Wageningen University
Meteorology and Air Quality Department


Summary
Highlights
We present the next release of a combined measurement and modeling system that keeps track of the emissions ("sources") and removal ("sinks") of atmospheric CO2 globally from January 2001 through December 2011. CarbonTracker Europe 2013 (released on 1 December 2013) incorporates several innovations compared to earlier systems, and provides a new view on the global carbon cycle.
  1. We have added new observation sites, and more complete records using ObsPack.
  2. We have assimilated two aircraft observation records (SAN, ULB) from the free troposphere.
  3. We have replaced the CASA-GFED2 prior biosphere model with the SIBCASA-GFED4 model that has a much higher (minute) timestep for calculating assimilation rates.
  4. We have estimated patterns of exchange within ecoregions over the northern hemisphere land-masses, to accommodate the large heterogeneity of the landscape
  5. We have used updated fossil fuel emissions products, with better seasonal cycles for fossil fuel burning emissions over Europe, the US, and Asia
  6. We have used a 1x1 degree zoom of the TM5 transport model over Europe and North America

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 2011 ecosystems in Europe have been a net sink of -0.42 ± 0.47 PgC/yr (1 Petagram Carbon equals 1015 gC, or 1 billion metric ton C, or 3.67 billion metric ton CO2), offsetting about one fourth of the emissions of 1.53 PgC/yr from the burning of fossil fuels in the EU27 countries and eastern European domain combined. The natural uptake is predominantly in non-EU countries, and is found in the forested areas (-280 TgC/yr) . Croplands appear as a net sink of carbon (-90 TgC/yr) in our system, but the spatial overlap with strong fossil fuel sources makes this estimate less robust than other ecosystems. It is more likely that this 90 TgC of cropland uptake is returned to the atmosphere after consumption of the goods produced, and are not part of a long-term net carbon sink. The same is true for other parts of the net sink: some processes can return carbon taken up over land back to the atmosphre outside the bounds of our monitoring capacity, such as for instance export of carbon by rivers.

Using two different version of our system (other transport, other biosphere fluxes, other fires, other fossil fuel emissions, other optimizable parameters), we place an uncertainty on the flux by putting it in a range of -360 to -440 TgC/yr. This range is smaller than the interannual variability which includes a large reduction in uptake due to the 2003 drought (+150 TgC/yr below average). The largest anomaly in our time series occurs in 2009 with an extra sink due to a very warm autumn (-290 TgC/yr). . The CarbonTracker Europe results are constructed such that they are consistent with over 150,000 CO2 observations from 97 sites in the atmosphere.


Figure 3: European sites from which data is used in CarbonTracker Europe
Word of caution about the biological flux maps
Figure 1 shows 1 x 1 degree detail for estimated fluxes. With the present observing network of about a dozen sites 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 similar ways (described by an exponential decay function) 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 predicted by CarbonTracker within ecosystems 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 2001-2011 time period, based on the optimized source/sink estimates. As a check on model transport properties, calculated CO2 mole fractions were compared with measurements of ~42,000 air samples taken by NOAA/ESRL at 30 aircraft sites and from the CONTRAIL program (NIES-MRI), as well as ~120,000 observations from the HIPPO I and II campaigns. These aircraft data had not been used in the estimation of sources/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 Europe
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 required several "educated guesses" for initial uncertainty estimates. The paper describing CarbonTracker (Peters et al. (2007), Proc. Nat. Acad. Sci. vol. 104, p. 18925-18930) and also that describing CarbonTracker Europe (Peters et al., 2009, Global Change Biology), present different uncertainty estimates, based on the sensitivity of the results to alternative yet plausible ways to construct the CarbonTracker system. For example, the 14 realizations used in the PNAs paper produce a range of estimates for net annual mean terrestrial uptake in North America from -0.40 to -1.01 PgC/yr, as given in the PNAS paper. The procedure is described in the Supporting Information Appendix to that paper, which is freely downloadable from the PNAS web site. In addition, the estimates do not take into account several additional factors noted below. The calculation was set up for sources/sinks to slowly revert, in the absence of observational data, to "first guesses" of close to zero net annual mean for ecosystems. This procedure may have produced a bias. Also due to the sparseness of measurements, we 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 Europe. Easily the largest single annual mean source of CO2 is emissions from fossil fuel burning, which are currently not estimated by CarbonTracker Europe. We use estimates from emissions inventories (economic accounting) and prescribe those to CarbonTracker Europe. 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 Europe. 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.

Overview tables
The tables below show our average flux extimates for the period 2001-2011. Table 1 shows the natural (= biosphere, ocean and fires) fluxes for the transcom regions, table 2 shows the natural fluxes for the extended transcom regions and table 3 shows the fossil fuel emissions.

Table 1: Average natural flux (biosphere + ocean + fire) estimated from CTE2013 [2001-2011]. The uncertainty is the one standard deviation from the filter estimate. Units are PgC/yr.
2001-2011
Transcom region CTE2013-EI
North American Boreal-0.15 ± 0.27
North American Temperate-0.52 ± 0.49
South American Tropical+0.08 ± 0.44
South American Temperate-0.00 ± 0.49
Northern Africa-0.17 ± 0.45
Southern Africa+0.04 ± 0.48
Eurasia Boreal-0.72 ± 0.77
Eurasia Temperate-0.20 ± 0.61
Tropical Asia-0.02 ± 0.21
Australia+0.03 ± 0.19
Europe-0.44 ± 0.47
North Pacific Temperate-0.37 ± 0.27
West Pacific Tropical+0.01 ± 0.01
East Pacific Tropical+0.38 ± 0.15
South Pacific Temperate-0.56 ± 0.30
Northern Ocean-0.26 ± 0.14
North Atlantic Temperate-0.35 ± 0.27
Atlantic Tropical+0.19 ± 0.10
South Atlantic Temperate-0.12 ± 0.16
Southern Ocean-0.16 ± 0.20
Indian Tropical+0.16 ± 0.08
Indian Temperate-0.69 ± 0.15
Non-optimized-0.00 ± 0.00

Table 2: Average natural flux (biosphere + ocean + fire) estimated from CTE2013 [2001-2011]. The uncertainty is the one standard deviation from the filter estimate. Units are PgC/yr.
2001-2011
Transcom region CTE2013-EI
Northern Land-2.02 ± 1.19
Northern Oceans-0.97 ± 0.41
Northern-2.99 ± 1.26
Tropical Land-0.10 ± 0.66
Tropical Ocean+0.74 ± 0.22
Tropical+0.64 ± 0.70
Southern Land+0.07 ± 0.71
Ocean in South-1.53 ± 0.42
Southern-1.46 ± 0.82
Americas-0.59 ± 0.85
Atlantic and Arctic-0.53 ± 0.38
Eurasia and Africa-0.56 ± 0.81
Indian Ocean-0.53 ± 0.17
Asia-0.90 ± 1.01
Pacific Ocean-0.54 ± 0.40
All Land-2.05 ± 1.53
All Ocean-1.76 ± 0.71
Global-3.81 ± 1.69
North America-0.67 ± 0.54
Extratropical Eurasia-1.35 ± 1.07
Temperate Northern Ocean-0.72 ± 0.39
Temperate Southern Ocean-1.37 ± 0.40
Tropical and Southern Land-0.03 ± 0.97
Tropical Pacific+0.39 ± 0.15
Temperate Oceans-2.08 ± 0.53
High Latitude Oceans-0.42 ± 0.23
Extratropical Oceans-2.50 ± 0.62
Tropical and Southern Oceans-0.78 ± 0.55
Northern Boreal Land-0.87 ± 0.81
Northern Temperate Land-1.15 ± 0.91
Tropical land minus northern land+1.92 ± 1.36
High North Ocean and Boreal Land-1.13 ± 0.82
Northern Temperate Land and Ocean-1.87 ± 0.99
Southern Temperate Land and Ocean-1.30 ± 0.82
Tropical and Southern Land and Ocean-0.82 ± 1.11
Tropics and south minus north+2.18 ± 1.67
Tropical and southern land minus northern land+1.99 ± 1.54

CarbonTracker Europe is a Wageningen University contribution
to the Integrated Carbon Observing System (ICOS)





Copyright Wageningen University, Dec 2013