the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Jul 2026
Observations of radiocarbon (14C) in atmospheric CO2 confirm decline in U.S. emissions of fossil fuel CO2 (FFCO2) between 2010 and 2015
Abstract. 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.
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Status: open (until 09 Sep 2026)
- RC1: 'Comment on egusphere-2026-2896', Anonymous Referee #1, 13 Aug 2026 reply
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- 1
Review of „Observations of radiocarbon (14C) in atmospheric CO2 confirm decline in U.S. emissions of fossil fuel CO2 (FFCO2) between 2010 and 2015”
This manuscript presents results from a dual-tracer 14C-CO2 inversion for the years 2010 and 2015, when sufficient observational coverage was available across the U.S. target region. The dual-tracer inversion framework was developed by authors of this study in Basu et al. (2016, 2020) and has been refined for the present study primarily through the use of updated observations, meteorological fields, and prior emission estimates. The core of the dual-tracer inversion consists of 14CO2 observations, mainly from the dense NOAA station network in North America, which are used to separate fossil and biogenic CO2 fluxes in the U.S., thereby providing valuable additional information compared to a CO2-only inversion.
The inversion results indicate a decline in the U.S. fossil fuel CO2 emissions of more than 5% between 2010 and 2015, consistent with trends based on independent emission inventories. The derived ffCO2 fluxes are higher than those from (gridded) prior flux estimates but show impressive agreement with an adjusted, U.S.-specific emission inventory product (Vulcan 3.0), even at the monthly timescale.
The authors present their findings in a well-structured and comprehensive manner, supported by clear figures and discussions. A range of sensitivity analyses, based on varying prior fluxes and prior error correlations, along with model performance evaluations and a detailed analysis of posterior flux correlations, are conducted to assess the reliability and robustness of their ffCO2 emission estimates.
The study clearly demonstrates the potential of atmospheric 14CO2 observations for estimating national ffCO2 emissions and for detecting inter-annual trends. It underscores the critical need for continuous 14CO2 measurements to verify emission inventories, enhance confidence in emission reporting, and enable the monitoring of ffCO2 emission trends, which is relevant for assessing the effectiveness of climate mitigation strategies.
Overall, this is an excellent study, which I strongly recommend for publication after addressing a few minor issues outlined below.
Specific comments
L. 63-66: Given that the 14C-based ffCO2 emission estimates show better agreement with the Vulcan product than the EPA product in 2010 and 2015, it would be interesting to know which additional information is incorporated in Vulcan but not explicitly considered by the EPA.
L. 132-134: Consecutive hourly CO2 observations from a given site might exhibit correlated errors in the model-data mismatch. Do the authors account for these temporal correlations in the model-data mismatch error covariance matrix? Implementing such correlations might also prevent the large number of CO2 observations from exerting a too strong influence on the dual-tracer inversion results compared to the sparser 14CO2 data.
L. 135-137: What is the integration time of the Δ14CO2 samples? Were they collected only during the afternoon?
L. 151-153: What is the temporal resolution of the prior fluxes?
L. 171-173: If I understand correctly, two posterior NBE estimates from NOAA’s CarbonTracker system are used as prior constraints in the current study. I’m not sure whether the CarbonTracker system assimilates similar CO2 observations that are also used as observational constraint in the current study. If this is the case, it raises a concern about the potential double use of the same observational information, i.e., once in deriving the CarbonTracker-based NBE prior and again in the flux estimation of the current study. I’m therefore wondering about the independence of the CarbonTracker-based NBE priors, particularly in Northern America. Could the authors comment on this?
L. 203-205: For easier interpretation, could the authors also express the change relative to the earlier results (i.e., +118TgC/yr) in relative terms (%)?
Tab. 1: Interestingly, the spread among the national total ffCO2 posterior estimates is larger than the spread among the ffCO2 priors when the same NBE prior is used. In contrast, the spread in the posterior ffCO2 (2015-2010) trend estimates is smaller than the spread in the prior trend estimates. Could this indicate that the 14C-CO2 observations provide stronger constraints on interannual trends than on absolute annual total fluxes?
Fig. 4&5: Please note that the light blue diamonds within the gray shaded region of the figures represent the prior ffCO2 emissions.
L. 496-510: Did the authors also evaluate how well the prior CO2 values fit the observed vertical profiles from the NOAA aircraft profiling sites? When comparing the prior vertical gradients with aircraft observations, the simulation of the seasonally-varying vertical structure of the profiles remains unaffected by surface flux adjustments that may arise from the assimilation of the (same) aircraft data. Such an analysis would strengthen the statement made in L. 504-506.
Fig. 8: Please indicate in the figure caption that the simulations are based on posterior fluxes.
Tab. 4: Please specify the units of the values shown in the table.
L. 620-629: This section could be moved to the Methods section. Additionally, it would be helpful to provide a table summarizing the spatial and temporal correlation lengths assumed for each flux category in the default setup and the three sensitivity experiments (ST1-ST3).
L. 727-729: Why is April 2025 an exception? For me, this is not immediately clear from Fig. 3, where the 1σ uncertainty ranges of the prior and posterior means are clearly separated from each other.
L. 729-730: How do the authors obtain “15 tri-monthly seasonal totals”? In the main text and in Tab. 4, only 10 tri-monthly seasonal totals are presented, 5 for 2010 and 5 for 2015.
Supplement:
First paragraph: How large is the transport model error compared to the observational error for Δ14CO2? Is the transport model error time dependent?
Tab. S6: Where does the strong increase in the CT16 global NBE estimate from -1334 TgC/yr in 2010 to -266 TgC/yr in 2015 come from? Is this a potential artefact?
Tab. S6: Where do the isotopic values for the cosmogenic production come from? For example, Miller et al. (2025) report a value of 6282 PgC‰/yr for 2010 (in their Tab. 1), which was subsequently scaled by a factor of 0.8 to achieve global flux balance, resulting in roughly 5000 PgC‰/yr. However, in Tab. S6 of the current study, a value of only 466 PgC‰/yr is assumed for 2010. How do these values align?
Technical corrections
L. 170: “to inform our prior fluxes, …”
L. 259: “increased” -> “increase”
L. 271: “FFCCO2” -> “FFCO2”
L. 317: “±26 TgC/y” -> “±26 TgC/yr”
L. 346: delete “a”
L. 583: “a prior” -> “a priori”
L. 646: delete one “(“
References:
Basu, S., Miller, J. B., and Lehman, S.: Separation of biospheric and fossil fuel fluxes of CO2 by atmospheric inversion of CO2 and 14CO2 measurements: Observation System Simulations, Atmos. Chem. Phys., 16, 5665-5683, 10.5194/acp-16-5665-2016, 2016.
Basu, S., Lehman, S. J., Miller, J. B., Andrews, A. E., Sweeney, C., Gurney, K. R., Xu, X., Southon, J., and Tans, P. P.: Estimating US fossil fuel CO2 emissions from measurements of 14C in atmospheric CO2, Proceedings of the National Academy of Sciences, 117, 13300-13307, 10.1073/pnas.1919032117, 2020.
Miller, J. B., Lehman, S. J., and Lindsay, C. M.: Numerical Representation of Contemporary Atmospheric Δ14CO2: 1. Time-Varying Global Fluxes and Atmospheric Mass Balance, Global Biogeochemical Cycles, 39, e2025GB008522, https://doi.org/10.1029/2025GB008522, 2025.