diff --git a/tools/transpile-engine/transpile.py b/tools/transpile-engine/transpile.py
index 5df57350..11da0be9 100644
--- a/tools/transpile-engine/transpile.py
+++ b/tools/transpile-engine/transpile.py
@@ -91,7 +91,7 @@ CLIMATE_DEFAULTS: dict[str, float] = {
     "lake_thermal_conductivity": 0.05,
     "river_moisture_transport": 0.075,
     "mountain_rain_shadow_block": 0.9,
-    "solar_min": 0.15,
+    "solar_min": 0.05,
     "solar_max": 0.70,
 }
 
@@ -145,7 +145,7 @@ const CLIMATE_DEFAULTS: Record<string, number> = {
   lake_thermal_conductivity: 0.05,
   river_moisture_transport: 0.075,
   mountain_rain_shadow_block: 0.9,
-  solar_min: 0.15,
+  solar_min: 0.05,
   solar_max: 0.70,
 }
 
@@ -714,6 +714,19 @@ def _emit_aerosol_forcing() -> str:
     const aerosol_cfg = ((this.spec['aerosol'] ?? {}) as Record<string, number>)
     if (Object.keys(aerosol_cfg).length === 0) return
     const { tiles, width: w, height: h } = grid
+
+    // Phase 0: Natural aerosol sources — background, desert dust, volcanic outgassing
+    const bg = this.p('aerosol_background', 0.002)
+    const dustRate = this.p('aerosol_desert_dust', 0.005)
+    const volcanicRate = this.p('aerosol_volcanic_outgas', 0.015)
+    for (let i = 0; i < tiles.length; i++) {
+      const tile = tiles[i]
+      let inject = bg
+      if (tile.biome_id === 'desert') inject += dustRate
+      else if (tile.biome_id === 'volcano') inject += volcanicRate
+      ;(tile as any).sulfate_aerosol = Math.max(0.0, ((tile as any).sulfate_aerosol ?? 0) + inject)
+    }
+
     let any_aerosol = false
     for (let i = 0; i < tiles.length; i++) {
       if (((tiles[i] as any).sulfate_aerosol ?? 0) > 0.001) { any_aerosol = true; break }
@@ -930,7 +943,7 @@ def assemble(fns: dict[str, dict[str, str]]) -> str:
     # temperature band is controlled by climate_params.json, not hardcoded.
     result = result.replace(
         "solarByRow(row, h)",
-        "solarByRow(row, h, this.p('solar_min', 0.15), this.p('solar_max', 0.70))",
+        "solarByRow(row, h, this.p('solar_min', 0.05), this.p('solar_max', 0.70))",
     )
     return result