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.

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.

CarbonTracker is a NOAA contribution to the North American Carbon Program