Abstract
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 aircraft profiles over Eurasia and North America are used to estimate the seasonal integral of net ecosystem exchange (NEE) between the atmosphere and the terrestrial biosphere in the Northern Hemisphere. We find that NEE is ∼25% larger than predicted by the Carnegie Ames Stanford Approach (CASA) model. We show that the estimates of NEE may have been biased low by too weak vertical mixing in the transport models used to infer seasonal changes in Northern Hemisphere CO2 mass from the surface measurements of CO2 mixing ratio.
Highlights
[1] 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 aircraft profiles over Eurasia and North America are used to estimate the seasonal integral of net ecosystem exchange (NEE) between the atmosphere and the terrestrial biosphere in the Northern Hemisphere
We further show, using vertically resolved observations of CO2 obtained at several sites in Eurasia and North America, that the TransCom models underestimate the seasonally-varying fluxes because they underestimate the efficiency of CO2 mixing throughout the free troposphere
Our column-based optimization implies that the true growing season net flux (GSNF) in the northern hemisphere is approximately 28% greater than that predicted by Carnegie Ames Stanford Approach (CASA)
Summary
[2] Forecasting CO2 levels in the atmosphere is needed to predict future climate. Accurate forecasts require an improved understanding of carbon sources and sinks [Intergovernmental Panel on Climate Change, 2001]. Surface fluxes are scaled within the framework of an atmospheric transport model to minimize the difference between the observed and simulated spatial and temporal gradients of atmospheric CO2 mixing ratio [Enting et al, 1995; Kaminski et al, 1999; Rayner et al, 1999; Bousquet et al, 2000; Krakauer et al, 2004; Baker et al, 2006] Estimates of both net ecosystem exchange (NEE) and the geographical distribution of fossil fuel carbon sinks vary substantially due in large part to errors in the atmospheric transport models used in these inversions [e.g., Gurney et al, 2004]. We further show, using vertically resolved observations of CO2 obtained at several sites in Eurasia and North America, that the TransCom models underestimate the seasonally-varying fluxes because they underestimate the efficiency of CO2 mixing throughout the free troposphere
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