{
  "schemaVersion": "carbon-scientific-context/v1",
  "version": "research-v2026.09.17",
  "reviewedOn": "2026-09-17",
  "boundary": "Original explanatory prose and source-verified scientific context. These records do not replace the frozen annual carbon account, fit a new model, or turn historical airborne fractions into forecasts.",
  "sources": [
    {
      "id": "JOOS-2013",
      "title": "Joos et al. (2013), Carbon dioxide and climate impulse response functions",
      "url": "https://acp.copernicus.org/articles/13/2793/2013/",
      "doi": "10.5194/acp-13-2793-2013",
      "locator": "Abstract and Figure 1: PD100 pulse experiment; the figure caption defines the multi-model mean and its plus/minus two-standard-deviation range.",
      "verification": "Primary abstract and original PDF Figure 1 caption checked. The number of contributing models changes with the time horizon.",
      "publicDocumentSha256": "e7132499026cb7d6d1a6c25a7572b85c09c32f38a37112c7653518e54723a1e7",
      "rights": "Factual extraction and original paraphrase; the source is linked, not redistributed."
    },
    {
      "id": "IPCC-AR6-WGI-SPM",
      "title": "IPCC AR6 WGI (2021), Summary for Policymakers",
      "url": "https://www.ipcc.ch/report/ar6/wg1/resources/spm-headline-statements/",
      "locator": "B.4: carbon-sink effectiveness under increasing emissions. D.1: cumulative CO2, at least net zero CO2, and reductions in other greenhouse gases.",
      "verification": "Official headline statements checked; these are assessed projections and physical requirements, not estimates from this page's annual data.",
      "rights": "Attributed factual paraphrase; report text and graphics are not redistributed."
    }
  ],
  "records": [
    {
      "id": "carbon-memory",
      "status": "Model experiment",
      "period": "PD100 experiment; 1000-year horizon",
      "heading": "Carbon has more than one clock.",
      "signal": "25 ± 9%",
      "label": "of an idealized pulse remains after 1,000 years",
      "finding": "Joos and colleagues compared models given a 100 GtC pulse on a constant 389 ppm CO2 background. Uptake is relatively rapid during the first decades, but the atmospheric response has a millennium-scale tail. Their reported atmospheric remainder after 1,000 years is 25 ± 9%.",
      "method": "This is the published PD100 experiment, not a pathway calculated on this page. Figure 1 gives an equally weighted multi-model mean and a plus/minus two-standard-deviation range. The contributing model count changes with the time horizon.",
      "limitation": "The pulse is measured in GtC, not GtCO2. Its response depends on pulse size and background conditions. The reported model spread is not an observational confidence interval for the annual airborne fraction above.",
      "sourceId": "JOOS-2013"
    },
    {
      "id": "carbon-sink-effectiveness",
      "status": "Assessed projection",
      "period": "AR6 increasing-emissions scenarios",
      "heading": "Historical uptake is not a fixed allowance.",
      "signal": "No fixed discount",
      "label": "natural uptake responds to changing conditions",
      "finding": "AR6 assesses that, under scenarios with increasing CO2 emissions, land and ocean sinks become less effective at slowing atmospheric accumulation. A historical retained share cannot safely be carried forward as an unchanging rule.",
      "method": "The assessment draws on carbon-cycle understanding and scenario experiments. It is separate from this page's descriptive ratio of summed atmospheric growth to summed emissions.",
      "limitation": "Lower effectiveness does not mean that every sink must immediately absorb fewer tonnes in absolute terms. Nor does one high annual airborne fraction identify permanent sink weakening.",
      "sourceId": "IPCC-AR6-WGI-SPM"
    },
    {
      "id": "carbon-net-zero",
      "status": "Assessed physical requirement",
      "period": "AR6 physical-science assessment, 2021",
      "heading": "The endpoint is different from the turning point.",
      "signal": "At least net zero CO2",
      "label": "required to limit human-induced warming to a specific level",
      "finding": "AR6 states that limiting human-induced warming to a specified level requires limiting cumulative CO2 emissions and reaching at least net zero CO2, alongside strong reductions in other greenhouse gases. A decline in a still-positive emissions flow is a step along that path, not evidence that the endpoint has been reached.",
      "method": "This is the IPCC's physical-science assessment, not an attribution result from the 45-year account. The naturally responding land and ocean sinks in the chart are not, by themselves, evidence that anthropogenic emissions have reached net zero.",
      "limitation": "A carbon account, a concentration target and a warming target are different statements. This page neither estimates a remaining carbon budget nor simulates temperatures after emissions cease.",
      "sourceId": "IPCC-AR6-WGI-SPM"
    }
  ],
  "sourceSha256": "c930d06efb966b228f23337fc5982f93d9f9e5435f1ca93b344ff4c40d8397dc",
  "checksumConvention": "SHA-256 of the reviewed UTF-8 extract with LF line endings."
}