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Executive Summary - CT2010

CarbonTracker 2010

CarbonTracker 2010 is the fifth release of NOAA's combined CO2 measurement and modeling system. CarbonTracker is designed to keep track of sources (emissions to the atmosphere) and sinks (removal from the atmosphere) of carbon dioxide around the world, using atmospheric CO2 observations from a host of collaborators. The current release of CarbonTracker, CT2010, provides global estimates of these surface fluxes of CO2 from January 2000 through December 2009.

Notice: methodological changes   CarbonTracker 2010 is a unique release due to a reanalysis over only part of the record and the use of components from differing land models to form the terrestrial first-guess fluxes. Details are below.

Estimates of CO2 sources and sinks over North America
From 2001 through 2009, ecosystems in North America have been a net sink of 0.66 ± 0.54 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 about 1.9 PgC yr-1 from the burning of fossil fuels in the U.S.A., Canada and Mexico combined (Figure 1).

Figure 1. Annual total emissions from North America. The bars in this figure represent CO2 emissions for each year in PgC yr-1 from the U.S.A., Canada, and Mexico combined. See map on this page.
CarbonTracker models four types of surface-to-amosphere exchange of CO2, each of which is shown in a different color: fossil fuel emissions (tan), terrestrial biosphere flux excluding fires (green), direct emissions from fires (red), and air-sea gas exchange (blue).
Negative emissions indicate that the flux removes CO2 from the atmosphere. The net surface exchange, computed as the sum of these four components, is shown as a thick black line.

Whereas fossil emissions are generally steady over this period, ranging between 1.8 and 1.9 PgC yr-1, the amount of CO2 taken up by the North American biosphere varies significantly from year to year. In terrestrial biosphere models, inter-annual variability in land uptake can be related to anomalies in large-scale temperature and precipitation patterns. While the CarbonTracker period of analysis is relatively short compared to the dynamics of slowly-changing pools of biospheric carbon, episodes of extremes in net ecosystem exchange (NEE) have been associated with climatic anomalies (see e.g. Peters et al., 2007). It is interesting to note that the inferred year-to-year variabilty (the "range") of land uptake is actually as big as the mean sink itself.

The year with the smallest annual uptake by terrestrial ecosystems in North America was 2002, when there was widespread drought in the U.S. west. During this year, land ecosystems accounted for a sink of only 0.2 PgC yr-1. This reduced land sink is largely responsible for 2002 being the year in the CarbonTracker record with the largest input of CO2 to the atmosphere from North America, when net emissions reached 1.7 ± 0.6 PgC yr-1.

In contrast, our observing system did not detect an effect from the 2007 drought in the U.S. Southeast. This is likely due to lack of coverage of the area (Figure 3) in our current observing network. New observations in CT2010 compared to CT2009, however, do lead to interesting differences in this region for 2008. For CT2010 in 2008, the U.S. Southeast represents a distinct source of carbon dioxide to the atmosphere, whereas CT2009 fluxes for this same place and time are equivocal.

In CT2010, we find that 2009 was the year with the largest North American land sink, with ecosystems taking up about 0.9 PgC yr-1—a land sink that is about 36% bigger than the average. Fossil fuel emissions from North America were also slightly reduced in 2009 compared to earlier years as a result of the economic downturn. As a result, ecosystems removed about half of fossil fuel emissions over North America in 2009, and this was the year with the lowest net input of CO2 to the atmosphere from North America. This 2009 net annual emission was 0.9 ± 0.4 PgC yr-1, about 25% smaller than the average of 1.3 ± 0.5 PgC yr-1. This is the first year since the beginning of the CarbonTracker record in 2000 for which the net flux from North America has fallen below 1 PgC yr-1.

Spatial distribution of surface fluxes
CarbonTracker flux estimates include sub-continental patterns of sources and sinks coupled to the distribution of dominant ecosystem types across the continent (Figure 2). We have greater confidence in countrywide totals than in estimates of regional sources and sinks, but we expect that such finer-scale estimates will become more robust with future expansion of the CO2 observing nework. Our results indicate that 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%).

Figure 2. Mean ecosystem fluxes. The pattern of net ecosystem exchange (NEE) of CO2 of the land biosphere averaged over 2001-2009, as estimated by CarbonTracker. This NEE represents land-to-atmosphere carbon exchange from photosynthesis and respiration in terrestrial ecosystems, and a contribution from fires. It does not include fossil fuel emissions. Negative fluxes (blue colors) represent CO2 uptake by the land biosphere, whereas positive fluxes (red colors) indicate regions in which the land biosphere is a net source of CO2 to the atmosphere. Units are gC m-2 yr-1.

Word of caution about high-resolution biological flux maps
Figure 2 shows estimated fluxes by ecoregion. While we also provide flux maps and data with a finer 1° x 1° spatial resolution, for example on our flux maps pages, these ecoregions define the actual scales at which CarbonTracker operates. With the present observing network, the detailed one-degree fluxes should not be interpreted as quantitatively meaningful for each block. Any within-ecoregion patterns come directly from results of the terrestrial biosphere model. Part of this high-resolution patterning comes from variations of temperature, precipitation, light, plant species, and soil type over each ecoregion. To spread the influence of measurements from the sparse observation network, CarbonTracker makes adjustments uniformly over an entire ecoregion. These adjustments scale the net ecosystem flux of CO2 predicted by the terrestrial biosphere model by the same factor across each ecoregion. Thus we caution that the 1° x 1° spatial detail in CarbonTracker land fluxes is based on the simulations of the terrestrial biosphere model and the assumption of large-scale ecosystem coherence. This has not been verified by observations.

The CarbonTracker observing system
CarbonTracker surface flux estimates are optimally consistent with measurements of ~31,500 flask samples of air from 81 sites across the world, ~27,800 four-hourly averages of continuously measured CO2 at 13 sites (10 in North America, plus observatories at Mauna Loa, Hawaii; Barrow, Alaska, South Pole; and American Samoa), and ~23,800 four-hourly averages from towers at 13 locations within the continent (see Figure 3). Eight of these towers sample air from heights more than 100m above ground level.
Figure 3. CarbonTracker 2010 Observational Network Click on any site marker for more information. Double-click on a site marker to center the map on that site.

Calculated time-dependent CO2 fields throughout the global 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 2000-2009 time period, based on the optimized source and sink estimates (Figure 4). As a check on model transport properties and CarbonTracker inversion performance, calculated CO2 mole fractions are regularly compared with measurements of ~31,000 air samples taken by NOAA/ESRL at 26 aircraft sites, which are not used in the estimation of optimized sources and sinks.

Since CarbonTracker simulates CO2 throughout the entire atmospheric column, the model atmosphere can be sampled exactly like satellite retrievals of CO2. Such estimates are generally more sensitive to the CO2 mole fraction in some parts of the atmosphere than in others, and by using a vertical profile of this sensitivity, a direct analog of the satellite estimate can be constructed.

Figure 4. Carbon dioxide weather Shown is the daily average of the pressure-weighted mean mole fraction of carbon dioxide in the free troposphere as modeled by CarbonTracker for March 20, 2009. Units are micromoles of CO2 per mole of dry air (μmol mol-1), and the values are given by the color scale depicted under the graphic. The "free troposphere" in this case is levels 6 through 10 of the TM5 model. This corresponds to about 1.2km above the ground to about 5.5km above ground, or in pressure terms, about 850 hPa to about 500 hPa. Gradients in CO2 concentration in this layer are due to exchange between the atmosphere and the earth surface, including fossil fuel emissions, air-sea exchange, and the photosynthesis, respiration, and wildfire emissions of the terrestrial biosphere. These gradients are subsequently transported by weather systems, even as they are gradually erased by atmospheric mixing.

Flux uncertainties
It is important to note that at this time the uncertainty estimates for CarbonTracker sources and sinks are themselves quite uncertain. They have been derived from the mathematics of the ensemble data assimilation system, which requires 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 is 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 the CO2 mole fraction signatures of those fluxes from observations. As a result, the biosphere and ocean fluxes estimated by CarbonTracker inherit error from the assumed fossil fuel emissions. While these emissions inventories may have a small relative error on global scales (perhaps 5 or 10%), any such bias translates into a larger relative error in the annual mean ecosystem sources and sinks, since those fluxes 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 robust and specific source/sink results at smaller spatial scales.

Consistency of modeled and observed atmospheric CO2 growth rates
Global atmospheric CO2 growth rates inferred directly from observed carbon dioxide at marine surface sites are consistent with those modeled by CarbonTracker, both in their average values and in their year-to-year variations (Figure 5). These global growth rates continue to hover at around 4 PgC yr-1, or around 1.9 ppm yr-1 (Figure 5). The 2009 growth rate modeled by CarbonTracker was below average at 3.4 ± 3.1 PgC yr-1. This was not due to a significant decrease in fossil fuel emissions, as can be seen from estimates of global fluxes used in CarbonTracker. Instead, the most notable difference for the 2009 atmospheric growth rate appears to be reduced biomass burning fluxes from the tropics and southern hemisphere, which at 1.4 PgC yr-1 are about 0.5 PgC yr-1 below their 2001-2009 mean of 1.9 PgC yr-1.

Figure 5. Atmospheric CO2 growth rates. Observed atmospheric CO2 growth rates (source: NOAA ESRL page on global trends in CO2) are plotted against the atmospheric CO2 growth rate inferred from CT2010 global fluxes. Note that error bars on the observed growth rates are relatively small and may not be visible on this plot.

CT2010 methodological differences

There are two main methodological differences between CT2010 and CT2009:
  1. The CarbonTracker 2010 release is a reanalysis covering the period 2007-2009. The CT2010 results prior to 2007 are those of the previous CT2009 release. We did this because the newest fossil fuel emissions estimates affect the years 2007 through 2009 only. These revisions do not impact fossil emissions estimates over 2000-2006. Furthermore, the stations which are newly assimilated in CT2010 mostly do not have CO2 measurements before 2007. As a result, all CT2010 results for the period 2000-2006 are identical to those from CT2009.
  2. The biological model we use to provide first-guess fluxes for terrestrial net ecosystem exchange (NEE) has been revised. This year the CASA-GFED team has transitioned to a model (version 3.1) driven by MODIS fPAR. The previous revision (version 2) of this model used AVHRR NDVI, scaled to represent MODIS fPAR. We have found that the seasonal cycle of globally-integrated NEE is reduced by about 10% in CASA-GFED3.1 compared to CASA-GFED2. CarbonTracker will require re-tuning of its optimization system to operate with this new model. This is a research problem we will be addressing over the next release cycle. Despite our inability to exploit CASA-GFED3.1 NEE fluxes in the current configuration of CarbonTracker, that model remains vitally important because it is the only means we have to assess fire emissions for 2009. We have chosen a hybrid approach for CT2010, combining elements of the old and new models. While the fire emissions come from CASA-GFED3.1, terrestrial NEE first guess fluxes derive from CASA-GFED2. The NEE fluxes have real estimated interannual variations for 2007 and 2008, but since CASA-GFED2 analysis stops in 2008, first-guess fluxes in 2009 are computed from the climatological NEE of CASA-GFED2. This is not unprecedented. For several previous releases, we have used climatological first-guess fluxes for the final year in the analysis cycle.
Details of these methodological differences are presented in the "What's New?" page and the CT2010 documentation.

CarbonTracker is a NOAA contribution to the North American Carbon Program