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MIT OpenCourseWare http://ocw.mit.edu 12.085 Seminar in Environmental ScienceSpring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.
Jessica Stanley
12.085
5/15/08
Terrestrial Carbon Sequestration
INTROD
Terrestrial carbon sequestration is th
UCTION
e practice of storing carbon within
biomass and soils. With rising concentrations of CO2 in the atmosphere causing
increased concern about global warming, techniques to sequester carbon or
otherwise lower the earth’s temperature are beginning to be considered. The ideas
of geoengineering, “the intentional large‐scale manipulation of the environment,
particularly manipulation that is intended to reduce undesired anthropogenic
climate change” (Keith, 2000) are being proposed, and the public is beginning to
consider that such action might be necessary.
The types of ideas for geoengineering vary greatly, and include such things as
fertilizing the ocean with iron to increase carbon uptake, placing mirrors or lenses
in space to reduce the amount of sunlight incident on the earth, injecting sulfur
aerosols or dust into the upper atmosphere to increase the albedo of the planet, and
changing land management strategies to increase terrestrial uptake of carbon
(Keith, 2000). Some options seem more drastic than others, and each comes with its
own set of benefits as well as social, political, and environmental consequences.
Terrestrial sequestration is appealing as an option for several reasons. First,
it seems like a “greener” or more natural option: by planting trees and making some
land management changes some of the carbon emissions can be offset. This idea is
easier to swallow for many people in comparison with some of the other proposed
solutions that resemble science fiction. Terrestrial sequestration also deals with the
problem more directly, by taking CO2 out of the atmosphere, rather than dealing
with the effects of the CO2 by changing the albedo or incident solar radiation and not
directly assessing the cause. The structure to manage land also already exists; in
many ways humans have been geoengineering for millennia in the form of
agriculture. Thus, if land use practices need to be changed or enhanced for
terrestrial sequestration a network or farmers and forestry workers already exists
to help put such changes in place.
Upon examination of terrestrial carbon sequestration it has several
downsides too. The dynamics and controls and the terrestrial carbon sinks,
especially soil, are not fully understood, which makes using these sinks challenging
because the magnitude and specific sequestration mechanisms are unknown. Also
carbon in these systems is much less fixed than in other systems, so even once
carbon is sequestered, there is no guarantee another change in land management
will not release it in to the atmosphere once more. It also is expensive, and gets
significantly more expensive the more carbon sequestered.
This paper will first give a brief overview of the carbon dynamics of the
earth, and how terrestrial sequestration fits in, then follow with some of the science
behind sequestering carbon terrestrially. Then it wiIl give a brief overview of cost
estimates, and the current methods for calculating the amount of carbon
sequestered in different types of soil environments. In conclusion, terrestrial carbon
sequestration by itself is not the complete solution to the climate problem, and the
science is not well enough understood to make accurate estimates for the amount of
carbon stored in terrestrial sinks.
There are five major carbon pools on the planet (Fig. 11. The ocean is the
largest, containing 38,000
Pg, followed by the
geologic with 5,000 Pg,
soils with 2,500, the
atmosphere with 760 Pg, Figure removed due to copyright restrictions.
and biota with 560 Pg
[Lal, 2004). The soil and
biotic pools together
comprise the terrestrial Figure 1. Carbon in each of the major pools. Geologic is broken up into coal (green), oil (red), and gas (blue). Soil is broken into soil inorganic
reservoir. carbon (red) and soil organic carbon (blue).
From 1850 to 1998,270 +/- 30 Pg COZ has been emitted from fossil fuel
combustion and cement production, while 136 +/- 55 Pg has been emitted as a
result of land use change 78 +/- 12 Pg of which was from anthropogenic decrease of
soil organic content (Lal, 2004). This means that a significant portion of the carbon
emitted since the industrial revolution has been from land use changes, and thus
come from the terrestrial reservoir. However, since 1980 most emission has been
from fossil fuels with 10‐30% of emissions from land use change and tropical
deforestation (Lal, 2004).
The carbon budget in the 1990s for emissions: 6.3 +/‐ .6 Pg C/yr emitted by
fossil fuels, and 1.6 +/‐ .8 Pg C/yr emitted by land use change. For absorption: 3.3
+/‐ .2 Pg C/yr absorbed by the atmosphere, 2.3 +/‐ .8 Pg C/yr absorbed by the
ocean, and 2.3 +/‐ 1.3 Pg C/yr absorbed by unknown terrestrial sinks (IPCC 2001).
The first thing to note is that emissions from land use change is still about 20% of
emissions. The second thing to note is that the uncertainties on both the emissions
by land use change and the absorption by terrestrial sinks is quite large, greater
than 50% uncertainties. Most of the emissions from land use change come from
tropical deforestation, as in most temperate countries the amount of deforestation is
at least compensated by new growth (NAS 1992). In the US about 40% of the
planting is done by the forestry industry (NAS 1992).
Estimates for how much impact on the global carbon cycle terrestrial
sequestration could have vary, but Moulton and Richards (1990), estimate that
more than 50% of carbon emissions could be compensated by reforestation.
S
OIL C S
Carbon in soils is stored in two forms, soil inorganic carbon (SIC) and soil
EQUESTRATION
organic carbon (SOC). SOC is more plentiful, easier to manipulate, and shorter lived.
SIC is mostly carbonate minerals, and composed of two sections, pedogenic and
lithogenic. The Lithogenic portion is derived from direct weathering of rocks. The
pedogenic portion forms from the reaction of Ca2+ and Mg2+ with atmospheric CO2,
and it is the most common form of carbon sequestration in arid and semiarid
environments (Lal, 2004).
The SOC mostly takes the form of humus, which is a dark, amorphous
material with a high surface area and charge density, which clings to clay sized
particles and is a mix of plant, animal, and microbial byproducts. The residence
time of SOC varies from 0.1 yr to 2200 yr (Lal, 2004). The SOC content of the soil is
the easier of the two forms of soil carbon to manipulate with land use practices, so
land practices which increase SOC are desirable, as well as land use practices which
increase the residence time for SOC. Carbon‐content of soil is linked to nitrogen,
phosphate, and water so perturbations of any of these systems can lead to carbon‐
loss. Depletion of SOC can come from physical, chemical, and biological processes.
Physical processes include a decrease in stable aggregates, crusting, compaction,
and erosion. Chemical processes include acidification, salinization, and changes in
the elemental balance. Biological processes include reduction of biologic activity and
change in species diversity (Lal, 2004).
Agricultural and land use practices can significantly affect the SOC content of
soil. Table 1 shows a list of the different types of practices which are beneficial and
harmful for carbon storage in the soil. Particularly, deforestation and biomass
burning, removal of biomass for fuel and foder, tillage, and the drainage of wetlands
release carbon into the atmosphere. Biomass burning produces charcoal, which is a
long‐lived form of carbon, so even though it releases carbon into the atmosphere, it
helps sequester some carbon for longer (Lal, 2004). The climate type, soil type, and
natural vegetation also have a very large impact on the amount of carbon which can
be stored in the soil, and how much effect different land use practices have on the
carbon content.
range of soil types and vegetation !Hendrix et al.1992". Practices that enhance C sequestration in ag-ricultural soils include conservation tillage, mulchfarming, growing cover crops and forages in rotationwith row crops, integrated nutrient management andadopting recommended agricultural practices !Lal1999, 2000, 2001, 2002, 2003a" !Table 2".
Impact of land use and management must be as-sessed on the basis of net C sequestration. This im-plies that the gross C sequestration in terrestrialecosystems must be adjusted for hidden C costs in allinput of fertilizers, pesticides, tillage operations, etc.
Net C sequestration=!gross terrestrial sequestration"! !hidden C costs" !3"
Hidden C costs are high for nitrogenous fertilizers,pesticides, and plow tillage. In addition to hidden Ccosts, some of these mitigation options may also leadto increased emission of N2O and CH4. Applicationof nitrogenous fertilizers may exacerbate emission of
N2O. Similarly, application of manure, compost andother biosolids accentuates emissions of CH4 andN2O !Minami et al. 1994". Therefore, improving use-efficiency of these inputs is important to enhancingnet C sequestration !Schlesinger 1999; Robertson etal. 2000; Smith et al. 2000".
The question of permanence must also be ad-dressed. Once sequestered in soil through adoption ofconservation tillage and other recommended manage-ment practices, how long will the C stay in the soil?On a global scale, the mean residence time !MRT" ofC, as computed by dividing the pool by flux, is morein soil than in trees but less than that in ocean. At thesoil/pedon or farm level, however, the permanence ofC sequestered in soil depends on the continuation ofthe recommended management practices. Once con-verted from plow till, the no-till system of seedbedpreparation must be continued in perpetuity. Reicoskyet al. !1999" showed that even a single plowing candrastically accentuate emission of CO2 from soil be-cause of an increase in the rate of mineralization.
110
Table 1. From Lal (2004)
Table removed due to copyright restrictions.
There is significant erosion of the organic portion of soil when land is
transitioned from the natural environment to cropland. The first stage of erosion is
breakup of soil aggregates by kinetic movement by wind or water. Soil aggregates
are composed of clay, organic matter, and polyvalent cations (Lal, 2001). The next
step is transport of detached and entrained particles by fluids, and finally deposition
of the material in the new locations. There are three main places where the eroded
material can end up. It can end up being redistributed over the land surface, which
leads to decomposition of organic matter, and probably a net emission of carbon,
estimated at 1.14 PgC/yr. The carbon that is transported by rivers is not well
understood, but it is thought that this ends up decomposing as well, one estimate is
that European rivers emit 30‐60 TgC/yr, 5‐10% of Europe’s total carbon emissions
(Lal, 2001). Another view is that rivers bring organic matter to the ocean for long‐
term storage (Lal 2004). Soils can also end up being buried in depressions, which is
thought to enhance aggregation and storage, it is estimated that 0.57‐1 PgC/yr is
stored this way. However if conditions are anaerobic, burial can lead to the
production of methane and the release of N2O (Lal, 2001).
The types of plants on the land also affect the amount of carbon sequestered
in the soil. Thus it is important to choose the correct plant type to maximize
sequestration. Different traits are more valuable for increasing soil sequestration
depending on the biome in which the plant resides (De Deyn et al., 2007). Rapidly
changing climate could change the biome in a specific location, and have effects on
the carbon stored in these ecosystems. There are two methods of enhancing carbon
storage, first by fast input of carbon through high primary productivity, and second
by slow output by slow decomposition. De Deyn et al. find that in ecosystems where
soil nutrients are the limiting factor, slow growth and long litter residence time tend
to dominate, while in rich‐soil ecosystem where light is the limiting factor the
opposite is true. De Deyn et al. also indicate many areas for further research, which
indicates that this part, as the other parts, of soil sequestration is not fully
understood.
BIOTIC STORAGE
There is also carbon that is stored in biomass itself, thus increasing the
amount of biomass takes carbon out of the atmosphere. This leads to the idea of
reforestation, or planting large plantations of trees. One of the problems with this
method though is that it is a relatively short‐term sink, as it is necessary to keep the
carbon from oxidizing to CO2 after the biomass dies. Lumber, soil and roots are
longer‐term storage, and one suggestion is to use the biomass to substitute for some
of the fossil fuel usage (NAS 1992). It generally takes 20 to 40 years for the biomass
to mature, and then the plantation no longer acts as a large sink, it reaches steady
state. Thus this method should be viewed more as a short‐term solution, a fix while
carbon emissions are reduced in other manners.
There are also other side effects of having plantations of trees, and these
effects differ based on the biome and the soil type, so some areas are much more
suited for plantations than others. Other aspects include effects on groundwater,
stream runoff, and soil. A study by Jackson et al. (2005) found that there were
dramatic changes in stream flow within the first few years of planting, an average of
38% decrease, with 13% of streams drying out for at least a year. Climate feedbacks
of reforestation can sometimes offset these effects by increased transpiration and
convective rainfalls, but convective rainfalls were only found to occur in Florida and
southern Georgia. The increased transpiration made for slightly lower summer
surface temperatures in some places. Increasing the biomass also changes the soil
chemistry significantly (Jackson et al., 2005). First most trees have acidic litter (i.e.
fallen leaves), and this causes a lowering of the pH of the soil, unless the trees are
growing on a well‐buffered medium like limestone. There is also an increase in Na in
the soil, which can cause saline ground water. Depending on the soil type and the
groundwater patterns, planting trees in an area which was not originally forested
can cause groundwater to become to salty for human use (Jackson et al., 2005). In
some areas though, for instance southern Australia and places which were once
forested and now cropland, reforestation can help the water quality by reducing
dryland salinization and reducing pesticide and fertilizer runoff. Tree plantations
are better in some areas than others, and that environmental planning is needed
before starting large‐scale projects.
Keith (2000) also briefly mentions the idea of genetically modifying
organisms to make capture more efficient, which could be an area of further
research.
C
OSTS
There were several estimates of the cost of implementing techniques of
biological sequestration. The National Academy of Sciences report (1992) found
reforestation to be one of the more expensive methods that they explored at $5.80
to $47.75 per ton carbon, and they state that it gets progressively more expensive
the more carbon it attempts to store.
Spearow et al. (2007) estimated the cost of carbon storage by setting aside
highly erodable cropland (HEL). To estimate the marginal costs, they took the profit
that value of the crop that would be grown on the land and compared it to land
rental prices. Some of the prices came from the USDA Farm Resource Regions Data.
The marginal cost could be looked at as a proxy for what farmers would have to be
paid to adopt the sequestration practices.
Carbon storage of the land was estimated using the method outlined by the
IPCC, which uses native soils as a reference for carbon storage potential. Depending
on the soil and how long it has been put to agricultural use, it is estimated that the
agricultural soil has lost 70‐75% of the potential carbon storage. Soil is thought to
reach equilibrium in about 20 years, when it becomes neither a net source nor a net
sink. The soil type and tillage practices also change the amount of carbon
sequestered, with mud based soils sequestering better than sandy soils (Spearow et
al., 2007).
Cost estimates ranged significantly, from $11 per Mg C for some cotton land
to $4492 per Mg C for soybeans on sandy soil. Average was $288 per Mg C. Costs
were elastic until about $200 per Mg C, after which they became inelastic, meaning
large costs for small increases. They also estimated the effect on total cost
production, saying for sorghum it had the most effect, 81% decrease of ending
stocks, while cotton had the least, with 17% of ending stocks. This would affect the
price of crops though, which would probably affect the marginal cost of carbon
sequestration, to make it more expensive (Spearow et al., 2007).
Because terrestrial sequestration gets incrementally more expensive, and is
relatively expensive anyway, it seems likely that terrestrial sequestration is not
suitable for a full‐scale geoengineering solution for the climate problem, it is likely
that is would only be used as an interim solution, or a partial solution.
Q
UANTI
The standard method of quantifying the amount of carbon sequestered in
FYING SEQUESTRATION
soils and agricultural land is the method outlined by the IPCC in 2006. This is a
report on the methodology of calculating the sequestered carbon for individual
counties. The first step is to choose Tier 1, 2, or 3. Tier 1 is the most general, using
only the tables of coefficients from the report, where as Tier 2 and 3 are more
empirical.
The Tier 1 technique is outlined specifically in the report, and worksheets are
included for the calculations. Generally the calculation is split up into very small
specific pieces starting with:
Where AFOLU is agriculture, forestry, and other land use; FL is forestry land; CL is
cropland; WL is wetlands; SL is settlement lands; and OL is other lands.
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Each of these is farther broken down:
Where LU is the specific land use, AB is above ground biomass, BB is below ground
biomass, DW is dead wood, LI is litter, SO is soils, and HWP is hard wood products.
Each of these categories is further spit up and calculated separately using the
worksheets in the annex of the report, and example of which is shown in Figure 2.
There are about 50 worksheets has a column for each value, and tells how to
calculate the value if it is a calculation, and what table to find a constant if it is a
constant. The tables of constants are found in the relevant chapters of the report.
This is useful, but the accuracy is somewhat questionable, estimating each piece
separately from generic coefficients may have large errors. The fact that the
calculation is parsed up so many times may be a relic of the fact that this not
completely understood.
The tables of coefficients used for calculating the carbon are based on
relevant portions of the scientific literature. For instance The table of default carbon
stocks for mineral soils with native vegetation is based largely on a paper by
Jabbagy and Jackson (2002) entitled “The Vertical Distribution of Soil Organic
Carbon and its Relation to Climate and Vegetation”. In that article they try to
characterize soil carbon in the top 3 m of soil based on more than 2700 profiles in
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three global databases, paying attention to variables such
igure 2. Example of a worksheet from the IPPC (2006) Report, Annex 1.
as vegetation type,
climate, and land use. They found that the plant type affected soil carbon
significantly, and had more of an affect than the direct effects of precipitation.
Eve et al., (2001) use the method set out by the IPCC (the 1996 precursor to
the 2006 version) to calculate the changes in carbon storage in U.S. croplands since
the 1990 baseline. They find that soil use practice changes have resulted in a net
sink to mineralized soils of 11.31 million metric tons carbon per year, however the
organic portion of soils is still a net source of 6.03 million metric tons per year. The
warm temperate moist mineral soils are the largest sink, whereas the sub‐tropical
moist organic soils have the largest emissions.
Another method besides the IPCC method was outlined Gross et al., (2001).
This approach has not been put into practice yet, but involves modeling and
monitoring carbon storage in U.S. farmland east of the Rockies. This was achieved
using a model called EPIC‐Erosion/Productivity Impact Calculator, which clusters
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Figure removed due to copyright restrictions.
soils and climate areas by their characteristics. The goal of this project is to
establish a baseline of the 1992 levels of soil carbon sequestration, and monitor
changes over time.
These are all method solely of estimating carbon sequestration. This is hard
to measure on the large scale, so it is challenging to tell the accuracy of these
techniques.
CONCLUSION
Terrestrial carbon sequestration is a set processes that involve the
interaction of soils, biota, and the climate system, and is not fully understood. It
could be a part of a multi‐faceted solution for the climate problem, but on it’s own it
does not represent a full‐scale geoengineering solution. This is partially because the
dynamics of carbon sequestration are not understood well enough to be controlled
and the amount of carbon sequestered accurately calculated. Also biotic and some
types of soil sequestration are not very long lived, so they are only effective as short‐
term solutions. In general terrestrial carbon sequestrations merits more research,
but is not a complete solution.
R
De Deyn, Gerlinde B., Johannes H. C. Cornelissen, Richard D. Bardgett. 2008. Plant
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