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Review
The potential for land-based biological CO2 removal to
lower future atmospheric CO2 concentration
Carbon Management (2010) 1(1), 145–160
Timothy M Lenton†
A combined approach of deliberate CO2 removal (CDR) from the atmosphere alongside reducing CO2
emissions is the best way to minimize the future rise in atmospheric CO2 concentration, and the only
timely way to bring the atmospheric CO2 concentration back down if it overshoots safe levels. Here, landbased biological CDR and storage methods are reviewed, including afforestation, biomass burial, biochar
production and bioenergy with CO2 capture and storage. The current and future CDR flux they could generate
and their total storage capacity for CO2 are quantitatively assessed. The results suggest that there is already
the potential to counterbalance land use change CO2 emissions. By mid-century, the CDR flux together with
natural sinks could match current total CO2 emissions, thus stabilizing atmospheric CO2 concentration. By
the end of the century, CDR could exceed CO2 emissions, thus lowering atmospheric CO2 concentration and
global temperature.
The global carbon cycle is currently perturbed by human
fossil fuel burning and land use change activities, with
atmospheric CO2 concentration rising (at ~2 ppm yr-1) and
carbon also accumulating in the ocean and on land [1] . In
order to minimize the risk of dangerous climate change,
as enshrined in Article 2 of the UN’s Framework Convention
on Climate Change (UNFCCC), the rise of atmospheric CO2
must be halted and potentially reversed. In simple terms,
stabilizing CO2 concentration demands that sinks match
sources, and lowering CO2 concentration demands that
sinks exceed sources (Figure 1) . The accepted policy
approach to achieving stabilization is to rapidly reduce
CO2 emissions to match natural (i.e., land and ocean)
sinks, and then to slowly reduce CO2 emissions to zero,
at the same rate that natural sinks decay. This should
maintain an approximately constant CO2 concentration
followed by a stabilization of global warming. However,
once the necessary rapid reductions in CO2 emissions
are underway, a safer strategy would be to carry on until
they are eliminated, thus limiting the cumulative carbon
emission. This will result in a ‘peak then slow decline’
of atmospheric CO2 concentration, with the cumulative
carbon emission determining the corresponding peak
in global temperature, termed the ‘cumulative warming
commitment’ [2] . Either way, we face an immediate and
profound collective challenge to transform the current
exponential increase in CO2 emissions (~2% yr-1 over
the past 25 years [3] and >3% yr-1 over the past decade [1])
into a comparable or greater rate of decrease in CO2 emissions. This transition must start soon and be completed
within decades, if global warming is to be restricted to
less than 2°C above preindustrial levels [2,4,5] . Already it
demands rates of technological and economic change that
may simply be unachievable [3] . So, what else can we do?
Rather than just trying to reduce anthropogenic
sources of CO2, if we can also actively create significant CO2 sinks, then we can halt the rise of CO2 concentration sooner, lowering the peak CO2 concentration (Figure 1) . Subsequently, if we can make the sum
of created and natural (i.e., land and ocean) CO2 sinks
exceed anthropogenic sources of CO2, we can bring
the CO2 concentration down, and we can do so faster
than by reducing emissions alone and relying on natural
sinks (Figure 1) . Creating sinks is termed ‘CO2 removal’
(CDR) and it is already implicit in several scenarios for
CO2 stabilization at relatively low levels [6] . Often it
†
School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK
Tel.: +44 1603 591 414; Fax: +44 1603 591 327; E-mail: [email protected]
future science group
10.4155/CMT.10.12 © Timothy M Lenton
ISSN 1758-3004
145
Review Lenton
takes the form of afforestation and
reforestation,
which has long been
Article 2 of The UN’s Framework
recognized by the UNFCCC. CDR
Convention on Climate Change
(UNFCCC): States the overarching
effectively reduces the cumulative
objective of “stabilization of greenhouse
carbon emission and hence should
gas concentrations in the atmosphere
reduce the corresponding cumulaat a level that would prevent dangerous
tive warming commitment [2] . An
anthropogenic interference with the
climate system. Such a level should be
additional long-term role for CDR
achieved within a time-frame sufficient
is that it could allow some ‘essento allow ecosystems to adapt naturally
tial’ or ‘unavoidable’ fossil fuel
to climate change, to ensure that food
production is not threatened and to
CO2 emissions to continue, without
enable economic development to
increasing the CO2 concentration,
proceed in a sustainable manner”.
by counter-balancing them.
Geoengineering: Deliberate large-scale
Whilst all this sounds promising
manipulation of the planetary
in
principle, it depends critically on
environment to counteract
how large the potential sink from
anthropogenic climate change. It can
be subdivided into methods of
CDR is in practice. Two factors are
reducing the absorption of sunlight
critical to determining the potential
(‘solar radiation management’) and
of CDR. First, the rate of CDR (the
methods of removing greenhouse
flux) that can be achieved at a given
gases from the atmosphere, especially
‘CO2 removal’.
time and, second, the total storage capacity for removed CO2 is of
CO2 removal (CDR): Subset of
geoengineering methods that involve
importance. The achievable CDR
actively removing CO2 from the ambient
flux, together with the anthropogenic
air, by biological, chemical or physical
emissions flux and natural sinks flux,
means, and storing the resulting carbon
in long-lived reservoirs.
determine whether CO2 concentration can be stabilized, reduced or
Afforestation: Establishment of forest
on land that has not recently been
will continue rising at a given time
forested; whereas, reforestation is the
(Figure 1) . The total storage capacreestablishment of forest after recent
ity for removed CO2, together with
removal. For afforestation to count as a
the total cumulative CO2 emission,
carbon store it must be permanent.
determines
how much anthropoBiochar: Charcoal created by the
genic CO2 will remain in the atmopyrolysis of biomass that is added to soil
to store carbon.
sphere–ocean system in the long
term and, therefore, the long-term
Bioenergy with carbon storage (BECS):
Technologies that use photosynthesis
concentration of CO2 and the corto remove carbon from the atmosphere,
responding
warming [2,7–9] . A third
making use of some of the energy in
important consideration, especially
the resulting biomass and capturing
some of the carbon in long-lived forms
in the long term, is whether there is
(either CO2 in geological storage
leakage of CO2 from the storage resor biochar).
ervoirs back to the atmosphere and,
if so, at what rate.
The various methods available for CDR have
been summarized in previous work [10] and a Royal
Society review [11] . They can be categorized into physical, chemical or biological approaches, and land- or
ocean-based approaches. Current assessments suggest that land-based methods of CDR either via biological (photosynthesis) or physical and chemical
means have greater potential than ocean-based methods [10,11] . Furthermore, existing economic assessment
suggests that land-based biological CDR has a better cost–benefit ratio than air capture of CO2 using
physical and chemical means [12] . Consequently, the
Key terms
146
Carbon Management (2010) 1(1)
chosen focus here is on quantifying land-based biological methods of CDR. At the outset, it is worth noting
that physical/chemical air capture would take up far
less land space than biological methods and could, in
principle, remove as much CO2 as societies were willing to pay for, but there is a distinct shortage of future
projections of the CDR flux it could generate (i.e., very
little to review).
Pathways & constraints
Plants and all other organisms performing photosynthesis are solar-powered carbon-capture devices.
Photosynthesis is actually a rather inefficient way of
converting sunlight into usable energy – approximately
0.5% efficient at best [13] . Solar thermal or solar photovoltaic methods are capable of approximately 20% efficiency [14] , but crucially these methods capture carbon
at the same time. So, whilst on purely physical grounds
biomass would only be expected to play a modest role
in future energy supply [13,14] , on chemical grounds it
could play a valuable role in various CDR pathways
(whilst also supplying carbon-based fuels), if sufficient
area is available to be devoted to it.
Currently, global terrestrial net primary production
(NPP) is approximately 60 PgC yr-1 [15,16] , whilst fossil fuel emissions (including cement production) are
approximately 8.5 PgC yr-1 and land use change emissions are approximately 1.5 PgC yr-1, totaling approximately 10 PgC yr-1 [1] . If land-based productivity is going
to be used to generate a carbon sink to match current
total emissions, it will require at least in the order of
15% of the world’s productive land surface. The productive (i.e., ice and desert free) land surface is approximately 10 Gha, so somwhere in the order of 1.5 Gha will
be required. If this sounds a lot, for reference, global
cropland currently totals approximately 1.5 Gha [17]
and managed grazing land more than 3.3 Gha [18] . The
area required could be considerably more, because we
have assumed that all carbon captured can be converted
to a permanent carbon store, whereas present global
NPP is mostly counterbalanced by heterotrophic respiration (and fires), leaving a net sink of approximately
2.5 PgC yr-1 [1] . The challenge is to lock carbon away
in permanent storage without causing carbon to be lost
elsewhere from ecosystems.
Figure 2 summarizes the main land-based biological
pathways of CDR, the conversion processes involved
and the destination reservoirs for carbon. The simplest
land-based biological CDR pathway is to accumulate
carbon in woody biomass through permanent afforestation, perhaps augmenting the sink by harvesting
some of the biomass as wood products and, thus,
maintaining the corresponding forestry plantations
in a high growth phase [19,20] . Alternative suggestions
future science group
The potential for land-based biological CO2 removal to lower future atmospheric CO2 concentration Review
are to deliberately bury wood [21] or
Time
crop residues [22,23] to store carbon.
None of these pathways make use
2010
2050
2100
of the chemical energy in biomass,
Peaking CO2
Rising CO2
hence they are referred to as ‘bio490 ppm
mass CDR’. Alternatively, if energy
525 ppm
Mitigation-only
is extracted from biomass, some of
policy path
the associated carbon can, in principle, be captured and stored as
CO2 (from fermentation processes
Rising CO2
or combustion flue gases) [24,25] or
390 ppm (+2 ppm yr-1)
Emissions
Natural
Emissions Natural sinks
as biochar (from pyrolysis of bio(declining)
sinks
(declining) (declining)
mass) [26] . The feedstocks for these
(declining)
‘bioenergy CDR’ pathways could
include deliberately grown energy
crops, forestry wood that is surplus
Emissions Land and
to other uses, and residues (i.e.,
(growing) ocean sinks
waste products) from agriculture,
Peaking CO2
Declining CO2
energy crops and forestry. Finally,
470 ppm
Mitigation and
there is the possibility that some of
365 ppm
CO2 removal
the biomass consumed in deliberpolicy path
ate anthropogenic vegetation fires
could be converted to biochar
(rather than released as CO2) [26] ,
Emissions CO2
Emissions CO2
Natural
Natural
which is referred to as ‘slash and
(declining) removal sinks?
(declining) removal sinks
char CDR’.
(growing) (declining)
(peaking)
Before getting into the specifics
of the different CDR pathways,
let us note some key over­a rching Figure 1. Contrasting the atmospheric CO2 balance on key time horizons following a
constraints on the potential for conventional, mitigation-only (reducing CO2 emissions) policy path, or a combined
management of the land biosphere approach of mitigating emissions and CO2 removal. The boxes represent CO2 in the
to generate a removal flux of CO2 . atmosphere (size gives approximate concentration) and the vertical arrows indicate the major
These are the supply of available fluxes of CO2 to and from the atmosphere (width gives approximate magnitude). Illustrative
land area, the yield of carbon (per numbers for CO2 concentration in 2050 and 2100 are based on the modeling discussed in this
unit area and time) and the con- article, but should not be taken as forecasts.
version efficiency to permanently
stored carbon. Of these, the supply of land area is abandoning of some land, whilst new land goes under
probably the strongest constraint on the achievable cultivation. Looking ahead, several Intergovernmental
CDR flux. Following others (and consistent with Panel on Climate Change Special Report Emissions
Article 2 of the UNFCCC), it is assumed that natural Scenarios (IPCC SRES) [28] project a net decline in
ecosystems should be protected because they provide cropland (A1, B1 and B2), whilst the A2 scenario
valuable services to humanity. Hence, for example, projects net growth [29] . All produce a supply of abanreplacing native forest with managed plantations is doned cropland, which is up to 0.6–1.3 Gha in 2050
not a permissible land use change in the pursuit of and approximately double this in 2100 [29] . Others
CDR. Many studies assume that abandoned agricul- estimate that up to 3.6 Gha of agricultural land could
tural land will be the key source of land for affor- in theory become available by 2050, if land use patestation and/or bioenergy crops. At first sight, this terns are optimized and very efficient agricultural
seems surprising: with a growing global population systems adopted [30] . Low-productivity land (includand changes in diet towards more land-intensive meat ing grazed grassland) is projected to dwindle in area
consumption, should we not expect expansion of agri- and to have negligible potential for deliberate biomass
cultural land? Historically, since the early 1960s, there growth [29] , although it has been suggested that large
has been little net change in the land area under culti- areas of desert in Australia and the Sahara could be
vation despite a doubling of population [27] . Yet, even irrigated by the desalination of seawater and forests
with constant area under cultivation, there can be grown there [31] .
future science group
www.future-science.com
147
Review Lenton
carbon is to be maintained in the
biomass of standing trees, then
the whole life cycle and harvestAgricultural
crops
ing regime must be considered and
average carbon sequestration correBurial
spondingly reduced, with values of
Buried
Residues
biomass
0.8–1.6 MgC ha-1 yr-1 being used [20] .
The conversion efficiency to
Ha
stored carbon varies significantly
rv
Energy crops
es
between methods. If one leaves
t
biomass in permanent forests and
their soils (where previously the
Energy
Stored CO2
land stored less carbon), convercrops
sion efficiency may approach 100%,
Ha
although natural disturbances such
Managed
rv
es
forests
as pests and fire that reduce carbon
t
storage cannot be completely prePyrolysis
vented [34] . Similarly, when burying
Biochar
Surplus wood
biomass, the conversion efficiency
is potentially close to 100%, but
st
decomposition cannot be come
v
Afforestation
ar
pletely prevented. We generously
H
assume a maximum 100% converAccumu
sion efficiency for either pathway.
Standing
biomass
lation
Other pathways that involve ferr
ha
c
mentation, combustion or pyrolynd
sis of biomass inevitably lead to
ha
Shifting
s
a
Sl
cultivation
greater losses of CO2. The energy
cost associated with capture and
storage of CO2, together with the
Figure 2. The main pathways of land-based biological CO2 removal. Carbon capture and
price earned on the carbon stored,
storage occurs either from fermentation or combustion. Return fluxes of CO2 to the atmosphere
will economically determine uptake
are not shown. The reservoirs on the right have different residence times for carbon and
of the associated technologies and
corresponding leakage rates back to the atmosphere.
the conversion efficiency achieved.
CCS: Carbon capture and storage.
Here, we will just consider the fraction of carbon that can in principle
To maximize the CDR flux on a given area, one be realistically captured. Biomass carbon that is turned
wants to maximize yield. For a given area of land, the into transport fuel represents a dispersed source of CO2
yield (expressed here in MgC ha-1 yr-1) varies with plant that cannot be captured, but the process of fermentatype, location (different locations have different climatic tion to produce ethanol yields a readily captured pure
and soil conditions) and harvesting regime. Yields are CO2 stream that contains approximately a third of the
often given in units of dry mass rather than carbon, so carbon in the feedstock [12] . If the remaining biomass
one also needs to know the carbon content of differ- carbon is combusted to generate electricity and heat,
ent biomass types. For tree plantations (e.g., Pinus and CO2 from the flue gasses can be captured with 60–80%
Eucalyptus – the two main species globally), achieved efficiency [101] . Together, CO2 capture from a mixture
yields range over approximately 3–14 Mg ha-1 yr-1 [32] , of fermentation and combustion can capture approxiwhich assuming approximately 0.5 gC g-1 average mately 50% of the carbon in the feedstock [101] . For
carbon content of wood [33] , gives approximately pyrolysis, approximately 50% yield of carbon in bio1.5–7 MgC ha-1 yr-1. Yet some projections for woody char is also achievable and can be exceeded in some
bioenergy crops have assumed global average yield levels circumstances [26] , but where energy output needs to be
ranging over 1.5–15 MgC ha-1 yr-1 (3–30 Mg ha-1 yr-1) [32] increased, biochar yield is inevitably reduced. In prinor 8–10.5 MgC ha-1 yr-1 (16–21 Mg ha-1 yr-1) [30] , which ciple, CO2 capture could be combined with pyrolysis to
seem ambitiously high [19] . Other energy crops gener- boost carbon recovery, but we opt for 50% achievable
ally have yields less than or equal to woody crops. If conversion efficiency for either pathway.
Land uses
Intermediaries
Carbon stores
Net primary production
Atmospheric CO2
S
CC
148
Carbon Management (2010) 1(1)
future science group
The potential for land-based biological CO2 removal to lower future atmospheric CO2 concentration Review
We now turn to consider the specific CDR pathways, their CDR flux potential and total carbon
storage capacities.
Afforestation & reforestation
The conversion of unforested land to permanent forest
creates a net carbon sink and a store of carbon in the
biomass of the trees and in the soil, although there can
be transient (and even net) losses of carbon from soil
depending on location. Once a forest reaches maturity,
the sink is thought to decline to zero, with respiratory
carbon losses matching photosynthetic carbon uptake,
although recent studies point to a persistent carbon
sink in old growth forests [35] . By harvesting carbon
in the form of wood products and replanting, forestry
plantations can be maintained in a higher average yield
state, thus increasing the CDR flux [20] . In addition, by
reducing demand for wood products from other land,
the total area planted and, hence, the cumulative carbon
storage can increase [20] .
Large afforestation programs have already been
undertaken, with an estimated 264 Mha afforested
in 2010, increasing at approximately 5 Mha yr-1 over
2005–2010 [36] . In China alone, the corresponding
CDR flux is estimated to have been 0.19 PgC yr-1
over 1988–2001 [37] . If the 264 Mha of existing plantations are accumulating carbon at an average rate
of 0.8–1.6 MgC ha-1 yr-1, as used in global projections [19,20] , then the corresponding CDR is already
0.21–0.42 PgC yr-1. Conceivably, this is an underestimate since yield can be considerably greater in the
tropics. If at maturity these plantations have a conservative yield of approximately 100 MgC ha-1, they will
store approximately 26 PgC globally. However, natural
disturbances have the potential to significantly reduce
carbon storage and the corresponding CDR flux [34] .
Many studies of the future CDR potential of afforestation and reforestation have been conducted over the
past 20 years, using quite different methods and underlying assumptions (Tables 1 & 2) . The key determinants of
the future global CDR flux achievable by afforestation
(Table 1) are the area that is afforested at a given time and
the yield (rate of carbon accumulation per unit area).
The afforestation CDR flux grows both as planted trees
approach their peak rates of carbon accumulation and as
progressively more land is subject to planting. Current
forecasts generally start from zero activity at the outset,
despite significant afforestation having been underway
Table 1. Existing forecasts of potential CO2 removal flux from afforestation, biochar production and bioenergy
with CO2 capture.
Method
Time
(year)
CDR flux
(PgC yr-1)
Key conditions/assumptions
Afforestation
2000
2010
2015
2025
2030
N/A
2035
0.19
0.21–0.42
0.53
0.83
0.12–0.24
0.4–0.8
10.0
1.0
0.2–1.5
~0.6–1.2
1.48
0.3–3.3
~1.5–3.3
0.56
1.2
1.74
3.15
5.5–9.5
0.19-0.23
6.16
2060
1.8
11.6
China only (1998–2001)
264 Mha × 0.8–1.6 MgC ha-1 yr-1
193 Mha (≡ 2.74 MgC ha-1 yr-1)
345 Mha in ~35 years
US$20–100 (tCO2)-1
As above, all forest products
1 Gha × 10 MgC ha-1 yr-1
550 Mha
SRES A2, B1, B2, A1b range
SRES B2, A1b
345 Mha
SRES A2, B1, B2, A1b range
SRES B2, A1b
Estimate of present potential
1 Gha, 2.5 MgC ha-1 yr-1 to biochar
1.5% yr-1 growth of above
Continued 1.5% yr-1 growth
180–310 EJ yr-1 all by pyrolysis
Present sugarcane and pulp mills
1.15 Gha sugarcane and switchgrass,
~10 MgC ha-1 yr-1, 53% captured
1 Gha forest, 2.5 MgC ha-1 yr-1, 60% captured
1.5% yr-1 growth of above
2035
2050
2055
2100
Biochar
production
CO2 capture from
bioenergy
N/A
2035
2060
2100
Ref.
[37]
[36], this study
[45]
[45]
[39,40]
[39,40]
[41,101]
[56]
[38]
[20]
[45]
[38]
[20]
[26]
[41,101]
[41,101]
[10]
[26]
[25]
[101]
[101], corrected
[101]
Note that the start years of future projections often differ between studies.
CDR: CO2 removal; SRES: Special Report Emissions Scenarios.
future science group
www.future-science.com
149
Review Lenton
assumed to undergo afforestation
within 25 years (40 Mha yr-1) [41,101] ,
approximately an order of magForm of
Location
Storage
Key assumptions/basis
Ref.
nitude above recent afforestation
carbon
capacity (PgC)
rates and several times the forecast
[42]
Biomass
Permanent new
50–100
supply of abandoned agricultural
[45]
forests
104
345 Mha in 100 years
land [20] , implying major conflicts
[47]
60–87
2050
with other land uses, notably food
[43]
51–113
2100, extrapolation of current rates
production and the preservation of
[44]
48–147
2105, demand for forest products
natural ecosystems.
[41]
120
2035, 1 Gha
Estimates of the total carbon
[20]
900
2100, physical potential
storage potential of permanent for[38]
17–146
2100, SRES A2, B1, B2, A1b range
est plantations are given in Table 2.
[20]
68–133
2100, social potential, SRES B2, A1b
Historical cumulative carbon emis~300
Long-term social potential
sions from land use change are
[26]
Biochar in Cropland
224
1.6 Gha, 140 MgC ha-1
approximately 200 PgC [40] , approxi[26]
soils
Grassland
175
1.25 Gha, 140 MgC ha-1
mately 150 PgC from deforestation
Abandoned
98–140
0.7–1 Gha by 2100, 140 MgC ha-1
This study
and this has often been used as an
cropland
upper limit on the amount of car~500
Total
bon that could be recaptured in
[60]
Liquid CO2 Oil and gas fields 184–245
forest biomass and soils in future.
[60]
Unmineable
4–55
However, in principle, this numcoal seams
ber can be far exceeded because, in
[60]
Deep saline
273–2700
many regions, managed plantations
formations
can store more carbon than native
~500–3000
Total
vegetation. A recent estimate of the
SRES: Special Report Emissions Scenarios.
‘physical potential’ of harvested
for decades. Hence, the forecasts tend to underestimate
plantations is that they could store
the CDR flux in the short term. Abandoned agricul- up to approximately 900 PgC by 2100 (on 3.8–4 Gha)
tural land is expected to continue to become avail- after an initial net emission of approximately 200 PgC
able throughout this century, at rates that range over in establishing them [20] . However, in practice, there are
0–17 Mha yr-1 across the SRES A2, B2, B1 and A1b very large constraints on converting land to permanent
scenarios [38] . The A2 scenario gives a low supply of forest, especially ongoing needs for food production,
0–2 Mha yr-1 throughout the century, but observed rates wood supply and conservation of natural habitats. If
of afforestation were approximately 5 Mha yr-1 in 2005– only allowing afforestation on abandoned agricultural
2010 and are already at the upper end of the SRES land, the ‘social potential’ for carbon storage is reduced
range. The ongoing supply of abandoned agricultural to 68–133 PgC by 2100 (on 695–1014 Mha) [20] . This
land potentially allows the afforestation CDR flux to estimate is in broad agreement with several other studies,
grow continuously to 2100 [20,38] .
despite very different methods and assumptions (Table 2),
Forecasts of the potential afforestation CDR flux including simply extrapolating current rates of afforestaon different time horizons are mostly of a comparable tion and deforestation [43] , or extrapolating increasing
order of magnitude (Table 1) : approximately 0.8 PgC yr-1 demand for wood products [44] . They all suggest that an
in 2030 [39,40] , approximately 1.5 PgC yr-1 in 2050 [38] upper limit of approximately 150 PgC could be stored
and approximately 3.3 PgC yr-1 in 2100 [20,38] . The one within 100 years.
exception is an estimate of approximately 10 PgC yr-1
There should be potential for significant additional
in 2035 [41,101] , which we have used as an upper limit in storage beyond the century timescale, because both
previous ana­lysis [10] . On reflection, it appears unrealis- the CDR flux and the cumulative uptake are projected
tic for two reasons. First, a key ‘physical’ constraint is to be growing in 2100 [20,38] . Many plantations are
set very high, the average rate of carbon accumulation in only forecast to be established late this century (as
new plantations is assumed to be 10 MgC ha-1 yr-1 [41,101] , land becomes available) and would not reach their
approximately an order of magnitude greater than peak carbon uptake until sometime next century [20,38] .
some observations [42] , or other projections, which However, the global CDR flux should eventually peak
use 0.8–1.6 MgC ha-1 yr-1 [19,20] . Second, there is a and then decline as the supply of land suitable for
lack of consideration of ‘social’ constraints; 1 Gha is afforestation dwindles and the trees in new plantations
Table 2. Estimates of the storage capacity for removed carbon in various forms
and reservoirs.
150
Carbon Management (2010) 1(1)
future science group
The potential for land-based biological CO2 removal to lower future atmospheric CO2 concentration Review
reach their maximum rate of accumulating carbon [45] .
Existing studies do not continue beyond 2100, but
approximately 300 PgC would seem conceivable in
the long term. Thus, it seems feasible that all the carbon that has been emitted by human land use change
activities in the past could, in the long-term future, be
recaptured by permanent afforestation.
Biomass burial
If standing plantations are harvested, can the resulting
wood supply provide an additional carbon sink? Global
wood removals from forests totaled 1.938 Pg dry matter or approximately 1 PgC yr-1 in the year 2000 [46] .
Approximately half of the roundwood extracted goes
to wood fuel and half to industrial uses. Most of this
wood is soon returned to the atmosphere as CO2
through combustion or heterotrophic decay and so cannot be considered to be a CDR flux. The global stock of
hardwood products is estimated to be 4.2 PgC, with an
average lifetime in the order of 10 years [47] . The stock
is estimated to be increasing at 0.026 PgC yr-1 [47] , representing a small CDR flux if the increase in stock is
permanently maintained, but even if this were to all go
into wooden buildings, they still have a typical lifetime
of less than a century.
The burial of wood deep in soil or of wood products in landfill sites has been suggested as a means of
slowing wood decomposition rates under the anaerobic conditions that prevail there. At the extreme, it has
been estimated that approximately 10 PgC yr-1 of dead
coarse wood (>10 cm diameter) is produced annually
in all the world’s forests, and this could be buried in
approximately 25-m deep trenches to create a corresponding CDR flux [21] . Aside from the serious practical constraints of managing all global forests, and the
biogeochemical and ecological implications of removing
nutrients and habitats in rotting wood, the fact that
anaerobic consumption of organic carbon can generate
a flux of methane, which is 20–30-times more potent
molecule-for-molecule than a greenhouse gas, does not
appear to have been considered [21] . This is a well-known
problem (or potential energy source) in landfill sites, but
only less than 3% of the carbon in solid wood buried in
landfill is estimated to be converted to CH4 and CO2
Table 3. Estimates of present, 2050 and 2100 global land biological CO2 removal flux potential by pathway.
Carbon source
Carbon store
CO2 removal flux Key conditions/assumptions
(PgC yr-1)
Standing
biomass
Burial
Biochar
Burial
Biochar
Biochar
Biochar
Biochar
Biochar
Various
0.21–0.42
264 Mha × 0.8–1.6 MgC ha-1 yr-1
[36], this study
0.33
0.16
0.18–0.6
0.18
0.16–0.34
0.18
0.21–0.35
0.01
1–1.5
0.65 Pg yr-1 of felling losses
All felling losses from forestry
1.5–5 Pg yr-1, 30% removed
50% of unused crop residues
Original-revised estimates
All biomass energy by pyrolysis
All shifting cultivation fires
All waste from charcoal making
Total potential
[46], this study
Standing
biomass
BECS CO2
BECS CO2
BECS CO2
Biochar
Various
0.2–1.5
All abandoned cropland
1.25–1.5
0.75–1.25
1.75
0.25–0.35
4–6
390–750 Mha, 8–15 Mg ha-1 yr-1 (dm)
60–100 EJ yr-1, 20 GJ Mg-1, 50%
~100 EJ yr-1, 15 GJ Mg-1, 50%
As today
Total potential
Standing
biomass
BECS CO2
Biochar
Various
0.3–3.3
SRES A2, B1, B2, A1b range
3–10.5
5.5–9.5
6–14
240–850 EJ yr-1, 20 GJ Mg-1, 50% captured
180–310 EJ yr-1 all by pyrolysis
Total potential
Ref.
Present
Afforestation
Forestry residues
Crop residues
All residues
Energy crops
Shifting cultivation
Charcoal making
All
[46], this study
[23,46], this study
[51]
[26,46], this study
[26]
[26,52], this study
[26]
2050
Afforestation
Energy crops
Surplus wood
All residues
Shifting cultivation
All
[20,38,45,56]
[32], this study
[59], this study
[59], this study
[52], this study
2100
Afforestation
All bioenergy
All
[20,38]
[29], this study
[26]
BECS: Bioenergy with carbon storage; SRES: Special Report Emissions Scenarios.
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Review Lenton
in an approximately approximately 1:1 ratio [48] . This
means that wood burial could still represent a net sink
of CO2 equivalents. However, the potential for dissolved
organic carbon losses also needs to be considered, and
the modest global flux of hardwood products limits their
CDR potential at present.
A related proposal is to bury agricultural crop residues
in the deep ocean [22,23] (which should minimize the
problem of a return flux of CH4 to the atmosphere).
In a calculation of the CDR flux potential, it has been
estimated that 5 Pg yr-1 of crop residues are produced
globally, corresponding to approximately 2 PgC yr-1 and
that approximately 0.6 PgC yr-1 (30%) of this could be
removed and buried in the ocean without drastically
affecting soil carbon stores (Table 3) [23] . However, more
rigorous published estimates of global biomass flows
in 2000 show that approximately 2.9 Pg yr-1 of crop
residues are already harvested (i.e., go to some other
use) and only approximately 1.5 Pg yr-1 are unused [46] .
Using the same assumption with carbon content, and
assuming that 30% is buried, produces a CDR flux of
0.18 PgC yr-1 (Table 3) . The residues from future bio­
energy crop production might provide a significant additional carbon source for burial. However, the removal
of crop residues could lead to a counteracting erosion
of soil organic carbon, as well as compromising other
ecosystem services [49] .
An alternative approach might be to bury the residues from forestry felling losses, either deep in soil
or in the deep ocean. Current felling losses in forests
total 0.65 Pg yr-1 [46] , which, if all buried, represents
a maximum CDR flux potential of approximately
0.33 PgC yr‑1 (Table 3) . In the future, if large-scale afforestation with harvesting is undertaken, there is considerable potential for the felling losses to increase. However,
removing all of this carbon from the forest floor could
conceivably lead to counteracting losses in soil carbon.
The capacity of deep ocean sediments to store additional biomass carbon is deemed large [23] , but it is
worth noting that a flux of approximately 0.5 PgC yr-1
is comparable to that currently reaching deep ocean
sediments from all marine productivity. An additional
flux of pure organic carbon without associated carbonate will tend to dissolve the carbonate already in the
sediments, adding alkalinity and carbon to the ocean in
a 2:1 ratio and, thus, increasing the long-term capacity
of the ocean to store dissolved inorganic carbon and
lower atmospheric CO2 [50] .
Biochar production & soil carbon
Whilst burying biomass locks carbon away, it makes no
use of its energy content. We now turn to CDR methods
that liberate some of the energy in biomass and retain
some of it to capture part of the carbon from the parent
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Carbon Management (2010) 1(1)
fuel. Biochar and CO2 capture pathways for bioenergy
can both use the same feedstocks; hence, one must be
careful to avoid double counting. They also both have
approximately 50% potential to capture carbon, so
the choice of pathway does not greatly alter the CDR
fluxes that are achievable. However, the end products
and their destinations are different, leading to different
ultimate constraints on the total amount of carbon that
can be stored. Furthermore, biochar can be produced in
shifting cultivation without yielding energy.
To illustrate the present CDR potential, we choose
biochar as an end product. It has been estimated that
if the currently available flows of carbon in biomass
waste from agriculture and forestry, biomass energy
production and human-induced fires were pyrolysed,
then 0.56 PgC yr-1 of biochar could be sequestered:
0.16 PgC yr-1 from agricultural and forest wastes,
0.18 PgC yr-1 from deriving all ‘modern’ biomass energy
by pyrolysis, 0.21 PgC yr-1 from ‘slash-and-char’ shifting
cultivation, and 0.01 PgC yr-1 from wastes of charcoal
production (Table 3) [26] . However, assessment of global
biomass flows in the year 2000 [46] , combined with life
cycle ana­lysis of biochar production [51] , suggests that the
potential from agricultural wastes is greater than originally estimated [26] ; using 50% of the 1.5 Pg yr-1 (dry
mass) of unused crop residues currently produced could
create a sink of 0.18 PgC yr-1 (Table 3) [51] . This would be
instead of (rather than additional to) the 0.18 PgC yr-1
estimated for deep ocean burial of 30% of crop residue,
but would come with the added benefits of yielding
some useful energy and improving soil. In addition,
the 0.65 Pg yr-1 (~0.325 PgC yr-1) of felling losses from
forestry [46] (if 50% can be converted to biochar) could
produce a CDR flux of approximately 0.16 PgC yr-1
(Table 3) . This is less than the effect of burial in the
deep ocean, but again comes with co-benefits rather
than potential costs. The estimate for slash-and-char
CDR may also be greater. Human-induced vegetation
fires release approximately 2 PgC yr-1, approximately
a third of which (0.5–0.7 PgC yr-1) is from shifting
cultivation [52] , and at least 50% of this carbon can
be converted to biochar using simple kilns [26] , giving
approximately 0.25–0.35 PgC yr-1 (Table 3) . The revised
total for the current physical potential of biochar CDR
is 0.77–0.87 PgC yr-1.
There are few forecasts of the future CDR flux
potential of biochar (Table 1) . Assuming a supply of
2.5 PgC yr-1 of woody biomass from 2035 onwards
and a 48% conversion efficiency yields 1.2 PgC yr-1
of biochar [41,101] . Assuming 1.5% per annum
growth, 1.74 PgC yr-1 could be produced in 2060
and 3.15 PgC yr-1 in 2100 [10] . However, this is predicated on the unrealistic afforestation scenario discussed previously, covering 1 Gha by 2035. A separate
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The potential for land-based biological CO2 removal to lower future atmospheric CO2 concentration Review
estimate is that if a forecast 180–310 EJ yr-1 bioenergy
supply in 2100 were all produced by pyrolysis then
5.5–9.5 PgC yr-1 of biochar would be removed [26] . We
reconsider this type of estimate in more detail below.
What about the storage capacity for biochar in soils?
It has been argued that loadings of up to 140 MgC ha-1
are not detrimental and, therefore, the current 1.6 Gha
of global cropland and 1.25 Gha of temperate grasslands can together accommodate approximately 400
PgC (Table 2) [26] . In addition, the 0.7–1 Gha of cropland forecast to be abandoned this century and potentially subject to either afforestation or bioenergy cropping could, at the same loading, accommodate 98–140
PgC, increasing the total to approximately 500 PgC
(Table 2) . Even with the aforementioned upper end estimates of biochar production, it would take a century
to fill this capacity, but it might conceivably begin
to limit biochar production by 2100. This carbon
storage capacity is greater than for afforestation and
comparable to the lower estimates for geological CO2
storage. If achieved, it would represent an approximate 25% increase in the carbon content of the world’s
soils. However, it should be carefully researched at to
whether loadings of 140 MgC ha-1 biochar everywhere
are really benign (e.g., for plant productivity).
At this point, it is worth briefly considering the
potential to also increase the organic carbon content of
soil. This is already factored into studies of afforestation,
but what about on cropland or other managed land?
Switching from conventional tillage to no-till farming
has been found to sequester carbon at shallow depths
at a mean rate of 0.57 ± 0.14 MgC ha-1 yr-1 across 67
long-term experiments, giving rise to an increase in
soil organic carbon storage of 7.1 ± 1.75 MgC ha-1, as
a new equilibrium is reached within approximately
15 years [53] . If this switch occurred globally on all
1.5 Gha of cropland, simple arithmetic suggests that
a change in soil carbon storage of approximately 11
PgC could be achieved with a maximum CDR flux
of approximately 0.9 PgC yr-1 over approximately
12.5 years. However, the few studies that have looked
deeper into the soil suggest that reducing tillage shifts
the distribution of carbon to shallower depths, but does
not increase the total storage [54] . A switch to no-tillage
might be augmented with increased inputs to the soil,
and others propose that a more sustained CDR flux of
0.4–0.6 PgC yr-1 over 50 years could achieve 20–30 PgC
storage in cropland soils [55,56] . Together with restoration
of degraded soils, a CDR flux of 0.6–1.2 PgC yr-1 over
50 years achieving 30–60 PgC storage has been proposed [57] . This represents an upper limit, since it is similar to historical losses of carbon from soil due to land use
change. It is an order of magnitude smaller than other
potential carbon stores that we have identified and,
future science group
more importantly, the required changes in agricultural
practices and land uses would have to be maintained for
this to represent permanent CDR. Reversion to earlier
practices could readily re-release the carbon as CO2.
Consequently, we do not consider this in our overall
estimates of CDR potential.
Bioenergy with carbon storage
The present potential for bioenergy CDR from sugar­
cane-based ethanol production and chemical pulp
mills has been estimated at 0.19–0.23 PgC yr-1 [25] .
Meanwhile, future projections of the physical potential for bioenergy CDR via capture and storage of CO2
and/or biochar production are in short supply, with
the exception of a few ‘back-of-envelope’-type calculations (Table 1) [26,41,58,101] . However, there are numerous
fairly detailed scenarios for future bioenergy production
reviewed elsewhere [32,59] , which provide a useful starting
point in trying to estimate the CDR potential. Estimates
of bioenergy potential range over 50–500 EJ yr-1 in 2050,
with the main contributors expected to be energy crops
(40–330 EJ yr-1), surplus forest biomass (60–100 EJ yr-1)
and the ‘residues’ (i.e., waste) that accompany agriculture and forestry (30–180 EJ yr-1) [59] . To convert bioenergy forecasts in energy units to carbon fluxes requires
knowing the corresponding energy density and carbon
content of the various fuel types, and these are often
not given in the published studies. An excellent source
in this regard is a review of 17 studies up to 2003 that
details the under­lying assumptions regarding planting areas and dry mass yields [32] . Inspection of these
assumptions suggests the upper end of the bioenergy
ranges should be treated with caution [32] .
Underlying the estimates for energy crops is the
assumption that the plantation area will range over
390–750 Mha in 2050, with yields of typically 8–15
Mg ha-1 yr-1 (dry mass) and global production estimates clustering at around approximately 5–6 Pg yr-1
(dry mass) [32] . Assuming that (at the upper limit) 0.5
gC g-1 gives 2.5–3 PgC yr-1 of energy crop biomass in
2050, and assuming that approximately 50% of this
can, in principle, be captured [101] , gives up to 1.25–
1.5 PgC yr-1 CDR via energy crops in 2050. A much
higher estimate of 6.16 PgC yr-1 has been made for 2035
(Table 1) [101] based on 1.15 Gha of plantation (0.43 Gha
sugar cane and 0.72 Gha switchgrass), but it implies
an average yield of approximately 20 Mg ha-1 yr-1,
which is well above other studies, and a supply of
land of 46 Mha yr-1, far in excess of the projections of
abandonment of agricultural land.
The forecast bioenergy supply from surplus forest
biomass of 60–100 EJ yr-1 in 2050 [59] , given that wood
has a typical energy content of 20 GJ Mg-1, corresponds
to 3–5 Pg yr-1 dry mass, or 1.5–2.5 PgC yr-1, suggesting
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153
Review Lenton
a CDR potential of 0.75–1.25 PgC yr-1 (Table 3) . An
alternative estimate is 1.8 PgC yr-1 in 2035 (Table 1) [101] ,
but this relies on 1 Gha of afforestation in 25 years.
The implied supply of carbon in 2050 should be critically compared with the yield from existing forests and
independent estimates of afforestation discussed earlier.
Current global wood removals are approximately
1 PgC yr-1 [46] , but only approximately half of this is
used as fuel and most of that does not count as ‘modern’
biomass energy amenable to large-scale bioenergy with
carbon storage (BECS). Meanwhile, the accumulation
of carbon in the biomass of new forests is only forecast to be approximately 1.5 PgC yr-1 in 2050 (Table 1) .
Thus, the upper end estimates of bioenergy from ‘surplus’ forest biomass in 2050 appear to be in excess of
the likely supply. This can be reconciled if higher yields
are being assumed and are achievable. Otherwise, either
net removal of carbon from standing biomass or larger
areas of afforestation are implied. In the first case, the
biomass would not be ‘surplus’ and it should not be considered as CDR (but rather a potential CO2 source). In
the second case, the implied afforestation may conflict
with other land uses.
A supply of agricultural and forest residues of 30–180
(mean: 100) EJ yr-1 in 2050 is deemed to be the most
certain source of bioenergy [59] . Assuming these residues have an average energy content of 15 GJ Mg-1, they
correspond to 2–12 (mean: ~7) Pg yr-1 dry mass or 1–6
(~3.5) PgC yr-1. Currently, all unused crop residues are
1.5 Pg yr-1 and felling losses in forests are 0.65 Pg yr-1,
totaling 2.15 Pg yr-1 [46] or approximately 1.1 PgC yr-1;
hence, the lower end of the range projected for 2050 is
certainly realistic. A growing population will lead to
more agricultural waste and, if large-scale afforestation and bioenergy cropping also occur by 2050, then
3.5 PgC yr-1 seems plausible. If this was all subject to
CO2 capture and storage, then CDR of up to approximately 1.75 PgC yr-1 may be achievable. However,
it may make more sense to devote agricultural and
forestry residues to biochar production resulting in a
comparable CDR flux and also helping to maintain
soil quality.
By 2100, integrated assessments of bioenergy potential constrained by the supply of suitable land area tend
to be approximately double what they are in 2050 [29,32] .
The greatest ‘geographical’ potential is on abandoned
agricultural land, with one study giving a range of
240–850 EJ yr-1 in 2100 for woody energy crops [29] .
Assuming a typical energy content of 20 GJ Mg-1 and
0.5 gC g-1, this corresponds to 6–21 PgC yr-1, which
if it was all subject to 50% efficient CO2 capture
gives a potential CDR flux of 3–10.5 PgC yr-1. This
compares reasonably well with an earlier estimate of
5.5–9.5 PgC yr-1 potential biochar CDR flux [26] , from
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Carbon Management (2010) 1(1)
180–310 EJ yr-1 biomass energy supply in 2100 [32] ;
although the assumed conversion efficiency is clearly
higher in that study.
The ultimate storage capacity for liquid CO2 is determined by the size of suitable geologic reserves, assuming
deep ocean injection will not be used (because of its
finite, although lengthy, residence time and fears about
impacts on deep sea ecosystems). Estimates of geologic
storage capacity range upwards from approximately
500 PgC to approximately 3000 PgC (Table 2) [60] .
The low end of this range could present a significant
constraint, as it would only take 100 years to produce
500 PgC with the upper estimates of CDR flux discussed
above. Consequently, lack of storage capacity could prevent the upper end CDR flux estimates for 2100 from
being realized. The problem would be exacerbated if
there is competition for storage capacity between liquid
CO2 captured at the point of emission (conventional
carbon capture and storage [CCS], which is a mitigation
approach), and liquid CO2 captured from the free air
by bioenergy or by chemical pathways. Consequently,
the uncertain but potentially large capacity of saline
aquifers to store CO2 (Table 2) is critical in determining
whether several centuries of ambitious CDR is feasible,
and whether they could accommodate all the carbon
from know fossil fuel reserves.
Overall potential
summarizes the total potential of land biological
pathways of CDR based on present biomass flows, and on
future projections of afforestation and bioenergy supply
for 2050 and 2100. In coming up with the total potential,
there is a danger of double-counting carbon if summing
apparently independent estimates of CDR flux derived
for different methods, because these often (implicitly)
take the same biomass carbon source to different end
products. A pertinent question, for example, is whether
estimates of bioenergy CDR are additional to the potential afforestation CDR? Comparison of studies conducted
with IMAGE 2.2 [20,29,38] suggests not; the same land
supply is being considered either for permanent afforestation [20,38] or for short-rotation woody energy crops [29] .
Given this, the upper estimates of the totals for 2050
and 2100 (Table 3) should be treated with caution, since
it is implying the use of land in addition to the supply
from abandoning of cropland. Strikingly, there is already
1–1.5 PgC yr-1 potential for land biological CDR, which
is comparable to the latest estimates of the CO2 emissions
due to land use change of 1.5 ± 0.7 PgC yr-1 over 1990–
2005 and 1.2 PgC yr-1 in 2008 [1] . In 2050, the potential
for land biological CDR increases to 4–6 PgC yr-1, which
is approximately 50% of the current total (fossil fuel and
land use change) emissions. Hence, if these are cut globally by approximately 50% by 2050, then land biological
Table 3
future science group
The potential for land-based biological CO2 removal to lower future atmospheric CO2 concentration Review
fossil fuel emissions are currently rising at the mean rate of 1.9% yr-1 over
550
the past 25 years, and that concerted
global mitigation activity starts
500
immediately in 2010. We further
assume that it will take 40 years to
450
undergo the economic and technological transition necessary to achieve
400
a maximum rate of decrease in fossil
fuel emissions of -1.9% yr-1. In 2050,
350
fossil fuel emissions return to today’s
level (~8.4 PgC yr-1), and from then
300
on they decline at the same rate at
250
which they recently grew, until they
2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
cease. This contrasts with our earYear
lier stabilization work [3] , but is the
B 3
same approach used by others [2] .
The results in a total fossil fuel emis2.5
sion of 903 PgC after 2000, which
added to 282 PgC over 1800–2000
2
gives 1185 PgC. Cumulative land
use change emissions after 2000
1.5
are assumed to be 100 PgC (following an exponential decay) and were
180 PgC over 1800–2000, giving
1
280 PgC. Total emissions are 1465
PgC throughout. We view this as an
0.5
economically and technologically
feasible mitigation scenario, which
0
2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
is still fairly ambitious, but falls short
Year
of what will be required to avoid 2°C
warming above preindustrial levFigure 3. Model results illustrating the potential for land-based
els. In response, atmospheric CO2
biological CO2 removal to lower (A) future atmospheric CO2 concentration concentration peaks at 523 ppm in
and (B) future global warming. Each plot shows baseline scenario with
2100 and global warming peaks at
no CDR (solid line), afforestation CDR scenario (dotted line), and bioenergy
2.50°C soon after, in 2113 (Figure 3) .
CDR scenario (dashed line). Scenarios are described in the text. In the
The 2°C target is exceeded in 2050
afforestation case, carbon removed from the atmosphere is added to the
as atmospheric CO2 concentration
vegetation pool in the model. In the bioenergy case, carbon removed from
reaches 490 ppm and cumulative
the atmosphere is taken out of the system.
total emissions reach 990 PgC (in
CDR: CO2 removal.
good agreement with the ‘trillion
tonnes of carbon’ target [2]).
CDR could match emissions and, with natural sinks presTo include CDR in a simple fashion, with as few free
ent, atmospheric CO2 concentration would have already parameters as possible, we assume that the flux of CDR
passed peak levels and would be declining. In 2100, the as a function of time, R(t), follows a Gaussian curve (a
potential for land biological CDR levels of 6–14 PgC yr-1 scaled normal distribution). The total storage capacity
could match or exceed current total emissions. So, even if for carbon (S) constrains the area under the curve, leavmitigation efforts have limited success and emissions are ing only the year of peak removal activity (the mean,
only back to present levels by 2100, the use of land bio- µ) and how narrow/tall to make the distribution (the
logical CDR could have atmospheric CO2 concentration variance, s2) to be specified:
declining rather than still rising.
- (t - n)
To illustrate this further, we use a simple coupled care 2v
=S
R
(
t
)
bon cycle–climate model [7] , with the same set up as in
2
2rv
recent work [3] . As a baseline scenario, we assume that
No CDR: solid line
Afforestation CDR scenario: dotted line
Bioenergy CDR scenario: dashed line
Global warming (°C)
CO2 (ppm)
A 600
2
2
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155
Review Lenton
A 14
Flux (PgC year -1)
12
Total emissions: solid line
Natural sinks: dotted line
CDR: dashed line
Total sinks: dot–dash line
10
8
6
4
2
0
2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Year
B 14
Flux (PgC year -1)
12
10
8
6
4
2
0
2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Year
C 14
Flux (PgC year -1)
12
10
8
6
4
2
0
2000 2010 2020 2030 2040 2050 2060 2070 2080 2090 2100
Year
Figure 4. CO2 fluxes to and from the atmosphere in (A) baseline scenario
with no CO2 removal, (B) afforestation CO2 removal scenario and (C)
bioenergy CO2 removal scenario. Total emissions is fossil fuel burning plus
land use change (identical in all three cases). Natural sinks is land sink plus
ocean sink (a dynamic response of the model). CDR is the prescribed scenarios.
Total removal is natural sinks plus CDR. When total removal matches total
emissions, atmospheric CO2 concentration is stabilized; when total removal
exceeds total emissions, atmospheric CO2 concentration is lowered.
CDR: CO2 removal.
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Carbon Management (2010) 1(1)
From the preceding review, CDR activity has the
capacity to increase throughout this century, but if it
is being maximized, then it is likely to become constrained by total carbon storage capacity by the end of
the century. Consequently, we set the peak CDR flux to
occur in µ = 2100. We consider two cases: afforestation
CDR, with a total removal of S = 300 PgC (Table 2) and
s = 40 yr; and bioenergy CDR, with a total removal of
1000 PgC (nominally 500 PgC biochar and 500 PgC
stored CO2, Table 2 ) and s = 32 yr. The values of s
(together with S) are chosen to produce CDR fluxes
comparable to the estimated potential (Table 3) : afforestation CDR is 0.24 PgC yr-1 in 2010 (low end of range),
1.4 PgC yr-1 in 2050 and peaks at 3.0 PgC yr-1 in 2100.
Bioenergy CDR is 0.24 PgC yr-1 in 2010 (i.e., it includes
current afforestation), 3.7 PgC yr-1 in 2050, and peaks
at 12.5 PgC yr-1 in 2100 (rather high).
The afforestation CDR scenario lowers peak atmospheric CO2 by 38 ppm to 485 ppm and brings the peak
forward by nearly 35 years to 2067 (Figure 3A) . Global
warming still exceeds the 2°C target, when CO2 reaches
480 ppm in 2054 and the ‘cumulative carbon loading’
(i.e., cumulative emissions minus cumulative CDR)
is again 990 PgC (only 36 PgC cumulative CDR has
occurred by this time). Peak warming is brought forward
by 35 years (to 2078) and lowered by 0.34°C to 2.16°C
(Figure 3B) . By 2100, atmospheric CO2 has been reduced
to 464 ppm (a lowering of 59 ppm) and global warming by 0.42°C to 2.07°C. The calculated drawdown of
atmospheric CO2 due to afforestation is comparable to
other estimates of up to 52 ppm in 2100 [20] .
The bioenergy CDR scenario lowers peak atmospheric CO2 by 54 ppm to 469 ppm and brings the peak
forward by nearly 50 years to 2053 (Figure 3A) . Peak
warming is brought forward by 50 years and lowered by
0.51°C, to 1.99°C in 2063, just staying below the 2°C
target (Figure 3B) . When temperature peaks, the cumulative carbon loading is 970 PgC, having had 123 PgC
of cumulative CDR (and emissions in excess of a trillion
tonnes). By 2100, atmospheric CO2 has been reduced
to 365 ppm (a lowering of 158 ppm) and global warming by 1.16°C to 1.34°C. At this point, 1283 PgC has
been emitted, but 500 PgC has been removed, giving a
cumulative carbon loading of 783 PgC.
Inspection of the fluxes of CO2 to and from the atmosphere (Figure 4) reveals some interesting behavior: CDR
suppresses the natural land and ocean carbon sinks and,
in the bioenergy CDR scenario, they actually become a
net carbon source in 2086 (the land becoming a source
in 2073 and the ocean in 2105). This is because any
anthropogenic perturbation to atmospheric CO2, be it
an addition or a removal, triggers a counterbalancing
response from the land and ocean, as detailed in our
previous work [10] . In the bioenergy CDR scenario, with
future science group
The potential for land-based biological CO2 removal to lower future atmospheric CO2 concentration Review
net carbon removal from the system via the atmosphere,
and atmospheric CO2 declining, the land and ocean
respond by out-gassing CO2, as they tend towards a
new equilibrium state with lower carbon storage. This
behavior emphasizes an inherent limitation for CDR
approaches and suggests that the rather high values for
bioenergy CDR flux assumed by the end of the century
may not be desirable anyway.
Our results come with several further caveats. They
represent an approximate upper limit on what land-based
biological CDR, using abandoned cropland, can achieve
this century. Bioenergy CDR appears to have significantly greater potential than afforestation CDR, despite
50% of the carbon being assumed to be lost as CO2. The
main reason is that in the reviewed studies, the productivity of short-rotation woody biomass energy crops is
modeled to be far greater (~10 MgC ha-1 yr-1) [29] than
the average yield of afforestation (~1 MgC ha-1 yr-1) [20] .
This warrants further scrutiny, as does whether high
yields of carbon can be removed from bioenergy eco­
systems (and only partially returned as biochar), without reducing soil organic carbon storage. In our modeling, we have not calculated non-CO2 climatic effects. In
particular, afforestation in the high latitudes can lead to
net warming due to shading snow and lowering surface
albedo and, if biochar is exposed on bare soil surfaces
(e.g., after cropping), it may also lower surface albedo,
causing warming.
Future perspective
Current pledges for national emissions targets under
the Copenhagen Accord cannot limit global warming to 2°C (despite this being the stated aim of the
Accord) [5,61] . Given this, if the 194 member states of
the UNFCCC are genuinely committed to its goal
(Article 2), then they will have to give much more
policy attention to methods of CDR, as an additional
means of trying to avoid dangerous climate change. Our
review and modeling suggests that land-based biological
methods of CDR could play a significant role in helping
limit global warming to no more than 2°C, but early
action is imperative, just as it is for efforts to reduce
CO2 emissions.
Encouragingly, significant afforestation is already
underway, and recent trends suggest that afforestation activities will continue to grow and deforestation to dwindle [36] . In an optimistic scenario, within
10 years, net land use change activities could be
approaching carbon neutrality (i.e., created sinks balancing land use sources). For land-based biological
CDR by means other than afforestation to become
significant in the future requires that credits be earned
for carbon removed, such as through biochar production or CO2 entering geological storage. Accreditation
future science group
requires verification that a given amount of carbon
has been removed and that it is staying where it is put.
Quantifying removed carbon should actually be easier
and potentially more reliable than quantifying avoided
emissions, especially from deforestation – because it is
an exercise in measuring an actual flux rather than an
avoided one. The biochar that is produced, for example, can simply be weighed. However, considerable
monitoring would be required to establish whether,
for example, biochar in soil was losing carbon and at
what rate. The accreditation problem for CO2 in geological storage is being tackled anyway, because of the
intended use of CCS at points of fossil fuel emission,
and this should aid the uptake of bioenergy with CO2
capture and storage.
More broadly, carbon markets will need to become
widespread and stable, and the price of CO2 pollution, or conversely the earning from CO2 removed,
will need to be set at a reasonably high level for there to
be large-scale uptake of CDR methods. This is also a
prerequisite for many methods of achieving meaningful reductions in CO2 emissions. The precise price of
carbon required to trigger significant activity will vary
between the technologies and needs further research.
The fact that significant afforestation is already happening suggests that the cost is not prohibitive, yet
existing studies range over orders of magnitude in
their cost estimates [19] . The cost of afforestation will
increase with the size of CDR flux being generated, but
most of the short term potential can be realized at less
than US$100 MgC -1 [38] . Biochar production has been
argued to be competitive with biomass combustion
(without CCS), at a relatively low cost of US$33–
59 MgC -1, if one factors in the co-benefits of applying
biochar to soil [62] . Subsequent life cycle ana­lysis puts
the breakeven price for biochar production at only
US$7 MgC -1 from yard waste, but US$147 MgC -1
from crop residues and US$227 MgC -1 from bioenergy crops [51] . BECS carries the cost of CCS, making
it relatively expensive (e.g., US$84–194 MgC -1) [25] .
Burial of biomass makes no use of embodied energy,
yet wood burial could still be relatively cheap at an
estimated US$50 MgC -1 [21] ; whereas burial of crop
residue in the deep ocean may be the most expensive
option, at an estimated US$340 MgC -1 [23] .
Further research is needed into several of the CDR
methods before large-scale deployment is considered, such as long term experiments on the effects of
wood burial deep in soil. In addition, the effects of
(unavoidable) climate change on photosynthesis-based
CDR need to be assessed. CO2 fertilization would be
expected to enhance CDR fluxes (as long as water or
nutrients are not limiting), whilst the effects of rising
temperatures could be positive or negative depending
www.future-science.com
157
Review Lenton
on the region and the magnitude of warming. Recent
trends of increasing forest disturbance and losses of soil
carbon suggest that it may become harder to generate
and maintain stores of carbon in forests and soils, at
least in some regions.
For land-based biological methods of CDR to play
a significant future role then they must not threaten
food production (Article 2 of the UNFCCC). Hence,
to a large extent, their potential rests in large part on
the wider scientific and societal challenge of increasing the efficiency of land use for food production.
Whilst large increases in the land use efficiency of
global food production seem eminently possible in
principle [30,63] , if they cannot be achieved in practice, then attention should be turned to physical and
chemical methods of CDR that make a much smaller
demand on land area. Exciting research is underway
on these [11] but, as noted at the outset, the costs are
comparatively high [12] .
Acknowledgements
I thank Ed Sears, Nem Vaughan and the anonymous referees for
helpful comments.
Financial & competing interests disclosure
This research was supported by the Norfolk Charitable Trust through
the GeoEngineering Assessment and Research (GEAR) initiative at the
University of East Anglia. The author has no other relevant affiliations
or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials
discussed in the manuscript apart from those disclosed. No writing
assistance was utilized in the production of this manuscript.
Executive summary
Pathways & constraints
ƒƒ Land-based biological CO2 removal (CDR) involves diverting carbon captured from the atmosphere by photosynthesis to long‑lived reservoirs.
ƒƒ These reservoirs include permanent forests, buried biomass (either deep in soil or in the deep ocean), biochar in soils and CO2 stored in
geological formations.
ƒƒ The CDR flux achievable is constrained by the supply of land area, the yield of carbon on that land area, and the efficiency of converting it to
long-lived carbon stores.
Afforestation & reforestation
ƒƒ Afforestation is already removing an estimated 0.21–0.42 PgC yr-1.
ƒƒ This could rise to approximately 1.5 PgC yr-1 in 2050 and approximately 3.3 PgC yr-1 in 2100.
ƒƒ The total storage capacity is approximately 300 PgC in standing trees and associated soil carbon.
Biomass burial
ƒƒ Burial of all existing forestry residues could remove 0.33 PgC yr-1.
ƒƒ Burial of 30% of existing crop residues could remove 0.18 PgC yr-1.
ƒƒ However, the potential for methane generation, erosion of soil carbon, and compromises to other ecosystem services suggest further
research is required.
Biochar production & soil carbon
ƒƒ Biochar production from existing biomass flows could remove 0.77–0.87 PgC yr-1.
ƒƒ It carries fewer risks and greater benefits than burying biomass.
ƒƒ The total storage capacity for biochar in soil is approximately 500 PgC.
ƒƒ The potential to increase soil organic carbon by changing agricultural practices is an order of magnitude smaller and would need to be
permanent in order to count as CDR.
Bioenergy with carbon storage
ƒƒ Bioenergy with capture and storage (as CO2 or biochar) could remove approximately 4 PgC yr-1 in 2050 and more than 10 PgC yr-1 in 2100.
ƒƒ The achievable CDR flux depends crucially on the supply of abandoned cropland, and could ultimately be constrained by lack of
storage capacity.
ƒƒ The total storage capacity for CO2 in geological formations is approximately 500–3000 PgC, but the upper estimates depend critically on the
uncertain capacity of saline aquifers.
Overall potential
ƒƒ There is already the potential for approximately 1–1.5 PgC yr-1 of CDR, by diverting or altering existing biomass flows.
ƒƒ This could counterbalance current CO2 emissions from land-use change.
ƒƒ By 2050, the potential CDR flux is 4–6 PgC yr-1.
ƒƒ Together with natural sinks, this could match current total CO2 emissions, thus stabilizing atmospheric CO2 concentration and lowering peak
global warming.
ƒƒ By 2100, the potential CDR flux is 6–14 PgC yr-1.
ƒƒ This could significantly exceed mitigated CO2 emissions, thus bringing down atmospheric CO2 concentration and reducing global warming.
ƒƒ The total amount of carbon that can be stored probably exceeds 1000 PgC.
ƒƒ This is sufficient to accommodate at least two centuries of CDR activity and could potentially contain all the fossil and land-use carbon that we
will emit.
158
Carbon Management (2010) 1(1)
future science group
The potential for land-based biological CO2 removal to lower future atmospheric CO2 concentration Review
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