diff --git a/engine/src/modules/climate/climate.gd b/engine/src/modules/climate/climate.gd
index 49027bd8..bcce6153 100644
--- a/engine/src/modules/climate/climate.gd
+++ b/engine/src/modules/climate/climate.gd
@@ -30,6 +30,7 @@ func process_turn(game_map: RefCounted, turn: int = 0, seed: int = 42) -> void:
 	_ensure_ocean_dist(game_map)
 
 	_collect_magic_forcing(game_map)
+	_apply_orbital_forcing(game_map, turn)
 	_apply_aerosol_forcing(game_map)
 	_update_temperatures(game_map)
 	_update_lake_thermal_effects(game_map)
@@ -38,7 +39,7 @@ func process_turn(game_map: RefCounted, turn: int = 0, seed: int = 42) -> void:
 	_update_lake_evaporation(game_map)
 	_update_deep_earth_water(game_map)
 	_update_precipitation(game_map)
-	# Quality evolution removed — now owned by ecosystem.gd
+	_check_terrain_evolution(game_map)
 	_tick_ley_residue(game_map)
 	EcologicalEventsScript.process_events(
 		game_map, turn, seed, _spec, DataLoader.get_ecological_events()
@@ -58,6 +59,36 @@ func _collect_magic_forcing(_game_map: RefCounted) -> void:
 	pass
 
 
+# -- Step 1a: Orbital forcing (Milankovitch-like cycles) --
+
+
+func _apply_orbital_forcing(game_map: RefCounted, turn: int) -> void:
+	## Superimpose deterministic orbital cycles onto the heat budget.
+	## Three overlapping sinusoidal cycles model obliquity, precession,
+	## and eccentricity at game timescale (1 turn = 10 years).
+	var t: float = float(turn)
+	var total_delta: float = 0.0
+	for i: int in range(1, 4):
+		var prefix: String = "orbital_cycle_%d_" % i
+		var period: float = _params.get(prefix + "period", 0.0)
+		if period <= 0.0:
+			continue
+		var amplitude: float = _params.get(prefix + "amplitude", 0.0)
+		var phase: float = _params.get(prefix + "phase", 0.0)
+		total_delta += amplitude * sin(TAU * (t / period) + phase)
+
+	if absf(total_delta) < 0.0001:
+		return
+
+	# Warm cycles increase evaporation → more moisture. The coupling factor
+	# controls how strongly temperature oscillation drives moisture oscillation.
+	var moisture_coupling: float = _params.get("orbital_moisture_coupling", 0.5)
+
+	for axial: Vector2i in game_map.tiles:
+		game_map.tiles[axial].magic_heat_delta += total_delta
+		game_map.tiles[axial].magic_moisture_delta += total_delta * moisture_coupling
+
+
 # -- Step 1b: Aerosol forcing (persistent stratospheric sulfate from volcanic/impact events) --
 
 
diff --git a/engine/src/modules/climate/climate_base.gd b/engine/src/modules/climate/climate_base.gd
index c20731db..b0b4dddd 100644
--- a/engine/src/modules/climate/climate_base.gd
+++ b/engine/src/modules/climate/climate_base.gd
@@ -220,6 +220,37 @@ func _update_moisture_wind(game_map: RefCounted) -> void:
 # -- Step 8: Terrain quality evolution --
 
 
+func _check_terrain_evolution(game_map: RefCounted) -> void:
+	var up_thresh: int = int(_params.get("quality_up_threshold", _DEFAULTS["quality_up_threshold"]))
+	var down_thresh: int = int(_params.get("quality_down_threshold", _DEFAULTS["quality_down_threshold"]))
+
+	for axial: Vector2i in game_map.tiles:
+		var tile: Variant = game_map.tiles[axial]
+		var tid: String = tile.biome_id
+
+		if tile.is_natural_wonder:
+			continue
+		if tid == "ocean" or tid == "coast" or tid == "lake" or tid == "volcano":
+			continue
+
+		var ideal: String = _ideal_terrain(tile)
+
+		if ideal == tid:
+			tile.quality_progress += 1
+			if tile.quality_progress >= up_thresh:
+				tile.quality_progress = 0
+				if tile.quality < 5:
+					tile.quality += 1
+		else:
+			tile.quality_progress -= 1
+			if tile.quality_progress <= -down_thresh:
+				tile.quality_progress = 0
+				if tile.quality > 1:
+					tile.quality -= 1
+				else:
+					tile.biome_id = ideal
+					tile.quality = 1
+					tile.quality_progress = 0
 
 
 # -- Internal helpers --