Background
The authors note that "CO2 fluxes measured on micrometeorological towers in many flux networks worldwide (Baldocchi et al., 2001) ... can only sample a very small fraction of the land surface as each can only represent a footprint area of about 1 km²." As a result, they say that they "seek ways to retrieve carbon cycle information from atmospheric CO2 concentration measurements, which have much larger footprints (10³-10⁴ km²) (Lin et al., 2003) than flux towers."

What was done
Chen et al. analyzed a 13-year (1990-1996, 1999-2004) hourly-averaged atmospheric CO2 concentration data base obtained from a 40-m tower at Fraserdale, Ontario, Canada (together with temperature, humidity and wind speed measured at 20 and 40 meters and precipitation at ground level), comparing their results with a marine boundary layer CO2 data set representing the free troposphere above the tower.

What was learned
The eight researchers found that in warmer years, the planetary boundary layer over their measurement site was more depleted of CO2, which suggests that the 10³-10⁴ km² land area of the boreal ecosystem upwind of the tower sequestered more carbon in such years; and they say this finding "suggests that gross primary productivity increased considerably faster with temperature than did ecosystem respiration," which relationship they found to be true for both annual temperatures (from year to year) and 10-day mean temperatures (throughout the growing season).

What it means
In the words of the scientists who conducted the research, "the fact that the temperature sensitivity of gross primary productivity is larger than that of ecosystem respiration suggests that global warming could lead to increased carbon sequestration in boreal ecosystems."

References
Baldocchi, D., Falge, E., Gu, L.H., Olson, R., Hollinger, D., Running, S., Anthoni, P., Bernhofer, C., Davis, K., Evans, R., Fuentes, J., Goldstein, A., Katul, G., Law B., Lee, X.H., Malhi, Y., Meyers, T., Munger, W., Oechel, W., Paw U, K.T., Pilegaard, K., Schmid, H.P., Valentini, R., Verma, S., Vesala, T., Wilson, K. and Wofsy, S. 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: 2415-2434.

Lin, C., Gerbig, C., Wofsy, S.C., Andrews, A.E., Daube, B.C., Davis, K.T. and Grainger, C.A. 2003. A near-field tool for simulating the upstream influence of atmospheric observations: The Stochastic Time-Inverted Lagrangian Transport (STILT) model. Journal of Geophysical Research 108: 10.1029/2002JD003161.