diff --git a/src/simulator/crates/mc-turn/Cargo.toml b/src/simulator/crates/mc-turn/Cargo.toml
index 31f47772..63f05994 100644
--- a/src/simulator/crates/mc-turn/Cargo.toml
+++ b/src/simulator/crates/mc-turn/Cargo.toml
@@ -19,6 +19,7 @@ mc-combat      = { path = "../mc-combat" }
 mc-economy     = { path = "../mc-economy" }
 mc-civics      = { path = "../mc-civics" }
 mc-trade       = { path = "../mc-trade" }
+mc-climate     = { path = "../mc-climate" }
 mc-tech        = { path = "../mc-tech" }
 mc-replay      = { path = "../mc-replay" }
 mc-comms       = { path = "../mc-comms" }
diff --git a/src/simulator/crates/mc-turn/src/processor.rs b/src/simulator/crates/mc-turn/src/processor.rs
index b705a698..d90576eb 100644
--- a/src/simulator/crates/mc-turn/src/processor.rs
+++ b/src/simulator/crates/mc-turn/src/processor.rs
@@ -491,6 +491,13 @@ impl TurnProcessor {
         // when a grid with lairs exists.
         self.process_fauna_encounters_inner(state, &mut result, true);
 
+        // Phase 5b: climate runtime (p3-26 gap 1). Tick the climate physics once per
+        // round so temperature / aerosol / precipitation evolve on the live grid in the
+        // headless sim — previously climate ran only in the GDScript live game. The grid
+        // carries the climate state across turns; default config matches the ecology
+        // bench (`ClimatePhysics::new("{}","[]","{}")`).
+        self.process_climate_phase(state);
+
         // Phase 5a-sentry: wake sentrying units that have enemies in vision range (2 hex).
         // Runs after movement so positions are current; runs before PvP so the
         // now-awoken unit's state is consistent when combat checks fire.
@@ -1007,6 +1014,21 @@ impl TurnProcessor {
     /// claim tiles — returning `Vec<usize>` from the pool is dropped here.
     /// The live game calls `mc_culture::CulturePool` via `GdCulture` and reacts
     /// to the ready list on the GDScript side.
+    /// p3-26 gap 1: tick the climate physics once per round on the live grid so
+    /// temperature / aerosol / precipitation evolve in the headless sim. The
+    /// physics operates on (and persists state in) `GridState`; a fresh
+    /// `ClimatePhysics` per round is fine — it is the operator, the grid is the
+    /// state. Default config (`"{}"`/`"[]"`/`"{}"`) matches the ecology bench;
+    /// `dt = 1.0` = one turn. No-op when there is no grid (pure unit-test states).
+    fn process_climate_phase(&self, state: &mut GameState) {
+        let turn = state.turn;
+        let seed = state.map_seed;
+        if let Some(grid) = state.grid.as_mut() {
+            let mut climate = mc_climate::physics::ClimatePhysics::new("{}", "[]", "{}");
+            climate.process_step(grid, turn, seed, 1.0);
+        }
+    }
+
     fn process_culture(&self, state: &mut GameState, pi: usize) {
         // Grid dims read before the &mut player borrow (needed for in-bounds
         // candidate filtering during border expansion).
@@ -5796,6 +5818,42 @@ mod tests {
         );
     }
 
+    #[test]
+    fn climate_phase_ticks_grid_deterministically() {
+        // p3-26 gap 1: the climate physics ticks the live grid each round, and is
+        // deterministic for a given (start, turn, seed). No-op without a grid.
+        use mc_core::grid::GridState;
+        let build = || {
+            let mut state = GameState::default();
+            state.turn = 3;
+            state.map_seed = 99;
+            let mut grid = GridState::new(8, 8);
+            for t in &mut grid.tiles {
+                t.biome_label_id = "temperate_grassland".into();
+                t.temperature = 0.5;
+                t.quality = 3;
+            }
+            state.grid = Some(grid);
+            state
+        };
+        let processor = TurnProcessor::new(100);
+
+        let mut a = build();
+        processor.process_climate_phase(&mut a);
+        let after_a: Vec<f32> = a.grid.as_ref().unwrap().tiles.iter().map(|t| t.temperature).collect();
+
+        let mut b = build();
+        processor.process_climate_phase(&mut b);
+        let after_b: Vec<f32> = b.grid.as_ref().unwrap().tiles.iter().map(|t| t.temperature).collect();
+
+        assert_eq!(after_a, after_b, "climate tick must be deterministic for same seed+turn");
+        assert_eq!(after_a.len(), 64, "all tiles processed");
+
+        // No grid → no panic.
+        let mut empty = GameState::default();
+        processor.process_climate_phase(&mut empty);
+    }
+
     #[test]
     fn processor_is_deterministic() {
         use mc_core::grid::GridState;