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  "schemaVersion": 1,
  "kind": "dated-modern-sea-ice-extent-reference",
  "title": "NSIDC monthly sea-ice extent · September 2026",
  "observationMonth": "2026-09",
  "date": "2026-09",
  "period": "2026-09",
  "displayDate": "September 2026",
  "dataset": {
    "name": "NSIDC Sea Ice Index, Version 4 (G02135)",
    "url": "https://nsidc.org/data/g02135/versions/4",
    "doi": "https://doi.org/10.7265/a98x-0f50"
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  "scope": "Dated modern monthly sea-ice extent at or above 15% concentration, separate from land ice and from the book impact illustration.",
  "poleHoleConvention": "The NSIDC Arctic extent product includes assumed ice above 15% concentration in the unobserved satellite pole hole; source geometry is preserved.",
  "sourceZIPs": [
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  "isClimatology": false,
  "isBookImpactEvidence": false,
  "source": {
    "dataset": "Sea Ice Index, Version 4",
    "datasetId": "G02135",
    "doi": "https://doi.org/10.7265/a98x-0f50",
    "landingPage": "https://nsidc.org/data/g02135/versions/4",
    "userGuide": "https://nsidc.org/sites/default/files/documents/user-guide/g02135-v004-userguide.pdf",
    "citation": "Fetterer, F., Knowles, K., Meier, W. N., Savoie, M., Windnagel, A. K. & Stafford, T. (2025). Sea Ice Index (G02135, Version 4). NSIDC. doi:10.7265/a98x-0f50. September 2026 monthly extent polygons, both hemispheres.",
    "attribution": "NOAA at NSIDC / National Snow and Ice Data Center; Sea Ice Index v4 (AMSR2-era product).",
    "usePolicy": "Publicly available for reuse with dataset citation and subset/date acknowledgement; no Creative Commons license inferred.",
    "usePolicyURL": "https://nsidc.org/about/data-use-and-copyright",
    "retrievedUTC": "2026-11-01T21:06:16.398367+00:00",
    "nominalSourceResolutionKm": 25
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  "selection": {
    "reason": "Latest completed monthly polygon files found in both hemispheres in this bounded acquisition; October 2026 files were not present.",
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    "height": 721,
    "west": -180,
    "east": 180,
    "north": 90,
    "south": -90,
    "longitudeStep": 0.25,
    "latitudeStep": 0.25,
    "registration": "node",
    "rowOrder": "north-to-south",
    "columnOrder": "west-to-east",
    "horizontalCoordinates": "Geographic longitude/latitude degrees; PROJ evaluation from EPSG:4326 into each source CRS.",
    "channels": [
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  "definition": {
    "extentThresholdConcentrationPercent": 15,
    "maskValues": "0=outside extent;255=inside source extent polygon. This is not sea-ice concentration or thickness.",
    "separateFromLandIce": "Greenland and Antarctica land ice come from the existing separate land-ice references. Source polygon holes are preserved.",
    "poleHole": "NSIDC monthly Arctic extent assumes the satellite pole-hole region is ice-covered above 15% concentration. The source polygon convention is preserved; it is not direct observation of every polar point."
  },
  "derivation": {
    "method": "Unmodified source polygon union; point membership at each global geographic grid node, transformed using each embedded .prj CRS. Polygon holes preserved; no geometric buffers or circular cap added.",
    "boundaryRule": "Source polygon covers/intersects the sample point, including boundaries.",
    "closure": "Each pole repeats its source longitude0 sample; eastern duplicate repeats western sample.",
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    "A dated September 2026 monthly snapshot, not live ice conditions and not a year-round ice cap. Arctic and Antarctic sea ice vary seasonally.",
    "Extent means at least 15% concentration; rendering every included cell solid white would overstate complete ice cover. It supplies no thickness, mass, age or ice-sheet elevation.",
    "The nominal 25 km source geometry is resampled to a 0.25 degree node mask for display. Small features and coastal detail are limited; no higher source accuracy is implied.",
    "PROJ reports an unspecified-accuracy ballpark WGS84-to-Hughes 1980 datum operation; the legacy source projection is preserved in metadata rather than claiming a precise modern datum shift.",
    "The Arctic pole-hole convention includes assumed coverage as documented by NSIDC. It is not a newly invented circular cap.",
    "This modern observational reference is independent of the book narrative and does not demonstrate cometary origin or reconstruct ancient sea ice."
  ],
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