# Source snapshot — arctic-sea-ice story

Retrieved **8 November 2026** by [`fetch.mjs`](fetch.mjs) (from the download folder: `node arctic-sea-ice-src/fetch.mjs`; no dependencies beyond Node and the system `unzip`). The raw files are archived here unchanged. [`calculate.mjs`](calculate.mjs) checks their SHA-256 against the values below, then writes `annual-extremes.csv` and every number quoted in the post to `statistics.json`; `../arctic-sea-ice-make-charts.mjs` builds the desktop and `-mobile.svg` charts from the same files. The daily file is updated every day upstream, so a re-fetch changes it; update the hashes in `calculate.mjs` and here together, deliberately.

## NSIDC Sea Ice Index, Version 4

- **Publisher:** National Snow and Ice Data Center (NSIDC), University of Colorado Boulder, distributed for NOAA (NOAA@NSIDC). Landing page: <https://nsidc.org/data/g02135/versions/4>.
- **Citation requested by NSIDC** (as a condition of use): Fetterer, F., Knowles, K., Meier, W. N., Savoie, M., Windnagel, A. K. & Stafford, T. (2025). *Sea Ice Index* (G02135, Version 4). [Data Set]. Boulder, Colorado USA. National Snow and Ice Data Center. <https://doi.org/10.7265/a98x-0f50>. Subset used: Northern Hemisphere daily extent, 1981–2010 daily climatology, September and March monthly extent, and the min/max rankings workbook. Date accessed: 8 November 2026.
- **Figure credit requested by NSIDC** (rankings workbook, Documentation sheet): "Credit: Sea Ice Index, National Snow and Ice Data Center."
- **Licence / terms:** NSIDC requires citation; no explicit public-domain dedication was verified. The data is NOAA-funded and distributed by NSIDC without access restrictions. NSIDC's [data use page](https://nsidc.org/about/data-use-and-copyright) states: "As a condition of use, NSIDC requests that authors include a data citation in the reference section of your publication"; the landing page says "As a condition of using these data, you must cite the use of this data set." The Sea Ice Index landing page names no specific licence. The citation and figure credit above are given in the post (each chart caption carries the credit).

| File | URL | Bytes | Upstream Last-Modified | SHA-256 |
|------|-----|-------|------------------------|---------|
| `N_seaice_extent_daily_v4.0.csv` | <https://noaadata.apps.nsidc.org/NOAA/G02135/north/daily/data/N_seaice_extent_daily_v4.0.csv> | 1,912,183 | Sat, 07 Nov 2026 11:24:58 GMT | `0b3b93d037a0737ea944ee5389c8a8e129e7b52ecf8e748ba4799baae54bbb1f` |
| `N_seaice_extent_climatology_1981-2010_v4.0.csv` | <https://noaadata.apps.nsidc.org/NOAA/G02135/north/daily/data/N_seaice_extent_climatology_1981-2010_v4.0.csv> | 34,520 | Mon, 02 Nov 2026 14:40:56 GMT | `3eba40748343468e81e5fd5c464be2046ab12bc818963c324098251390212942` |
| `N_09_extent_v4.0.csv` | <https://noaadata.apps.nsidc.org/NOAA/G02135/north/monthly/data/N_09_extent_v4.0.csv> | 2,352 | Mon, 02 Nov 2026 14:40:56 GMT | `3968dfdbe73b8361712da9136cfeff6dbc9a6d87ab3e75daa30cc254615eab1f` |
| `N_03_extent_v4.0.csv` | <https://noaadata.apps.nsidc.org/NOAA/G02135/north/monthly/data/N_03_extent_v4.0.csv> | 2,352 | Mon, 02 Nov 2026 14:40:56 GMT | `01c4fa25d1a4a35aee9558eeecd652564fc91582e35e7b2fcaf8c19ebf56aff7` |
| `Sea_Ice_Index_Min_Max_Rankings_G02135_v4.0.xlsx` | <https://noaadata.apps.nsidc.org/NOAA/G02135/seaice_analysis/Sea_Ice_Index_Min_Max_Rankings_G02135_v4.0.xlsx> | 86,153 | Sat, 07 Nov 2026 11:25:50 GMT | `f1bacdc103d531146c7e1373d3c04e53eca2997ae642ddfdf3c0755fdecb8642` |

The daily file's last row is 6 November 2026. Its first two lines are a header and a units line; extents are in 10^6 km², three decimals. Rows from 2025 on come from AMSR2 (NSIDC-0803) input, earlier rows from NSIDC-0051 (SMMR, SSM/I, SSMIS); see DATA.md.

## Derived files (written by scripts in this folder)

| File | Written by | Contents | SHA-256 |
|------|-----------|----------|---------|
| `nsidc-rankings-nh-5day.csv` | `fetch.mjs` | Sheet "NH-Annual-5-Day-Extent" of the rankings workbook, 1979–2025: `year, min_5day, min_rank, min_date, max_5day, max_rank, max_date` (values as published) | `fe6f5f68440cb933c27784635d64becd09e2453ce1467b0a77f4d70a7cb57f57` |
| `annual-extremes.csv` | `calculate.mjs` | 1979–2026: `year, min_extent_mkm2, min_date, max_extent_mkm2, max_date` (5-day trailing mean, 3 decimals) | `cd1fc99ac639c7b1e7c6afd691f25b7b90e38d1b79589eb544cb94d438fcd0bb` |
| `statistics.json` | `calculate.mjs` | Every number in the post, plus cross-checks | (regenerated; byte-identical across runs) |

## Context documents cited, not snapshotted

- NSIDC, "User Notice: Sea Ice Index transitioning to Version 4 with AMSR2 data", 23 July 2025, <https://nsidc.org/data/user-resources/data-announcements/user-notice-sea-ice-index-transitioning-version-4-amsr2-data>.
- NSIDC, "Arctic sea ice has reached minimum extent for 2026", <https://nsidc.org/news-analyses/news-stories/arctic-sea-ice-has-reached-minimum-extent-2026-antarctic-sea-ice-maximum-most-likely-reached-well> (4.60 million km² on 12 September; tenth lowest, tied with 2008, 2010 and 2025).
- NSIDC, "Arctic sea ice record low maximum strikes again", <https://nsidc.org/news-analyses/news-stories/arctic-sea-ice-record-low-maximum-strikes-again> (14.29 million km² on 15 March; statistically tied with 2025's 14.31; values within 40,000 km² are considered tied).
- NSIDC Special Report 28, "Sea Ice Index Version 4 Analysis", July 2025, <https://nsidc.org/sites/default/files/documents/technical-reference/nsidc-special-report-28.pdf>.
- NASA Earth Observatory, "Arctic Sea Ice Shrinks to Its 2026 Minimum", <https://science.nasa.gov/earth/earth-observatory/arctic-sea-ice-shrinks-to-its-2026-minimum/> (minimum on 12 September, about 4.6 million km², tied for 10th with 2008, 2010 and 2025; each of the past 20 summers among the 20 lowest).
- Met Office, "Briefing on Arctic sea ice minimum and Antarctic sea ice maximum report – September 2026", <https://www.metoffice.gov.uk/research/approach/monitoring/sea-ice/2026/briefing-on-arctic-sea-ice-minimum-and-antarctic-sea-ice-maximum-report---september-2026> (OSI SAF: 5.06 million km² on 12 September, 14th lowest since 1979; NSIDC: 11th lowest).
- England, M. R., Polvani, L. M., Screen, J. & Chan, A. C. (2025), "Minimal Arctic Sea Ice Loss in the Last 20 Years, Consistent With Internal Climate Variability", *Geophysical Research Letters* 52, e2025GL116175, <https://doi.org/10.1029/2025GL116175> (CC BY).