Scientific Background

    The atmospheric burden of CO₂ is increasing in response to anthropogenic combustion of fossil fuels. A large portion of this carbon is absorbed by the oceans and terrestrial ecosystems. Approximately one-half of this large and variable sink is due to net uptake by terrestrial ecosystems (Ciais et al., 1995; Battle et al., 2000). To better predict the future response of the carbon cycle to climate and land use change, we need to understand the causes of the terrestrial sink of carbon, which is hampered by our limited ability to quantify the terrestrial carbon cycle on appropriate spatial and temporal scales. Measurements of ecosystem-atmosphere CO₂ exchange that integrate over domains of similar ecosystem and climate, and across seasons would greatly extend our understanding of the terrestrial carbon cycle.

Current status of CO2 flux measurements on different scales

    To understand fully the role of terrestrial ecosystems in climate change, we need to observe definitively and quantitatively the exchange of CO2 between the atmosphere and terrestrial ecosystems over various scales, e.g., from a forest stand area (about 1 km²) to the entire globe. The current approaches, however, yield results of CO₂ fluxes representing disparate spatial scales (right figure). Tower-based eddy covariance measurements provide estimates of net ecosystem-atmosphere exchange (NEE) at scales ranging from 0.01km² to 10 km² (e.g. Wofsy et al, 1993), whereas global CO₂ mixing ratio measurements provide the means to infer (by inversion of atmospheric tracer transport) NEE at hemispheric and perhaps continental scales (e.g. Enting et al, 1995; Fan et al, 1998; Rayner et al, 1999; Bousquet et al, 2000; Gurney et al, 2002).  The former provides an understanding of specific mechanisms that govern carbon fluxes in particular ecosystems (e.g. Goulden et al, 1998; Baldocchi et al, 2001), while the latter integrate the effects of all the physical, biogeochemical, and anthropogenic processes operating over large scales. 

Estimated uncertainty of annual carbon budget from global inversion simulation using different scenarios of North American observation networks. Courtesy of  A.S. Denning.

    The inverse estimate of NEE currently provides information only at very large (continental) scales because of the sparse network of background of CO₂ measurements. This is inadequate for model testing because many of the processes responsible for long-term carbon sinks are not amendable to eddy covariance measurements or represented in the ecophysiological models. Testing those processes will require independent estimate of carbon exchange on regional scales, which can be implemented by aircraft- and tower-based measurements of CO2 mixing ratio.  The latter one can provide higher temporal-resolution of  the measurements.

   To improve the accuracy as well as the spatial and temporal resolution of the atmospheric inversion approach over North America, we will implement and maintain a network of well-calibrated CO2 mixing ratio measurements on AmeriFlux towers by the VTT methodology (see the sections of methods and projects). Preliminary results show that the uncertainty of seasonal and annual fluxes from inversion models can be reduced by increasing the spatial and temporal resolution of the measurements. For example, Denning and Skidmore at Colorado State suggest, shown in the left figure, that the addition of only 5 VTTs in two clusters, midwest and southeast, reduce the total uncertainty of the inversed flux to just over 0.2 GtC/year.

    Studies have shown the potential for estimation of regional sources and sinks by sampling the temporal and spatial variations of CO₂ mixing ratio in the atmosphere over the continents (Davis et al. 2003; Hurwitz et al, in press; Bakwin, 2003 etc.). Because differences among sites can be very small, the measurements of CO₂ mixing ratio must be made very precisely, and intercalibration among different sites and laboratories is crucial (Masarie et al., 2001)

What will be done?

(1) build a system to measure CO₂ precisely and accurately.

(2) build and deploy a network of continuous, high-precision CO₂ analyzers for the estimate of regional NEE in N. Wisconsin.

(3) merge direct flux measurements over the region using the University of Wyoming King Air flux aircraft and flux towers to map spatial variability in NEE of CO₂.

(4) establish a North American network of well-calibrated CO₂ mixing ratio measurements at approximately 10 AmeriFlux towers using the ‘virtual tall tower’ (VTT) methodology.

References

Baldocchi D, E. Falge, L.H. Gu, R. Olson, D. Hollinger, S. Running, P. Anthoni, C. Bernhofer, K. Davis, R. Evans, J. Fuentes, A. Goldstein, G. Katul, B. Law, X.H. Lee, Y. Malhi, T. Meyers, W. Munger, W. Oechel, K.T. Paw,, K. Pilegaard, H. P. Schmid, R. Valentini, S. Verma, T. Vesala, K. Wilson, and S. Wofsy, 2001: FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities.. Bulletin of the American Meteorological Society, 82(11), 2415–2434.

Battle, M., M.L. Bender, P.P. Tans, J.W.C. White, J.T. Ellis, T. Conway and R.J. Francey, (2000) Global carbon sinks and their variability inferred from atmospheric O₂ and d¹³C, Science, 287, 2467-2470.

Bousquet, P., P. Peylin, P. Ciais, C. LeQuere, P. Friedlingstein, and P. Tans, 2000. Regional Changes in Carbon Dioxide Fluxes of Land and Oceans Since 1980. Science, 290, 1342-1346

Ciais, P., P. Tans, M. Trolier, J. White, and R. Francey, (1995) A large northern hemisphere terrestrial sink indicated by the ¹³C/¹²C ratio of atmospheric CO2, Science, 269, 1098-1102.

Enting, I. G., Trudinger, C. M. and Francey, R. J., 1995. A synthesis inversion of the concentration and delta ¹³C of atmospheric CO₂ . Tellus, 47B, 35-52.

Fan, S., M. Gloor, J. Mahlman, S. Pacala, J. Sarmiento, T. Takahashi, and P. Tans, 1998. A large terrestrial carbon sink in North America implied by atmospheric and oceanic carbon dioxide data and models.  Science, 282, 442-446.

Goulden, M L., S. C. Wofsy, J. W. Harden, S E. Trumbore, P M. Crill, S T. Gower, T Fries, B C. Daube, S-M. Fan, D J. Sutton, A Bazzaz, and J. W. Munger, Sensitivity of boreal forest carbon balance to soil thaw, Science, 279, 214-217, 1998.

Gurney, K.R., R. M. Law, A. S. Denning, P. J. Rayner, D. Baker, P. Bousquet, L. Bruhwiler, Y.-H. Chen, P. Ciais, S. Fan, I.Y. Fung, M. Gloor, M. Heimann, K. Higuchi, J. John, T. Maki, S. Maksyutov, K. Masarie, P. Peylin, M. Prather, B.C. Pak, J. Randerson, J. Sarmiento, S. Taguchi, T. Takahashi and C.-W. Yuen, 2001: Towards robust regional estimates of CO2 sources and sinks using atmospheric transport models.  Nature, 415, 626-630, Feb. 2002.

Masarie, K. A. and Tans, P. P., 1996.  Extension and integration of atmospheric carbon dioxide data into a globally consistent measurement record (1996) Jour. Geophys. Res., 100, 11593-11610.

Rayner, P. J., I. G. Enting, R. J. Francey, and R. Langenfelds, 1999. Reconstructing the recent carbon cycle from atmospheric CO₂, ¹³C, and O₂/N₂ observations. Tellus, 51B, 213-232.

Wofsy, S. C., M. L. Goulden, and J. W. Munger, Net exchange of CO2 in a mid-latitude forest, Science, 260, 1314-1317, 1993.