Global Carbon Budget 2021
Authors: Friedlingstein Pierre , Jones Matthew W. , O'Sullivan Michael , Andrew Robbie M. , Bakker Dorothee C. E. , Hauck Judith , Le Quéré Corinne , Peters Glen P. , Peters Wouter , Pongratz Julia , Sitch Stephen , Canadell Josep G. , Ciais Philippe , Jackson Rob B. , Alin Simone R. , Anthoni Peter , Bates Nicholas R. , Becker Meike , Bellouin Nicolas , Bopp Laurent , Chau Thi T. T. , Chevallier Frédéric , Chini Louise P. , Cronin Margot , Currie Kim I. , Decharme Bertrand , Djeutchouang Laique , Dou Xinyu , Evans Wiley , Feely Richard A. , Feng Liang , Gasser Thomas , Gilfillan Dennis , Gkritzalis Thanos , Grassi Giacomo , Gregor Luke , Gruber Nicolas , Gürses Özgür , Harris Ian , Houghton Richard A. , Hurtt George C. , Iida Yosuke , Ilyina Tatiana , Luijkx Ingrid T. , Jain Atul K. , Jones Steve D. , Kato Etsushi , Kennedy Daniel , Klein Goldewijk Kees , Knauer Jürgen , Korsbakken Jan Ivar , Körtzinger Arne , Landschützer Peter , Lauvset Siv K. , Lefèvre Nathalie , Lienert Sebastian , Liu Junjie , Marland Gregg , McGuire Patrick C. , Melton Joe R. , Munro David R. , Nabel Julia E. M. S. , Nakaoka Shin-Ichiro , Niwa Yosuke , Ono Tsuneo , Pierrot Denis , Poulter Benjamin , Rehder Gregor , Resplandy Laure , Robertson Eddy , Rödenbeck Christian , Rosan Thais M. , Schwinger Jörg , Schwingshackl Clemens , Séférian Roland , Sutton Adrienne J. , Sweeney Colm , Tanhua Toste , Tans Pieter P. , Tian Hanqin , Tilbrook Bronte , Tubiello Francesco , van der Werf Guido , Vuichard Nicolas , Wada Chisato , Wanninkhof Rik , Watson Andrew , Willis David , Wiltshire Andrew J. , Yuan Wenping , Yue Chao , Yue Xu , Zaehle Sönke , Zeng Jiye 
Published: November 2021

Abstract. Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe and synthesize data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy statistics and cement production data, while emissions from land-use change (ELUC), mainly deforestation, are based on land-use and land-use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly, and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) is estimated with global ocean biogeochemistry models and observation-based data-products. The terrestrial CO2 sink (SLAND) is estimated with dynamic global vegetation models. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the first time, an approach is shown to reconcile the difference in our ELUC estimate with the one from national greenhouse gases inventories, supporting the assessment of collective countries’ climate progress. For the year 2020, EFOS declined by 5.4 % relative to 2019, with fossil emissions at 9.5 ± 0.5 GtC yr−1 (9.3 ± 0.5 GtC yr−1 when the cement carbonation sink is included), ELUC was 0.9 ± 0.7 GtC yr−1, for a total anthropogenic CO2 emission of 10.2 ± 0.8 GtC yr−1 (37.4 ± 2.9 GtCO2). Also, for 2020, GATM was 5.0 ± 0.2 GtC yr−1 (2.4 ± 0.1 ppm yr−1), SOCEAN was 3.0 ± 0.4 GtC yr−1 and SLAND was 2.9 ± 1 GtC yr−1, with a BIM of −0.8 GtC yr−1. The global atmospheric CO2 concentration averaged over 2020 reached 412.45 ± 0.1 ppm. Preliminary data for 2021, suggest a rebound in EFOS relative to 2020 of +4.9 % (4.1 % to 5.7 %) globally. Overall, the mean and trend in the components of the global carbon budget are consistently estimated over the period 1959–2020, but discrepancies of up to 1 GtC yr−1 persist for the representation of annual to semi-decadal variability in CO2 fluxes. Comparison of estimates from multiple approaches and observations shows: (1) a persistent large uncertainty in the estimate of land-use changes emissions, (2) a low agreement between the different methods on the magnitude of the land CO2 flux in the northern extra- tropics, and (3) a discrepancy between the different methods on the strength of the ocean sink over the last decade. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding of the global carbon cycle compared with previous publications of this data set (Friedlingstein et al., 2020; Friedlingstein et al., 2019; Le Quéré et al., 2018b, 2018a, 2016, 2015b, 2015a, 2014, 2013). The data presented in this work are available at https://doi.org/10.18160/gcp-2021 (Friedlingstein et al., 2021).