Abstract
We have compared a suite of recent global CO2 atmospheric inversion results to independent airborne observations and to each other, to assess their dependence on differences in northern extratropical (NET) vertical transport and to identify some of the drivers of model spread. We evaluate posterior CO2 concentration profiles against observations from the High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER) Pole-to-Pole Observations (HIPPO) aircraft campaigns over the mid-Pacific in 2009–2011. Although the models differ in inverse approaches, assimilated observations, prior fluxes, and transport models, their broad latitudinal separation of land fluxes has converged significantly since the Atmospheric Carbon Cycle Inversion Intercomparison (TransCom 3) and the REgional Carbon Cycle Assessment and Processes (RECCAP) projects, with model spread reduced by 80% since TransCom 3 and 70% since RECCAP. Most modeled CO2 fields agree reasonably well with the HIPPO observations, specifically for the annual mean vertical gradients in the Northern Hemisphere. Northern Hemisphere vertical mixing no longer appears to be a dominant driver of northern versus tropical (T) annual flux differences. Our newer suite of models still gives northern extratropical land uptake that is modest relative to previous estimates (Gurney et al., 2002; Peylin et al., 2013) and near-neutral tropical land uptake for 2009–2011. Given estimates of emissions from deforestation, this implies a continued uptake in intact tropical forests that is strong relative to historical estimates (Gurney et al., 2002; Peylin et al., 2013). The results from these models for other time periods (2004–2014, 2001–2004, 1992–1996) and reevaluation of the TransCom 3 Level 2 and RECCAP results confirm that tropical land carbon fluxes including deforestation have been near neutral for several decades. However, models still have large disagreements on ocean–land partitioning. The fossil fuel (FF) and the atmospheric growth rate terms have been thought to be the best-known terms in the global carbon budget, but we show that they currently limit our ability to assess regional-scale terrestrial fluxes and ocean–land partitioning from the model ensemble.
Highlights
Current appraisals of the global atmospheric carbon budget are informed by surface fluxes computed by inverse transport models (e.g., Newsam and Enting, 1988; Tans et al, 1990; Rayner et al, 1999; Gurney et al, 2002, 2003, 2004; Peylin et al, 2013)
The fossil fuel (FF) and the atmospheric growth rate terms have been thought to be the best-known terms in the global carbon budget, but we show that they currently limit our ability to assess regional-scale terrestrial fluxes and ocean–land partitioning from the model ensemble
We first evaluate if the spread of retrieved land fluxes over different zonal bands is correlated with northern extratropical (NET) vertical CO2 gradients and if the modeled gradients match observations, as was previously done for the T3L2 models by Stephens et al (2007)
Summary
Current appraisals of the global atmospheric carbon budget are informed by surface fluxes computed by inverse transport models (e.g., Newsam and Enting, 1988; Tans et al, 1990; Rayner et al, 1999; Gurney et al, 2002, 2003, 2004; Peylin et al, 2013). The most prominent community-wide inverse result intercomparison that included comparisons of posterior concentrations to independent observations was the TransCom 3 study (Gurney et al, 2002, 2004), which studied fluxes for the 1992–1996 period. This comparison could focus on the impact of transport model differences by optimizing the fluxes using a common method over the same regions (11 land and 11 ocean). The atmospheric inversion component of RECCAP was a comprehensive intercomparison that analyzed long-term mean, long-term trend, interannual variations, and mean seasonal variations of CO2 fluxes using common post-processing (Peylin et al, 2013). When the fluxes were analyzed for the years 2001 to 2004, Peylin et al
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