Jens Terhaar

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).

By absorbing large amounts of anthropogenic carbon from the atmosphere, the ocean strongly buffers the magnitude of warming caused by CO₂ emissions. This ocean carbon uptake is often conceptually separated into two components, quantified by the feedback parameters β and γ: The uptake of anthropogenic CO₂ driven by rising atmospheric concentration (β), and the effect of climate warming (γ), which reduces this anthropogenic uptake and drives a loss of natural carbon already stored in the ocean. Together, β and γ determine how effectively the ocean absorbs CO₂ emissions and shape how much emitted carbon remains in the atmosphere. Thus, β and γ directly affect how much the climate system warms in response to emitted carbon, i.e., the transient climate response to cumulative CO₂ emissions (TCRE). However, estimates of ocean β and γ from Earth system models differ widely, leading to substantial uncertainty in the TCRE. Here, we extend and apply a previously identified emergent constraint linking ocean anthropogenic carbon uptake to observable indicators of the formation of Southern Ocean mode and intermediate waters, North Atlantic deep water formation, and the global surface ocean chemical uptake capacity. In addition to these three predictors, we add an additional indicator for bottom water formation via open-ocean deep convection, defined as the volume of waters below 3000 m that is ventilated via extremely deep mixed layer depths. Using observations of these four indicators, the constrained β is 0.96±0.05 Pg C ppm−1 at doubling of atmospheric CO₂ and 0.85±0.04 Pg C ppm−1 at quadrupling of atmospheric CO₂. The observationally constrained β is around 4-5% larger and around 35-47% less uncertain than previously estimated. This larger ocean β is further supported by simulated historical anthropogenic carbon uptake in most models being lower than observation-based estimates, although uncertainties in the latter remain too large to directly constrain β. Beyond constraining ocean β, we show that ocean β and γ become increasingly correlated over time. This relationship emerges because models with higher anthropogenic carbon uptake, and hence larger β, also show a stronger suppression of that uptake in response to climate change. The stronger suppression is driven by a larger reduction in the indicators for ocean carbon uptake identified above in models with higher indicators at the beginning of the simulations, e.g., a model with stronger ventilation at present will also see a stronger reduction of that ventilation in the future. As atmospheric CO₂ increases, the climate-driven reduction in anthropogenic carbon uptake increasingly dominates over the climate-driven loss of natural carbon, causing γ to become primarily controlled by reductions in anthropogenic carbon uptake over time. As a result, the same observable indicators that constrain ocean β also constrain ocean γ at high CO₂. At doubling of atmospheric CO₂, this yields a constrained ocean γ of -10.1±2.4 Pg C ◦C−1, which is 23% larger in magnitude but has the same uncertainty as the unconstrained multi-model mean. Reflecting the strengthening of this relationship with increasing atmospheric CO₂, the constraint tightens further at quadrupling of CO₂ to -24.5±3.6 Pg C °C−1, which is 28% larger and 42% less uncertain. Overall, we have extended a previous constraint for the ocean carbon uptake by (1) adding an additional parameter that removed a known bias from unrealistic open-ocean deep convection, (2) transferring the constraint to γ and providing a mechanistical explanation for that constraint and evidence for that mechanism, (3) applying the constraint regionally, (4) identifying substantial biases in β and γ and reducing uncertainties, and (5) improving process understanding of drivers of changes in the ocean carbon sink. Although both the increase in atmospheric CO₂ uptake and the increase in carbon loss with climate change were biased across the model ensembles and could be constrained, the bias corrections of β and γ offset each other, leaving the best estimate of the ocean carbon sink relatively unchanged (2–3% increase compared to this ensemble mean). However, the uncertainties of β and γ are reduced and hence is the overall uncertainty of the carbon sink, which will propagate into smaller uncertainties of important climate metrics such as the TCRE.

By absorbing large amounts of anthropogenic carbon from the atmosphere, the ocean strongly buffers the magnitude of warming caused by CO2 emissions. This ocean carbon uptake is often conceptually separated into two components, quantified by the feedback parameters β and γ: The uptake of anthropogenic CO2 driven by rising atmospheric concentration (β), and the effect of climate warming (γ), which reduces this anthropogenic uptake and drives a loss of natural carbon already stored in the ocean. Together, β and γ determine how effectively the ocean absorbs CO2 emissions and shape how much emitted carbon remains in the atmosphere. Thus, β and γ directly affect how much the climate system warms in response to emitted carbon, i.e., the transient climate response to cumulative CO2 emissions (TCRE). However, estimates of ocean β and γ from Earth system models differ widely, leading to substantial uncertainty in the TCRE. Here, we extend and apply a previously identified emergent constraint linking ocean anthropogenic carbon uptake to observable indicators of the formation of Southern Ocean mode and intermediate waters, North Atlantic deep water formation, and the global surface ocean chemical uptake capacity. In addition to these three predictors, we add an additional indicator for bottom water formation via open-ocean deep convection, defined as the volume of waters below 3000 m that is ventilated via extremely deep mixed layer depths. Using observations of these four indicators, the constrained β is 0.96±0.05 Pg C ppm−1 at doubling of atmospheric CO2 and 0.85±0.04 Pg C ppm−1 at quadrupling of atmospheric CO2. The observationally constrained β is around 4-5% larger and around 35-47% less uncertain than previously estimated. This larger ocean β is further supported by simulated historical anthropogenic carbon uptake in most models being lower than observation-based estimates,although uncertainties in the latter remain too large to directly constrain β. Beyond constraining ocean β, we show that ocean β and γ become increasingly correlated over time. This relationship emerges because models with higher anthropogenic carbon uptake, and hence larger β, also show a stronger suppression of that uptake in response to climate change. The stronger suppression is driven by a larger reduction in the indicators for ocean carbon uptake identified above in models with higher indicators at the beginning of the simulations, e.g., a model with stronger ventilation at present will also see a stronger reduction of that ventilation in the future. As atmospheric CO2 increases, the climate-driven reduction in anthropogenic carbon uptake increasingly dominates over the climate-driven loss of natural carbon, causing γ to become primarily controlled by reductions in anthropogenic carbon uptake over time. As a result, the same observable indicators that constrain ocean β also constrain ocean γ at high CO2. At doubling of atmospheric CO2, this yields a constrained ocean γ of -10.1±2.4 Pg C ◦C−1, which is 23% larger in magnitude but has the same uncertainty asthe unconstrained multi-model mean. Reflecting the strengthening of this relationship with increasing atmospheric CO2, the constraint tightens further at quadrupling of CO2 to -24.5±3.6 Pg C °C−1, which is 28% larger and 42% less uncertain. Overall, we have extended a previous constraint for the ocean carbon uptake by (1) adding an additional parameter that removed a known bias from unrealistic open-ocean deep convection, (2) transferring the constraint to γ and providing a mechanistical explanation for that constraint and evidence for that mechanism, (3) applying the constraint regionally, (4) identifying substantial biases in β and γ and reducing uncertainties, and (5) improving process understanding of drivers of changes in the ocean carbon sink. Although both the increase in atmospheric CO2 uptake and the increase in carbon loss with climate change were biased across the model ensembles and could be constrained, the bias corrections of β and γ offset each other, leaving the best estimate of the ocean carbon sink relatively unchanged (2–3% increase compared to this ensemble mean). However, the uncertainties of β and γ are reduced and hence is the overall uncertainty of the carbon sink, which will propagate into smaller uncertainties of important climate metrics such as the TCRE.

Although the Arctic Ocean is relatively small in volume, its extensive coastline delivers large quantities of terrigenous material from rivers and coastal erosion. As a result, the Arctic Ocean is impacted more strongly by terrigenous material than most other parts of the global ocean. Yet the effect of this material on carbon cycling and ocean acidification remains poorly quantified. In this study, we use an ocean biogeochemical model driven by observation-based estimates of terrigenous carbon, alkalinity, and nutrients to evaluate their contribution to the mean state, depth pattern, and seasonal cycle of ocean acidification, as measured by the aragonite saturation state. Riverine alkalinity generally mitigates acidification, whereas organic carbon from coastal erosion intensifies it. Nutrients from both sources mitigate ocean acidification at the surface by stimulating primary production, but intensify it at depth through subsequent remineralisation. Together, riverine and erosion-derived inputs account for about 20–40 % of the seasonal variability in the saturation state of the surface ocean. This amplification of the natural seasonal cycle is primarily caused by an increase in the summertime maximum of the saturation state. Terrigenous inputs also reduce the Arctic Ocean's capacity to absorb atmospheric CO₂ by 17–25 %. Accurately representing carbon and nutrient inputs from rivers and coastal erosion in biogeochemical models is therefore important for reliable assessments of ocean acidification, ecosystem health, and carbon budgets in the Arctic Ocean.

Although the Arctic Ocean is relatively small in volume, its extensive coastline delivers large quantities of terrigenous material from rivers and coastal erosion. As a result, the Arctic Ocean is impacted more strongly by terrigenous material than most other parts of the global ocean. Yet the effect of this material on carbon cycling and ocean acidification remains poorly quantified. In this study, we use an ocean biogeochemical model driven by observation-based estimates of terrigenous carbon, alkalinity, and nutrients to evaluate their contribution to the mean state, depth pattern, and seasonal cycle of ocean acidification, as measured by the aragonite saturation state. Riverine alkalinity generally mitigates acidification, whereas organic carbon from coastal erosion intensifies it. Nutrients from both sources mitigate ocean acidification at the surface by stimulating primary production, but intensify it at depth through subsequent remineralisation. Together, riverine and erosion-derived inputs account for about 20–40 % of the seasonal variability in the saturation state of the surface ocean. This amplification of the natural seasonal cycle is primarily caused by an increase in the summertime maximum of the saturation state. Terrigenous inputs also reduce the Arctic Ocean's capacity to absorb atmospheric CO₂ by 17–25 %. Accurately representing carbon and nutrient inputs from rivers and coastal erosion in biogeochemical models is therefore important for reliable assessments of ocean acidification, ecosystem health, and carbon budgets in the Arctic Ocean.

The Paris Agreement encourages member states to keep global warming below 2.0C, and more ambitiously, limit warming to 1.5C. Recent coupled Earth system model simulations adaptively adjusted emissions to reach these stabilization temperatures show that fossil fuel CO2 reductions cannot reach the goals of the Paris Agreement alone. Building upon this framework, we show how varying the relative contribution of cumulative sustained methane emissions reductions is independent of pathway and modifies the allowable emissions from fossil fuels to reach 1.5C, 2.0C, and 3.0C of warming. Our results show that methane emissions reductions can have a large impact on the allowable emissions budget, and with mitigation goals >75% methane emissions reductions combined with rapid decarbonization, may reach the Paris Agreement goals. Despite much shorter atmospheric perturbation lifetimes of methane compared to CO2, we find a surprisingly linear relationship of the climate impacts of a one-time release of CO2 versus the cumulative impact of sustained reduction in methane emissions at 2663.1 kgCO2/kgCH4/yr. The 5000-year timescale of atmospheric CO2 removal by ocean sediments translates into a methane emission equivalency of 0.53x CO2, much smaller than the currently proposed emissions trading 1 system value of 25x and the Global Warming Potential approach 100-year horizon of 27.8x. Thus, methane emissions must be treated as a sustained reduction versus an instantaneous reduction like CO2, and should be part of the most urgent decarbonization towards the Paris goals, while the concept of a market exchange value between methane and fossil fuel CO2 must be viewed critically. While acknowledging the indefinite commitment of rapid methane emissions mitigation, this combined with rapid decarbonization may help to prevent an overshoot of Paris Agreement temperature goals.

Although the Arctic Ocean is relatively small in volume, its extensive coastline delivers large quantities of terrigenous material from rivers and coastal erosion. As a result, the Arctic Ocean is impacted more strongly by terrigenous material than most other parts of the global ocean. Yet the effect of this material on carbon cycling and ocean acidification remains poorly quantified. In this study, we use an ocean biogeochemical model driven by observation-based estimates of terrigenous carbon, alkalinity, and nutrients to evaluate their contribution to the mean state, depth pattern, and seasonal cycle of ocean acidification, as measured by the aragonite saturation state. Riverine alkalinity generally mitigates acidification, whereas organic carbon from coastal erosion intensifies it. Nutrients from both sources mitigate ocean acidification at the surface by stimulating primary production, but intensify it at depth through subsequent remineralisation. Together, riverine and erosion-derived inputs account for about 20–40 % of the seasonal variability in the saturation state of the surface ocean. This amplification of the natural seasonal cycle is primarily caused by an increase in the summertime maximum of the saturation state. Terrigenous inputs also reduce the Arctic Ocean's capacity to absorb atmospheric CO₂ by 17–25 %. Accurately representing carbon and nutrient inputs from rivers and coastal erosion in biogeochemical models is therefore important for reliable assessments of ocean acidification, ecosystem health, and carbon budgets in the Arctic Ocean.