Abstract
Abstract. Regional land carbon budgets provide insights into the spatial distribution of the land uptake of atmospheric carbon dioxide and can be used to evaluate carbon cycle models and to define baselines for land-based additional mitigation efforts. The scientific community has been involved in providing observation-based estimates of regional carbon budgets either by downscaling atmospheric CO2 observations into surface fluxes with atmospheric inversions, by using inventories of carbon stock changes in terrestrial ecosystems, by upscaling local field observations such as flux towers with gridded climate and remote sensing fields, or by integrating data-driven or process-oriented terrestrial carbon cycle models. The first coordinated attempt to collect regional carbon budgets for nine regions covering the entire globe in the RECCAP-1 project has delivered estimates for the decade 2000–2009, but these budgets were not comparable between regions due to different definitions and component fluxes being reported or omitted. The recent recognition of lateral fluxes of carbon by human activities and rivers that connect CO2 uptake in one area with its release in another also requires better definitions and protocols to reach harmonized regional budgets that can be summed up to a globe scale and compared with the atmospheric CO2 growth rate and inversion results. In this study, using the international initiative RECCAP-2 coordinated by the Global Carbon Project, which aims to be an update to regional carbon budgets over the last 2 decades based on observations for 10 regions covering the globe with a better harmonization than the precursor project, we provide recommendations for using atmospheric inversion results to match bottom-up carbon accounting and models, and we define the different component fluxes of the net land atmosphere carbon exchange that should be reported by each research group in charge of each region. Special attention is given to lateral fluxes, inland water fluxes, and land use fluxes.
Highlights
The objective of this paper is to define the land–atmosphere CO2 or total carbon (C) fluxes to be used in the REgional Carbon Cycle Assessment and Processes-2 (RECCAP2) project
Accurate and consistent observation-based estimates of terrestrial carbon budgets at regional scales are needed to understand the global land carbon sink, to evaluate land carbon models used for carbon budget assessments and future climate projections, and to define baselines for land-based mitigation efforts
RECCAP1, regional data from inventories were compared with global models output from atmospheric inversions and process-based land models, with the results for 9 land regions in the period 2000–2009 being synthesized in a special issue
Summary
The objective of this paper is to define the land–atmosphere CO2 or total carbon (C) fluxes to be used in the REgional Carbon Cycle Assessment and Processes-2 (RECCAP2) project. RECCAP1, regional data from inventories were compared with global models output from atmospheric inversions and process-based land models, with the results for 9 land regions in the period 2000–2009 being synthesized in a special issue (https://bg.copernicus.org/articles/special_ issue107.html, last access: November 2021). The RECCAP1 synthesis spurred efforts to provide new global analysis of inland water CO2 fluxes (Raymond et al, 2013). Ciais et al (2020) collected bottom-up inventory estimates from RECCAP1 papers and completed them with other components to derive the first global bottomup estimate of the net land atmosphere C exchange, which compared well with the independent top-down estimate obtained from the CO2 growth rate minus fossil fuel emissions and ocean uptake
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