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<front>
<journal-meta>
<journal-id journal-id-type="publisher">EGUsphere</journal-id>
<journal-title-group>
<journal-title>EGUsphere</journal-title>
<abbrev-journal-title abbrev-type="publisher">EGUsphere</abbrev-journal-title>
<abbrev-journal-title abbrev-type="nlm-ta">EGUsphere</abbrev-journal-title>
</journal-title-group>
<issn pub-type="epub"></issn>
<publisher><publisher-name>Copernicus Publications</publisher-name>
<publisher-loc>Göttingen, Germany</publisher-loc>
</publisher>
</journal-meta>
<article-meta>
<article-id pub-id-type="doi">10.5194/egusphere-2026-2896</article-id>
<title-group>
<article-title>Observations of radiocarbon (&lt;sup&gt;14&lt;/sup&gt;C) in atmospheric CO&lt;sub&gt;2&lt;/sub&gt; confirm decline in U.S. emissions of fossil fuel CO&lt;sub&gt;2&lt;/sub&gt; (FFCO&lt;sub&gt;2&lt;/sub&gt;) between 2010 and 2015</article-title>
</title-group>
<contrib-group><contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Islam</surname>
<given-names>S. M. Nazrul</given-names>
</name>
<xref ref-type="aff" rid="aff1">
<sup>1</sup>
</xref>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="aff" rid="aff8">
<sup>8</sup>
</xref>
<xref ref-type="aff" rid="aff9">
<sup>9</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Lehman</surname>
<given-names>Scott J.</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Basu</surname>
<given-names>Sourish</given-names>
<ext-link>https://orcid.org/0000-0001-8605-5894</ext-link>
</name>
<xref ref-type="aff" rid="aff4">
<sup>4</sup>
</xref>
<xref ref-type="aff" rid="aff5">
<sup>5</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Andrews</surname>
<given-names>Arlyn E.</given-names>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
<xref ref-type="aff" rid="aff7">
<sup>7</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>McKain</surname>
<given-names>Kathryn</given-names>
<ext-link>https://orcid.org/0000-0002-8323-5758</ext-link>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Sweeney</surname>
<given-names>Colm</given-names>
<ext-link>https://orcid.org/0000-0002-4517-0797</ext-link>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Xu</surname>
<given-names>Xiaomei</given-names>
<ext-link>https://orcid.org/0000-0003-3678-2748</ext-link>
</name>
<xref ref-type="aff" rid="aff6">
<sup>6</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Wolak</surname>
<given-names>Chad</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Morgan</surname>
<given-names>Stephen P.</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Cappa</surname>
<given-names>Patrick</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Southon</surname>
<given-names>John R.</given-names>
</name>
<xref ref-type="aff" rid="aff6">
<sup>6</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Tans</surname>
<given-names>Pieter P.</given-names>
</name>
<xref ref-type="aff" rid="aff3">
<sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author" xlink:type="simple"><name name-style="western"><surname>Miller</surname>
<given-names>John B.</given-names>
<ext-link>https://orcid.org/0000-0001-8630-1610</ext-link>
</name>
<xref ref-type="aff" rid="aff2">
<sup>2</sup>
</xref>
</contrib>
</contrib-group><aff id="aff1">
<label>1</label>
<addr-line>Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309</addr-line>
</aff>
<aff id="aff2">
<label>2</label>
<addr-line>NOAA Global Monitoring Laboratory, Boulder, CO 80305</addr-line>
</aff>
<aff id="aff3">
<label>3</label>
<addr-line>Institute of Arctic and Alpine Research, University of Colorado Boulder, Boulder, CO 80309</addr-line>
</aff>
<aff id="aff4">
<label>4</label>
<addr-line>Global Modeling and Assimilation Office, NASA Goddard Space Flight Center, Greenbelt, MD 20771</addr-line>
</aff>
<aff id="aff5">
<label>5</label>
<addr-line>Earth System Science Interdisciplinary Center, University of Maryland, College Park, MD 20740</addr-line>
</aff>
<aff id="aff6">
<label>6</label>
<addr-line>Keck Carbon Cycle AMS Facility, University of California, Irvine, CA 92697</addr-line>
</aff>
<aff id="aff7">
<label>7</label>
<addr-line>now at: SilverLining, Washington, DC 20001</addr-line>
</aff>
<aff id="aff8">
<label>8</label>
<addr-line>now at: Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, CO 80309</addr-line>
</aff>
<aff id="aff9">
<label>9</label>
<addr-line>now at: NOAA Chemical Sciences Laboratory, Boulder, CO 80305</addr-line>
</aff>
<pub-date pub-type="epub">
<day>06</day>
<month>07</month>
<year>2026</year>
</pub-date>
<volume>2026</volume>
<fpage>1</fpage>
<lpage>33</lpage>
<permissions>
<copyright-statement>Copyright: &#x000a9; 2026 S. M. Nazrul Islam et al.</copyright-statement>
<copyright-year>2026</copyright-year>
<license license-type="open-access">
<license-p>This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit <ext-link ext-link-type="uri"  xlink:href="https://creativecommons.org/licenses/by/4.0/">https://creativecommons.org/licenses/by/4.0/</ext-link></license-p>
</license>
</permissions>
<self-uri xlink:href="https://egusphere.copernicus.org/preprints/2026/egusphere-2026-2896/">This article is available from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-2896/</self-uri>
<self-uri xlink:href="https://egusphere.copernicus.org/preprints/2026/egusphere-2026-2896/egusphere-2026-2896.pdf">The full text article is available as a PDF file from https://egusphere.copernicus.org/preprints/2026/egusphere-2026-2896/egusphere-2026-2896.pdf</self-uri>
<abstract>
<p>All emissions pathways aimed at stabilizing global temperatures at the internationally agreed target of 1.5&amp;ndash;2 &amp;deg;C above pre-industrial levels require steep cuts in global CO&lt;sub&gt;2&lt;/sub&gt; emissions. Reliable emissions tracking is therefore essential to monitoring progress towards related mitigation goals, especially for the world&amp;rsquo;s largest emitters. Here we make use of atmospheric measurements of &amp;Delta;&lt;sup&gt;14&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; and the dual-tracer &amp;Delta;&lt;sup&gt;14&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt;:CO&lt;sub&gt;2&lt;/sub&gt; assimilation and inversion system previously developed by our group to estimate annual and monthly CO&lt;sub&gt;2&lt;/sub&gt; emissions from fossil fuel use and cement production (FFCO&lt;sub&gt;2&lt;/sub&gt;) for the U.S. for 2010 and 2015, the first two individual years for which large numbers of atmospheric &amp;Delta;&lt;sup&gt;14&lt;/sup&gt;CO&lt;sub&gt;2&lt;/sub&gt; measurements are available. Ensemble-mean national FFCO&lt;sub&gt;2&lt;/sub&gt; 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 2&amp;sigma; 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 1&amp;sigma;, ensemble wide. Central estimates of the change in U.S. FFCO&lt;sub&gt;2&lt;/sub&gt; 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 FFCO&lt;sub&gt;2&lt;/sub&gt; 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.</p>
</abstract>
<counts><page-count count="33"/></counts>
<funding-group>
<award-group id="gs1">
<funding-source>National Oceanic and Atmospheric Administration</funding-source>
<award-id>NA19OAR4310166</award-id>
</award-group>
<award-group id="gs2">
<funding-source>National Aeronautics and Space Administration</funding-source>
<award-id>80NSSC23M0011</award-id>
</award-group>
</funding-group>
</article-meta>
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