diff --git a/tools/transpile-engine/ecology_assembly.py b/tools/transpile-engine/ecology_assembly.py
index f2238b90..9266d4cb 100644
--- a/tools/transpile-engine/ecology_assembly.py
+++ b/tools/transpile-engine/ecology_assembly.py
@@ -135,9 +135,9 @@ def _classifier() -> str:
 
 function classifyBiome(tile: TileState): string {
   const sub = tile.substrate_id
-  // Aquatic tiles keep their terrain_id
+  // Aquatic tiles keep their biome_id
   if (sub === 'deep_water' || sub === 'shallow_water' || sub === 'lake_bed') {
-    return tile.terrain_id
+    return tile.biome_id
   }
 
   const temp = tile.temperature
@@ -182,7 +182,7 @@ function isWater(tile: TileState): boolean {
   if (sub) {
     return sub === 'deep_water' || sub === 'shallow_water' || sub === 'lake_bed'
   }
-  return tile.terrain_id === 'ocean' || tile.terrain_id === 'coast'
+  return tile.biome_id === 'ocean' || tile.biome_id === 'coast'
 }
 
 function climateMatch(tile: TileState, biome: BiomeDef): number {
diff --git a/tools/transpile-engine/mapgen_assembly.py b/tools/transpile-engine/mapgen_assembly.py
index 668dfac8..0d8c5e7a 100644
--- a/tools/transpile-engine/mapgen_assembly.py
+++ b/tools/transpile-engine/mapgen_assembly.py
@@ -233,6 +233,7 @@ class GenMap {
   readonly width: number
   readonly height: number
   readonly tiles: Map<string, GenTile> = new Map()
+  sea_level = 0.0
 
   constructor(width: number, height: number) {
     this.width = width
@@ -300,6 +301,8 @@ class GenMap {
       height: this.height,
       global_avg_temp: 0.5,
       ocean_dead_fraction: 0.0,
+      ecosystem_health: 1.0,
+      sea_level: this.sea_level,
     }
   }
 }
@@ -630,6 +633,7 @@ function assignSeaLevel(
     Math.max(Math.round(oceanTarget * allElevs.length), 0), allElevs.length - 1,
   )
   const seaLevel = allElevs[seaIdx]
+  gm.sea_level = seaLevel
 
   for (const tile of gm.tiles.values()) {
     const elev = elevation.get(axialKey(tile.axial)) ?? 0.0
diff --git a/tools/transpile-engine/transpile.py b/tools/transpile-engine/transpile.py
index afc3dfa6..5df57350 100644
--- a/tools/transpile-engine/transpile.py
+++ b/tools/transpile-engine/transpile.py
@@ -356,6 +356,7 @@ export class ClimatePhysics {
     this.stepDeepEarthWater(grid)
     this.stepPrecipitation(grid)
     this.stepTerrainEvolution(grid)
+    this.stepSeaLevel(grid)
     const events = this.stepEcologicalEvents(grid, turn, seed)
     this.stepAnchorDecay(grid)
     this.stepGlobalStats(grid)
@@ -771,7 +772,31 @@ def _emit_terrain_evolution() -> str:
       const tid = tile.biome_id
 
       if (tile.is_natural_wonder) continue
-      if (tid === 'ocean' || tid === 'coast' || tid === 'lake' || tid === 'volcano') continue
+
+      // Water freezing/thawing — ice forms below freeze threshold, thaws above
+      const freezeTemp = this.p('water_freeze_threshold', 0.12)
+      const thawTemp = freezeTemp + 0.03  // hysteresis prevents oscillation
+      const isWater = tid === 'ocean' || tid === 'coast' || tid === 'lake' || tid === 'inland_sea'
+      if (isWater) {
+        if (tile.temperature < freezeTemp) {
+          tile.original_biome_id = tid
+          tile.biome_id = 'ice'
+          tile.quality = 1
+          tile.quality_progress = 0
+        }
+        continue
+      }
+      if (tid === 'ice') {
+        if (tile.temperature > thawTemp && tile.original_biome_id) {
+          tile.biome_id = tile.original_biome_id
+          tile.original_biome_id = ''
+          tile.quality = 1
+          tile.quality_progress = 0
+        }
+        continue
+      }
+
+      if (tid === 'volcano') continue
 
       const ideal = idealTerrain(tile, this.spec)
 
@@ -800,6 +825,52 @@ def _emit_terrain_evolution() -> str:
 """
 
 
+def _emit_sea_level() -> str:
+    return """\
+  private stepSeaLevel(grid: GridState): void {
+    // Sea level responds to global temperature: warmer → thermal expansion + ice melt → higher.
+    // Each turn, sea_level drifts toward an equilibrium determined by global_avg_temp.
+    // Tiles below sea_level flood to coast/ocean; tiles above expose to land.
+    const sensitivity = this.p('sea_level_temp_sensitivity', 0.0008)
+    const eqTemp = this.p('sea_level_equilibrium_temp', 0.45)
+    const { tiles, width: w, height: h } = grid
+
+    // Adjust sea level: positive when warmer than equilibrium, negative when cooler
+    const tempAnomaly = grid.global_avg_temp - eqTemp
+    grid.sea_level += tempAnomaly * sensitivity
+
+    // Flood low land / expose high water
+    let changed = false
+    for (let i = 0; i < tiles.length; i++) {
+      const tile = tiles[i]
+      const isWater = tile.biome_id === 'ocean' || tile.biome_id === 'coast' ||
+        tile.biome_id === 'lake' || tile.biome_id === 'inland_sea'
+
+      if (!isWater && tile.elevation < grid.sea_level) {
+        // Land tile floods — becomes coast if adjacent to land, ocean otherwise
+        if (tile.is_natural_wonder) continue
+        tile.original_biome_id = tile.biome_id
+        tile.biome_id = 'coast'
+        tile.quality = 1
+        tile.quality_progress = 0
+        changed = true
+      } else if (tile.biome_id === 'coast' && tile.elevation >= grid.sea_level + 0.02) {
+        // Coast tile exposed — reclassify as land based on climate
+        // Small hysteresis buffer (0.02) prevents oscillation at the boundary
+        tile.biome_id = classifyTerrain(tile.temperature, tile.moisture, tile.elevation)
+        tile.quality = 1
+        tile.quality_progress = 0
+        changed = true
+      }
+    }
+
+    // Invalidate ocean distance cache if coastline changed
+    if (changed) this.oceanDistGridId = -1
+  }
+
+"""
+
+
 def _emit_anchor_decay() -> str:
     return """\
   private stepAnchorDecay(grid: GridState): void {
@@ -848,6 +919,7 @@ def assemble(fns: dict[str, dict[str, str]]) -> str:
             parts.append(f"  // NOTE: {gd_name} not found in climate.gd\n")
             parts.append(f"  {visibility} {ts_name}(grid: GridState): void {{ }}\n\n")
 
+    parts.append(_emit_sea_level())
     parts.append(_emit_ecological_events())
     parts.append("\n")
     parts.append(_emit_anchor_decay())