Grand Line Four-Island-Type Climate System

Structural lore — what Oda has confirmed about this. Mysteries (unsolved questions) live in Vault/Threads/; this is what the world IS.

What we know

(Bullet-list of established canonical facts. Each fact cites the chapter where it was confirmed. No speculation in this section.)

• Ch. 139: Per-island endemic-fauna axis canonized. Kureha: “They live in hot, steaming jungles.” / “Were you traipsing around in a jungle on some prehistoric island with your belly exposed?” (pp. 13-14) — explicitly contrasts Drum (snow-island, an illness eradicated 100 years ago) with Little Garden (jungle-island, where the Keschia ticks still live). Confirms the Grand Line’s per-island-type biome system has island-specific endemic fauna including extinct species preserved by isolation; reinforces the each island is a closed ecosystem register. See Keschia Five-Day-Sickness Insect Bite Hot Jungles Eradicated-100-Years-Ago.
• Ch. 132: The Grand Line’s islands divide into four climate types — summer islands, spring islands, autumn islands, winter islands (Vivi, pp. 6-7).
• Ch. 132: Each island, regardless of its baseline type, still cycles through four sub-seasons of its own — so sailing a four-island cluster passes through 16 distinct seasonal states in total (Vivi, p. 7).
• Ch. 132: This climate-mosaic is the structural reason inter-island Grand Line seas are so violent — “It’s no wonder the seas between them are so turbulent” (Vivi, p. 7).
• Ch. 132: A practical navigation corollary — stable weather means there is an island nearby (Vivi, pp. 7-8); confirmed immediately by the sighting of Drum Island.
• Ch. 132: Exceptions and unknown weather patterns exist on top of the four-type model (“of course there are exceptions to this, as well as unknown weather patterns”, p. 7).

Chronology / structure

The four island-types — summer, spring, autumn, winter — each hold a baseline climate but still cycle four sub-seasons of their own. A traverse of one four-island cluster therefore passes through 16 seasonal states (summer-on- summer-island → … → winter-on-winter-island). The model complements the seven-magnetic-routes system as the second structural reason Grand Line islands are isolated and the inter-island seas are chaotic.

Open questions about this topic

(One-line summary of each unresolved question. Each links to a Threads/ entry where the mystery is tracked.)

• Grand Line Island Isolation Doctrine — how the climate-mosaic and the magnetic-route system together isolate Grand Line islands.