Colm Sweeney

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).

Amplified Arctic warming could substantially increase methane (CH4) fluxes from high-latitudes by accelerating soil thaw and releasing soil organic carbon into the atmosphere. Yet, the magnitude and spatiotemporal patterns of current and future high-latitude CH4 fluxes remain highly uncertain. We estimate growing season (May to September) CH4 fluxes in Alaska from 2012 to 2017 using atmospheric observations from two NASA campaigns: the Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE) and Arctic-Carbon Atmospheric Profiles (Arctic-CAP). We couple these observations with an atmospheric transport model and a geostatistical inverse model to estimate the geographic distribution, seasonality, and inter-annual variability of CH4 fluxes. As part of this approach, we evaluate how well different state-of-the-art, process-based flux models compare to the inverse estimate of CH4 fluxes. We find that estimated statewide fluxes are a factor of 1.3 to 2.9 higher compared to different ensembles of process-based models. This discrepancy is largest across the North Slope of Alaska – up to a factor of 6.5 when compared to one of the model ensembles. We do not find any statistically significant inter-annual variability (IAV) during years 2012–2017, and the IAV in our estimate is generally consistent with process-based models. In addition, our analysis reveals that differences in inundation fraction mapping contribute to the magnitude discrepancies in current process-based models, and future improvements in inundation maps are essential to reduce uncertainties in CH4 flux estimates across high latitudes. These results provide an improved atmospheric constraint on regional CH4 flux estimates from northern wetlands under accelerated Arctic warming.

The Arctic has experienced the largest increase in atmospheric CO ₂ seasonal cycle amplitude (SCA) since the 1960s. Previous studies primarily attributed this to enhanced regional warming. Here, we show that at numerous locations in the Arctic, atmospheric CO ₂ SCAs have increased at a relative rate similar to atmospheric CO ₂ abundance over the past five decades, suggesting a likely strong role of CO ₂ fertilization. Using a tagged‐tracer transport experiment and long‐term atmospheric observations of CO ₂ , carbonyl sulfide (COS), and the ¹³ C: ¹² C ratio of CO ₂ (), we quantified high northern latitude (HNL, 53°–90°N) ecosystem process changes. Results suggest that HNL terrestrial ecosystems drive Arctic CO ₂ SCA increases, with gross primary productivity (GPP) exerting a more dominant impact than ecosystem respiration. Using atmospheric COS measurements and mass balance calculation, we estimated 2000–2020 growing‐season (May–August) average HNL stomatal conductance at 0.024 ± 0.004 mol m ⁻² s ⁻¹ . This exhibited a slight negative trend of −4.3 ± 3.8 (1) %, consistent with CO ₂ –induced reductions in stomata observed previously at smaller scales. However, accounting for regional leaf area expansion, leaf‐area‐integrated ecosystem conductance was estimated to be 0.044 ± 0.006 mol m ⁻² s ⁻¹ with a small positive trend of (1) %. Furthermore, observations suggest the growing‐season intercellular‐to‐ambient CO ₂ ratio in HNL ecosystems remained constant. Integrating these estimates, we estimate growing‐season GPP in HNL ecosystems averaged 33 ± 3 (1 ) PgC yr ⁻¹ from 2000 to 2020 and increased by 12 ± 2 (1) % over this period, driven primarily by CO ₂ fertilization.

NOAA’s AirCore atmospheric sampling program in Boulder, Colorado has provided one of the longest continuous stratospheric records of calibrated carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) measurements in the world from 2012 to present day. Further, the AirCore technology has been adopted by several institutions worldwide for atmospheric vertical trace gas profiling. Over the past decade, this program has seen multiple technical improvements including the development of smaller, more lightweight AirCores, which has allowed for new methodologies to routinely assess the quality of vertical profile observations of trace gases. The development of new analytical techniques over the past decade, along with new developments in methodological and analytical approaches to expand the vertical coverage and trace gases concurrently analyzed in AirCore samples, expanded the AirCore profiling suite to additional species such as nitrous oxide (N2O). Disseminating these methodologies is essential for establishing new AirCore programs abroad and for maintaining consistency and reproducibility with existing ones. Here, we highlight a 13-year observational record of CO2, CH4 and CO, and a more recent record of N2O (circa 2019) over Boulder, Colorado, from monthly profiling efforts within the NOAA AirCore Program, examining trends in CO2 and seasonal variability in all species. Same-string AirCore comparisons have allowed for sampling system repeatability to be assessed throughout the atmospheric column from Boulder, Colorado, which is typically less than 0.1 ppm, 2 ppb, 5 ppb, and 1 ppb for CO2, CH4, CO and N2O, respectively, at pressure level intervals commensurate with molecular diffusion length scales. These high-quality profiles uniquely capture both seasonal and transport variability in atmospheric composition derived from changes in surface fluxes, long-range transport, potential vorticity displacement and stratosphere-troposphere exchange, providing access to new understanding of stratospheric composition, chemistry and dynamics. Other new aspects of the AirCore program described here include both additional trace gas species that provide further insight into stratospheric transport time scales and a UAS-based platform for automated AirCore recovery and expansion of profiling capabilities to under-sampled regions.

All emissions pathways aimed at stabilizing global temperatures at the internationally agreed target of 1.5–2 °C above pre-industrial levels require steep cuts in global CO₂ emissions. Reliable emissions tracking is therefore essential to monitoring progress towards related mitigation goals, especially for the world’s largest emitters. Here we make use of atmospheric measurements of Δ¹⁴CO₂ and the dual-tracer Δ¹⁴CO₂:CO₂ assimilation and inversion system previously developed by our group to estimate annual and monthly CO₂ emissions from fossil fuel use and cement production (FFCO₂) for the U.S. for 2010 and 2015, the first two individual years for which large numbers of atmospheric Δ¹⁴CO₂ measurements are available. Ensemble-mean national FFCO₂ totals obtained from a 9-member suite of inverse results are larger than reported by the U.S. Environmental Protection Agency (EPA), but overlap at their respective 2σ ensemble-wide model spreads (inversions) and reported 95 % confidence intervals (EPA) in both years. In contrast, the inverse results agree with both annual totals and 16 of 24 derived monthly totals from the Vulcan 3.0 emissions data product with 1σ, ensemble wide. Central estimates of the change in U.S. FFCO₂ emission between 2010 and 2015 range from -5.1 % (EPA), -5.8 % (Vulcan), and -7.8 % (this work), providing a first confirmation of an expected national FFCO₂ decline based on atmospheric observations. The development of a reliable emissions tracking system based directly on atmospheric observations, as detailed here, may take on additional scientific and policy relevance given the recent interruption of EPA reporting.

All emissions pathways aimed at stabilizing global temperatures at the internationally agreed target of 1.5–2 °C above pre-industrial levels require steep cuts in global CO₂ emissions. Reliable emissions tracking is therefore essential to monitoring progress towards related mitigation goals, especially for the world’s largest emitters. Here we make use of atmospheric measurements of Δ¹⁴CO₂ and the dual-tracer Δ¹⁴CO₂:CO₂ assimilation and inversion system previously developed by our group to estimate annual and monthly CO₂ emissions from fossil fuel use and cement production (FFCO₂) for the U.S. for 2010 and 2015, the first two individual years for which large numbers of atmospheric Δ¹⁴CO₂ measurements are available. Ensemble-mean national FFCO₂ totals obtained from a 9-member suite of inverse results are larger than reported by the U.S. Environmental Protection Agency (EPA), but overlap at their respective 2σ ensemble-wide model spreads (inversions) and reported 95 % confidence intervals (EPA) in both years. In contrast, the inverse results agree with both annual totals and 16 of 24 derived monthly totals from the Vulcan 3.0 emissions data product with 1σ, ensemble wide. Central estimates of the change in U.S. FFCO₂ emission between 2010 and 2015 range from -5.1 % (EPA), -5.8 % (Vulcan), and -7.8 % (this work), providing a first confirmation of an expected national FFCO₂ decline based on atmospheric observations. The development of a reliable emissions tracking system based directly on atmospheric observations, as detailed here, may take on additional scientific and policy relevance given the recent interruption of EPA reporting.

We present the results of a Very Large Methane Release (VLMR) experiment evaluating methane retrievals from the Geostationary Operational Environmental Satellites (GOES) Advanced Baseline Imagers (ABIs) and multiple low-Earth-orbit satellite instruments with high point-source detection thresholds. The experiment coordinated observations of a US gas pipeline blowdown with nine satellites, an aircraft, and a truck-based mobile laboratory. We used the GOES-16, -18, and -19 ABIs with revisits every 10 min to 7 s to quantify release magnitude and associated uncertainty. Best methane retrieval precision of 7% was achieved in the 30-s mesoscale scan modes averaged to 5 min, yielding an estimated methane plume detection threshold of 15 to 30 t h ⁻¹ per m s ⁻¹ of wind. GOES detected total emissions of 370 ± 30 t over 42 to 63 min from two release points. Source rate and mass estimates are broadly consistent across measurement platforms, but the total detected release mass is ~25% lower than that reported by the operator based on pipeline volume and pressure. This discrepancy may reflect late-stage emissions below satellite detection thresholds and indicates a potential low bias in satellite estimates of total emissions from large transient releases. Coordinated field experiments such as VLMR can complement existing controlled-release satellite evaluation programs by providing a framework to validate observations of very large methane point sources.