7.3.2.3 Regional Fluxes

Quantifying present-day regional carbon sources and sinks and understanding the underlying carbon mechanisms are needed to inform policy decisions. Furthermore, by analysing spatial and temporal detail, mechanisms can be isolated.

7.3.2.3.1 The top-down view: atmospheric inversions

The atmosphere mixes and integrates surface fluxes that vary spatially and temporally. The distribution of regional fluxes over land and oceans can be retrieved using observations of atmospheric CO₂ and related tracers within models of atmospheric transport. This is called the ‘top-down’ approach to estimating fluxes. Atmospheric inversions belong to that approach, and determine an optimal set of fluxes that minimise the mismatch between modelled and observed concentrations, accounting for measurement and model errors. Fossil fuel emissions have small uncertainties that are often ignored and, when considered (e.g., Enting et al., 1995; Rodenbeck et al., 2003a), are found to have little influence on the inversion. Fossil fuel emissions are generally considered perfectly known in inversions, so that their effect can be easily modelled and subtracted from atmospheric CO₂ data to solve for regional land-atmosphere and ocean-atmosphere fluxes, although making such an assumption biases the results (Gurney et al., 2005). Input data for inversions come from a global network of about 100 CO₂ concentration measurement sites,^[3] with mostly discrete flask sampling, and a smaller number of in situ continuous measurement sites. Generally, regional fluxes derived from inverse models have smaller uncertainties upwind of regions with denser data coverage. Measurement and modelling errors and uneven and sparse coverage of the network generate random errors in inversion results. In addition, inverse methodological details, such as the choice of transport model, can introduce systematic errors. A number of new inversion ensembles, with different methodological details, have been produced since the TAR (Gurney et al., 2003; Rödenbeck et al., 2003a,b; Peylin et al., 2005; Baker et al., 2006). Generally, confidence in the long-term mean inverted regional fluxes is lower than confidence in the year-to-year anomalies (see Section 7.3.2.4). For individual regions, continents or ocean basins, the errors of inversions increase and the significance can be lost. Because of this, Figure 7.7 reports the oceans and land fluxes aggregated into large latitude bands, as well as a breakdown of five land and ocean regions in the NH, which is constrained by denser atmospheric stations. Both random and systematic errors are reported in Figure 7.7.

Figure 7.7. Regional ocean-atmosphere and land-atmosphere CO₂ fluxes for the NH (top) and the globe (bottom) from inversion ensembles and bottom-up studies. Fluxes to the atmosphere are positive and uptake has a negative sign. Inversion results all correspond to the post-Pinatubo period 1992 to 1996. Orange: Bottom-up terrestrial fluxes from Pacala et al. (2001) and Kurz and Apps (1999) for North America, from Janssens et al. (2003) for Europe and from Shvidenko and Nilsson (2003) plus Fang et al. (2001) for North Asia (Asian Russia and China). Cyan (filled circles): Bottom-up ocean flux estimates from Takahashi, et al. (2002). Blue: ocean fluxes from atmospheric inversions. Green: terrestrial fluxes from inversion models. Magenta: total inversion fluxes. Red: fossil fuel emissions. The mean flux of different inversion ensembles is reported. Inversion errors for regional fluxes are not reported here; their values usually range between 0.5 and 1 GtC yr–1. Error bar: range of atmospheric inversion fluxes from the TAR. Squares: Gurney et al. (2002) inversions using annual mean CO₂ observations and 16 transport models. Circles: Gurney et al. (2003) inversions using monthly CO₂ observations and 13 transport models. Triangles: Peylin et al. (2005) inversions with three transport models, three regional breakdowns and three inversion settings. Inverted triangles: Rödenbeck et al. (2003a) inversions where the fluxes are solved on the model grid using monthly flask data.

7.3.2.3.2 The bottom-up view: land and ocean observations and models

The range of carbon flux and inventory data enables quantification of the distribution and variability of CO₂ fluxes between the Earth’s surface and the atmosphere. This is called the ‘bottom-up’ approach. The fluxes can be determined by measuring carbon stock changes at repeated intervals, from which time-integrated fluxes can be deduced, or by direct observations of the fluxes. The stock change approach includes basin-scale in situ measurements of dissolved and particulate organic and inorganic carbon or tracers in the ocean (e.g., Sabine et al., 2004a), extensive forest biomass inventories (e.g., UN-ECE/FAO, 2000; Fang et al., 2001; Goodale et al., 2002; Nabuurs et al., 2003; Shvidenko and Nilsson, 2003) and soil carbon inventories and models (e.g., Ogle et al., 2003; Bellamy et al., 2005; van Wesemael et al., 2005; Falloon et al., 2006). The direct flux measurement approach includes surveys of ocean CO₂ partial pressure (pCO₂) from ship-based measurements, drifters and time series (e.g., Lefèvre et al., 1999; Takahashi et al., 2002), and ecosystem flux measurements via eddy covariance flux networks (e.g., Valentini et al., 2000; Baldocchi et al., 2001).

The air-sea CO₂ fluxes consist of a superposition of natural and anthropogenic CO₂ fluxes, with the former being globally nearly balanced (except for a small net outgassing associated with the input of carbon by rivers). Takahashi et al. (2002) present both surface ocean pCO₂ and estimated atmosphere-ocean CO₂ fluxes (used as prior knowledge in many atmospheric inversions) normalised to 1995 using National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) 41-year mean monthly winds. Large annual CO₂ fluxes to the ocean occur in the Southern Ocean subpolar regions (40°S–60°S), in the North Atlantic poleward of 30°N and in the North Pacific poleward of 30°N (see Figure 7.8). Ocean inversions calculate natural and anthropogenic air-sea fluxes (Gloor et al., 2003; Mikaloff Fletcher et al., 2006), by optimising ocean carbon model results against vertical profiles of DIC data. These studies indicate that the Southern Ocean is the largest sink of anthropogenic CO₂, together with mid- to high-latitude regions in the North Atlantic. This is consistent with global ocean hydrographic surveys (Sabine et al., 2004a and Figure 5.10). However, only half of the anthropogenic CO₂ absorbed by the Southern Ocean is stored there, due to strong northward transport (Mikaloff Fletcher et al., 2006). The tropical Pacific is a broad area of natural CO₂ outgassing to the atmosphere, but this region is a sink of anthropogenic CO₂.

Models are used to extrapolate flux observations into regional estimates, using remote-sensing properties and knowledge of the processes controlling the CO₂ fluxes and their variability. Rayner et al. (2005) use inverse process-based models, where observations are ‘assimilated’ to infer optimised fluxes. Since the TAR, the global air-sea flux synthesis has been updated (Takahashi et al., 2002 and Figure 7.8), and new syntheses have been made of continental-scale carbon budgets of the NH continents (Pacala et al., 2001; Goodale et al., 2002; Janssens et al., 2003; Shvidenko and Nilsson, 2003; Ciais et al., 2005a), and of tropical forests (Malhi and Grace, 2000). These estimates are shown in Figure 7.7 and compared with inversion results.

Comparing bottom-up regional fluxes with inversion results is not straightforward because: (1) inversion fluxes may contain a certain amount of prior knowledge of bottom-up fluxes so that the two approaches are not fully independent; (2) the time period for which inversion models and bottom-up estimates are compared is often not consistent, in the presence of interannual variations in fluxes^[4] (see Section 7.3.2.4); and (3) inversions of CO₂ data produce estimates of CO₂ fluxes, so the results will differ from budgets for carbon fluxes (due to the emission of reduced carbon compounds that get oxidized into CO₂ in the atmosphere and are subject to transport and chemistry) and carbon storage changes (due to lateral carbon transport, e.g., by rivers) (Sarmiento and Sundquist, 1992). Some of these effects can be included by ‘off-line’ conversion of inversion results (Enting and Mansbridge, 1991; Suntharalingam et al., 2005). Reduced carbon compounds such as volatile organic compounds (VOCs), carbon monoxide (CO) and CH₄ emitted by ecosystems and human activities are transported and oxidized into CO₂ in the atmosphere (Folberth et al., 2005). Trade of forest and crop products displaces carbon from ecosystems (Imhoff et al., 2004). Rivers displace dissolved and particulate inorganic and organic carbon from land to ocean (e.g., Aumont et al., 2001). A summary of the main results of inversion and bottom-up estimates of regional CO₂ fluxes is given below.

7.3.2.3.3 Robust findings of regional land-atmosphere flux

• Tropical lands are found in inversions to be either carbon neutral or sink regions, despite widespread deforestation, as is apparent in Figure 7.7, where emissions from land include deforestation. This implies carbon uptake by undisturbed tropical ecosystems, in agreement with limited forest inventory data in the Amazon (Phillips et al., 1998; Malhi and Grace, 2000).

• Inversions place a substantial land carbon sink in the NH. The inversion estimate is –1.7 (–0.4 to –2.3) GtC yr^–1 (from data in Figure 7.7). A bottom-up value of the NH land sink of –0.98 (–0.38 to –1.6) GtC yr^–1 was also estimated, based upon regional synthesis studies (Kurz and Apps, 1999; Fang et al., 2001; Pacala et al., 2001; Janssens et al., 2003; Nilsson et al., 2003; Shvidenko and Nilsson, 2003). The inversion sink value is on average higher than the bottom-up value. Part of this discrepancy could be explained by lateral transport of carbon via rivers, crop trade and emission of reduced carbon compounds.

• The longitudinal partitioning of the northern land sink between North America, Europe and Northern Asia has large uncertainties (see Figure 7.7). Inversions give a very large spread over Europe (–0.9 to +0.2 GtC yr^–1), and Northern Asia (–1.2 to +0.3 GtC yr^–1) and a large spread over North America (–0.6 to –1.1 GtC yr^–1). Within the uncertainties of each approach, continental-scale carbon fluxes from bottom-up and top-down methods over Europe, North America and Northern Asia are mutually consistent (Pacala et al., 2001; Janssens et al., 2003). The North American carbon sink estimated by recent inversions is on average lower than an earlier widely cited study by Fan et al. (1998). Nevertheless, the Fan et al. (1998) estimate remains within the range of inversion uncertainties. In addition, the fluxes calculated in Fan et al. (1998) coincide with the low growth rate post-Pinatubo period, and hence are not necessarily representative of long-term behaviour.

7.3.2.3.4 Robust findings of regional ocean-atmosphere flux

• The regional air-sea CO₂ fluxes consist of a superposition of natural and anthropogenic CO₂ fluxes, with the former being globally nearly balanced (except for a small net outgassing associated with the input of carbon by rivers), and the latter having a global integral uptake of 2.2 ± 0.5 GtC yr^–1 (see Table 7.1).

• The tropical oceans are outgassing CO₂ to the atmosphere (see Figure 7.8), with a mean flux of the order of 0.7 GtC yr^–1, estimated from an oceanic inversion (Gloor et al., 2003), in good agreement with atmospheric inversions (0 to 1.5 GtC yr^–1), and estimates based on oceanic pCO₂ observations (0.8 GtC yr^–1; Takahashi et al., 2002).

Figure 7.8. Estimates (4° × 5°) of sea-to-air flux of CO₂, computed using 940,000 measurements of surface water pCO₂ collected since 1956 and averaged monthly, together with NCEP/NCAR 41-year mean monthly wind speeds and a (10-m wind speed)² dependence on the gas transfer rate (Wanninkhof, 1992). The fluxes were normalised to the year 1995 using techniques described in Takahashi et al. (2002), who used wind speeds taken at the 0.995 standard deviation level (about 40 m above the sea surface). The annual flux of CO₂ for 1995 with 10-m winds is –1.6 GtC yr^–1, with an approximate uncertainty (see Footnote 1) of ±1 GtC yr^–1, mainly due to uncertainty in the gas exchange velocity and limited data coverage. This estimated global flux consists of an uptake of anthropogenic CO₂ of –2.2 GtC yr^–1 (see text) plus an outgassing of 0.6 GtC yr^–1, corresponding primarily to oxidation of organic carbon borne by rivers (Figure 7.3). The monthly flux values with 10-m winds used here are available from T. Takahashi at http://www.ldeo.columbia.edu/res/pi/CO2/carbondioxide/pages/air_sea_flux_rev1.html.

• The extratropical NH ocean is a net sink for anthropogenic and natural CO₂, with a magnitude of the order of 1.2 GtC yr^–1, consistent among various estimates.

• The Southern Ocean is a large sink of atmospheric CO₂ (Takahashi et al., 2002; Gurney et al., 2002) and of anthropogenic CO₂ (Gloor et al., 2003; Mikaloff Fletcher et al., 2006). Its magnitude has been estimated to be about 1.5 GtC yr^–1. This estimate is consistent among the different methods at the scale of the entire Southern Ocean. However, differences persist with regard to the Southern Ocean flux distribution between subpolar and polar latitudes (T. Roy et al., 2003). Atmospheric inversions and oceanic inversions indicate a larger sink in subpolar regions (Gurney et al., 2002; Gloor et al., 2003), consistent with the distribution of CO₂ fluxes based on available ∆pCO₂ observations (Figure 7.8 and Takahashi, 2002).