Abstract
Accurate assessment of anthropogenic carbon dioxide (<span classCombining double low line"inline-formula">CO2</span>) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere - the "global carbon budget" - is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we
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describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil <span classCombining double low line"inline-formula">CO2</span> emissions (<span classCombining double low line"inline-formula">EFF</span>) are based on energy statistics and cement production data, while emissions from land use and land-use change (<span classCombining double low line"inline-formula">ELUC</span>), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric <span classCombining double low line"inline-formula">CO2</span> concentration is measured directly and its growth rate (<span classCombining double low line"inline-formula">GATM</span>) is computed from the annual changes in concentration. The ocean <span classCombining double low line"inline-formula">CO2</span> sink (<span classCombining double low line"inline-formula">SOCEAN</span>) and terrestrial <span classCombining double low line"inline-formula">CO2</span> sink (<span classCombining double low line"inline-formula">SLAND</span>) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (<span classCombining double low line"inline-formula">BIM</span>), 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 <span classCombining double low line"inline-formula">±1σ</span>. For the last decade available (2008-2017), <span classCombining double low line"inline-formula">EFF</span> was <span classCombining double low line"inline-formula">9.4±0.5</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, <span classCombining double low line"inline-formula">ELUC</span> <span classCombining double low line"inline-formula">1.5±0.7</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, <span classCombining double low line"inline-formula">GATM</span> <span classCombining double low line"inline-formula">4.7±0.02</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, <span classCombining double low line"inline-formula">SOCEAN</span> <span classCombining double low line"inline-formula">2.4±0.5</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, and <span classCombining double low line"inline-formula">SLAND</span> <span classCombining double low line"inline-formula">3.2±0.8</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, with a budget imbalance <span classCombining double low line"inline-formula">BIM</span> of 0.5 GtC yr<span classCombining double low line"inline-formula">ĝ'1</span> indicating overestimated emissions and/or underestimated sinks. For the year 2017 alone, the growth in <span classCombining double low line"inline-formula">EFF</span> was about 1.6 % and emissions increased to <span classCombining double low line"inline-formula">9.9±0.5</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>. Also for 2017, <span classCombining double low line"inline-formula">ELUC</span> was <span classCombining double low line"inline-formula">1.4±0.7</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, <span classCombining double low line"inline-formula">GATM</span> was <span classCombining double low line"inline-formula">4.6±0.2</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, <span classCombining double low line"inline-formula">SOCEAN</span> was <span classCombining double low line"inline-formula">2.5±0.5</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, and <span classCombining double low line"inline-formula">SLAND</span> was <span classCombining double low line"inline-formula">3.8±0.8</span> GtC yr<span classCombining double low line"inline-formula">ĝ'1</span>, with a <span classCombining double low line"inline-formula">BIM</span> of 0.3 GtC. The global atmospheric <span classCombining double low line"inline-formula">CO2</span> concentration reached <span classCombining double low line"inline-formula">405.0±0.1</span> ppm averaged over 2017. For 2018, preliminary data for the first 6-9 months indicate a renewed growth in <span classCombining double low line"inline-formula">EFF</span> of <span classCombining double low line"inline-formula">+</span>2.7 % (range of 1.8 % to 3.7 %) based on national emission projections for China, the US, the EU, and India and projections of gross domestic product corrected for recent changes in the carbon intensity of the economy for the rest of the world. The analysis presented here shows that the mean and trend in the five components of the global carbon budget are consistently estimated over the period of 1959-2017, but discrepancies of up to 1 GtC yr<span classCombining double low line"inline-formula">ĝ'1</span> persist for the representation of semi-decadal variability in <span classCombining double low line"inline-formula">CO2</span> fluxes. A detailed comparison among individual estimates and the introduction of a broad range of observations show (1) no consensus in the mean and trend in land-use change emissions, (2) a persistent low agreement among the different methods on the magnitude of the land <span classCombining double low line"inline-formula">CO2</span> flux in the northern extra-tropics, and (3) an apparent underestimation of the <span classCombining double low line"inline-formula">CO2</span> variability by ocean models, originating outside the tropics. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding the global carbon cycle compared with previous publications of this data set (Le Quéré et al., 2018, 2016, 2015a, b, 2014, 2013).
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