TerraKeepKeeping Earth within its limits
Cloud-free satellite view around Arctic sea ice, with the finding area outlined
75.00°N 0.00°ENASA Blue Marble via GIBS · outline = finding area
Published

Arctic sea ice at 5.169 M km² — below the 1981–2010 bottom tenth for this date

Arctic sea ice · seen · published

On 25 September 2026, Arctic sea-ice extent measured 5.169 million km². For that day of the year in 1981–2010, nine years in ten had more ice: the 10th-percentile line sits at 5.329 M km², so the reading is 0.16 M km² below it and 1.371 M km² below the 1981–2010 average of 6.54 M km². A later day confirms it is not a one-day glitch: 26 September came in at 5.164 M km² against a p10 of 5.345 M km², 0.181 M km² short.

What this is not: extent is not volume. It counts any ocean cell at least 15% covered, so thin new ice weighs the same as thick multi-year ice, and this number says nothing about how much ice is actually there. Passive-microwave sensors can misread melt ponds and coastal cells. The comparison line is fixed to 1981–2010, so in a warming world it is crossed more often and a crossing is not itself news of an extreme. Context: ENSO is in an El Niño phase (ONI 1.8), which is recorded here, not proven as a cause.

What would change our mind: a revision of the NSIDC daily series, a sensor or coastal-mask problem, or extent climbing back above the p10 line and staying there.

Written by AI, then checked by a separate reviewer before publication. Every claim points at the evidence below.

What we saw

  • nsidc-g02135-v4 — Arctic daily extent 5.169 M km² on 2026-09-25; 1981–2010 p10 for day 268 = 5.329 M km², average 6.54 M km². (Sep 25, 2026) See the source ↗
  • nsidc-g02135-v4 — Arctic daily extent 5.164 M km² on 2026-09-26; 1981–2010 p10 for day 269 = 5.345 M km², average 6.583 M km². (Sep 26, 2026) See the source ↗
  • nsidc-g02135-v4 — Arctic daily extent 5.228 M km² on 2026-09-27; 1981–2010 p10 for day 270 = 5.417 M km², average 6.635 M km². (Sep 27, 2026) See the source ↗
  • nsidc-g02135-v4 — Arctic daily extent 5.333 M km² on 2026-09-28; 1981–2010 p10 for day 271 = 5.435 M km², average 6.691 M km². (Sep 28, 2026) See the source ↗
  • nsidc-g02135-v4 — Arctic extent still below p10 on a later day: 5.164 M km² vs p10 5.345 M km² (2026-09-26). (Sep 26, 2026) See the source ↗

What it might not be

  • Extent measures area with ≥15% ice cover — not thickness or volume.
  • Passive microwave can misread melt ponds and coastal cells.
  • The p10 line is fixed to 1981–2010, so it is crossed more often as the world warms.
  • Confirmation is a later day from the same NSIDC series (a revisit), not an independent sensor.
  • Daily values are a 5-day trailing mean and may be revised.
  • El Niño (ONI 1.8) is context only; no causal link is claimed.
  • Sea ice: extent is area with ≥ 15% ice, not volume or thickness — thin new ice counts the same as old multi-year ice.
  • Sea ice: passive microwave misreads melt ponds and coastal cells; the p10 line is 1981–2010, so a warming world crosses it more often.
  • All: global zero-key feeds can go stale or be revised; a threshold crossing is a signal to look, not a verdict on cause or impact.

What would change our mind

Newer satellite images that show no change, a correction from the data source, or a reply with evidence from the ground. If that happens, the case is marked “Turned out wrong” — in public — and it stays on this site.

Why this was published

Two separate sources agreed — nsidc-g02135-v4, then nsidc-g02135-v4 — and a reviewer checked the evidence before it went up.

Replies

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Ledger id 01a0e1ef-d7fe-7071-b4b7-6d463f02d02d

Technical details rule, evidence ids, history, proof
Rule
indicator_threshold v1.0
Tier
T1
Tags
sea-ice, polar
Confirmed
Yes — independent revisit signal, 2026-09-27
Rule FP rate
11 % · 1 of 9 decided
Location
75.00°N 0.00°E
Observed
2026-09-25
Opened
2026-09-27 by automatic rule indicator_threshold v1.0
  1. candidate
  2. confirmed
  3. published
  4. resolved

Evidence5

seriesnsidc-g02135-v4

Arctic daily extent 5.169 M km² on 2026-09-25; 1981–2010 p10 for day 268 = 5.329 M km², average 6.54 M km².

anomaly_mkm2 -1.371below_p10_mkm2 0.16extent_mkm2 5.169p10_mkm2 5.329
· indicator_threshold v1.0 · nsidc-north-extent-2026-09-25
Source ↗
seriesnsidc-g02135-v4

Arctic daily extent 5.164 M km² on 2026-09-26; 1981–2010 p10 for day 269 = 5.345 M km², average 6.583 M km².

anomaly_mkm2 -1.419below_p10_mkm2 0.181extent_mkm2 5.164p10_mkm2 5.345
· indicator_threshold v1.0 · nsidc-north-extent-2026-09-26
Source ↗
seriesnsidc-g02135-v4

Arctic daily extent 5.228 M km² on 2026-09-27; 1981–2010 p10 for day 270 = 5.417 M km², average 6.635 M km².

anomaly_mkm2 -1.407below_p10_mkm2 0.189extent_mkm2 5.228p10_mkm2 5.417
· indicator_threshold v1.0 · nsidc-north-extent-2026-09-27
Source ↗
seriesnsidc-g02135-v4

Arctic daily extent 5.333 M km² on 2026-09-28; 1981–2010 p10 for day 271 = 5.435 M km², average 6.691 M km².

anomaly_mkm2 -1.358below_p10_mkm2 0.102extent_mkm2 5.333p10_mkm2 5.435
· indicator_threshold v1.0 · nsidc-north-extent-2026-09-28
Source ↗
seriesnsidc-g02135-v4Confirming signal

Arctic extent still below p10 on a later day: 5.164 M km² vs p10 5.345 M km² (2026-09-26).

below_p10_mkm2 0.181extent_mkm2 5.164p10_mkm2 5.345
· indicator_threshold v1.0 · nsidc-north-extent-2026-09-26-persist
Source ↗

Context

ENSO El Niño · ONI +1.8
  • Dataset: NSIDC Sea Ice Index v4 (G02135), daily extent vs the 1981–2010 day-of-year climatology (5-day trailing mean, passive microwave). Window: 2026-09-25.

Publication gates

  • Evidence — 4 pieces from nsidc-g02135-v4 — met
  • Independent confirmation — a different revisit (nsidc-g02135-v4), 2026-09-27 — met
  • Human approval — 1 of 1 needed at tier 1 — met
  • Narration — AI-drafted by claude-opus-5 — met
Published
2026-09-30 by AI model
Narrated By
AI model
Reviewed By
AI model
Policy
2026-09-26-autonomous
Note
autonomous publish: narrated by model:claude-opus-5, reviewed by model:claude-sonnet-5 (policy 2026-09-26-autonomous)

History8

  1. status changed → publishedAI model
  2. reviewed → confirmedAI model
  3. narrated → confirmedAI model
  4. evidence added → confirmedautomatic rule indicator_threshold v1.0
  5. evidence added → confirmedautomatic rule indicator_threshold v1.0
  6. evidence added → confirmedautomatic rule indicator_threshold v1.0
  7. confirmed → confirmedautomatic rule indicator_threshold v1.0
  8. created → candidateautomatic rule indicator_threshold v1.0

Verify

Checkpoint
earthdeck.dev/findings/v1 · 971 entries
Signed root
65becba35e7b…cfb1ac
This case
8 signed events
Leaf
#967 of 971 — this case’s latest event
Leaf hash
530f8db39541…64ea3e
Proof
8-hash audit path
  1. 6ad54db39ebed92a9a5f1f729c1d65db20d0a95c0866fe729a57ff7d9aabcae5
  2. ca6ee0407a8f0248ceca855319c34f7e0e75ed44e167e7dccb528404c8b858b9
  3. dfed8ea5f9a2938ff735b488850f0af7e829587f069073051c5e18396ed15c92
  4. f03d09f61778113da25d1abb1fd48d266ab0d3a8ba35563ff54eb6dd55459da7
  5. 86d57304ddba91cb44fb33ac9e4294873940805fa57f40f04b5b131c5eec4764
  6. d4a69eb9bd4aba2b04a0c06f8365d5c370a5591fa80d0e8a9962a94eea222b01
  7. b151de5eaefddf3d1cfc86f94cdbc3995ed5d406a6c0acfc99649a5d48d50794
  8. 5f7580b7f5a7f09ff7675565fb36c19876079a6c101f721419a5a07a7a23b1c1
Last event
a8499d2d3da8…ae16dc
Note
Checkpoint text
earthdeck.dev/findings/v1
971
Zb7Lo157ettQqPmu0tm88BnWSkMC3ZJMEFFRbmvPsaw=

— earthdeck TkCXvy4ab1W8aoaOb+pzCGCDnG5hglritooG+HSh5E2eK5bX/GOa8ngRawRBFFq9xkoAkQHY2WV8CTtkhuUX2a2EhA8=

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