Global Carbon Budget 2018 Academic Article uri icon

abstract

  • <p><strong>Abstract.</strong> Accurate assessment of anthropogenic carbon dioxide (<span class="inline-formula">CO<sub>2</sub></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 describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil <span class="inline-formula">CO<sub>2</sub></span> emissions (<span class="inline-formula"><i>E</i><sub>FF</sub></span>) are based on energy statistics and cement production data, while emissions from land use and land-use change (<span class="inline-formula"><i>E</i><sub>LUC</sub></span>), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric <span class="inline-formula">CO<sub>2</sub></span> concentration is measured directly and its growth rate (<span class="inline-formula"><i>G</i><sub>ATM</sub></span>) is computed from the annual changes in concentration. The ocean <span class="inline-formula">CO<sub>2</sub></span> sink (<span class="inline-formula"><i>S</i><sub>OCEAN</sub></span>) and terrestrial <span class="inline-formula">CO<sub>2</sub></span> sink (<span class="inline-formula"><i>S</i><sub>LAND</sub></span>) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (<span class="inline-formula"><i>B</i><sub>IM</sub></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 class="inline-formula">±1<i>?</i></span>. For the last decade available (2008–2017), <span class="inline-formula"><i>E</i><sub>FF</sub></span> was <span class="inline-formula">9.4±0.5</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, <span class="inline-formula"><i>E</i><sub>LUC</sub></span> <span class="inline-formula">1.5±0.7</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, <span class="inline-formula"><i>G</i><sub>ATM</sub></span> <span class="inline-formula">4.7±0.02</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, <span class="inline-formula"><i>S</i><sub>OCEAN</sub></span> <span class="inline-formula">2.4±0.5</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, and <span class="inline-formula"><i>S</i><sub>LAND</sub></span> <span class="inline-formula">3.2±0.8</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, with a budget imbalance <span class="inline-formula"><i>B</i><sub>IM</sub></span> of 0.5<span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span> indicating overestimated emissions and/or underestimated sinks. For the year 2017 alone, the growth in <span class="inline-formula"><i>E</i><sub>FF</sub></span> was about 1.6<span class="thinspace"></span>% and emissions increased to <span class="inline-formula">9.9±0.5</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>. Also for 2017, <span class="inline-formula"><i>E</i><sub>LUC</sub></span> was <span class="inline-formula">1.4±0.7</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, <span class="inline-formula"><i>G</i><sub>ATM</sub></span> was <span class="inline-formula">4.6±0.2</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, <span class="inline-formula"><i>S</i><sub>OCEAN</sub></span> was <span class="inline-formula">2.5±0.5</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, and <span class="inline-formula"><i>S</i><sub>LAND</sub></span> was <span class="inline-formula">3.8±0.8</span><span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span>, with a <span class="inline-formula"><i>B</i><sub>IM</sub></span> of 0.3<span class="thinspace"></span>GtC. The global atmospheric <span class="inline-formula">CO<sub>2</sub></span> concentration reached <span class="inline-formula">405.0±0.1</span><span class="thinspace"></span>ppm averaged over 2017. For 2018, preliminary data for the first 6–9 months indicate a renewed growth in <span class="inline-formula"><i>E</i><sub>FF</sub></span> of <span class="inline-formula">+</span>2.7<span class="thinspace"></span>% (range of 1.8<span class="thinspace"></span>% to 3.7<span class="thinspace"></span>%) 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<span class="thinspace"></span>GtC<span class="thinspace"></span>yr<span class="inline-formula"><sup>?1</sup></span> persist for the representation of semi-decadal variability in <span class="inline-formula">CO<sub>2</sub></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 class="inline-formula">CO<sub>2</sub></span> flux in the northern extra-tropics, and (3) an apparent underestimation of the <span class="inline-formula">CO<sub>2</sub></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). All results presented here can be downloaded from <span class="uri">https://doi.org/10.18160/GCP-2018</span>.</p>

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