Global carbon budget 2013

Accurate assessment of anthropogenic carbon dioxide (CO₂) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere 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 a methodology to quantify all major components of the global carbon budget, including their uncertainties, based on the combination of a range of data, algorithms, statistics and model estimates and their interpretation by a broad scientific community. We discuss changes compared to previous estimates, consistency within and among components, alongside methodology and data limitations. CO₂ emissions from fossil-fuel combustion and cement production (E_FF) are based on energy statistics, while emissions from land-use change (E_LUC), mainly deforestation, are based on combined evidence from land-cover change data, fire activity associated with deforestation, and models. The global atmospheric CO₂ concentration is measured directly and its rate of growth (G_ATM) is computed from the annual changes in concentration. The mean ocean CO₂ sink (S_OCEAN) is based on observations from the 1990s, while the annual anomalies and trends are estimated with ocean models. The variability in S_OCEAN is evaluated for the first time in this budget with data products based on surveys of ocean CO₂ measurements. The global residual terrestrial CO₂ sink (S_LAND) is estimated by the difference of the other terms of the global carbon budget and compared to results of independent dynamic global vegetation models forced by observed climate, CO₂ and land cover change (some including nitrogen–carbon interactions). All uncertainties are reported as ±1σ, reflecting the current capacity to characterise the annual estimates of each component of the global carbon budget. For the last decade available (2003–2012), E_FF was 8.6 ± 0.4 GtC yr⁻¹, E_LUC 0.9 ± 0.5 GtC yr⁻¹, G_ATM 4.3 ± 0.1 GtC yr⁻¹, S_OCEAN 2.5 ± 0.5 GtC yr⁻¹, and S_LAND 2.8 ± 0.8 GtC yr⁻¹. For year 2012 alone, E_FF grew to 9.7 ± 0.5 GtC yr⁻¹, 2.2% above 2011, reflecting a continued growing trend in these emissions, G_ATM was 5.1 ± 0.2 GtC yr⁻¹, S_OCEAN was 2.9 ± 0.5 GtC yr⁻¹, and assuming an E_LUC of 1.0 ± 0.5 GtC yr⁻¹ (based on the 2001–2010 average), S_LAND was 2.7 ± 0.9 GtC yr⁻¹. G_ATM was high in 2012 compared to the 2003–2012 average, almost entirely reflecting the high E_FF. The global atmospheric CO₂ concentration reached 392.52 ± 0.10 ppm averaged over 2012. We estimate that E_FF will increase by 2.1% (1.1–3.1%) to 9.9 ± 0.5 GtC in 2013, 61% above emissions in 1990, based on projections of world gross domestic product and recent changes in the carbon intensity of the economy. With this projection, cumulative emissions of CO₂ will reach about 535 ± 55 GtC for 1870–2013, about 70% from E_FF (390 ± 20 GtC) and 30% from E_LUC (145 ± 50 GtC).