Estimating the Size of Terrestrial Carbon Sinks

In most early summaries of the global carbon budget, the size of the terrestrial sink was determined by difference. That is, estimates of land use emissions (typically from Houghton's studies) were subtracted from the net uptake of the terrestrial biosphere inferred from atmospheric measurements (e.g., Table 14.2, Table 14.3). That is, the terrestrial carbon sink was inferred from the difference of other fluxes—and was not described explicitly. This is why it was (until recently) usually referred to as the residual or missing sink—or the sink required to "balance the books" of the global carbon cycle.

But the CCMLP exercise simulated both terrestrial carbon sources (from land use) and potential terrestrial carbon sinks. The CCMLP models assumed that terrestrial sinks may have resulted from a combination of three factors: (1) ecosystem recovery land use/land cover change; (2) the effects of increasing CO2 concentrations on plant productivity (the so-called CO2 fertilization effect); and (3) the effects of recent climate variability and climatic change on ecosystems. The CCMLP study did not, however, include the potential effects of other ecological drivers, such as changing nitrogen deposition, forest management practices, or fire disturbances.

The CCMLP exercise was based on process-based terrestrial ecosystem models. These models use, to the extent possible, a "first principles" approach to simulating ecological and biogeochemical processes. The models generally include detailed calculations of ecosystem physiological processes (including photosynthesis, plant respiration, and microbial respiration), as well as heuristic representations of vegetation dynamics and disturbances. The models used in the CCMLP activity have also been tested against a wide variety of data, including in situ measurements of carbon and water fluxes, primary o

net flux, atmospheric methods: | CO2 and O2 (IPCC Prentice et al., 2001)

model results:

land-use change only, Houghton land use change only, CCMLP CO2 fertilization only, CCMLP climate change only, CCMLP net flux, CCMLP

Figure 14.2. Estimates of the terrestrial carbon balance. Here the net carbon balance of the terrestrial biosphere (as inferred from atmospheric measurements) is presented from Prentice et al. (2001). Estimates of land use carbon emissions from Houghton (1999) are also reported. For comparison, recent modeling estimates of terrestrial carbon balance (arising from combinations of CO2, climate, and land use change) are shown from the CCMLP exercise (McGuire et al. 2001). Unit is PgC y-1. Figure adapted from House et al. (2003).

productivity, and soil moisture balance; satellite-based measurements of vegetation cover and plant phenology; and (by linking the output the ecosystem models to atmospheric transport models) measurements of seasonal atmospheric CO2 concentrations gathered across the globe. Although the models still have many deficiencies, they represent our current understanding of how terrestrial ecosystems work and how they respond to climate, land use, and changing atmospheric chemistry.

As illustrated in Appendix 14.2, terrestrial carbon sinks can be generated through a variety of different mechanisms: (1) those that increase ecosystem productivity; (2) those that increase the residence time of carbon in vegetation, litter, or soil; and (3) those that change the loss of carbon through disturbances (both natural or anthropogenic). Although there is still a running debate on the relative importance of these different processes, the CCMLP simulations considered the possible carbon sinks associated with CO2 fertilization (through enhancing NPP), climatic variability (through changes in NPP and residence time of carbon), and land use/land cover change (through changes in the average carbon residence time).

In the CCMLP results, the simulated net uptake of carbon for the 1980s was roughly -0.3 to -1.5 billion metric tons (McGuire et al. 2001). This range of estimates is in rough agreement (albeit on the high side) with net land-atmosphere flux estimates derived from atmospheric measurements and ocean modeling studies (Prentice et al. 2001; House et al. 2003)-see Figure 14.2.

Table 14.4. Breakdown of the terrestrial carbon budget for the 1980s, based on the CCMLP simulations

CCMLP terrestrial carbon fluxes, 1980s

Flux

estimates

Net land-atmosphere flux

-0.3

to

-1.5

Source from cropland change

+0.6

to

+ 1.0

Sink from CO2 fertilization

-1.5

to

-3.1

Sink from climatic variability

-0.2

to

+0.9

Source: See McGuire et al. 2001; Prentice et al. 2001; House et al. in press. Note: Units are in PgC (1015g of carbon) per year. Note that this reports only the changes in carbon fluxes from cropland change, not all land use changes.

Source: See McGuire et al. 2001; Prentice et al. 2001; House et al. in press. Note: Units are in PgC (1015g of carbon) per year. Note that this reports only the changes in carbon fluxes from cropland change, not all land use changes.

The CCMLP results can be broken down into their individual components (Table 14.4). These results suggest that these two factors (ecosystem recovery from past land use, and CO2 fertilization) could be responsible for the "missing" terrestrial carbon sink. Climatic variability actually appears to stimulate (on average) a net source of carbon from terrestrial ecosystems during the 1980s. Other processes not included in the CCMLP study—including increasing nitrogen deposition, shifting disturbances, and changing forest practices—may also be partly responsible for the sink. Interestingly, this "multifactor" explanation of the terrestrial carbon sink differs greatly from the explanation offered during the 1970s and 1980s—that the terrestrial carbon sink resulted only from the effects of CO2 fertilization (Sabine et al., Chapter 2, this volume).

Here, it is important to remember two things. First, process-based ecosystem models are able to mimic the magnitude and timing (between the 1980s and 1990s) of the net land-atmosphere carbon flux, assuming that carbon sources are driven by land use and climatic variability and that carbon sinks are driven by ecosystem recovery from past land use and CO2 fertilization. Second, this does not mean that these mechanisms have actually been "proven" to be responsible for the terrestrial carbon sink. The models are still largely based on local- to regional-scale representations of ecological processes and have had only limited testing against global data. Further work is needed to rigorously test different hypothesis regarding the generation of carbon sources and sinks in the terrestrial biosphere.

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