The Missing “Missing Sink”

 2003 Canadian Institute of Forestry/
Institut forestier du Canada (CIF/IFC)
The Forestry Chronicle
http://pubs.nrc-cnrc.gc.ca/tfc/TFC.html
The Missing “Missing Sink”
by Sten Nilsson1, Matthias Jonas1, Vladimir Stolbovoi1, Anatoly Shvidenko1,
Michael Obersteiner1 and Ian McCallum1
To assess CO2 fluxes of the terrestrial biosphere, ground-based inventories, flux measurements and bottom-up modeling approaches are used. In most cases inventory-based approaches are not able to produce a full carbon account (FCA). The FCA refers to a carbon budget that is complete, encompasses all components, and is applied continuously in time. Atmospheric inversion modeling implicitly measures the sum of all fluxes, meaning a FCA. Eddy-covariance measurements have huge variations and are difficult to scale up
to regional and decadal levels. Bookkeeping up to more complex process-based models rely on land-use change estimates over time,
which have large uncertainties. To overcome the accounting gap between top-down and bottom-up measurements, the IPCC introduced
the terrestrial “missing sink” concept by taking long-term land-use changes into account to further break down the global carbon budget. IIASA has developed a bottom-up FCA approach that breaks down the terrestrial carbon balance of Russia for 1990 (1988–1992)
resulting in a sink. This was then combined with the terrestrial sink strength of the extra-tropical Northern Hemisphere (approximately > 30°N) determined via top-down atmospheric inversion. Using this approach, the remainder, the terrestrial sink strength of the extratropical Northern Hemisphere without Russia, could then be determined with a relative uncertainty that is smaller (i.e., < 100%) than
the uncertainties exhibited by inverse models. From the analysis it can be concluded that the “missing sink” issue can be reduced to an
issue of relevant accounting due to the fact that the combined top-down/bottom-up approach does not identify any missing sink.
Key words: carbon balance, flux, missing sink, inverse modeling, inventory approaches, full carbon accounting, top-down/bottom-up
approaches, terrestrial ecosystems, Russia
Les inventaires au sol, les mesures des variations et les approches de modélisation stratifiée sont utilisés pour mesurer les variations
de CO2 dans la biosphère terrestre. Dans la plupart des cas, les approches basées sur les inventaires ne sont pas capables d’établir une
comptabilité complète du carbone (CCA). La CCA fait référence à un décompte du carbone qui est complet, englobant toutes les composantes et qui est appliqué de façon continue dans le temps. La modélisation de l’inversion atmosphérique mesure implicitement la somme
de toutes les variations, soit une CCA. Les mesures de covariance indiquent d’importantes variations et sont difficiles à adapter à une
région et pour une décennie. L’enregistrement des données pour des modèles reposant sur des processus plus complexes tient compte
de l’estimation des changements dans l’utilisation du territoire au cours du temps, ce qui comporte d’importantes incertitudes. Pour surmonter l’écart dans la comptabilité des mesures du haut en bas et des mesures de bas en haut, l’IPCC a introduit le concept du « réservoir manquant » terrestre en prenant en considération les changements dans l’utilisation du territoire afin de morceler encore plus en
détail le budget global du carbone. L’IIASA a élaboré une approche de CCA de bas en haut qui morcèle le décompte du carbone terrestre pour la Russie en 1990 (1988–1992), qui pour effet de créer un réservoir. Ces résultats ont été par la suite combinés à la force du
réservoir terrestre de l’Hémisphère Nord au-dessus des tropiques (à environ 30ºN et plus) déterminée par l’inversion atmosphérique de
haut en bas. Au moyen de cette approche, la différence, la force du réservoir terrestre de l’Hémisphère Nord à partir des tropiques sans
la Russie, a pu être déterminée avec un niveau d’incertitude inférieure (c’est-à-dire, < 100%) aux incertitudes atteintes par les modèles
d’inversion. On peut conclure à partir de cette analyse que la question du « réservoir manquant » peut être ramenée à une question de
comptabilité adéquate par le fait que l’approche combinée de haut en bas/du bas en haut n’identifie aucun réservoir manquant.
Mots clés: budget du carbone, variation, réservoir manquant, modélisation inverse, approches selon l’inventaire, approches de comptabilité complète de haut en bas/de bas en haut, écosystèmes terrestres, Russie
Different methods are available for measuring atmospheric CO2 concentrations and fluxes at various scales. These
CO2 measurements in combination with measurements of
O2, 13CO2, 14CO2, and N2O permit global-scale assessments
of CO2 emissions caused by man or nature as well as assessments of the CO2 sequestered by land and oceans. Modeling
based on atmospheric observations (inverse modeling) enables
the land/ocean–atmosphere fluxes to be partitioned for largescale latitudinal regions. However, higher-order regional
breakdowns are difficult owing to year-to-year variations in the
rate of atmospheric CO2 accumulation. These variations are of
the same magnitude as the decadal mean annual accumulation
and are caused by shifts in natural carbon fluxes. These yearto-year changes are one reason for conflicting results from inverse
modeling (Battle et al. 2000, Bousquet et al. 2000, Prentice
et al. 2001).
1Forestry
Project, International Institute for Applied Systems Analysis,
Schlossplatz 21, A-2361 Laxenburg, Austria (e-mail: [email protected]).
For assessing the fluxes of the terrestrial biosphere, groundbased inventories, flux measurements (eddy-covariance techniques), and bottom-up modeling approaches (ranging from bookkeeping models to more complex process-based models) are
used. However, inventory-based approaches generally are not
able to produce a full carbon account (FCA). The FCA refers
to a carbon budget that is complete, encompasses all components, and is applied continuously in time (Steffen et al. 1998).
Atmospheric inversion modeling implicitly measures the sum
of all fluxes, including all impacts such as land-use change, management interventions, and disturbances, as well as environmental conditions and indirect human impacts such as increased
atmospheric CO2 concentrations. Thus, inverse modeling
reflects FCA in the form of the net flux into the atmosphere.
Eddy-covariance measurements have huge variations and are
difficult to scale up to regional and decadal levels. Models (ranging from bookkeeping models to more complex process-based
models) rely on land-use change estimates over time, which have
large uncertainties. The last two approaches should in theory
deliver FCAs. However, they will require further development
and refinement before they are actually able do so. Currently,
NOVEMBER/DECEMBER 2003, VOL. 79, NO. 6, THE FORESTRY CHRONICLE
1071
Fig. 1. Terrestrial FCA for Russia (1988–1992): Fluxes and pool changes, including uncertainties (in parentheses), in TgC yr-1, where:
A:
Atmosphere
Ant:
Sum of Dis and Con
H:
Hydrosphere
Con:
Consumption of biomass (agricultural products, wood harvest, etc.)
L:
Lithosphere
CSRO:
Combined surface runoff (sum of Dep_H and SRO)
P:
Pedosphere
Dep:
Deposition (precipitation)
V:
Vegetation
Det:
Detritus production
Dis:
Disturbances (fire and insects)
DOS:
Dissolved organic substances (transported out of the pedosphere)
HR:
Heterotrophic respiration
Leak:
Leakage (translocation of DOS into the lithosphere)
NPP:
Net primary production
RO:
Runoff
SRO:
Surface runoff (transport of suspended vegetation tissues into the hydrosphere)
URO:
Underground runoff (translocation of DOS into the hydrosphere)
there are substantial differences in assessments of carbon fluxes using the methods discussed above (e.g., House et al. 2003).
To overcome the accounting gap between top-down (atmospheric) and bottom-up (ground-based) measurements, the
IPCC (Bolin et al. 2000, Prentice et al. 2001) introduced the terrestrial “missing sink” concept by taking long-term land-use changes
into account to further break down the global carbon budget. The
global terrestrial “missing sink” is assessed to range from –4.0
to –0.3 Gt C · yr–1 for the 1980s, and from –4.8 to –1.6 Gt C ·
yr–1 for the 1990s (Prentice et al. 2001, House et al. 2003; see
also Bolin et al. 2000, Schimel et al. 2001). The location of the
dominating part of the “missing sink” is assessed to be in the
extratropical Northern Hemisphere (e.g., Prentice et al. 2001).
There are concerns regarding the introduction of the landuse change concept in breaking down the global carbon budget. This concept considers changed fluxes due to land conversion
and vegetation replacement, but not those due to changed
biomass production and changed consumption of products from
1072
converted land. In addition, the concept raises concerns
because the approach seeks to balance one component of the
global system—the terrestrial system—with knowledge of
only a single component of that balance, namely, the land-use
change emissions. Currently, we can only assume we know how
many components are needed for the reconstruction of the full
terrestrial balance.
IIASA has developed a bottom-up full accounting approach
that breaks down the terrestrial carbon balance using a bottomup concept. It focuses (by making use of measured and
derived data) on pool changes and fluxes of and between all
relevant terrestrial components following the concepts discussed
by, for example, Steffen et al. (1998) and Schulze et al.
(2002). This approach was applied for Russia for the year 1990
(1998–1992) with scenarios up to 2010 (Nilsson et al. 2000)
and for Austria centering on the year 1990 (Jonas and Nilsson 2001). In the context of the “missing sink” debate, particularly
in relation to exchange processes of carbon concerning the hydro-
NOVEMBRE/DECEMBRE 2003, VOL. 79, NO. 6, THE FORESTRY CHRONICLE
Fig. 2. Comparison of atmospheric inversion modeling and scaled-up bottom-up sink strength for Russia.
sphere–lithosphere system (Brye et al. 2001, Nemani et al. 2002,
Siemens 2003), we have revised and improved our carbon account
for Russia for the year 1990 by introducing more process-driven accounting and quantifying all known carbon fluxes at a more
detailed level (see Nilsson et al. 2003). The revised aggregated
terrestrial organic carbon balance for Russia for 1990 based
on detailed analyses of all known carbon fluxes is presented
in Fig. 1.
The overall balance shows that in 1990 the terrestrial
ecosystems were a net sink of 351 ± 176 Tg · yr–1 (±50%). This
figure can be compared with the earlier Nilsson et al. (2000)
estimate of 149 ± 200 Tg C · yr–1. By introducing more
detailed fluxes in the new assessment, particularly with respect
to exchange processes of carbon in the gaseous, soluble and solid
(humus) phases in the hydrosphere–lithosphere system, the net
terrestrial sink strength more than doubled and its uncertainty was substantially reduced. The comparison illustrates how
sensitive the assessment of the atmospheric balance and its uncertainties are in relation to small changes in surface and subsurface
fluxes and which fluxes are considered in the accounting.
The results underline the need for a thorough FCA or, rather,
full greenhouse gas accounting.
In the next step of the analyses, we make use of our Russian
bottom-up approach in combination with top-down approaches that are based on atmospheric inversion assessments for the
extratropical Northern Hemisphere (approximately > 30°N).
In doing so, we take advantage of the findings that atmospheric inversion results for latitudinal partitioning of sources
and sinks between northern and southern mid- to high latitudinal and tropical regions are considered “robust” in comparison with the results for regions exhibiting greater regional breakdown (Prentice et al. 2001).
As illustrated in Fig. 2, our scaled-up bottom-up terrestrial
sink strength for Russia is in agreement with the inverse modeling (top-down) results for Eurasia and the extratropical
Northern Hemisphere.
3House et al. 2003
We have also assessed the uncertainties for subregions of
the extratropical Northern Hemisphere by making use of the
terrestrial sink strength of the extratropical Northern Hemisphere, determined top-down via atmospheric inversion, and
the terrestrial sink strength of Russia, determined bottom-up.
The remainder, the terrestrial sink strength of the northern extratropical belt without Russia, can now be determined with a relative uncertainty that is smaller than 100%, which is substantially
smaller than the uncertainties exhibited by inverse models
(Schimel et al. 2001, House et al. 2003).
Based on the analysis, the following conclusions can be made:
• The “missing sink” issue can be reduced to an issue of relevant accounting because our top-down/bottom-up linkage
does not identify any missing sink.
• By linking regional-scale, bottom-up full accounts with atmospheric inversion assessments, the uncertainties of terrestrial sink estimates using inverse modeling can be reduced
considerably, which will help to overcome controversial estimates of regional sink/source strengths.
• To achieve a satisfactory terrestrial FCA, the hydrosphere–lithosphere system must be included adequately in
the account.
References
Battle, M., M.L. Bender, P.P. Tans, J.W.C. White, J.T. Ellis, T. Conway and R.J. Francey. 2000. Global carbon sinks and their variability
inferred from atmospheric O2 and ∂13C. Science 287: 2467–2470.
Bolin, B., R. Sukumar, P. Ciais, W. Cramer, P. Jarvis, H. Kheshgi, C. Nobre, S. Semenov and W. Steffen. 2000. Global perspective. In R.T. Watson, I.R. Noble, B. Bolin, N.H. Ravindnanakh, D.J.
Verardo and D.J. Dokken (eds.). Land use, land-use change, and forestry.
Special Report of the Intergovernmental Panel on Climate Change.
pp. 23–51. Cambridge University Press, Cambridge, UK.
Bousquet, P., P. Peylin, P. Ciais, C. Le Quéré, P. Friedlingstein and P.P. Tans. 2000. Regional Changes in Carbon Dioxide Fluxes of Land and Oceans since 1980. Science 290: 1342–1346.
Brye, K.R., J.M. Norman, L.G. Bundy and S.T. Gower. 2001.
Nitrogen and Carbon Leaching in Agroecoystems and Their Role in
Denitrification Potential. J. Environ. Quality 30: 58–70.
NOVEMBER/DECEMBER 2003, VOL. 79, NO. 6, THE FORESTRY CHRONICLE
1073
House, J.I., I.C. Prentice, N. Ramankutty, R.A. Houghton and M.
Heimann. 2003. Reducing apparent inconsistencies in estimates of
terrestrial CO2 sources and sinks. Tellus 55B: 345–363.
Jonas, M. and S. Nilsson. 2001. The Austrian Carbon Database (ACDb)
Study – Overview, IIASA Interim Report IR-01-064, International
Institute for Applied Systems Analysis, Laxenburg, Austria.
http://www.iiasa.ac.at/Publications/Documents/IR-01-064.pdf
Nemani, R., M. White, P. Thornton, K. Nishika, S. Reddy, J. Jenkins and S. Running. 2002. Recent trends in hydrologic balance
have enhanced the terrestrial carbon sink in the United States. Geophysical Research Letters (2002 GL 014867) Published online.
Nilsson, S., M. Jonas, A. Shvidenko, V. Stolbovoi and I. McCallum. 2003. Monitoring, Verification and Permanence of Carbon
Sinks, Abstract from the CarboEurope Conference “The Continental
Carbon Cycle,” 19–21 March 2003, Lisbon, Portugal, CarboEurope.
Nilsson, S., A. Shvidenko, V. Stolbovoi, M. Gluck, M. Jonas and
M. Obersteiner. 2000. Full carbon account for Russia, IIASA Interim Report IR-00-021, International Institute for Applied Systems Analysis, Laxenburg, Austria. http://www.iiasa.ac.at/Publications/Documents/IR-00-021.pdf
Prentice, I.C., G. Farquhar, M. Fasham, M. Goulden, M. Heimann,
V. Jaramillo, H. Kheshgi, C. Le Quéré, R.J. Scholes and D.W.R.
Watson. 2001. The carbon cycle and atmospheric carbon dioxide. In
J.T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden,
X. Dai, K. Maskell and C.A. Johnson (eds.). Climate Change 2001:
The Scientific Basis. Contribution of Working Group I to the Third
Assessment Report of the Intergovernmental Panel on Climate
Change. pp. 183–237. Cambridge University Press, Cambridge.
1074
Schimel, D., J.I. House, K.A. Hibbard, P. Bousquet, P. Chiais, P.
Peylin, B.H. Braswell, M.J. Apps, D. Baker, A. Bondeau, J.
Canadell, G. Churkina, W. Cramer, A.S. Denning, C.B. Field, P.
Friedlingstein, C. Goodale, M. Heimann, R.A. Houghton, J.M. Melillo, B. Moore III, D. Murdiyarso, I. Noble, S.W. Pacala, I.C.
Prentice, M.R. Raupach, P.J. Rayner, R.J. Scholes, W.L. Steffen
and C. Wirth. 2001. Recent patterns and mechanisms of carbon exchange
by terrestrial ecosystems. Nature 414: 169–172.
Schulze, E.-D., R. Valentini and M.-J. Sanz. 2002. The long way
from Kyoto to Marrakesh: Implications of the Kyoto Protocol negotiations for global ecology. Global Change Biology 8: 505–518.
Siemens, J. 2003. Soil Carbon Losses by Doc and Dic Leaching, Abstract
from the CarboEurope Conference “The Continental Carbon Cycle,”
19–21 March 2003, Lisbon, Portugal, CarboEurope.
Steffen, W., I. Noble, J. Canadell, M. Apps, E.-D. Schulze, P.G.
Jarvis, D. Baldocchi, P. Ciais, W. Cramer, J. Ehleringer, G.
Farquhar, C.B. Field, A. Ghazi, R. Gifford, M. Heimann, R.
Houghton, P. Kabat, C. Körner, E. Lambin, S. Linder, H.A.
Mooney, D. Murdiyarso, W.M. Post, I.C. Prentice, M.R. Raupach,
D.S. Schimel, A. Shvidenko and R. Valentini. 1998. The terrestrial carbon cycle: Implications for the Kyoto Protocol. Science 280:
1393–1394.
NOVEMBRE/DECEMBRE 2003, VOL. 79, NO. 6, THE FORESTRY CHRONICLE