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Executive Summary (CT2009)
Highlights
We present the fourth release of a combined measurement and modeling system that keeps track of the emissions ("sources") and removal ("sinks") of atmospheric CO2 globally from January 2000 through December 2008. CarbonTracker 2009 (released on 9 November 2009) incorporates several improvements over the previous release.
  1. We have added several observation sites within North America. Quasi-continuous measurements at Lac Labiche, Alberta, Canada and East Trout Lake, Saskatchewan, Canada have been provided by Environment Canada (EC). ESRL also adds quasi-continuous measurements from towers in Boulder, Colorado; Beech Island, South Carolina in collaboration with Savannah River National Laboratory and University of Georgia; Shenandoah National Park, Virginia in collaboration with University of Virginia, Department of Environmental Sciences; and Walnut Grove, California in collaboration with DOE Environmental Energy Technologies Division at Lawrence Berkeley National Laboratory.
  2. Global resolution of the TM5 atmospheric transport model used in CarbonTracker has improved from 6° longitude x 4° latitude to 3° x 2°.
  3. First-guess fluxes from the terrestrial biosphere model and the air-sea flux model are now available in binary format, so that the changes that CarbonTracker estimates can be analyzed by interested users.
  4. Optimized fluxes are presented at two different scales: the 1° longitude by 1° latitude distribution used in previous CarbonTracker releases, and a distribution at "ecoregion" scale. Ecoregions, described in detail here, are the native scale at which CarbonTracker performs its optimization. Showing fluxes at this scale--without the within-region patterning derived from underlying models--is in some ways a more direct way of presenting CarbonTracker's results.

Figure 1: The long term mean biological uptake
Figure 2: The long term mean fossil fuel emissons

Estimates of CO2 sources and sinks
From 2001 through 2008 ecosystems in North America have been a net sink of 0.59 ± 0.55 PgC yr-1 (1 Petagram Carbon equals 1015 gC, or 1 billion metric ton C, or 3.67 billion metric ton CO2). This natural sink offsets about one-third of the emissions of 1.9 PgC yr-1 from the burning of fossil fuels in the U.S.A., Canada and Mexico combined. Our estimates include sub-continental patterns of sources and sinks coupled to the distribution of dominant ecosystem types across the continent (see Figures 1 and 2). The sinks are mainly located in the agricultural regions of the Midwest (36%), deciduous forests along the East Coast (33%), and boreal coniferous forests (17%). There also appears to be substantial year to year variation of the sink, with the net natural emissions to the atmosphere ranging from -0.2 to -0.8 PgC yr-1. These variations are strongly correlated with large scale temperature and moisture variations. We estimated the lowest net annual uptake by ecosystems in 2002, when there were widespread drought conditions through most of the West. In contrast, our observing system did not detect an effect from the 2007 drought in the Southeast. This is likely due to lack of coverage of the area (Figure 3) in our current observing network. The estimates are optimally consistent with measurements of ~26,000 flask samples of air from 72 sites across the world, ~20,000 daily averages of continuously measured CO2 at 9 sites (6 in North America, plus observatories at Mauna Loa, Hawaii; South Pole; and American Samoa), and ~22,000 daily averages from towers at 13 locations within the continent (see Figures 3 and 4). Eight of these towers sample air from heights more than 100m above ground level.


Figure 3: North American sites only
Word of caution about the biological flux maps
Figure 1 shows 1° x 1° detail for estimated fluxes. With the present observing network, the detailed 1° x 1° degree fluxes should not be interpreted as quantitatively meaningful for each block. To spread the influence of sparse observing sites we make the assumption that large ecosystem regions respond in the same way to variations of temperature and light. However, temperature and light are not uniform in an entire region, and thus the same response function does not produce a uniform flux over the region. Thus we caution that the spatial detail is only predicted by CarbonTracker based on the assumption of large-scale ecosystem coherence, but has not been verified by observations.

Calculated time-dependent CO2 fields throughout the atmosphere
A "byproduct" of the data assimilation system, once sources and sinks have been estimated, is that the mole fraction of CO2 is calculated everywhere in the model domain and over the entire 2000-2008 time period, based on the optimized source and sink estimates. As a check on model transport properties, calculated CO2 mole fractions are regularly compared with measurements of ~27,000 air samples taken by NOAA/ESRL at 26 aircraft sites, which are not used in the estimation of sources and sinks. Column averages of the CO2 mole fraction have been calculated as well, and they can be compared to satellite measurements of the same quantity when the averaging is done in the same way as for the satellite results.


Figure 4: All observation sites used in CarbonTracker
Uncertainties
It is important to note that at this time the uncertainty estimates for the sources/sinks are themselves quite uncertain. They have been derived from the mathematics of the data assimilation system, which require several "educated guesses" for initial uncertainty estimates. The paper describing CarbonTracker (Peters et al. (2007), Proc. Nat. Acad. Sci. vol. 104, p. 18925-18930) presents different uncertainty estimates based on the sensitivity of the results to 14 alternative yet plausible ways to construct the CarbonTracker system. For example, the 14 realizations produce a range of the net annual mean terrestrial emissions in North America of -0.40 to -1.01 PgC -1 (negative emissions indicate a sink). The procedure is described in the Supporting Information Appendix to that paper, which is freely downloadable from the PNAS web site. Furthermore, the estimates do not take into account several additional factors noted below. The calculation was set up for sources and sinks to slowly revert, in the absence of observational data, to "first guesses" of net ecosystem exchange, which are close to zero on an annual basis. This set-up may result in a bias. Also due to the sparseness of measurements, we have had to assume coherence of ecosystem processes over large distances, giving existing observations perhaps an undue amount of weight. The process model for terrestrial photosynthesis and respiration was very "basic", and will likely be greatly improved in future releases of CarbonTracker. Easily the largest single annual mean source of CO2 is emissions from fossil fuel burning, which are currently not estimated by CarbonTracker. We use estimates from emissions inventories (economic accounting) and subtract those from the total sources derived by CarbonTracker. A small relative error in the inventories would thus translate into a larger relative error in the annual mean ecosystem sources/sinks that have smaller magnitudes. We expect to add a process model of fossil fuel combustion in future releases of CarbonTracker. Finally, additional measurement sites are expected to lead to the greatest improvements, especially to more credible and specific source/sink results at smaller spatial scales.

CarbonTracker is a NOAA contribution to the North American Carbon Program