Reconciling estimates of the contemporary North American carbon balance among terrestrial biosphere models, atmospheric inversions, and a new approach for estimating net ecosystem exchange from inventory‐based data

Abstract

We develop an approach for estimating net ecosystem exchange (NEE) using inventory‐based information over North America (NA) for a recent 7‐year period (ca. 2000–2006). The approach notably retains information on the spatial distribution of NEE, or the vertical exchange between land and atmosphere of all non‐fossil fuel sources and sinks of CO2, while accounting for lateral transfers of forest and crop products as well as their eventual emissions. The total NEE estimate of a −327 ± 252 TgC yr−1 sink for NA was driven primarily by CO2 uptake in the Forest Lands sector (−248 TgC yr−1), largely in the Northwest and Southeast regions of the US, and in the Crop Lands sector (−297 TgC yr−1), predominantly in the Midwest US states. These sinks are counteracted by the carbon source estimated for the Other Lands sector (+218 TgC yr−1), where much of the forest and crop products are assumed to be returned to the atmosphere (through livestock and human consumption). The ecosystems of Mexico are estimated to be a small net source (+18 TgC yr−1) due to land use change between 1993 and 2002. We compare these inventory‐based estimates with results from a suite of terrestrial biosphere and atmospheric inversion models, where the mean continental‐scale NEE estimate for each ensemble is −511 TgC yr−1 and −931 TgC yr−1, respectively. In the modeling approaches, all sectors, including Other Lands, were generally estimated to be a carbon sink, driven in part by assumed CO2 fertilization and/or lack of consideration of carbon sources from disturbances and product emissions. Additional fluxes not measured by the inventories, although highly uncertain, could add an additional −239 TgC yr−1 to the inventory‐based NA sink estimate, thus suggesting some convergence with the modeling approaches.

Reviews and syntheses: An empirical spatiotemporal description of the global surface–atmosphere carbon fluxes: opportunities and data limitations

“…The impact of this lateral C transport on fluxes can be seen at the country scale in the form of import and exports, but even more so at subregional scales where the movement of crop biomass to feed livestock and humans is evident (Hayes et al, 2012;West et al, 2011). To capture the spatial distribution of CO 2 fluxes from agricultural harvest, we used livestock and human CO 2 emissions estimates (Wolf et al, 2015b) that are available from 2005 to 2011 at 0.05 • spatial resolution.…”

Atmospheric inversion of surface carbon flux with consideration of the spatial distribution of US crop production and consumption

“…Similarly, lateral transfer of carbon also occurs with forest products . Hayes et al (2012) successfully reconciled a large portion of this discrepancy using crop and forest product information for North America. Based on this study, it can be inferred that these lateral transports of carbon (Olivier and Aardenne, 2005) Fire GFEDv2 Interannual n/a Biosphere BEPS model Interannual, seasonal, 2.0 Pg C yr −1 (Gurney et al, 2003) Ju et al, 2006 diurnal distributed globally over land surfaces regions based on spatial pattern of the GPP Ocean OPA-PISCES-T model Seasonal 0.67 Pg C yr −1 distributed over (Buitenhuis et al, 2006) ocean regions (Deng and Chen, 2011) need to be considered in both bottom-up and top-down modeling in order for them to converge on similar spatial patterns of the carbon sink and source distribution.…”

“…With a simple atmospheric budgeting approach applied to CO 2 measurements in the inflows and outflows through the troposphere over the conterminous US, Crevoisier et al (2010) deduced a sink of 0.5 ± 0.4 Pg C yr −1 in [2004][2005][2006]. Seven other inversion studies, on average, indicate that temperate North America was a sink of 0.685 ± 0.574 Pg C yr −1 in 2000-2006 (see summary in Hayes et al, 2012). Although these estimates have large uncertainties, they generally indicate that the major sink in North America is located in the temperate region while the boreal region is either a small sink or source.…”

“…However, the magnitude and the spatial distribution of carbon sinks and sources over the continent are still highly uncertain Hayes et al, 2012). In terms of the sink magnitude, atmospheric inversion studies (referred to as "top-down") performed for different time periods from 1992 to 2007 suggest that North America has been a sink ranging from 0.54 to 1.06 Pg C yr −1 (Gurney et al, 2002(Gurney et al, , 2003Rödenbeck et al, 2003;Baker et al, 2006;Peters et al, 2007;Deng et al, 2013;Peylin et al, 2013), while 19 biospheric models (referred to as "bottomup") produced an average sink of 0.6 Pg C yr −1 , with a range from −0.7 to 2.2 Pg C yr −1 , for North America over the period from 2000 to 2005 .…”

An Assessment of Carbon Storage in China’s Arboreal Forests

“…Carbon stored in forest vegetation accounts for 60% of the carbon in the Earth's terrestrial biosphere. Thus, forests are the largest terrestrial ecosystem in terms of carbon storage [1][2][3][4]. Forests absorb and store the greenhouse gas CO 2 , and play an irreplaceable role in balancing CO 2 in the atmosphere.…”