New constraints on Northern Hemisphere growing season net flux

Zhonghua Yang, Division of Geological and Planetary Science, California Institute of Technology, yangzh@gps.caltech.edu
Rebecca A. Washenfelder, Earth System Research Laboratory, NOAA, Rebecca.Washenfelder@noaa.gov
Gretcher Keppel-Aleks, Division of Engineering and Applied Science, California Institute, of Technology
Paul O Wennberg, Division of Geological and Planetary Science & Environmental Science and Engineering, California Institute of Technology, wennberg@gps.caltech.edu (Presenting)
N. Y. Krakauer, Department of Earth and Planetary Science, University of, California, niryk@berkeley.edu
J. T. Randerson, Department of Earth System Science, University of California, Irvine, jranders@uci.edu

Observations of the column-averaged dry molar mixing ratio of CO2 above both Park Falls, Wisconsin and Kitt Peak, Arizona, together with partial columns derived from six aircraft profiles over Eurasia and North America are used to estimate the Net Ecosystem Exchange (NEE) between the atmosphere and the terrestrial biosphere in the Northern Hemisphere. We find that NEE is approximately 25% larger than predicted by the Carnegie Ames Stanford Approach (CASA) model. We show that the earlier estimates of NEE based on surface CO2 observations alone were biased low by the overly sluggish vertical exchange in the transport models. To the extent that this transport error also underestimates the rate of mixing of the fossil fuel carbon into the free troposphere, the inter-hemispheric gradient in the atmospheric CO2 mass is also likely underestimated in the previous TransCom studies. If so, the uptake of anthropogenic carbon in the Northern Hemisphere is smaller than previous estimates.