Özgür Gürses

Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their partitioning among the atmosphere, ocean, and terrestrial biosphere in a changing climate is critical to better understand the global carbon cycle, supporting the development of climate policies, and project future climate change. Here we describe and synthesise datasets and methodologies to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFOS) are based on energy and cement production data. Emissions from land-use change (ELUC) are estimated by bookkeeping models based on land-use data. The global atmospheric CO2 growth rate (GATM) is computed from changes in concentration measured at surface stations. The global net uptake of CO2 by the ocean (SOCEAN) is estimated with global ocean biogeochemistry models and observation-based fCO2-products. The global net uptake of CO2 by the land (SLAND) is estimated with dynamic global vegetation models. Additional lines of evidence are provided by atmospheric inversions, atmospheric oxygen measurements, ocean interior observation-based estimates, and Earth System Models. The sum of all sources and sinks results in the carbon budget imbalance (BIM), a measure of imperfect data and incomplete understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the year 2025, EFOS increased by 1.2% relative to 2024, with fossil emissions at 10.5 ± 0.5 GtC yr-1 (including the cement carbonation sink, 0.2 GtC yr-1), ELUC was 1.2 ± 0.7 GtC yr-1, for total anthropogenic CO2 emissions of 11.7 ± 0.9 GtC yr-1 (42.8 ± 3.2 GtCO2 yr-1). Also, for 2025, GATM was 4.4 ± 0.2 GtC yr-1 (2.06 ± 0.1 ppm yr-1), 3.6 GtC lower than the record high 2024 growth rate. SOCEAN was 3.2 ± 0.7 GtC yr-1 and SLAND was 3.6 ± 0.9 GtC yr-1, with a small BIM (0.5 GtC yr-1). The global atmospheric CO2 concentration averaged over 2025 reached 425.6 ± 0.1 ppm. Preliminary data for 2026 suggest a decrease in EFOS relative to 2025 of 0.40% (–1.5% to +0.6%) globally, and atmospheric CO2 concentration increasing by 2.2 ppm reaching 427.7 ppm, 54% above the pre-industrial level (around 278 ppm in 1750). Overall, the mean and trend in the components of the global carbon budget are consistently estimated over the period 1959–2025, with a small positive overall budget imbalance, (0.6 GtC yr-1 averaged over the period 1959–2025), although discrepancies of up to around 1 GtC yr-1 persist for the representation of annual to 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 change emissions, (2) a low agreement between the different estimates on the magnitude of the land CO2 flux, particularly in the northern extra-tropics, and (3) a discrepancy between the different estimates on the mean ocean sink and its trend. This living data update documents changes in methods and datasets applied to this most-recent global carbon budget as well as evolving community understanding of the global carbon cycle. The data presented in this work are available at https://doi.org/10.18160/GCP-2026 (Friedlingstein et al., 2026b).

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

Accurate assessment of anthropogenic carbon dioxide (CO ₂ ) 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 synthesise datasets and methodologies to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO ₂ emissions ( E _FOS ) are based on energy and cement production data. Emissions from land-use change ( E _LUC ) are estimated by bookkeeping models based on land-use data. The global atmospheric CO ₂ growth rate ( G _ATM ) is computed from changes in concentration measured at surface stations. The global net uptake of CO ₂ by the ocean ( S _OCEAN ) is estimated with global ocean biogeochemistry models and observation-based f CO ₂ -products. The global net uptake of CO ₂ by the land ( S _LAND ) is estimated with dynamic global vegetation models. Additional lines of evidence are provided by atmospheric inversions, atmospheric oxygen measurements, ocean interior observation-based estimates, and Earth System Models. This year, we introduced corrections on the E _LUC , S _OCEAN and S _LAND estimates. The sum of all sources and sinks results in the carbon budget imbalance ( B _IM ), a measure of imperfect data and incomplete understanding of the contemporary carbon cycle. All uncertainties are reported as ± 1 σ . For the year 2024, E _FOS increased by 1.1 % relative to 2023, with fossil emissions at 10.3 ± 0.5 GtC yr ⁻¹ (including the cement carbonation sink, 0.2 GtC yr ⁻¹ ), E _LUC was 1.3 ± 0.7 GtC yr ⁻¹ , for total anthropogenic CO ₂ emissions of 11.6 ± 0.9 GtC yr ⁻¹ (42.4 ± 3.2 GtCO ₂ yr ⁻¹ ). Also, for 2024, G _ATM was 7.9 ± 0.2 GtC yr ⁻¹ (3.73 ± 0.1 ppm yr ⁻¹ ), 2.2 GtC above the 2023 growth rate. S _OCEAN was 3.4 ± 0.4 GtC yr ⁻¹ and S _LAND was 1.9 ± 1.1 GtC yr ⁻¹ , leaving a large negative B _IM (−1.7 GtC yr ⁻¹ ), suggesting that the total sink or G _ATM is strongly overestimated in 2024. The global atmospheric CO ₂ concentration averaged over 2024 reached 422.8 ± 0.1 ppm. Preliminary data for 2025 suggest an increase in E _FOS relative to 2024 of +1.0 % (0.2 % to 1.7 %) globally, and atmospheric CO ₂ concentration increasing by 2.1 ppm reaching 425.6 ppm, 53 % above the pre-industrial level (around 278 ppm in 1750). Overall, the mean and trend in the components of the global carbon budget are consistently estimated over the period 1959–2024, with a near-zero overall budget imbalance, although discrepancies of up to around 1 GtC yr ⁻¹ persist for the representation of annual to decadal variability in CO ₂ fluxes. Comparison of estimates from multiple approaches and observations shows: (1) a persistent large uncertainty in the estimate of land-use change emissions, (2) a low agreement between the different methods on the magnitude of the land CO ₂ flux in the northern extra-tropics, and (3) a discrepancy between the different methods on the mean ocean sink. This living data update documents changes in methods and datasets applied to this most-recent global carbon budget as well as evolving community understanding of the global carbon cycle. The data presented in this work are available at https://doi.org/10.18160/GCP-2025 (Friedlingstein et al., 2025c).

The global ocean and terrestrial carbon dioxide (CO2) sinks have removed approximately half of the total anthropogenic carbon emissions emitted to the atmosphere since 1850. Robust estimates of future carbon uptake are paramount to determine Paris Agreement compatible remaining greenhouse gas emission budgets including negative emission pathways to balance hard to abate emissions. Missing carbon-climate feedbacks in state-of-the-art greenhouse gas concentration-driven Earth System Models (ESMs), however, render future carbon cycle estimates uncertain. Here, historical and future ocean and land carbon uptake estimates from emissions-driven CMIP6 experiments conducted with AWI-ESM-1-REcoM are presented.In the emissions-driven model setup, carbon-climate feedbacks and differences in the initial distribution of terrestrial vegetation lead to a reduced carbon source from anthropogenic land use changes, a smaller atmospheric CO2 growth and a substantially weaker oceanic and terrestrial carbon uptake increase until the 1970s, compared to the concentration-driven model setup. Thereafter, the terrestrial CO2 sink increases stronger in the emissions-driven setup, leading to similar atmospheric CO2 growth in both model setups by the end of the historical period. In the future, ocean and land carbon sinks respond distinctively to both model setup and scenario forcing before peak emissions, between peak emissions and peak atmospheric CO2, and before and after net zero emissions. The land sink in particular continues to increase stronger than the ocean sink after peak atmospheric CO2. By the end of the 21st century, carbon-climate feedbacks yield atmospheric CO2 concentrations considerably lower by 17 to 42 ppm and a weaker ocean carbon sink in the emissions-driven model setup, with the largest differences in strong mitigation scenarios. As emissions-driven ESM setups are recommended for the upcoming CMIP7, these model results stress the need to improve our understanding of the future evolution of the global carbon sinks.

We present an assessment of the coupled ocean–sea ice–biogeochemistry model FESOM2.1–REcoM3, in which we integrated state equations for dissolved acidic polysaccharides (PCHO) and transparent exopolymer particles (TEP), as proposed by Engel et al. (2004), to explicitly describe these two organic carbon pools in the Arctic Ocean. PCHO is simulated as one fraction of the phytoplankton exudates, which can then aggregate to form larger particles, TEP. Since observational datasets on TEP are rare in time and space, we systematically assess the novel model implementation by stepwise discussing the essential components of the organic carbon cycle. Firstly, the simulated phytoplankton biomass yields good results when compared to in situ and remote-sensing products of total Chlorophyll a and particulate organic carbon. Secondly, we compare PCHO to observations in the Fram Strait, as an exemplary data-rich region, and to datasets in other regions of the Arctic Ocean. The model realistically reproduces a high phytoplankton exudation rate of PCHO under nutrient-depleted conditions. Thirdly, we assess simulated TEP concentrations by comparing them to in situ measurements from several campaigns to the Arctic Ocean. The simulation provides a first estimate of mean TEP concentrations of 200–400 µg C L⁻¹ on the continental shelves and 10–50 µg C L⁻¹ in the central basins (0–30 m depth range). Lastly, we put the model performance into a global context for TEP concentrations in the upper ocean layer. As such, the implementation of PCHO exudation, aggregation to TEP, and their remineralization processes into FESOM2.1–REcoM3 offers a reasonably good agreement with observations, on which further modeling work can build upon.