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.
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