a reforestation
policy. First, there are land use commitments. For reforestation to
be pursued on a large scale, planting would have to take place on
marginal agricultural lands. This represents a long-term commitment
to nonagricultural uses.
Second, there may be resource constraints (e.g., water). Great
care and understanding would be required to select tree species,
species mixes, and management strategies to maximize the potential
of sites with widely different available resources.
Policy Options
Public policy decisions to increase carbon storage through
reforestation involve such silviculture issues as replanting,
selection of species to be planted, and land management practices
such as fertilization. To implement these reforestation options,
however, someone must pay for the reforestation itself and for the
cost of maintaining land in forest cover. Landowners face a variety
of alternative opportunities and liabilities. Policies to increase
and maintain forests to store carbon must therefore address
questions of economics as well as silviculture.
The cost of sequestration, considered apart from the value to be
recaptured by sales of timber, ranges upward from zero. The
economic cost—which can, of course, be negative if the
investment in sequestration is less than the return from the sale
of harvested products and other benefits—is determined both by
the value of forest products and by the value of alternative land
uses. Because forests have a biological time scale on the order of
decades, significant uncertainty is the norm in economic analyses.
Public policies can affect management choices by changing, for
example, the taxes levied on timber harvest, the regulations that
govern forest practices, or real estate taxes.
To ensure long-term sequestering of wood on private lands,
governments may need to purchase title or limited property
rights.
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Other Benefits and Costs
Reforestation can have many other positive benefits, including
enhanced biodiversity, wildlife, air and water quality, aesthetics,
forest products, and recreational opportunities. Reforestation can
also raise environmental concerns, and there is some apprehension
about the implications if planting were to occur as broad expanses
of monocultural plantations.
Research and Development
A recent National Research Council (1990) report entitled
Forestry Research: A Mandate for Change provides a number of
research recommendations relative to societal concerns regarding
the relationship of forests and climate, biological diversity loss,
forest product demand, "pristine" forest area demand, sustainable
wood production in conjunction with environmental protection, and
maintenance of forest health. The NRC Forestry Research Committee
(1990) recommends
• improving understanding of the basic biology and ecology
of forests,
• developing information to sustain productivity of forests
as well as to protect their inherent biological diversity, and
• understanding the economic and policymaking processes
that affect the fate of forests.
Conclusions
Reforestation has the potential to offset a large amount of
CO2 emissions but at a cost that
increases as the amount of offset increases. This analysis
recognizes the large land resource required and adopts a
conservative approach with respect to the U.S. Forest Service
analysis of the amount of carbon that might be sequestered. It also
recognizes that forests will mature and that reforestation is thus
an interim approach to the long-term concerns of greenhouse
warming. In addition, if a forest is harvested, the only true
CO2 offset is the amount of carbon
stored in soil, roots, and as lumber or other long-lived products.
Furthermore, there is some apprehension about the implications for
biodiversity if planting were to occur as broad expanses of
monocultural plantations. Overall, however, reforestation seems to
provide a method of storing carbon with little adverse societal
impact and a number of benefits.
Increasing Ocean Absorption of Carbon
Dioxide
The Approach
The oceans already play an enormous role in establishing
planetary climate, both through the transport of heat and supply of
water vapor and
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through the absorption of a large fraction of fossil fuel
CO2. Estimates of the net ocean sink
for CO2 range from the traditionally
accepted value of some 40 percent of fossil fuel CO2 emissions (through reaction of CO2 gas with carbonate ion over the entire
ocean surface, and based upon models derived from Oeschger et al.
(1975)), which today gives close to 3 Gt C/yr, to the much lower
value of 0.6 Gt C/yr recently reported by Tans et al. (1990). No
realistic model of earth's climate can escape simulation of the
oceans in some form.
While the oceanic role in moderating the present-day fossil fuel
increase depends almost totally on the rate of mixing and the
alkalinity, the potential future role of ocean biota cannot be
neglected. The potential amount of total carbon that could be
utilized by oceanic photosynthesis has been estimated to be 35
Gt/yr. However, this figure represents the gross fixation of carbon
in the ocean; the net effect on the atmosphere will depend on the
return flow from decomposition and will eventually reach steady
state. Ice core records (Neftel et al., 1982; Barnola et al., 1987)
show that in the past, the atmospheric CO2 level has fluctuated independently of
the activities of man, with ice age CO2 concentrations some 30 percent lower
than the most recent preindustrial value. A key question is thus,
Can this state be achieved today?
In 1984, three independent research groups published hypotheses
on this phenomenon (Knox and McElroy, 1984; Sarmiento and
Toggweiler, 1984; Siegenthaler and Wenk, 1984). Each reached the
conclusion that the key lay in the surface nutrient concentrations
in polar ocean regions. In areas such as the far North Pacific and
the antarctic circumpolar ocean, high concentrations of nitrate and
phosphate (the key ingredients for plant growth) are unused. The
problem did not seem to be insufficient light, or bitter cold, but
some other variables not yet recognized. The 1984 models showed
that if these nutrients were assimilated, the conversion of CO2 to organic carbon could readily account
for the ice age signal. These nutrients can be regarded as an
important unused chemical capacity of the ocean, one of a scale to
significantly affect the global carbon balance. A radical solution
to this ice age CO2 puzzle has been
proposed by Martin and co-workers (Martin and Fitzwater, 1988;
Martin and Gordon, 1988; Martin, 1990; Martin et al., 1990). These
scientists achieved the first reliable measurements of "dissolved"
iron, at the nanomolar level, in ocean waters through stringent
avoidance of the all-pervasive contamination problem. They further
showed that addition of trace amounts of iron to natural
populations of phytoplankton stimulated photosynthesis, and they
hypothesized that iron limited the phytoplankton growth in these
areas. Thus trace inputs from atmospheric dust events could trigger
blooms of the plankton and ultimately lower atmospheric CO2. Finally, the ice core record shows that
glacial times, with dry and dusty continents, are characterized by
strong dust input to oceans.
The route to contemporary utilization of this unused oceanic
potential is
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(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
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is proposed that CCN emissions should be released over the
oceans, that the release should produce an increase in the
stratocumulus cloud albedo only, and that the clouds should remain
at the same latitudes over the ocean where the surface albedo is
relatively constant and small.
Albrecht (1989) estimates that a roughly 30 percent increase in
CCN would be necessary to increase the fractional cloudiness or
albedo of marine stratocumulus clouds by 4 percent. Albrecht's
idealized stratocumulus cloud, which he argues is typical, has a
thickness of 375 m, a drizzle rate of 1 mm per day, and a mean
droplet radius of 100 mm, and he assumes
that each droplet is formed by the coalescence of 1000 smaller
droplets. The rate at which the CCN are depleted by his model is
1000/cm3 per day. Consequently,
about 300/cm3 per day (30 percent
of 1000) of additional CCN would have to be discharged per day at
the base of the cloud to maintain a 4 percent increase in
cloudiness. This assumes that the perturbed atmosphere would also
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.
Mass Estimates of Cloud Condensation
Nuclei
With Albrecht's assumption in mind that cloudiness in a typical
ocean region is limited by the small number of CCN, we now
extrapolate to the entire globe. 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 n of
CCN that need to be added per day is 1.8 × 1025 CCN/day. The mass of a CCN is equal
to 4/3pr3 × density, and it is assumed
that the mean radius r is equal to 0.07 × 10-4 cm (Charlson et al., 1987). Because
the density of sulfuric acid (H2SO4) is
1.841 g/cm3, the CCN mass is 2.7
× 10-15 g. The total weight
of H2SO4 to be added per day is 31 ×
103 t per day SO2 if all 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. Consequently, the equivalent
emissions of 365 U.S. coal-burning power plants, distributed
homogeneously, would be needed to produce sufficient CCN.
To estimate the value of the sulfur directly, the total weight
of SO2 to be added per day would
equal 32 × 103 t, or about
16 × 103 t of sulfur (S) per
day, which is equivalent to about 6 × 106 t S/yr. If the average market price of
sulfur delivered at the mine or plant is taken as $96.60/t for the
years 1983 to 1987, the cost would be about $580 million per year.
Equating this yearly cost to the 300 parts per million by volume
(ppmv) of CO2 necessary for full
compensation gives $580 × 106/yr/(3890 × 106 t C/ppmv CO2 × 300 ppmv CO2), or about a fraction of 1 cent/t
CO2. To obtain an equivalence to
conserved carbon, known emissions of carbon in 1978, 1979, and
1980
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have been compared with the total measured increase of CO2 to obtain the equivalence: 3890 ×
106 t 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 Gt C or 4400 Gt CO2.
Cost Estimates
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 sulphur. 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 would be 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 perhaps cannot
now be defined) could present a difficult problem for such a system
to solve.
From the above, 16 × 103
t per day or 6 × 106 t/yr
would be needed. If we allocate 102 t per ship per day, and a ship stays
out 300 days each year, roughly 200 ships of 10,000-t capacity (one
reprovisioning stop every 100 days) are required. At a cost of $100
million per ship (surely generous), the capital cost of the fleet
is $20 billion. Amortized over 20 years, the annual capital cost is
$1 billion. Sulfur will cost another $0.6 billion per year, and $2
million per ship per year for operating costs (this is $10,000 per
operating day), giving a total cost of $2 billion per year. Over 40
years (until 2030), this gives a cost of $80 billion, or
approximately $100 billion. This continuously mitigates 103 Gt, for a cost of $0.10/t C/yr, or
$0.025/t CO2/yr. This provides a
cost estimate in the range of $0.03 to $1/t CO2. Of course, this continues to be a
yearly cost of $2 billion per year.
The SO2 could also be emitted
from power plants. These plants could be built out in the ocean
near the equator (the Pacific gives more room than the Atlantic)
and could furnish power for nearby locations (e.g., South America).
Transmission or use of the power in the form of refined materials,
or possibly by the use of superconducting power transmission
systems, could be considered. It would likely require eight large
power plants using "spiked" coal (with 4 times the normal amount of
sulfur), at a cost of $2 to $2.5 billion per plant. Most of the
cost might be borne by those buying the power; so imagining a cost
of, at most, 10 percent per year (the interest on the investment),
total cost would be $2 billion per year (with the above conversion,
$2 × 109/3890 ×
106 × 300 
$0.0005/t CO2).
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Possible Acid Deposition
One must consider whether the injection of this much additional
SO2 into the atmosphere would cause
an acid deposition problem. It must be kept in mind that the
principal component of naturally occurring CCN is sulfate 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 of about 6 × 109 kg/yr, one-quarter of the total
natural amount, is being considered, although locally much more
would be added to the amount naturally present. The oceans have an
enormous buffering capacity (Stumm and Morgan, 1970), so that 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 would be 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
stated above.
Another possible way of dealing with the problem of acid rain
would be to introduce sulfate in the form of ammonium sulfate or
bisulfate, both of which are neutral salts. This would avoid the
acid question from the start. These salts are frequently used 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 C mitigated per year or $0.04/t CO2.
It may also be sensible to consider using ships that pump a
seawater aerosol into the air above the ocean, thus increasing the
density of sea salt aerosol crystals, which can act as CCN (Latham
and Smith, 1990).
Atmospheric Chlorofluorocarbon
Removal
Another option for mitigating greenhouse warming could be to
remove chlorofluorocarbons (CFCs) from their principal reservoir,
the lower atmosphere or troposhere. The expected tropospheric
residence time for CFCs exceeds 65 years (cf. Table 19.2);
evidently these highly inert gases disappear only by very slow loss
to the stratosphere, where ultraviolet rays from the sun cause
molecular decomposition. A reasonable query is whether this natural
process of CFC depletion can be significantly enhanced by
large-scale technical means.
It has been suggested that extremely powerful lasers might be
used to break up tropospheric CFCs (Stix, 1989). Vast arrays of
pulsed lasers at
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mountain altitudes would launch intense infrared beams into the
atmosphere. The laser beams would then selectively destroy
chlorofluorocarbon molecules in the atmosphere through the process
of multiphoton dissociation. Due to the low atmospheric
concentration of the CFCs (less than one part per billion by
volume), any process to remove them must be highly selective. That
is, the process cannot afford to waste energy in reactions
involving any of the far more abundant non-CFC molecules in the
atmosphere. The suggested laser scheme then depends first upon
finding bands of strong laser-light absorption by CFC molecules.
Second, within these bands, one must find ''spectral windows" where
absorption of the laser light by non-CFC molecules in the
atmosphere is virtually absent. Computer calculations making use of
an extensive atmospheric-gas infrared cross-section data base
suggest that 90 percent transmission over 50-km paths would be
possible through dry atmospheres.
Nevertheless, a large number of questions remain unexplored,
among them laser and optical technology, electro-optical conversion
efficiency, anomalous or unexpected laser-light absorption channels
including excited-state processes and stimulated rotational Raman
scattering, infrared bandpass mirrors, adequate laser selectivity,
pulse shaping benefits, wind velocity and atmospheric humidity
patterns, site availabilities, and safety and ecology. Even making
very optimistic assumptions about the resolution of these and other
questions, the expense associated with the installation and
operation of the elaborate and extensive laser facilities would be
prohibitive: to remove 10 percent of the atmospheric CFCs per year,
the electric power bill alone is estimated to exceed $10 billion.
Nevertheless, if technological breakthroughs were to introduce a
factor of 10 to 20 improvement in overall efficiency, the cost of
such processing of the atmosphere, although very large indeed,
might be worth evaluating.
In conclusion, the panel does not believe that the use of lasers
to remove CFCs from the atmosphere is currently feasible.
Conclusions
Several of the geoengineering possibilities discussed in this
chapter, including atmospheric CFC removal, space mirrors, and the
multiple balloon stratospheric screen, appear, with current
technology or that expected to be available soon, to be either
impractical, too cumbersome to manage, or too expensive. These
ideas might merit some further study to be certain of this
conclusion but do not now seem worth great effort. They should be
kept in mind, however, because technological changes may make them
more attractive.
Reforestation is a low-cost, ecologically attractive option that
could be adopted rapidly as an expanded program. It is, however,
limited in its low-cost
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form by the easy availability of appropriate land. Therefore the
panel hesitates to look beyond its initial potential mitigation of
240 Mt CO2/yr. In addition, a number
of years would be required to build reforestation to its full
mitigation potential.
Stimulation of ocean biomass with iron may be feasible and would
be a relatively low-cost option. Its application appears to be
limited at most to the mitigation of about 7 Gt CO2 equivalent per year (about 1.5 times
U.S. annual CO2 emissions). The
biological, ecological, and ocean chemical and physical dynamics of
this possibility are not well understood and should be investigated
further, both theoretically and experimentally. There continue to
be questions as to whether iron is the limiting nutrient.
Furthermore, the circulation dynamics of the antarctic ocean might
severely limit the effect. If feasible, the mitigation potential of
the possibility—storage of CO2
in a standing crop and as dissolved CO2 with slow sequestering of carbon to the
ocean bottom—could probably be established over several years.
If applications of iron were stopped, the standing crop would be
expected to die within days or weeks, thus ending the mitigation
effect.
Cloud stimulation by provision of cloud condensation nuclei
appears to be a feasible and low-cost option capable of being used
to mitigate any quantity of CO2
equivalent per year. Details of the cloud physics, verification of
the amount of CCN to be added for a particular degree of
mitigation, and the possible acid rain or other effects of adding
CCN over the oceans need to be investigated before such system is
put to use. Once a decision has been made, the system could be
mobilized and begin to operate in a year or so, and mitigation
effects would be immediate. If the system were stopped, the
mitigation effect would presumably cease very rapidly, within days
or weeks, as extra CCN were removed by rain and drizzle.
Several schemes depend on the effect of additional dust (or
possibly soot) in the stratosphere or very low stratosphere
screening out sunlight. Such dust might be delivered to the
stratosphere by various means, including being fired with large
rifles or rockets or being lifted by hydrogen or hot-air balloons.
These possibilities appear feasible, economical, and capable of
mitigating the effect of as much CO2
equivalent per year as we care to pay for. (Lifting dust, or soot,
to the tropopause or the low stratosphere with aircraft may be
limited, at low cost, to the mitigation of 8 to 80 Gt CO2 equivalent per year.) Such systems could
probably be put into full effect within a year or two of a decision
to do so, and mitigation effects would begin immediately. Because
dust falls out naturally, if the delivery of dust were stopped,
mitigation effects would cease within about 6 months for dust (or
soot) delivered to the tropopause and within a couple of years for
dust delivered to the midstratosphere.
Such dust would have a visible effect, particularly on sunsets
and sunrises, and would heat the stratosphere at the altitude of
the dust. The
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heating would have an effect on the chemistry of the
stratospheric ozone layer, and this possibility must be considered
before major use of such a mitigation system. The amount of dust to
be added is within the range of that added from time to time by
volcanic eruption, so the effects on climate would not be expected
to go beyond those experienced naturally. However, either the
natural or the artificial effects on the chemistry might be very
serious under conditions of increased CFC chlorine in the
stratosphere, and the result of having these effects continuously
must be considered, so the option might not be usable. Better
specification of dust characteristics and size for best effect and
better data on the fallout rate of dust from various altitudes as
well as on chlorine chemistry are needed. It wil be important to
observe the effects on stratospheric chemistry of any volcanic
eruptions that occur, with special attention to separating the
effects of dust, aerosol, and hydrochloric acid.
Of these systems to alter the planetary albedo, the increase of
low-level marine clouds by increasing CCN and the delivery of dust
to the stratosphere by using large rifles seem the most promising.
The rifle system appears to be inexpensive, to be relatively easily
managed, and to require few launch sites. However, the possible
effect of the additional stratospheric dust on ozone chemistry may
be a serious problem, and the noise of the rifles would have to be
managed. Balloons also appear to be a good possibility, but the
return of the balloons to ground level would require
management.
Sunlight screening systems would not have to be put into
practice until shortly before they were needed for mitigation,
although research to understand their effects, as well as design
and engineering work, should be done now so that it will be known
whether these technologies are available if wanted.
Perhaps one of the surprises of this analysis is the relatively
low costs at which some of the geoengineering options might be
implemented. If, however, further analyses support the preliminary
conclusions, it will bear further inquiry to decide if they can
produce the targeted responses without unacceptable additional
effects. The level at which we are currently able to evaluate the
cost-effectiveness of engineering the global mean radiation balance
leaves great uncertainty in both technical feasibility and
environmental consequences. This analysis does suggest that further
inquiry is appropriate.
Notes
1. Throughout this report, tons (t) are metric; 1 Mt = 1 megaton
= 1 million tons; 1 Gt = 1 gigaton = 1 billion tons.
2. The ships can distribute material across the lane by towing
hoses spread away from the ship with paravanes, a well-known
minesweeping technology.
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Page 461
3. 4p(6.6)2 × 106 km2
×(103 m/km)2 = 547 × 1012 m2
@ 5 × 1014 m2.
The screening only requires covering the illuminated disk, or pr2,
but in many of the cases treated it will not be possible to
maneuver the screening material so as to remain only above the
sunlit side of the disk, therefore 4pr 2
is used.
4.The correct parasol coverage area may be 1.4 percent because
the Ramanathan computation is for 1 percent increase in the 30
percent albedo, but this change will have slight effect on the
estimates. See also Penner et al. (1984), who estimate a dust
requirement of 1.168 × 1010
kg.
5.The current space transportation system costs about $5200/kg;
$130 million per launch with a capacity of 55,000 pounds to 160
nautical miles at 28.5 degrees. A Delta rocket costs $45 million
per launch for 11,000 pounds to 100 nautical miles, or $9000/kg.
Even at $100/kg, the cost of the material would be only 2 percent
of the cost of putting the material into orbit.
6. If the correct equivalence is 1 percent to 1200 Gt, for
example, all material quantities, etc., are smaller by a factor of
10/12; if the equivalence is 800 Gt, the numbers are larger by a
factor of 10/8, etc. For costs per ton of CO2, all costs per ton of carbon should be
divided by 44/12 @ 4.
7. In this connection, see also Early (1989). Estimates range
between $1/t CO2/yr for the lower launch cost and long-lived
mirrors and $10/t CO2/yr for the
higher cost and annual replenishment.
8. From Toon and Pollack (1976), one can make a crude estimate
of the mass of lower stratospheric dust by noting that they give
the density at 20 km as about 1 mg/m3. The "all sizes" curve in
their Figure 9 suggests a reasonably constant concentration in the
8 km from 12 to 20 km. So the mass can be taken to be roughly 5
× 1014 m2 × 8 × 103 m × 10-6 g/m3
= 40 × 1011 g = 4 ×
1012 g = 4 × 109 kg, or roughly half the amount to be
injected to form the screen.
9. It is interesting that this is the same mass as that computed
above for the space mirror, given an assumed density of 2 g/cm3 for dust instead of the 1 g/cm3 used previously (clay has a density of
1.8 to 2.6, alumina of 4, basaltic lava of 2.8 to 3.0).
10. Another suggestion is to shape the dust into highly
conductive needles about 0.1 m in radius by 0.5 m long, the
scattering at an optical wavelength of 1 m would be dipole
scattering with an effective scattering cross section 100 times
greater than for spheres, thus requiring 100 times less material.
Mack and Reiffen (1964) computed these effects in connection with
the West Ford project. The maximum cross section expected for a
perfectly conducting half-wave resonant dipole is 0.86l. In the case of a dipole such as that
specified above, which has an area of 1/2 × 1/5 l a scattering cross section enhancement of
0.86 × 10 = 8.6 is obtained. This enhancement would be
decreased by averaging over all angles of the dipole to the
incoming radiation. Mack and Reiffen compute this effect to be
about 0.1 for backscattering for several polarizations of the
incoming light. In addition, highly conducting dipoles would have Q
values too large to cover the necessary optical bandwidth
effectively. It appears that assuming Mie scattering of dust with a
size spectrum optimized to the scattering of the visible part of
the solar spectrum, roughly comparable to the estimate of Ramaswamy
and Kiehl (1985), is fairly efficient.
11. Material from staff at Naval Surface Weapons Center,
Dahlgren, Virginia, was furnished to John I. Connally, Jr., vice
president of Scientific Applications
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Page 462
International Corporation, and by him in a letter to Lee Hunt,
executive director, Naval Studies Board, National Research
Council.
12. The cost factors will scale proportionately except for any
economies of scale, which have not been considered.
13. Data for Nike Orion and scientific balloons from Ray Pless
(Wallops Island, NASA, private communication, 1990).
14. This estimate uses the value given in Table 7.2 of National
Research Council (1985) for the column (Turco et al., 1983) at 11
to 13 km.
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