Gregor Rehder

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).

Estimating air–sea CO₂ exchange in coastal and semi-enclosed seas is challenging due to limited spatial and temporal observational coverage. Atmospheric inverse modelling offers a complementary approach, although its ability to resolve regional marine carbon fluxes is still uncertain. In the Baltic Sea, intensive shipping further complicates the regional carbon budget through substantial anthropogenic CO₂ emissions. This study presents one of the first assessments combining air–sea CO₂ exchange and ship-derived CO₂ emissions in the Baltic Sea using both observation-based and atmospheric inversion approaches. Air–sea CO₂ fluxes for 2019 to 2021 were estimated using two independent methods. The first method combined a bulk flux calculation, which includes sea–air pCO₂ differences, wind-speed-dependent gas transfer velocity, solubility, and sea-ice cover, with shipping emissions simulated by the Ship Traffic Emission Assessment Model (STEAM). The second method used atmospheric CO₂ observations together with atmospheric transport modelling in the CIF-FLEXPART inversion framework to estimate surface CO₂ fluxes, providing an alternative observation-based approach. The results were analyzed for the entire Baltic Sea and for 13 HELCOM sub-basins. The STEAM-corrected bulk method indicated that the Baltic Sea acted as a net CO₂ sink during 2019 to 2021, with a mean flux of -1.79 mmol m⁻² d⁻¹. Without shipping emissions, the estimated flux was -3.76 mmol m⁻² d⁻¹, showing that ship traffic increased the net flux by approximately 1.97 mmol m⁻² d⁻¹. In contrast, the CIF-FLEXPART inversion estimated a net source of 3.92 mmol m⁻² d⁻¹ for the Baltic Sea. The climatological bulk-flux estimate for 2003 to 2021, excluding shipping emissions, gave a near-neutral mean flux of 0.25 mmol m⁻² d⁻¹. The basin-scale monthly fluxes showed moderate overall agreement between the two methods (mean Pearson correlation r = 0.49), with stronger correlations (r = 0.62–0.79) in several southern basins where observational coverage was greatest. The contrasting estimates of the Baltic Sea carbon balance show that considerable uncertainties remain in both bulk method and inversion method approaches and that shipping emissions need to be taken into account. More continuous and reliable observations, improved representation of coastal carbon processes, and further development of atmospheric inversion systems are needed to better constrain regional marine carbon budgets.

Estimating air–sea CO₂ exchange in coastal and semi-enclosed seas is challenging due to limited spatial and temporal observational coverage. Atmospheric inverse modelling offers a complementary approach, although its ability to resolve regional marine carbon fluxes is still uncertain. In the Baltic Sea, intensive shipping further complicates the regional carbon budget through substantial anthropogenic CO₂ emissions. This study presents one of the first assessments combining air–sea CO₂ exchange and ship-derived CO₂ emissions in the Baltic Sea using both observation-based and atmospheric inversion approaches. Air–sea CO₂ fluxes for 2019 to 2021 were estimated using two independent methods. The first method combined a bulk flux calculation, which includes sea–air pCO₂ differences, wind-speed-dependent gas transfer velocity, solubility, and sea-ice cover, with shipping emissions simulated by the Ship Traffic Emission Assessment Model (STEAM). The second method used atmospheric CO₂ observations together with atmospheric transport modelling in the CIF-FLEXPART inversion framework to estimate surface CO₂ fluxes, providing an alternative observation-based approach. The results were analyzed for the entire Baltic Sea and for 13 HELCOM sub-basins. The STEAM-corrected bulk method indicated that the Baltic Sea acted as a net CO₂ sink during 2019 to 2021, with a mean flux of -1.79 mmol m⁻² d⁻¹. Without shipping emissions, the estimated flux was -3.76 mmol m⁻² d⁻¹, showing that ship traffic increased the net flux by approximately 1.97 mmol m⁻² d⁻¹. In contrast, the CIF-FLEXPART inversion estimated a net source of 3.92 mmol m⁻² d⁻¹ for the Baltic Sea. The climatological bulk-flux estimate for 2003 to 2021, excluding shipping emissions, gave a near-neutral mean flux of 0.25 mmol m⁻² d⁻¹. The basin-scale monthly fluxes showed moderate overall agreement between the two methods (mean Pearson correlation r = 0.49), with stronger correlations (r = 0.62–0.79) in several southern basins where observational coverage was greatest. The contrasting estimates of the Baltic Sea carbon balance show that considerable uncertainties remain in both bulk method and inversion method approaches and that shipping emissions need to be taken into account. More continuous and reliable observations, improved representation of coastal carbon processes, and further development of atmospheric inversion systems are needed to better constrain regional marine carbon budgets.

Estimating air–sea CO2 exchange in coastal and semi-enclosed seas is challenging due to limited spatial and temporal observational coverage. Atmospheric inverse modelling offers a complementary approach, although its ability to resolve regional marine carbon fluxes is still uncertain. In the Baltic Sea, intensive shipping further complicates the regional carbon budget through substantial anthropogenic CO2 emissions. This study presents one of the first assessments combining air–sea CO2 exchange and ship-derived CO2 emissions in the Baltic Sea using both observation-based and atmospheric inversion approaches. Air–sea CO2 fluxes for 2019 to 2021 were estimated using two independent methods. The first method combined a bulk flux calculation, which includes sea–air pCO2 differences, wind-speed-dependent gas transfer velocity, solubility, and sea-ice cover, with shipping emissions simulated by the Ship Traffic Emission Assessment Model (STEAM). The second method used atmospheric CO2 observations together with atmospheric transport modelling in the CIF-FLEXPART inversion framework to estimate surface CO2 fluxes, providing an alternative observation-based approach. The results were analyzed for the entire Baltic Sea and for 13 HELCOM sub-basins. The STEAM-corrected bulk method indicated that the Baltic Sea acted as a net CO2 sink during 2019 to 2021, with a mean flux of -1.79 mmol m−2 d−1. Without shipping emissions, the estimated flux was -3.76 mmol m−2 d−1, showing that ship traffic increased the net flux by approximately 1.97 mmol m−2 d−1. In contrast, the CIF-FLEXPART inversion estimated a net source of 3.92 mmol m−2 d−1 for the Baltic Sea. The climatological bulk-flux estimate for 2003 to 2021, excluding shipping emissions, gave a near-neutral mean flux of 0.25 mmol m−2 d−1. The basin-scale monthly fluxes showed moderate overall agreement between the two methods (mean Pearson correlation r = 0.49), with stronger correlations (r = 0.62–0.79) in several southern basins where observational coverage was greatest. The contrasting estimates of the Baltic Sea carbon balance show that considerable uncertainties remain in both bulk method and inversion method approaches and that shipping emissions need to be taken into account. More continuous and reliable observations, improved representation of coastal carbon processes, and further development of atmospheric inversion systems are needed to better constrain regional marine carbon budgets.

The decreasing water transparency of the Baltic Sea over the last century has been documented by in situ measurements of the Secchi disk depth ( z _SD ) as well as by satellite measurements over recent decades. While the decrease in transparency for the Baltic Sea in general has become negligible from about 2013 onwards, satellite-detected light attenuation of the violet and blue light ( k _d 412) has increased, particularly from 2013 onwards. In the Gulf of Bothnia (particularly in the Bothnian Bay), eastern Gulf of Finland and eastern Gulf of Riga, k _d 412 has increased significantly, indicating a dramatic increase in the concentration of colored dissolved organic matter (CDOM). These findings are partly supported by in situ CDOM data. The increase in CDOM concentration is probably due to climate change induced shifts in hydrology and greening of the land, all related to global warming. The increase in CDOM concentration and its effect on light availability may cause significant ecological changes and deterioration of the aquatic food chains.

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).

Large-scale Euro-Atlantic variability, shaped by the polar jet stream, governs weather and climate in the Baltic Sea region, thereby impacting the physical and biogeochemical properties of the Baltic Sea ecosystem. This review synthesizes how key atmospheric circulation features and modes of climate variability, including the North Atlantic Oscillation, atmospheric blocking and the Atlantic Multidecadal Variability, influence the Baltic Sea region. By integrating evidence from the published literature, observational datasets, and both global and regional climate model simulations, we assess established as well as potential linkages to key climatic variables, including temperature, precipitation, and storm activity, across temporal scales ranging from synoptic events to multidecadal variability. We then evaluate how these climate controls cascade into ecosystem-relevant processes, namely oxygen dynamics, primary productivity and ocean acidification. Although physical links are already established, the pathways connecting large-scale atmospheric patterns to biogeochemistry are still poorly constrained, partly because dedicated field studies and targeted model experiments are limited. We outline priority research needs to enhance near-term predictability and reduce uncertainty in future projections for the Baltic Sea.