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Phil. Trans. R. Soc. A (2011) 369, 2113–2132
doi:10.1098/rsta.2011.0016
The atmospheric signature of carbon capture
and storage
BY RALPH F. KEELING1, *, ANDREW C. MANNING2
AND MANVENDRA K. DUBEY3
1 Scripps
Institution of Oceanography, University of California, San Diego,
9500 Gilman Drive, La Jolla, CA 92093-0244, USA
2 School of Environmental Sciences, University of East Anglia,
Norwich NR4 7TJ, UK
3 Earth System Observations, Earth and Environmental Sciences Division, Los
Alamos National Laboratory, MS D462, Los Alamos, NM 87545, USA
Compared with other industrial processes, carbon capture and storage (CCS) will have
an unusual impact on atmospheric composition by reducing the CO2 released from
fossil-fuel combustion plants, but not reducing the associated O2 loss. CO2 that leaks
into the air from below-ground CCS sites will also be unusual in lacking the O2
deficit normally associated with typical land CO2 sources, such as from combustion or
ecosystem exchanges. CCS may also produce distinct isotopic changes in atmospheric
CO2 . Using simple models and calculations, we estimate the impact of CCS or leakage
on regional atmospheric composition. We also estimate the possible impact on global
atmospheric composition, assuming that the technology is widely adopted. Because of
its unique signature, CCS may be especially amenable to monitoring, both regionally
and globally, using atmospheric observing systems. Measurements of the O2 /N2 ratio
and the CO2 concentration in the proximity of a CCS site may allow detection of point
leaks of the order of 1000 ton CO2 yr−1 from a CCS reservoir up to 1 km from the
source. Measurements of O2 /N2 and CO2 in background air from a global network
may allow quantification of global and hemispheric capture rates from CCS to the
order of ±0.4 Pg C yr−1 .
Keywords: carbon capture and storage; geosequestration; leak detection; atmospheric oxygen;
carbon-13; carbon-14
1. Introduction
From a geochemical perspective, the processes contributing to increases in
atmospheric CO2 can be grouped into discrete categories, each with distinct
geochemical fingerprints. Fossil-fuel burning, for example, releases CO2 and
consumes O2 in characteristic ratios depending on the fuel type, leading to
small but detectable deficits in the atmospheric O2 /N2 ratio in proportion to
*Author for correspondence ([email protected]).
One contribution of 17 to a Discussion Meeting Issue ‘Greenhouse gases in the Earth system:
setting the agenda to 2030’.
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the CO2 excess. Fossil-fuel burning also enriches the 12 C content of atmospheric
CO2 relative to the 13 C and 14 C contents of CO2 , leading to declines in the 13 C/12 C
and 14 C/12 C ratios [1]. Similarly, sources or sinks of CO2 from the land biosphere
lead to changes in atmospheric CO2 that are strongly anticorrelated to changes
in the atmospheric O2 /N2 ratio and in the 13 C/12 C ratio of CO2 , while sources or
sinks of CO2 from the ocean, mostly involving inorganic chemical reaction in sea
water, have little impact on the atmospheric O2 /N2 ratio or the 13 C/12 C ratio of
CO2 . Programmes to measure changes in atmospheric O2 /N2 , 13 C/12 C or 14 C/12 C
have exploited these differing fingerprints to separate CO2 sources and sinks into
contributions from different geochemical processes such as land or ocean uptake,
or fossil-fuel burning (e.g. [2–5]).
Carbon capture and storage (CCS), a technology that may allow continued use
of fossil fuels as an energy source while avoiding the associated CO2 emissions
[6–8], will also have distinct geochemical fingerprints. CCS involves capturing the
CO2 that would otherwise be emitted from large stationary CO2 sources, such as
power plants, and storing it in permanent or quasi-permanent geologic reservoirs,
such as saline aquifers, depleted oil reservoirs or below the sea floor. A related
idea is to capture CO2 directly from the atmosphere, an approach known as direct
air capture (DAC) [9–11]. CCS and DAC represent novel geochemical processes,
largely without parallel on Earth today.
The effect of CCS on atmospheric composition will be more complex than
simply the avoided CO2 concentration increases. While reducing CO2 emissions
from combustion, CCS will not reverse the atmospheric O2 loss and the associated
impact on the atmospheric O2 /N2 ratio. The exhaust plume of a power plant
equipped with CCS will thus contain an O2 deficit comparable to an unmodified
power plant, but without the accompanying normal excess of CO2 . CCS will
thereby reduce the spatial gradients and short-term variability in CO2 while
leaving the spatial gradients and short-term variability in O2 /N2 ratio unchanged,
compared with a world without CCS. CCS will also reduce the gradients and
variability in the isotopic composition of CO2 . These reductions will occur both in
the near-field of a power plant, as well as at much larger continental, hemispheric
and global scales.
Globally, the net effect of CCS will be to reduce CO2 buildup without any
effect on O2 loss. As a storage mechanism for CO2 , therefore, CCS will resemble
oceanic CO2 uptake, which similarly reduces CO2 buildup without impact on
O2 [12]. However, CCS will leave a strong signature on the isotopes of CO2 by
lessening the strong isotopic imprint of fossil-fuel burning. By contrast, oceanic
CO2 uptake has weaker isotopic influences.
CO2 pumped below ground will typically form a buoyant supercritical fluid,
tending to migrate upwards unless contained below a continuous caprock [6].
Even where the caprock appears continuous, leakage could occur through the
injection well if not properly sealed, leading to a point source at the well, or via
permeation through the caprock, leading to widespread areal leakage. Leakage
could also occur through old abandoned wells that are not properly sealed or
through faults in the caprock, leading to a source spread along a line at the
surface or at hot-spots [13]. Leaked CO2 will have characteristic atmospheric
fingerprints, typically resembling fossil-fuel CO2 in its isotopic composition, but
not accompanied by the normal atmospheric O2 deficit expected from a fossil-fuel
CO2 source [14].
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Signature of carbon capture and storage
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Developing methods to detect leakage from geological CO2 sequestration sites
is critical to ensure that the objectives of the sequestration process are being
met and to avoid safety concerns. Haefeli et al. [15] note that proper accounting
of both capture efficiency and leakage is required under the United Nations
Framework Convention on Climate Change, and leakage may need to be kept near
or below 0.01 per cent per year to meet targets for stabilization of atmospheric
CO2 concentrations in all cases. For a large-scale storage site containing 10 Mton
CO2 , this corresponds to a leakage rate of 1000 ton CO2 yr−1 or 32 g CO2 s−1
[16]. Seismic or electromagnetic methods may prove useful for tracking CO2 flow
below ground, and soil sampling may help detect CO2 plumes near the surface,
but these may not be sufficient to quantify leakage accurately or to rule out
leakage through faults or wells. Soils naturally contain excess CO2 from plant
and microbial respiration, which may mask any excess from leakage. Chemical
tracers, such as SF6 or perfluorocarbons, may be injected along with the CO2 to
aid in leak detection [17]. Tracer injection has additional up-front and detection
costs, however. Also, the migration of the tracer below ground may not exactly
track the migration of CO2 , and its presence may raise other environmental
concerns [16]. If full advantage is taken of the unique geochemical fingerprints
of CO2 leaking from geological sequestration, however, tracer injection might not
be necessary.
The introduction of CCS will present both new challenges and new
opportunities for atmospheric measurement programmes. Existing approaches
to calculating natural land and ocean carbon sinks using measurements of O2
concentration or carbon isotopes involve accounting for direct human impacts
(e.g. from fossil-fuel burning and land use) on atmospheric composition from
bottom-up accounting methods. The natural sinks are then calculated from the
residual changes in atmospheric composition not accounted for by the direct
human impact [3,18]. The extension of these methods into an era with CCS will
require that rates of CCS and leakage from CCS sites also be well quantified.
How well this will work in practice remains unclear. Bottom-up reporting may
become less, not more, reliable once CO2 has a ‘price tag’, with built-in financial
incentives to under-report CO2 sources, such as fossil-fuel burning, and to overreport sinks, such as CCS [19]. On the other hand, with steady improvements
in our ability to understand and quantify land and ocean processes, it may be
possible to turn the calculations around, calculating the financially relevant direct
human components, such as fossil-fuel burning, land use, or CCS, from the change
in atmospheric composition not accounted for by the (independently known) land
and ocean sinks. CCS may be especially amenable to such an approach given its
unique atmospheric signatures.
In this paper, we consider in detail two aspects of CCS’s atmospheric
signature. First, we illustrate the potential use of atmospheric measurements
for both assessing capture efficiency and detecting leakage from below-ground
sequestration sites. Second, we use simulations to illustrate the complex
implications of CCS on global trends in CO2 , O2 /N2 and CO2 isotopes. Even
though wide adoption of CCS to mitigate CO2 emissions is at least a few
decades away, both of these subjects already have relevance—the first because
leak detection is a major issue already for pilot CCS projects, and the second
because the possible growth of CCS is already relevant for the global observing
systems planned to extend over decades.
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2. Near-field monitoring of CO2 capture and leakage
To explore atmospheric signatures near a CCS site, we turn to the atmospheric
modelling study of Leuning et al. [16] who simulate changes in atmospheric
composition from a hypothetical leak at the Otway Basin Pilot Project in
southeastern Australia. The simulations, using a three-dimensional transport
model [20], assume a leak rate of 1000 ton CO2 yr−1 and also allow for a secondary
source from a processing plant of 8800 ton CO2 yr−1 . The simulations reproduced
here assume that the leakage involves a point source. Atmospheric variations are
examined at a ‘detection site’ 700 m northeast of the leak. The processing plant is
assumed to be 2000 m due north of the source. Although the Otway Basin Project
will inject CO2 of magmatic origin (d13 C ≈ −6‰) extracted from natural gas, we
assume here, for illustrative purposes, that the CO2 is derived from a coal-fired
power plant equipped with CCS with a capture efficiency of 90 per cent, a more
typical target for scaled-up future applications.
By convention, 13 C/12 C variations are expressed as deviations from a standard
in delta notation, d13 C = (R/Rstd − 1), where R and Rstd are the 13 C/12 C ratio
of the sample and standard, respectively, and d13 C is multiplied by 1000 and
expressed in parts per thousand (‰). O2 /N2 ratios are also expressed in delta
notation, d(O2 /N2 ), except that the relative deviation from the O2 /N2 standard is
multiplied by 106 and expressed as ‘per meg’. For coal, d13 C = −26‰ is assumed,
with 1.15 mol of O2 consumed per mole of C produced [21], leading to 11.5 mol of
O2 consumed for every mole of CO2 that escapes the capture process, considering
that 90 per cent of the CO2 is captured. The capture process is assumed to
be associated with a −5‰ fractionation in 13 C/12 C, roughly consistent with
fractionation associated with CO2 capture by lime, which is dominated by kinetic
effects [22]. With this assumption, mass balance requires that the captured CO2
has an isotopic composition of −26.5‰ and CO2 released from the processing
plant has an isotopic composition of −21.5‰. The isotopic composition of the
captured CO2 is very similar to the original coal, owing to the high efficiency
of the capture process. The background atmosphere is assumed to have a CO2
concentration of 374 ppm, d13 C of −8.1‰ and d(O2 /N2 ) of −350 per meg.
Simulated changes at the detection site over a 6-day period are shown in
figure 1 under generally southwesterly wind conditions. The CO2 simulations
are as published by Leuning et al. [16]; here, we also show changes in d13 C
and O2 /N2 computed directly from the changes in CO2 , taking into account the
characteristic O2 : C and d13 C signatures of the sources given above. Discrete CO2
peaks are seen in the simulations corresponding to troughs in d13 C and O2 /N2 .
These peaks occur when the exhaust plumes from the leak or the processing
plant pass directly over the detection site. There is minimal intermixing of the
two plumes, so that individual peaks are directly attributable to one source
or the other. The peaks from the processing plant are in the 5–10 ppm range,
whereas those from the leak are in the 1–3 ppm range. The processing plant
peaks are accompanied by declines (300–600 per meg) in the O2 /N2 ratio, whereas
those from the leak produce no changes in the O2 /N2 ratio. The magnitude of
the peaks and their characteristics are highly dependent on the wind regime
for the 6-day period. Under light winds or highly stratified conditions (e.g. at
night), much larger changes would be expected, with the plumes intermixing to
some degree.
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Signature of carbon capture and storage
Otway Basin model
CO2 (ppm)
390
point source
processing plant
385
380
d(O2/N2) (per meg)
d 13C (‰)
375
–8.0
–8.1
–8.2
–8.3
–8.4
–8.5
–8.6
–300
–400
–500
–600
–700
–800
–900
–1000
0
24
48
72
time (h)
96
120
144
Figure 1. Modelled changes in CO2 , d13 C and O2 /N2 , based on CO2 simulations reported in Leuning
et al. [16]. The Otway Basin model simulates changes downwind of a point source of below-ground
leakage as well as from a processing plant, here assumed to be a coal-fired power plant equipped
for carbon capture. See text for details.
The model depicts only part of the full variability to be expected at the
detection site, as it neglects variations driven by local vegetation or other
local sources and variation in the background atmosphere. CO2 exchange with
vegetation and soils typically drives a diurnal cycle in CO2 concentrations near
the ground, driven by the day–night cycle of net photosynthesis/respiration and
the day–night cycle in vertical mixing [23]. In a stable nocturnal boundary layer,
CO2 concentrations can rise above the free-tropospheric background by tens to
hundreds of parts per million, while day-time values typically drop below free
tropospheric values only by a few parts per million. Field studies show that the
near-surface CO2 variations are typically strongly correlated with variations in
the 13 C/12 C ratio of CO2 and the O2 /N2 ratio [23–26]. The correlations depend,
in the case of 13 C/12 C, on the average isotopic composition of the CO2 that is
added or removed by vegetation, and in the case of O2 /N2 on the oxidative state
of the organic matter that is being created or destroyed as well as the cycling of
nitrogen [27].
In figure 2, we superimpose the model simulations of the Otway Basin on
a scatter plot from observed variations in CO2 and 13 C/12 C of CO2 from a
coniferous forest in Colorado, USA [24]. This superposition illustrates certain
features to be expected downwind of a generic sequestration site where natural
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–7.5
–6‰
source
d 13C (‰)
–8.0
–21.5
–8.5
–26.5‰
source
–9.0
Niwot Ridge data
Otway Basin model
background (assumed)
–9.5
–10.0
410
400
390
380
CO2 (ppm)
370
Figure 2. Scatter plot of d13 C versus CO2 concentrations measured every 6 min from a height of 5 m
over a 24 h period from midnight on 9–10 July 2003 from the Niwot Ridge AmeriFlux Tower site
(40.03◦ N, 105.55◦ W, 3050 m above sea level) [24]. Also shown is the scatter plot from the Otway
Basin model, from figure 1, with the background value assumed for the model. The vector insert
shows the covariations in d13 C and CO2 expected for sources with different isotopic compositions.
The Niwot Ridge data and the Otway Basin leak both conform to a source of around −26‰. The
nonlinear x-axis coordinate is scaled in units of 1/CO2 , as appropriate to yield linear vector slopes
for a process adding CO2 to the atmosphere.
variability is also present. The Colorado data fall roughly along a straight line
with a slope corresponding to the addition of CO2 with d13 C ≈ −26‰ and with
day-time values on the upper right (lower CO2 ) of the plot and night-time values
distributed over the remainder of the plot. The scatter of the data about the line
is dictated mostly by instrumental precision in the isotopic measurement. The
covariations in CO2 and d13 C from the CCS leak lie along a straight line that has
virtually the same slope as the ecosystem-driven exchanges, as expected since
both sources have nearly the same isotopic signature of −26‰. For detecting
leakage, it can be seen that the d13 C data would provide little information not
already contained in the CO2 data.
For the d13 C data to be useful for leak detection, the exogenous CO2 from CCS
must have a distinctly different isotopic composition from CO2 of other nearby
sources. However, the isotopic composition of most coals and petroleum lies in the
range of −24 to −30‰ [28], similar to the range for most ecosystems [29]. d13 C
data may nevertheless have value in some specific situations, such as detecting
emission of thermogenic CO2 , which often must be removed as a contaminant
from natural gas, and may have d13 C as high as −6‰. Based on the scatter in
figure 2, it would appear that a thermogenic plume of the order of 4–5 ppm would
be readily distinguishable from the natural variability. CO2 from natural-gas
combustion, which can be as low as −45‰, would be readily detectable. Isotopic
measurements would also be more helpful if the local ecosystem were dominated
by C4 grasses (d13 C ≈ −13‰), and may have value for distinguishing residual CO2
emissions from CCS processing plants (as shown in figure 2), since the residual
CO2 will be offset by the fractionation associated with the capture process.
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Signature of carbon capture and storage
–200
ground leakage
–300
d(O2/N2) (per meg)
–400
–500
–600
–700
processing
plant emissions
–800
Siberian field data
Otway Basin model
background (assumed)
–900
–1000
360
370
380
390
400
CO2 (ppm)
410
420
Figure 3. Scatter plot of changes in atmospheric O2 /N2 ratio versus CO2 mole fraction measured
with roughly 20 min resolution from the Zotino Tall Tower Observatory (ZOTTO) in the boreal
forest of central Siberia from a height of 4 m, from approximately midnight on 3–5 July 2006 [26].
See text for explanation of d units used for O2 /N2 ratio. Molar fluxes of O2 can be compared with
CO2 using a conversion factor of 4.8 per meg ppm−1 . Also shown are results from the Otway Basin
model, from figure 1, with the background value assumed for the model.
Figure 3 shows a similar superposition of the Otway Basin simulation on a
scatter plot of O2 /N2 versus CO2 from a site in central Siberia [26]. Again,
we consider this juxtaposition of synthetic and real data to illustrate the main
features to be expected downwind of a generic sequestration site, allowing also for
variability from local vegetation and the background atmosphere. The Siberian
data tend to fall along a straight line with a slope of around −5.03 per meg
ppm−1 , corresponding to a molar O2 /CO2 exchange ratio of −1.05, with daytime data in the upper left and night-time data spread across the rest of the
plot. Similar measurements for a forest site in Wisconsin also show very strong
covariations with similar slopes (O2 /C approx. 1.01–1.10) [25]. Urban areas may
have different ratios reflecting fossil-fuel combustion, but even in urban settings,
tight correlations are observed [30]. The plumes from the leak and the processing
plant have characteristic slopes on the O2 : CO2 plot that are different from each
other as well as different from that of the ecosystem. Based on the scatter of the
data about a linear relationship, a leakage detection limit of around 2–4 ppm CO2
is implied by the Siberian and Wisconsin data.
The processing plant plume would be especially easy to detect because it
involves readily detectable declines in O2 /N2 ratio that deviate markedly from
the natural relationship. This plume lies along a line on the plot with a slope,
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when expressed in molar (O2 : C) units, equal to rf /(1 − e), where rf is the O2 : C
combustion ratio of the coal and e is the processing plant capture efficiency
(0 < e < 1). The combustion ratio rf of coals can be directly determined from their
elemental composition. Combining elemental composition data with atmospheric
measurements thus provides a means to measure e remotely. This approach
may prove more accurate than assessments based on stack emissions [31]. One
complication, however, is that releases of O2 -deficient (i.e. N2 -rich) waste air and
CO2 from capture plants may not always be perfectly collocated in space and
time, leading to several distinct plumes from the plant itself, one depleted in O2
and the other enriched in CO2 .
The leakage plumes as simulated for Otway Basin (solid line in figure 3) would
be somewhat difficult to detect against the variability seen in the Siberian data.
This scatter is mostly due to instrument imprecision, which could be improved
by a factor of three or more. Ultimate sensitivities to CO2 leakage of the order of
1–2 ppm seem feasible, which would be sufficient for detecting point source leaks
of the order of 1000 ton CO2 yr−1 at distances of the order of 1 km under modest
wind conditions, as depicted for the Otway Basin simulation [16].
Obtaining high-precision field measurements will be essential for definitive
leak detection. Bowling et al. [24] report a precision for 6 min average d13 C
determinations using a tunable diode laser system of ±0.15‰ or better. For
O2 /N2 , an expanding range of high-precision techniques has been developed in
recent years. Kozlova & Manning [32] report a precision of ±1.5 per meg for
approximately 20 min determinations using a paramagnetic technique. Stephens
et al. [25] report precisions for 1 min average O2 /N2 ratio of ±2.5 per meg based
on a modified commercial fuel-cell analyser, while Stephens et al. [33] report a
precision of ±1.1 per meg for a 5 min average using a technique based on light
absorption at 147 nm. Fessenden et al. [14] report promising results from a pilot
study to detect leakage of CO2 using combined O2 /N2 and CO2 measurements.
These techniques would appear to be ripe for considerable further refinements and
cost reductions, particularly if economies of scale come into play. With suitable
characterization of local variability, it appears possible to detect signals of a few
parts per million CO2 from leakage under general conditions.
3. The global signature of carbon capture and storage
We now turn to the question of how global atmospheric trends of CO2
concentration, O2 /N2 ratio and the carbon isotopes of CO2 would be impacted
by wide adoption of CCS in the future. Our approach involves comparing a
baseline scenario, which assumes no mitigation efforts, with alternative scenarios
that incorporate mitigation. For the baseline, we adopt the B1-IMAGE scenario
from the Intergovernmental Panel on Climate Change Special report on emissions
scenarios [34]. The B1-IMAGE scenario assumes a future world in which energy
and land usage are guided by a high level of environmental consciousness, but in
which no specific measures are taken to mitigate climate change. B1-IMAGE is
an appropriate baseline scenario because mitigation efforts such as CCS are not
likely to be implemented on a wide scale unless other measures are also taken to
reduce emissions, such as energy conservation.
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We also consider four alternative ‘mitigation’ scenarios in which CO2 emissions
are reduced (compared with B1-IMAGE) by an amount that grows linearly from
0 to 2 Pg C yr−1 over the 50-year period from 2010 to 2060 and remains constant
(at 2 Pg C yr−1 ) thereafter. In other words, the mitigation scenarios effectively
assume two ‘wedges’ of mitigation, according to Pacala & Socolow [35]. (A ‘wedge’
refers to a 1 Pg yr−1 reduction in emissions in over 50 years [35].) The four
mitigation scenarios differ from each other in how the emission reductions are
achieved: (1) CCS, (2) energy conservation, i.e. averted fossil-fuel usage, (3) land
carbon sequestration, or (4) DAC of CO2 . Land carbon sequestration is assumed
to occur via reforestation or afforestation, etc. Although our emphasis here is
primarily on the atmospheric imprint of CCS (scenario 1), the other scenarios
are useful for comparison purposes.
To predict the future changes in CO2 and related tracers, we use a simple
coupled reservoir model, in which the ocean is described by a ‘box diffusion’ type
of model [36,37], and the land biosphere and atmosphere are each described as
single well-mixed reservoirs. The model accounts for ocean uptake of CO2 due to
inorganic carbon chemistry and ocean mixing, and it accounts for CO2 uptake
by the land biota by assuming that net primary production increases linearly
with CO2 concentration. This model provides a means to account for the effects
of human emissions (fossil-fuel burning, land-use change, mitigation measures),
for land and ocean carbon sink processes, and for so-called isofluxes between
the atmosphere and the land biosphere and oceans, which involve swapping of
atoms between reservoirs, without any net gain or loss in any reservoir. Isofluxes
influence the 13 C/12 C and 14 C/12 C ratios of CO2 without changing atmospheric
CO2 abundance.
Simulations are carried out allowing for variations in emissions according to
the B1-IMAGE scenario starting in year 2005, with emissions prior to 2005
based on records compiled by the Carbon Dioxide Information and Analysis
Center [38,39]. The simulations for CO2 , for the 13 C/12 C ratio of CO2 and for
atmospheric O2 /N2 ratio involve ‘forward’ model runs, in which concentrations
(or ratios) are computed for specified human emissions. For the 14 C/12 C ratio,
the model is run up to year 2005, with the atmospheric 14 C/12 C ratio specified
to match observations, thus implicitly accounting for the input of ‘bomb’ 14 C
from the atmospheric nuclear weapons tests in the mid-twentieth century. After
2005, the simulated 14 C/12 C ratio is carried out in ‘forward’ mode, assuming
no additional anthropogenic 14 C source. The parameters governing ocean and
land carbon uptake and isotopic exchanges are tuned to match the observed
changes over the twentieth century in CO2 concentration, to match the ocean
radiocarbon inventory and to match the observed atmospheric 14 C/12 C trend
since year 2000 without any bomb input. The model uncertainties have little
impact on our analysis, which focuses on the differences between the scenarios,
rather than the absolute predicted changes.
The simulations account for temporal changes in the composition of global
average fossil fuel based on the changing emissions of discrete fuel classes from the
B1-IMAGE scenario [34] and emission factors (for 13 C/12 C and O2 : C) of different
fuel classes [21,28]. For the CCS and energy conservation scenarios, the reduced
emissions are assumed to involve fossil fuel with global average composition. Land
sequestration is assumed to involve photosynthesis with a molar O2 : C ratio of 1.1
and an isotopic fractionation in 13 C/12 C of −18‰. The CO2 source from land-use
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atmospheric CO2 concentration
550 (a)
CO2 (ppm)
500
450
400
CO2 (Pg C yr–1)
350
16 (b)
14
12
10
8
6
4
2
0
2000
combined fossil-fuel and land-use CO2 source
baseline scenario
mitigation scenario(s)
2020
2040
2060
2080
2100
year
Figure 4. (a) Projected changes in atmospheric CO2 from the IPCC B1-IMAGE scenario
(baseline scenario) and from a mitigation scenario in which emissions are reduced (see text).
(b) Anthropogenic CO2 emissions assumed for the scenarios. Simulations are based on a simple
reservoir model (see text).
change is assumed to have a 13 C/12 C ratio identical to the contemporaneous land
biosphere and with an O2 /C ratio of 1.1. CCS and DAC are assumed to occur
with 0‰ and −5‰ fractionation in 13 C/12 C, respectively. The 14 C/12 C changes
are simulated following the formalism of Siegenthaler & Oeschger [40], leading
to results in standard delta notation (D14 C), where, for example, D14 C = 100‰
corresponds to a 14 C/12 C ratio 10 per cent larger than the isotopic standard,
after correcting for the offset in the 13 C/12 C ratio. Model results for 13 C/12 C are
similarly reported in standard delta notation.
The projected net anthropogenic emissions and the associated rise in CO2
are shown in figure 4, for both the baseline and mitigation scenarios. All four
mitigation scenarios yield identical changes in CO2 concentration, reflecting the
identical net anthropogenic CO2 emissions. In the baseline scenario, fossil-fuel and
land-use emissions rise to a peak of approximately 12 Pg C yr−1 around year 2040,
falling thereafter (figure 4). CO2 rises at a roughly linear rate until mid-century,
slowing thereafter, and virtually stabilizing at approximately 500 ppm by year
2100. In the mitigation scenarios, the rise in CO2 is similar until mid-century, but
is slower thereafter, peaking in year 2084, with a very slight fall thereafter. Despite
having identical trajectories in CO2 concentration, the mitigation scenarios have
different trajectories in O2 /N2 ratio and carbon isotopes, which we discuss
in turn.
Phil. Trans. R. Soc. A (2011)
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Signature of carbon capture and storage
–1000
baseline scenario
carbon capture and storage
conservation
land sequestration
direct air capture
d(O2/N2) (per meg)
–1500
–2000
LND
FOS
–2500
–3000
–3500
380
OCE,
CCS,
DAC
400
420
440
460
CO2 (ppm)
480
500
520
Figure 5. Projected future changes in atmospheric O2 /N2 ratio versus CO2 concentration for the
baseline scenario and mitigation scenario depicted in figure 4. The mitigation scenario is shown with
four alternatives, depending on the type of mitigation, with different consequences for O2 /N2 . Also
shown are vectors indicating the influence of processes in isolation: land uptake (LND), fossil-fuel
burning (FOS), ocean CO2 uptake (OCE), carbon capture and storage (CCS), and direct air
capture (DAC).
(a) O2 /N2 ratio
Figure 5 shows the simulated changes in atmospheric O2 /N2 ratio relative to
CO2 for the different scenarios. The inset arrows show the vector contributions
from the various processes. The simulated trajectories are seen to have a steeper
slope on the O2 /N2 versus CO2 plot than expected from fossil-fuel burning alone
(FOS vector). The main reason, regardless of scenario, is that the CO2 rise is
reduced by oceanic uptake of CO2 , which has no effect on the decline in O2 /N2
ratio. The slope is also slightly influenced for all scenarios by land-use emissions
and by the modelled land sink, which combine to yield a small net land CO2
sink and O2 source. This sink has a lower O2 : C ratio than fossil-fuel burning,
steepening the slope of the overall atmospheric trajectory. These same effects
influence past trends [4]. The simulated atmospheric trajectory is also impacted
by CCS and DAC, which act similarly to the ocean sink, offsetting the CO2 rise
but not the O2 decline.
The fraction of CO2 emissions absorbed by the oceans increases as the CO2
growth rate slows, allowing the oceanic CO2 excess to be mixed into a larger
volume of the ocean. As a result, during the latter half of the twenty-first century,
when CO2 growth rate is much lower, the ocean plays a larger relative role in
slowing the CO2 rise. Because this behaviour of the ocean sink has no effect on
O2 , the O2 /N2 ratio declines increasingly faster for all scenarios compared with
the CO2 rise, with the changes most apparent late in the century. The O2 /N2
decline continues even after CO2 has peaked.
In the CCS and DAC mitigation scenarios, the change in O2 /N2 ratio is even
greater than in the baseline scenario without mitigation. This behaviour results
because the production of O2 by the natural land CO2 sink is reduced in scenarios
Phil. Trans. R. Soc. A (2011)
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R. F. Keeling et al.
100
baseline scenario
carbon capture and storage
conservation
land sequestration
direct air capture
D14C (‰)
50
CCS
ISO
0
DAC, LND,
OCE
FOS
–50
–100
380
400
420
440
460
CO2 (ppm)
480
500
520
Figure 6. Same as figure 5 for D14 C versus CO2 concentration. Vectors as in figure 5, plus
isofluxes (ISO).
with lower CO2 . In the scenario with land carbon sequestration, the change in
O2 /N2 ratio is smaller than in the baseline scenario because of photosynthetic
O2 production. The energy conservation scenario has an even smaller drop in
O2 /N2 because of the higher O2 : C ratio of fossil-fuel burning as compared with
land photosynthesis.
(b) Atmospheric
14 C
Figure 6 shows simulated changes in atmospheric D14 C relative to changes in
CO2 concentration along with a vector inset showing the influence of the dominant
processes. Most vector slopes can only be roughly indicated, because they are not
strictly constant in time, but vary with absolute CO2 concentrations and D14 C
value. Fossil-fuel burning adds CO2 that is devoid of 14 C to the atmosphere, thus
driving the 14 C/12 C ratio downwards, a process known as the Suess effect [41].
All scenarios yield trajectories on the plot with a slope smaller than expected
from fossil-fuel burning alone.
Land and ocean CO2 uptake have no effect on D14 C because the isotopic
fractionation associated with these processes is corrected for when expressing
the 14 C/12 C ratio in D notation. Land and ocean uptake thus tend to steepen the
trajectories in figure 6 compared with the fossil-fuel burning vector. This effect is
more than offset, however, by the isofluxes of 14 C with the land and oceans, which
buffer the decline via exchanges of C atoms between the atmosphere and the land
and oceans. The D14 C isoflux effect includes a diminishing contribution from the
redistribution of bomb 14 C excess. The main contribution, however, is from the
redistribution of 14 C-deficient fossil-fuel carbon. For all scenarios, the isoflux term
increases in relative importance with time, causing the upward curvature of all
scenarios in figure 6. This behaviour results because the isoflux buffering is more
efficient than the ocean buffering of CO2 concentration, reflecting the different
impulse response functions of atmospheric CO2 and carbon isotopes (e.g. [42]).
Phil. Trans. R. Soc. A (2011)
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Signature of carbon capture and storage
LND,
CCS
–8.2
baseline scenario
carbon capture and storage
conservation
land sequestration
direct air capture
ISO
DAC
–8.4
OCE
d 13C (‰)
–8.6
FOS
–8.8
–9.0
–9.2
–9.4
–9.6
380
400
420
440
460
CO2 (ppm)
480
500
520
Figure 7. Same as figure 5 for d13 C versus CO2 concentration. Vectors as given in figures 5 and 6.
In all scenarios, the main factor driving the D14 C changes is the amount of
fossil-fuel CO2 released to the atmosphere. In figure 6, the CCS and energy
conservation scenarios are indistinguishable, with D14 C declining less than the
baseline scenario owing to the avoided emission of 14 C-devoid fossil carbon. The
land sequestration and DAC scenarios are also indistinguishable from one another.
While both are displaced in figure 6 from the baseline scenario, the displacement
is mostly due to differences in the CO2 trajectory rather than the D14 C trajectory.
Examined closely, however, the changes in D14 C for the land sequestration and
DAC scenarios are also very slightly displaced (downwards) compared with the
baseline scenario. The displacement is a dilution effect: while the same amount
of 14 C-devoid carbon is released by fossil-fuel burning, the impact on D14 C is
greater if there is less CO2 in the atmosphere. A similar dilution effect impacts
every mitigation scenario relative to the baseline scenario.
(c) 13 C/12 C ratio of atmospheric CO2
Figure 7 shows the simulated changes in d13 C relative to the changes in CO2
concentration along with a vector inset showing the influence of the dominant
processes. In most respects, the impacts on d13 C are similar to those on D14 C. As
for D14 C, the vector slopes for d13 C can only be roughly indicated, owing to their
sensitivity to the concentration and d13 C level of atmospheric CO2 . Fossil-fuel CO2
has a lower d13 C level than atmospheric CO2 . Fossil-fuel burning (FOS vector)
thus drives the atmospheric d13 C level downwards. The downward trajectory in
d13 C in all scenarios is smaller, however, than expected from fossil-fuel burning
alone mainly because of the impact of isofluxes. The isofluxes grow in relative
importance with time, just as for 14 C, leading to upward curvature in the plots.
The trajectories for d13 C are qualitatively different from those of D14 C mainly
because d13 C is additionally sensitive to isotopic fractionation associated with
the land, ocean and DAC sinks. The ocean CO2 sink is associated with only
weak fractionation, leading to a vector slope that is only slightly different from
Phil. Trans. R. Soc. A (2011)
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R. F. Keeling et al.
zero (OCE vector). The land sink (LND vector), however, involves strong isotopic
fractionation associated with photosynthesis. For the land sequestration scenario,
this fractionation offsets the effect of fossil-fuel addition on both CO2 and
d13 C, leading to smaller changes in d13 C overall. All four mitigation scenarios
involve sequestering or reducing emissions of 13 C-depleted carbon. The CCS and
energy conservation scenarios, in fact, have identical trajectories in figure 7. The
land sequestration scenario differs very slightly from these, but the difference is
not significant compared with uncertainties in model parameters, which govern
this difference (future photosynthetic fractionation and the d13 C of fossil fuels).
The DAC scenario has a d13 C trajectory similar to the baseline scenario, the
main difference being the change in CO2 . The small d13 C change from the
baseline scenario reflects two competing effects: the assumed −5‰ fractionation
associated with DAC that drives d13 C higher, and a dilution effect equivalent to
that mentioned above for D14 C that drives d13 C lower.
4. Global verification of carbon capture and storage
The simulations suggest the possibility of detecting and quantifying rates of
carbon capture at the global scale through combined tracer measurements. The
CCS scenario stands out as being characterized by maximal changes in O2 /N2
and minimal changes in d13 C and D14 C compared with the other scenarios.
The DAC scenario, in contrast, stands out by having maximal changes in all
three tracers.
To assess the ability to quantify global capture rates, it is useful to consider
the composite tracer ‘atmospheric potential oxygen’ (APO) [17,43] defined
according to
1.1
(XCO2 − 350),
dAPO = d(O2 /N2 ) +
0.2095
where 1.1 is an estimate of the relevant global average O2 : C ratio for land
photosynthesis and respiration, 0.2095 converts from mole fraction to per meg
units, XCO2 is the CO2 mole fraction in parts per million, 350 is an arbitrary
reference value of the CO2 mole fraction and dAPO is expressed in per meg units.
dAPO is largely conserved by land photosynthesis and respiration, which produce
compensating changes in O2 and CO2 . A process that captures 1 Pg C of CO2
from the global atmosphere without impacting O2 will decrease the global average
CO2 concentration and dAPO by 0.47 ppm and 2.47 per meg, respectively.
The main contributions to changes in dAPO are fossil-fuel burning, and air–
sea exchanges of O2 , N2 and CO2 , and possibly also carbon capture. Fossil-fuel
burning reduces dAPO because it is characterized by an O2 : C ratio higher
than 1.1. Subtracting fossil-fuel and air–sea gas exchange contributions from the
observed total trend in dAPO will yield the contribution from CCS and DAC.
Table 1 summarizes estimates of the impact on dAPO of various processes, along
with estimates of their feasible observational accuracy. Our calculation suggests
that it should be possible to estimate global CCS or DAC rates (or their sum)
to an accuracy of around ±0.4 Pg C yr−1 based on present capabilities, provided
that an adequate ocean observing system is in place. A key element of such
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Signature of carbon capture and storage
Table 1. Modelled contributions to dAPO change from year 2010 to 2060 expressed on an annual
basis, i.e. total change divided by 50 years. Units are in per meg per year.
oceanic CO2 uptake
fossil-fuel burning
capture process
air–sea N2 exchangee
air–sea O2 exchangee
total (observable)
baseline
scenario
CCS or
DAC scenarios
target observational
accuracya
−8.31
−8.44
0
0
0
−16.75
−7.88
−8.44
−2.47
0
0
−18.78
±0.4b
±0.5c
±0.8d
±0.1f
n.a.g
±0.4c
a Rough
estimate of feasible target, assuming an adequate observing system is in place.
direct determination by repeat hydrography; also accounts for uncertainty in air–sea
O2 exchange termg .
c Estimated from contribution to ocean sink in table 5 of Manning & Keeling [18].
d Estimated from quadrature sum of other errors, as required to balance global budget, assuming
an additional contribution of ±0.15 per meg yr−1 from uncertainty in aB according to
Manning & Keeling [18]. A precision of ±0.8 per meg yr−1 corresponds to a resolution in the
global capture rate of ±0.4 Pg C yr−1 .
e Neglected by model, but required for complete budget.
f Based on uncertainty in ocean warming rate according to Manning & Keeling [18].
g As clarified by Keeling et al. [44], repeat hydrographic measurements will provide a more direct
constraint on the relevant combined fluxes of O2 and CO2 than either separately.
b Requires
an observing system is repeat hydrographic measurements to quantify temporal
changes in dissolved O2 and dissolved inorganic carbon species, which are directly
related, at the global scale, to the relevant air–sea gas exchange terms.
5. Impact on interhemispheric gradient
Most fossil-fuel burning occurs in the northern extra-tropics. Given the
approximate 1 year time scale for CO2 in the northern extra-tropics to mix into
the Southern Hemisphere (SH) via atmospheric circulation [45], an excess in the
annual mean CO2 concentration of around 7 ppm in the Northern Hemisphere
(NH) relative to the SH is expected based on current emissions. The observed
excess, however, is only approximately 3 ppm as a result of other processes,
such as CO2 uptake by the land and oceans as well as by the natural north–
south transport of carbon by ocean circulation. Fossil-fuel burning also tends
to cause deficits in O2 /N2 , d13 C and D14 C in the NH compared with the SH,
and the gradients in these tracers are also strongly influenced by land and
oceanic exchanges.
Any widespread implementation of CCS would also necessarily occur mostly
in the northern extra-tropics. CCS will change the gradients in CO2 , d13 C and
D14 C in a way that is essentially indistinguishable from that of reduced fossil-fuel
burning. CCS would have no impact, however, on the O2 /N2 gradient.
By reducing the CO2 gradient but not the O2 /N2 gradient, CCS will change
the dAPO gradient, causing dAPO to decrease in the NH compared with the SH.
Based on the 1 year interhemispheric exchange time and the atmospheric mass,
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R. F. Keeling et al.
Table 2. Nominal exchange ratios for CO2 source/sink processes.
fossil-fuel burning
land biospheric sink
ocean sink
isoflux
carbon capture and storage
direct air capture
d(O2 /N2 ) : CO2
(per meg ppm−1 )
d13 C : CO2
(‰ ppm−1 )a
D14 C : CO2
(‰ ppm−1 )
−6.7
−5.2
0
n.a.
0
0
−0.045
−0.048
−0.005
∞
−0.045
−0.013
−2.5
0
0
∞
−2.5
0
a Estimates based on a CO concentration of 400 ppm, and fractionation of −18‰ and −5‰ for
2
the land sink and DAC, respectively.
the effect would decrease dAPO in the NH relative to the SH by approximately 5
per meg for each Pg C of CCS in the northern extra-tropics. The dAPO gradient
has been measured over the past two decades to a precision of around ±2 per
meg [44,46,47]. The mean gradient is small (approx. 5 per meg NH deficit),
but appears to have quasi-decadal variability at the ±5 per meg level [44]. If
the natural variability in the dAPO gradient can be adequately understood,
continued measurements could constrain hemispheric CCS rates to the level
of ±0.4 Pg C yr−1 .
The dAPO gradient would also be impacted by DAC, depending on its
distribution. DAC does not need to be collocated with fuel usage, however,
allowing a more uniform distribution between the hemispheres. In a world with
both DAC and CCS, the dAPO gradient will constrain the combined spatial
distributions of CCS and DAC.
6. Discussion
Our goal was to examine the possibility of verifying rates of CCS via observed
changes in the atmosphere, both in the near-field, i.e. within a few kilometres of
processing plants and storage sites, and at global and hemispheric scales through
multiple tracer measurements. Atmospheric detection of CCS (and leakage)
appears feasible by exploiting differences in the exchange ratios (O2 : C, 13 C/12 C
and 14 C/12 C) of CCS relative to other processes, as summarized in table 2.
In the near field, for example, combined measurements of CO2 concentration
with measurements of O2 /N2 ratio and/or d13 C can help to resolve leakage or
capture efficiency in the face of variability due to vegetation and soils. At the
global scale, combined measurements of CO2 , O2 /N2 , d13 C and D14 C may provide
constraints for understanding rates of CCS in relation to other processes, such
as improvements that allow reduced fossil-fuel usage, DAC or land sequestration.
On both scales, the most powerful constraint on CCS (or DAC) comes from
combined measurements of CO2 and O2 /N2 . d13 C and D14 C data can also provide
useful constraints in many circumstances, but are more limited by measurement
precision or smaller relative signals.
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Signature of carbon capture and storage
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In the near field, detecting any CO2 excess from leakage is only the first step.
A critical additional step is to interpret the concentration changes in terms of
fluxes, which requires some form of inverse modelling. This is an active field, and
much progress can be expected in the near future [16]. Another critical step is to
better define the O2 : C and 13 C/12 C exchange ratios of local ecosystems and their
natural variability. This could be done as an integral part of a field programme,
by placing field instrumentation in proximity to potential leakage sites and in
nearby locations as controls. Based on present understanding of land ecosystem
exchange ratios and available measurement technology, it would appear possible
to detect CO2 leakage to the level of approximately 2 ppm under a wide range of
circumstances, even with active vegetation. In sites with less active vegetation,
such as arid regions, even lower detection limits might be feasible. The ability to
discriminate between leakage and ecosystem CO2 exchanges opens the possibility
to resolve emissions under near-stagnant conditions, when CO2 excesses from all
sources will build up to higher levels. This could be especially critical for leak
detection and addressing issues of public safety. The ability to exploit O2 /N2 and
d13 C measurements to resolve leakage hinges on the rapid progress that has been
made over the past decade in field-deployable sensors for these species. Although
D14 C measurements could also be useful for near-field CCS monitoring, present
technology requires flask sampling, with high costs per data point.
Other processes besides land ecosystem exchange may produce atmospheric
signatures that may interfere with the signatures of CCS in near-field applications.
Fossil-fuel burning can occur in varying O2 : C ratios, depending on fuel type,
and this may cause variability in many settings, particularly in urban or
industrial areas. Cement manufacture, which releases CO2 produced via thermal
decomposition of limestone, may interfere in some settings. Increased use of
hydrogen gas (H2 ) as an energy carrier, e.g. derived ultimately from fossil fuels,
will alter patterns of CO2 production and O2 consumption from fossil fuels by
separating the point of CO2 production (associated with H2 production) from
the O2 consumption (associated with energy end use). Hydrogen usage cannot
affect global CO2 and O2 budgets, however, because it does not alter the total
O2 consumed and CO2 produced.
At global and hemispheric scales, the signature of CCS will be superimposed
on other processes impacting CO2 , O2 /N2 , d13 C and D14 C. How to make best
use of observations of these parameters will depend on which processes are best
known from independent information. Our analyses suggest that, with present
uncertainties in fossil-fuel emissions, and assuming an improved ocean observing
system is implemented, it should be feasible to detect global and hemispheric
rates of CCS via the tracer dAPO to the ±0.4 Pg C yr−1 level. The observations
would constrain, not the gross capture rate, but the net rate, discounting
leakage, which is the appropriate measure for assessing the effectiveness of
CCS in reducing CO2 buildup in the atmosphere. If both CCS and DAC are
implemented on a wide scale, the observations would constrain the combination
of both processes. Obtaining a detection limit near ±0.4 Pg C yr−1 will require
independent development of an ocean observing system and maintaining the
current accuracy in estimating fossil-fuel emissions, despite financial incentives
for fraud. In the absence of an adequate ocean observing system or with degraded
fossil-fuel estimates, dAPO measurements would serve to constrain the combined
exchanges from CCS, the oceans and fossil-fuel burning, but not each separately.
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R. F. Keeling et al.
We thank David Bowling (University of Utah) and Elena Kozlova (University of East Anglia) for
providing data shown in figures 2 and 3, respectively. We thank David Etheridge for providing
access to the Otway modelling results and other helpful comments. A.C.M. is supported by a UK
NERC/QUEST Advanced Fellowship (ref. no. NE/C002504/1). R.F.K. has been supported under
grants from the US NSF (ATM-0632770, ATM-0651834), DoE (DE-FG02–07ER64362) and NOAA
(2007-2000636). M.K.D. thanks LANL’s Laboratory Directed Research and Development Program
for nurturing his work in direct air capture of CO2 and CCS.
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