Global Carbon Budget 2018

Atmospheric CO₂ concentration is measured directly and its growth rate (G_ATM) is computed from the annual changes in concentration. The ocean CO₂ sink (S_OCEAN) and terrestrial CO₂ sink (S_LAND) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (B_IM), 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 last decade available (2008–2017), E_FF was 9.4±0.5 GtC yr⁻¹, E_LUC 1.5±0.7 GtC yr⁻¹, G_ATM 4.7±0.02 GtC yr⁻¹, S_OCEAN 2.4±0.5 GtC yr⁻¹, and S_LAND 3.2±0.8 GtC yr⁻¹, with a budget imbalance B_IM of 0.5 GtC yr⁻¹ indicating overestimated emissions and/or underestimated sinks.

For the year 2017 alone, the growth in E_FF was about 1.6 % and emissions increased to 9.9±0.5 GtC yr⁻¹. Also for 2017, E_LUC was 1.4±0.7 GtC yr⁻¹, G_ATM was 4.6±0.2 GtC yr⁻¹, S_OCEAN was 2.5±0.5 GtC yr⁻¹, and S_LAND was 3.8±0.8 GtC yr⁻¹, with a B_IM of 0.3 GtC. The global atmospheric CO₂ concentration reached 405.0±0.1 ppm averaged over 2017.

For 2018, preliminary data for the first 6–9 months indicate a renewed growth in E_FF of +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⁻¹ persist for the representation of semi-decadal variability in CO₂ 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 CO₂ flux in the northern extra-tropics, and (3) an apparent underestimation of the CO₂ 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