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Highlights
We present the first European specific 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 2007. CarbonTracker Europe 2008 (released on 31 October 2008) incorporates several innovations to specifically address the European carbon cycle.
- We have added a number of European observation sites.
- We have added more ecoregions over Europe to accommodate the larger heterogeneity of the landscape
- We have added specific European seasonal cycles for fossil fuel burning emissions
- We have used a 1x1 degree zoom of the TM5 transport model over Europe
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Figure 1: The long term mean biological uptake
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Figure 2: The long term mean fossil fuel emissons
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Estimates of CO2 sources and sinks
From 2001 through 2007 ecosystems in Europe have been a net sink of -0.16 ± 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 tenth of the emissions of 1.6 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 northern
coniferous (-94 TgC/yr) and boreal mixed forests (-30 TgC/yr) as well as the
forest and field complexes of eastern Europe (-85 TgC/yr). Croplands appear as
a net source of carbon (+60 TgC/yr) in our system, but the spatial overlap with
strong fossil fuel sources makes this estimate less robust than other
ecosystems. From a suite of three different biosphere model priors we place an
uncertainty on the mean flux, bringing it in a range of -122 (+21%) to -258
(-46%) TgC/yr. This range is smaller than the interannual variability which
includes a large reduction in uptake due to the 2003 drought (+147 TgC/yr below
average) followed by three years of more than average uptake
that more than compensated the drought year. The largest
anomaly in our time series occurs in 2005 and is driven by increased seasonal
cycles in the observations, resulting from favorable summer moisture
conditions. The CarbonTracker Europe results are constructed such that they are consistent with over 70,000 CO2 observations in the atmosphere.
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Figure 3: European sites from which data is used in CarbonTracker Europe
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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 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.
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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 2000-2007 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 ~23,000 air samples taken by NOAA/ESRL at 30 aircraft sites, which 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.
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Figure 4: All observation sites used in CarbonTracker Europe
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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, submitted to 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.
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CarbonTracker Europe is a Wageningen University contribution
to the Integrated Carbon Observing System (ICOS)
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