Appendix 1: CO2 removals in residual combustion products (charcoal)

Appendix 1

CO2 Removals in Residual Combustion Products(charcoal): Basis for Future Methodological
Development

Background
Charred materials are the product of the incomplete combustion of vegetation and fossil fuels (Goldberg, 1985).
The continuum of combustion products such as char, ash, soot and charcoal are commonly referred to as black
carbon (BC). BC is a heterogeneous mixture of residues that have contrasting chemistry and thus resistance tofurther biological or chemical degradation. Together they occur ubiquitously in soils and other terrestrial
sediments, and in marine sediments as well.
A large portion (>80%) of the BC that is produced by a fire event remains proximal to the site where it wasformed. It is then incorporated into the soil where it can remain for long periods of time. However, BC can also
be transported via fluvial and atmospheric pathways to marine sediments, with the majority moving through thefluvial system. This results in most of the particulate BC transported to the oceans being deposited on the coastal
shelves, while a smaller portion continues on to the deeper ocean sediments. Another fraction of the particulate
BC produced is dispersed into the atmosphere. With residence times that can exceed 7days, much of thiscomponent of BC is transported to the oceans and ultimately contributes to the BC fraction of deep ocean
sediments, where it is very stable.
Over the past few decades BC concentrations in the earth’s atmosphere and biosphere have become of interest
because, in aerosol form, they are strong absorbers of solar radiation. They can provide a record of palaeoenvironments in sediment and ice cores, and they may also be a sizable contributor of oxygen to the atmosphere
over geological time frames. BC, in particular the charcoal component, is also important because it represents
one of the few ways that carbon can be rendered relatively inert, such that it can not easily recombine withoxygen to form CO2. Hence, there is a strong potential for BC to act as a significant removal (sink) of carbon
from the more rapid bio-atmospheric carbon cycle to the slower (long-term) geological carbon cycle (e.g., Graetzand Skjemstad, 2003; Schmidt, 2004; Druffel, 2004).

Role of black carbon in the global C budget
In a recent review of the formation and persistence of BC in terrestrial ecosystems, Forbes et al. (2006) provided
a revised estimate of the formation of BC from vegetation fires and fossil fuel burning of 50 - 270 Tg yr-1. This isa very large C flux and a key question is thus whether the rate of annual BC formation exceeds the amount of C
released from the large pool of BC that is already accumulated in terrestrial and marine ecosystems. Whilst it iscurrently not possible to definitively answer this question, it is important to continue research that will enable amethodology to be developed in the future for accounting for BC in greenhouse gas inventories, and for better
understanding the role of BC in the global C budget.
Forbes et al. (2006) also identified a set of important issues to be addressed in order to make development of areliable methodology possible. They identified the need to describe rates of BC formation in a consistent way
and suggested it should be expressed as a percentage of the amount of C consumed (CC) by fire. They found thatwhen expressed this way (BC/CC), the rates of BC formation were <3% for grass and savanna fires, and 4-5%
for forest fires. The authors concluded that estimates of BC formation based solely on physical measures arevery unreliable (lead to significant over estimates) because they are unable to accurately identify and quantify
the BC component of post-fire residues that also contain a range of partly combusted materials.
BC is subject to slow rates of degradation by photochemical processes and by microbial processes in soils andsediments, but knowledge of longer-term rates and of the factors affecting them is very limited. Research such as
incubation studies has shown that BC degradation through biological processes is very slow. Other evidence alsosuggests very slow degradation of BC; BC can comprise up to 40% of the organic C in terrestrial soils and 1231% of the very large pool of organic C in deep ocean sediments, and has radiocarbon ages in soils in excess ofthousands of years. Hence, BC appears to have a significant half-life, in the order of thousands of years and isthus the most stable biomass-derived material in the biospheric C cycle. This relative inertness means that theestimated 3-5% of the carbon converted to BC during forest, savanna and grassland fires, must be considered asignificant component of the global carbon cycle with a very slow turnover.

Conclusions
In order to better gauge the influence of BC on the global carbon cycle, an improved understanding of theproduction and degradation rates of BC are required for those ecosystems exposed to extensive vegetation fires.
Further, the assessment of fluvial and aeolian transport of BC needs to be understood at fine scales, and a better
understanding gained of degradation rates of BC in land and ocean sediments. This will allow a methodology for2006 IPCC Guidelines for National Greenhouse Gas Inventories

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accounting for BC in greenhouse gas inventories to be developed, and for uncertainties and discrepancies
regarding estimates of BC fluxes between the atmosphere, biosphere and oceans to be minimised. The result will
be a more accurate global black carbon budget, and better understanding of the role of BC as a potential sink inthe global C cycle.

References
Druffel, E.R.M. (2004). Comments on black carbon in the global carbon cycle. Marine Chem.; 92:197-200.
Forbes, M.S., Raison, R.J., and Skjemstad, J.O. (2006). Formation, transformation and transport of black carbon(charcoal) in terrestrial and aquatic ecosystems. Journal of the Science of the Total Environment (in press).
Goldberg, E.D. (1985). Black Carbon in the Environment: Properties and Distribution. John Wiley and Sons,
New York; 198 pp.
Graetz, R.D. and Skjemstad, J.O. (2003). The charcoal sink of biomass burning on the Australian continent.
CSIRO Atmospheric Research Technical Paper; no 64.
Schmidt, M.W.I (2004). Carbon budget in the black. Nature; 427:305-306.

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