Strong Atmospheric 14C Signature of Respired CO2 Observed over Midwestern United States

The Science

A recent study demonstrates a novel methodology for constraining the net exchange of carbon dioxide (CO₂) between the landscape and atmosphere using ¹⁴CO₂ observed from a tall tower in the midwestern United States. Exchanges include net ecosystem respiration (including belowground carbon), fires, and anthropogenic sources.

The Impact

The study determined that soil respiration of carbon drives variability in ¹⁴CO₂ during the summer months and that simulations from the Carnegie-Ames-Stanford Approach (CASA) model underestimate the biospheric ¹⁴CO₂ source compared to observations at the Wisconsin Tall Tower. This approach has the potential to better constrain the long-term carbon balance of terrestrial ecosystems and the short-term impact of disturbance-based loss of carbon to the atmosphere, highlighting areas for continued land-surface/biogeochemistry model development.

Summary

A recent study found that during the summer months the biospheric component dominates the atmospheric ¹⁴CO₂ budget at the Park Falls AmeriFlux/WLEF Tall Tower in northern Wisconsin. Respiration of carbon from soils is an important component of the global carbon cycle, returning carbon previously taken up via photosynthesis over decadal time scales back to the atmosphere. For 2010, observations from 400 m above ground indicate that the terrestrial biosphere was responsible for a 2 to 3 times higher contribution to total ¹⁴CO₂ than predicted by the CASA terrestrial ecosystem model. This finding indicates that the model is underpredicting ecosystem respiration and net primary production. Based on back-trajectory analyses, this bias likely includes a substantial contribution from the North American boreal ecoregion, but transported biospheric emissions from outside the model domain cannot be ruled out. The ¹⁴CO₂ enhancement also appears somewhat decreased in observations made over subsequent years, suggesting that 2010 may be anomalous. Going forward, this isotopic signal could be exploited as an important indicator to better constrain both the long-term carbon balance of terrestrial ecosystems and the short-term impact of disturbance-based loss of carbon to the atmosphere.

Principal Investigator(s)

Karis McFarlane
Lawrence Livermore National Laboratory
[email protected]

Brian LaFranchi
Now at Aclima
[email protected]

Funding

This work was funded by the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory (LLNL) under Contract DE-AC52-07NA27344 and by Sandia National Laboratories, operated by Sandia Corporation, a Lockheed Martin Company, for the DOE National Nuclear Security Administration under contract DEAC04-94AL85000. Funded also by the Terrestrial Ecosystem Science program  of the Climate and Environmental Science Division (SCW1447) of the Office of Biological and Environmental Research (BER), within the DOE Office of Science; LLNL Laboratory-Directed Research and Development (ERD-14-038); National Oceanic and Atmospheric Administration’s (NOAA) Earth System Research Laboratories (ESRL) Global Monitoring Division; and NOAA Climate Program Office’s Atmospheric Chemistry, Carbon Cycle.

References

LaFranchi, B.W., K.J. McFarlane, J.B. Miller, S.J. Lehman, C.L. Phillips, A.E. Andrews, P.P. Tans, H. Chen, Z. Liu, J.C. Turnbull, X. Xu, and T.P. Guilderson. “Strong regional atmospheric ¹⁴C signature of respired CO₂ observed from a tall tower over the midwestern United States.” Journal of Geophysical Research: Biogeosciences 121(8), 2275–2295 (2016). [DOI:10.1002/2015JG003271]