Black carbon (BC) is produced by incomplete combustion of biomass by wildfires and burning of fossil fuels. BC is environmentally persistent over centuries to millennia, sequestering carbon in marine and terrestrial environments. However, its production, storage and dynamics, and therefore its role in the broader carbon cycling during global change, are poorly understood. In this Review, we discuss BC cycling across the land-to-ocean continuum. Wildfires are the main source of BC, producing 128 ± 84 teragrams per year. Negative climate–BC feedbacks could arise as wildfire increases with anthropogenic warming, producing more BC, which in turn will sequester carbon, but the magnitude of these effects are unknown. Most BC is stored in terrestrial systems with some transported to the ocean via rivers and the atmosphere. However, the oceanic BC budget is not balanced, with known BC removal fluxes exceeding BC inputs. We demonstrate these observed inconsistencies using a simple ocean box model, which highlights key areas of future research. Measurements of BC mineralization and export rates along the land-to-ocean continuum and quantification of previously unexplored sources of oceanic BC are needed to close the global BC budget.
Black carbon (BC) is produced from incomplete combustion of biomass and fossil fuels and persists for centuries to millennia in the environment.
BC production is expected to increase with increasing fire activity under anthropogenic warming and could act as a negative feedback to climate change.
BC is often divided into particulate and dissolved BC, which can have different environmental transport mechanisms, residence times and fates.
The largest BC pool is in the soil (160–200 petagrams (Pg) C globally). Rivers transport 43 ± 15 teragrams (Tg) BC per year; BC is cycled in other inland aquatic systems, but the global relevance of these processes is unknown.
Oceans store 12–14 Pg C of dissolved BC. The observed age of this BC (4,800 ± 620 14C years in the surface ocean, >20,000 14C years in the deep ocean) does not match expected ages based on mass balance estimates.
Future research must further explore the possibility that some of the dissolved BC in marine waters is not derived from terrestrial fires.
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The fluxes of PBC, radiocarbon ages of PBC and mapped river basins derive from ref.62. The fluxes of DBC within latitudinal ranges derive from ref.2. The magnitude of the global stocks, fluxes, and radiocarbon ages of BC shown in Fig. 4 derive from the studies cited in Table 1. The data set is on the PANGAEA data repository (www.pangaea.de) under the username alyshacoppola (connected to the ORCID ID https://orcid.org/0000-0002-9928-2786).
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A.I.C. acknowledges support from the Swiss National Science Ambizione grant ‘Fire in the Ocean’ (PZ00P2_185835) and thanks T. Eglinton, A. Varkalis and L. Tinkham. M.S. and T.D. acknowledge funding by the German Science Foundation (DFG) within the Cluster of Excellence EXC 2077 ‘The Ocean Floor — Earth’s Uncharted Interface’ (project number 390741603). N.D.W. acknowledges funding from the US Department of Energy (DOE)-funded COMPASS-FME project. Pacific Northwest National Laboratory (PNNL) is operated by Battelle for the US DOE under contract DE-AC05-76RL01830. Additional support was provided by the National Science Foundation OCE #2017577 (to S.W.). M.W.J. was funded by an independent research fellowship from the UK Natural Environment Research Council (NERC) (grant NE/V01417X/1).
The authors declare no competing interests.
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- Black carbon
(BC). A carbonaceous, polycondensed aromatic product (>60% organic carbon) derived from the incomplete combustion of biomass and fossil fuels, with greater environmental persistence than its unburned biomass source.
Compounds that experience rapid turnover within hours to days of release and do not accumulate.
- Environmentally persistent
Compounds that resist rapid microbial degradation, accumulating on land and in the ocean for centuries to millennia.
- Dissolved black carbon
(DBC). The black carbon fraction measured within dissolved organic carbon.
- Particulate black carbon
(PBC). The black carbon fraction in solid environmental matrices, such as particulate organic carbon, soils and sediments.
- Dissolved organic carbon
(DOC). Organic carbon dissolved in water that passes through a filter, usually with a pore size of 0.1 to 0.7 µm.
- Particulate organic carbon
(POC). Organic carbon suspended in water that is retained on a filter, usually with a pore size of 0.1 to 0.7 µm.
Fine particle matter 2.5 µm or smaller in size.
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Coppola, A.I., Wagner, S., Lennartz, S.T. et al. The black carbon cycle and its role in the Earth system. Nat Rev Earth Environ 3, 516–532 (2022). https://doi.org/10.1038/s43017-022-00316-6