Civ settlements: environment-driven dynamic growth (droughts, harvests, storms)

Settlements were static -- every town grew with one global rate to the same
habitability cap (frozen in Live World), so they grew "the same amount
everywhere". Now stepCivilization(dtHours, liveTime) is environment-driven and
time-varying. All drivers are deterministic functions of (region, year, seed),
so step-back replays them with no new saved/snapshot state (population is
already snapshotted).

- Differentiated growth: rate scales with habitability (fertile boom, marginal
  crawl); capacity K = maxPop * habitability * siteQuality * conditions.
- Site quality: max city size varies by an order of magnitude with location --
  a great river (log-scaled discharge) or coast hosts a metropolis, a dry inland
  cell a town (civSiteVariety). This spreads final sizes ~8-9x instead of all
  saturating equally.
- Conditions (per ~20deg region, per year): multi-year droughts (worse in arid
  regions), year-to-year harvests (bigger swings inland), rare cold years, river
  floods, and the existing volcano ash. Regional, so different parts of a
  continent have different fortunes.
- Storms kill directly: a system within its radius of a town deals deaths scaled
  by strength (hurricanes x civHurricaneDeathMult) -- a parked hurricane can gut
  a coastal city. Reads the already-snapshotted storms().
- Famine: accelerated loss when food < population; sustained drought / acute
  disaster can collapse a settlement to ruins (revives when conditions return).
- Viewer: markers withered-tinted by hardship (3D + 2D), cell-info "drought/
  conditions" line, and cause-attributed kind=3 events ("Hurricane X devastates
  Y", "Famine shrinks Y to a Town", tier up/down/abandon).

New civ* knobs (self-describing config -> no save bump). test_civ extended:
sizes vary widely (not uniform), bad-year declines, a hurricane over a town
kills its population. All 10 suites pass; GUI build clean. Docs updated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

BUILD.md

@@ -296,6 +296,18 @@ population grows/declines on the Live World clock toward a food-driven carrying
     civHabFoodWeight     0.40  habitability weight of food (flora/fauna + ecoregion productivity)
     civHabTempOpt        18 C  most comfortable annual-mean temperature
     civHabElevPenalty    2500 m high terrain steeply penalised above this
+    civSiteVariety       1.0   0 = flat city sizes; higher = big rivers/coasts host far larger cities
+    civGrowthMin         0.25  growth-rate floor at habitability 0 (rate scales with habitability)
+    civHarvestVar        0.25  year-to-year harvest swing (scaled by continentality)
+    civDroughtStrength   0.70  how hard a full drought cuts a region's carrying capacity
+    civDroughtPeriod     8 yr  drought-noise epoch length (drought duration scale)
+    civDroughtThresh    -0.15  drought-onset threshold (lower = rarer)
+    civDroughtArid       0.50  extra drought-proneness in arid regions
+    civColdYearStrength  0.50  crop loss in a rare cold year, x the cell's cold exposure
+    civFloodBonus        0.25  fertile-silt bonus on river cells (rare flood disaster)
+    civFamineRate        0.15  /yr accelerated population loss when food < population
+    civStormDeathRate    0.50  /yr deaths from a full-strength storm over a settlement
+    civHurricaneDeathMult 3.0  extra storm-death multiplier for a hurricane/typhoon
 
 ## Headless logic test (no display)
 

CLAUDE.md

@@ -133,13 +133,22 @@ on the Live World clock). **Steps 1–2 of the roadmap are done (plus a derived
   coast) + food (flora/fauna density + ecoregion productivity), gated by freezing winters / high
   terrain; colour mode `Habitability`, key `I`). On key **`U`** ("the dawn") `placeSettlements()` seeds
   a fixed set **once** on the best, well-spaced (`civMinSpacingRadians`) fertile cells (separate
-  `sCivRng`; named from the continent's `NameGen` bank). `stepCivilization()` runs each live frame:
+  `sCivRng`; named from the continent's `NameGen` bank). `stepCivilization(dtHours, liveTime)` runs each
-  each settlement's population moves **logistically toward a food-driven carrying capacity** `K =
+  live frame: population moves **logistically toward a food-driven carrying capacity**, but everything is
-  civMaxPopulation·habitability` (cut transiently where an active volcano ashes the area), so it
+  **environment-driven and dynamic** (not the old "grow the same everywhere"): the growth **rate** scales
-  **grows / declines / is abandoned** (kept in the set, can revive). Tiers village→town→city by
+  with habitability (fertile cells boom, marginal crawl); the capacity `K = civMaxPopulation·habitability·
-  population; markers (3D + 2D) sized by tier + a **Civ** tab + cell-info line + kind=3 `WorldEvent`s
+  **siteQuality**·conditions` where **siteQuality** spreads max size by orders of magnitude (a great river
-  ("X grew into a city", "X was abandoned"). Since the set is fixed, the step-back snapshot only
+  — `discharge` is log-scaled — or a coast hosts a metropolis, a dry inland cell a town); and
-  restores the per-settlement **population** vector. Saved (**v20**). Knobs `civ*`.
+  **conditions** are deterministic time-varying drivers (pure functions of (region, year, seed) → constant
+  within a year, reversible on step-back): **regional droughts** (multi-year, worse in arid regions),
+  year-to-year **harvests** (bigger swings in continental interiors), rare **cold years** and river
+  **floods**, plus **volcano ash**. **Storms** over a town kill people directly (hurricanes worst,
+  `civStormDeathRate`/`civHurricaneDeathMult`); sustained famine / acute disasters can **collapse** a
+  settlement to ruins (kept in the set, can revive). Tiers village→town→city; markers (3D + 2D) sized by
+  tier and **withered-tinted** by hardship + a **Civ** tab + cell-info "conditions/drought" line + kind=3
+  `WorldEvent`s ("X grew into a city", "Hurricane <name> devastates X", "Famine shrinks X to a Town", "X
+  was abandoned"). The set is fixed, so the step-back snapshot only restores the per-settlement
+  **population** vector; conditions recompute. Saved (**v20**). Knobs `civ*`.
   *Next steps (not yet built): territory + borders, kingdoms/empires, culture + beliefs, conflict +
   diplomacy.*
 
@@ -858,9 +867,15 @@ triangles (plates are fixed in phase 1).
   (2500 m, high terrain steeply penalised above this). Population — `civSeedPopulation` (250, initial
   village), `civGrowthRate` (0.02/yr logistic rate), `civMaxPopulation` (2e6, the carrying capacity at
   habitability 1), tier thresholds `civTownPop` (5000) / `civCityPop` (100000), `civAbandonPop` (50,
-  below = abandoned/ruins but can revive). An active volcano's ash within ~1.5× its blast radius cuts a
+  below = abandoned/ruins but can revive). **Dynamics** — `civSiteVariety` (1.0; 0 = flat capacities,
-  settlement's carrying capacity (the "ashed-out region" decline). Marker sizes/colours are render
+  higher = big rivers/coasts host far larger cities → wide size spread), `civGrowthMin` (0.25, growth-rate
-  constants (ViewerRender.cpp).
+  floor at habitability 0), `civHarvestVar` (0.25, year-to-year harvest swing, scaled by continentality),
+  `civDroughtStrength` (0.70) / `civDroughtPeriod` (8 yr) / `civDroughtThresh` (−0.15) / `civDroughtArid`
+  (0.50, drought-proneness in arid regions), `civColdYearStrength` (0.50), `civFloodBonus` (0.25, river
+  silt), `civFamineRate` (0.15, accelerated loss when food < population), `civStormDeathRate` (0.50) /
+  `civHurricaneDeathMult` (3.0, deaths from a storm/hurricane over a town). Droughts/harvests are
+  deterministic per (~20° region, year, seed); an active volcano's ash within ~1.5× its blast radius also
+  cuts capacity. Marker sizes/colours + hardship tint are render constants (ViewerRender.cpp).
 - `upliftGain` (PlanetConfig) — m/tick per unit convergence stress; main
   knob for how fast/high relief builds.
 - `relax` (PlanetConfig) — isostatic relaxation toward base elevation. Peaks

docs/design-notes.md

@@ -366,16 +366,35 @@ seeds a **fixed** set once — greedily the highest-habitability cells with a mi
 
 Because placement is one-time, the settlement *set* never changes, so the only mutable per-step state
 is each settlement's **population** — which is all the step-back snapshot stores (a `vector<double>` in
-`WeatherSnapshot`, restored in `restoreWeather`; no per-frame string churn). `stepCivilization(dtHours)`
+`WeatherSnapshot`, restored in `restoreWeather`; no per-frame string churn). `stepCivilization(dtHours,
-runs in `liveAdvance` after `stepVolcanoes`: each population moves logistically toward a carrying
+liveTime)` runs in `liveAdvance` after `stepVolcanoes`. It is **environment-driven and dynamic** (the
-capacity `K = civMaxPopulation·habitability(cell)`, cut transiently where an active volcano's ash plume
+first cut grew every town uniformly to the same cap):
-(`ashTimer>0`, within ~1.5× blast radius) overlaps — so towns **grow, decline, and are abandoned**
+- growth **rate** `r = civGrowthRate·(civGrowthMin + (1−civGrowthMin)·habitability)` so fertile cells
-(floored at 1 so a site can revive when K recovers). Tier (village/town/city) is derived from
+  grow far faster than marginal ones;
-population. The viewer compares before/after populations in `detectLiveEvents` to log kind=3
+- carrying capacity `K = civMaxPopulation · habitability · siteQuality · conditions`, where
-`WorldEvent`s (tier crossings, abandonment), draws markers (3D spheres + 2D dots, sized by tier; city/
+  **siteQuality** = `0.45 + civSiteVariety·(coastBonus + log10(1+discharge/20))` makes max city size vary
-town names as 3D labels), adds a **Civ** tab (7th) and a cell-info line, and a `Habitability` colour
+  by an order of magnitude (a continental river or coast → a metropolis, a dry inland cell → a town —
-mode (key `I`). `buildGeometry()` clears the set on reseed (like geography/volcanoes). Save **v20**
+  this is what spreads final sizes instead of all saturating equally);
-appends the settlement records (population included); `sCellSettlement` is rebuilt on load. Knobs `civ*`.
+- **conditions** = `harvest · drought · coldYear · flood · ash`, all **deterministic functions of
+  (≈20° region bucket, integer year, seed)** — constant within a year, region-correlated, recomputed
+  on a step-back (pure, so no extra saved/snapshot state): year-to-year harvests (swing scaled by the
+  seasonal-amplitude/continentality field), multi-year droughts (a slow noise interpolated across
+  `civDroughtPeriod`-year epochs, threshold raised by aridity `1−sMoist`), rare cold years (× the cell's
+  near-freezing-winter exposure), river floods (silt bonus / rare disaster), and the volcano-ash cut.
+- **Storms** read live `storms()` (already snapshotted): a system within its `radius` of a town deals
+  direct deaths `civStormDeathRate·strength·overlap·(hurricane? civHurricaneDeathMult)` — a parked
+  hurricane can gut a coastal city.
+- logistic step + an accelerated `civFamineRate` loss when `K<P` + the storm deaths; floored at 1 so a
+  site revives. So towns **grow, fluctuate, shrink in droughts, and collapse/abandon** under sustained
+  famine or an acute disaster.
+Per-settlement derived `sCivCond` (combined multiplier) + `sCivDrought` (severity) drive the viewer:
+markers are **withered-tinted** by hardship, cell-info shows "drought/conditions", and `detectLiveEvents`
+logs kind=3 events with the **cause** — a storm over the shrunk town → "Hurricane <name> devastates X"
+(reuses `weatherEventName`), else "Famine shrinks X to a Town" when `sCivDrought` is high, else tier
+up/down/abandon. Markers (3D spheres + 2D dots, sized by tier; city/town 3D labels), a **Civ** tab (7th),
+a cell-info line, and a `Habitability` colour mode (key `I`). `buildGeometry()` clears the set on reseed.
+Save **v20** stores the settlement records (population included); `sCellSettlement` is rebuilt on load.
+Knobs `civ*`.
 
 ## Headless testing
 

src/render/Panels.cpp

@@ -95,6 +95,14 @@ static std::vector<std::string> cellInfo(const Planet& p, int i, double elev, do
                                                     : TextFormat("%.0f", s.population);
             L.push_back(std::string(alive ? settleTierName(t) : "Ruins of") + " " + s.name
                         + " (pop " + pop + ")");
+            // Live conditions: drought / hardship / boom (derived each civ step).
+            const auto& cond = p.settlementCondition(); const auto& dro = p.settlementDrought();
+            if (si < (int)cond.size()) {
+                double cd = cond[si], dr = (si < (int)dro.size()) ? dro[si] : 0.0;
+                if (dr > 0.15) L.push_back(std::string(TextFormat("  drought %.0f%%   conditions %.0f%%", dr * 100.0, cd * 100.0)));
+                else           L.push_back(std::string(TextFormat("  conditions %.0f%% (%s)", cd * 100.0,
+                                            cd > 1.05 ? "good harvest" : cd < 0.8 ? "hardship" : "normal")));
+            }
         }
     }
     // Climate (derived; present once computeClimate() has run).

src/render/Viewer.cpp

@@ -372,6 +372,20 @@ void Viewer::detectLiveEvents(const std::vector<WeatherSystem>& beforeStorms,
         else if (aliveA && (int)ta > (int)tb)
             appendEvent(3, 1, liveTime, s.cell, s.id,
                         s.name + " grew into a " + settleTierName(ta), popLine(s));
+        else if (aliveA && (int)ta < (int)tb) {       // tier DOWN -- attribute the cause
+            const char* stormName = nullptr;
+            for (const auto& ws : planet.storms()) {
+                double ang = std::acos(std::clamp(planet.cells[s.cell].unit.dot(Vec3{ws.pos.x, ws.pos.y, ws.pos.z}), -1.0, 1.0));
+                if (ang < ws.radius && ws.strength > 0.35) { stormName = weatherEventName(ws, planet); break; }
+            }
+            double dr = (k < planet.settlementDrought().size()) ? planet.settlementDrought()[k] : 0.0;
+            std::string title, detail = popLine(s);
+            if (stormName) title = std::string(stormName) + " devastates " + s.name;
+            else if (dr > 0.25) { title = std::string("Famine shrinks ") + s.name + " to a " + settleTierName(ta);
+                                  detail = std::string(TextFormat("drought %.0f%%, ", dr * 100.0)) + popLine(s); }
+            else title = s.name + " declined to a " + settleTierName(ta);
+            appendEvent(3, 2, liveTime, s.cell, s.id, title, detail);
+        }
     }
 }
 
@@ -661,7 +675,7 @@ void Viewer::liveAdvance(double dtClock, double dtWeather) {
     // Volcanoes are stateful lifecycle agents; step-back restores their snapshot, then dt=0 here
     // reasserts restored terrain/biome state without advancing the lifecycle.
     VolcanoUpdate vu = planet.stepVolcanoes(dtWeather);
-    CivUpdate cu = planet.stepCivilization(dtWeather);   // population grows/declines on the clock
+    CivUpdate cu = planet.stepCivilization(dtWeather, liveTime);   // env-driven growth/decline on the clock
     if (dtWeather > 0.0) detectLiveEvents(beforeStorms, beforeVolcanoes, beforeSettlements);
     if (vu.breach)      refreshView();
     else if (vu.recolor || cu.recolor) recolor();

src/render/ViewerRender.cpp

@@ -9,6 +9,17 @@
 #include <cmath>
 #include <vector>
 
+// Tint a settlement marker by its live environmental condition (1 = thriving, <1 = hardship/drought):
+// blend toward a dull withered brown-red and darken as conditions worsen.
+static Color witherColor(Color base, double cond) {
+    double h = std::clamp((0.8 - cond) / 0.6, 0.0, 1.0);   // 0 above 0.8 .. 1 at/below 0.2
+    if (h <= 0.0) return base;
+    const unsigned char w[3] = { 130, 80, 70 };            // withered brown-red
+    double dim = 1.0 - 0.35 * h;
+    auto L = [&](unsigned char b, unsigned char wc) { return (unsigned char)std::clamp((b * (1.0 - h) + wc * h) * dim, 0.0, 255.0); };
+    return Color{ L(base.r, w[0]), L(base.g, w[1]), L(base.b, w[2]), base.a };
+}
+
 // Label style for a geographic feature: font size + colour, returns true if it's a "minor" feature
 // (peaks/rivers/lakes/seas/small islands) -- those are drawn only when zoomed in, to declutter.
 static bool labelStyle(const GeoFeature& f, int& font, Color& col) {
@@ -211,7 +222,9 @@ void Viewer::renderGlobe3D() {
     // Settlement markers (civilization): a dot per settlement, sized + coloured by tier; dim for ruins.
     if (showSettlements && !planet.settlements.empty()) {
         const double townP = planet.cfg.civTownPop, cityP = planet.cfg.civCityPop, abP = planet.cfg.civAbandonPop;
-        for (const Settlement& s : planet.settlements) {
+        const auto& cond = planet.settlementCondition();
+        for (size_t k = 0; k < planet.settlements.size(); ++k) {
+            const Settlement& s = planet.settlements[k];
             if (s.cell < 0 || s.cell >= (int)planet.cells.size()) continue;
             const Cell& c = planet.cells[s.cell];
             float r = visBase + (float)c.elevation * elevExagg + 0.006f;
@@ -223,6 +236,7 @@ void Viewer::renderGlobe3D() {
                       : t == SettleTier::City ? Color{250, 220, 110, 255}
                       : t == SettleTier::Town ? Color{225, 170, 90, 255}
                                               : Color{210, 130, 85, 255};
+            col = witherColor(col, k < cond.size() ? cond[k] : 1.0);   // hardship -> withered tint
             DrawSphere(p, rad, col);
             if (alive && t != SettleTier::Village) {     // a ring marks notable settlements
                 float rr = visBase + (float)c.elevation * elevExagg + 0.01f;
@@ -355,8 +369,10 @@ void Viewer::renderMap2D() {
     }
     if (showSettlements && !planet.settlements.empty()) {
         const double townP = planet.cfg.civTownPop, cityP = planet.cfg.civCityPop, abP = planet.cfg.civAbandonPop;
+        const auto& cond = planet.settlementCondition();
         float zf = (float)std::min(2.0, mapZoom);
-        for (const Settlement& s : planet.settlements) {
+        for (size_t k = 0; k < planet.settlements.size(); ++k) {
+            const Settlement& s = planet.settlements[k];
             if (s.cell < 0 || s.cell >= (int)planet.cells.size()) continue;
             double lon, lat; dirToLonLat(planet.cells[s.cell].unit, lon, lat);
             Vector2 sp = projLonLat(lon, lat, mapLon, vr);
@@ -367,6 +383,7 @@ void Viewer::renderMap2D() {
                       : t == SettleTier::City ? Color{250, 220, 110, 255}
                       : t == SettleTier::Town ? Color{225, 170, 90, 255}
                                               : Color{210, 130, 85, 255};
+            col = witherColor(col, k < cond.size() ? cond[k] : 1.0);
             DrawCircleV(sp, rad, col);
             if (alive && t != SettleTier::Village) DrawCircleLines((int)sp.x, (int)sp.y, rad + 2.0f, Color{255, 245, 210, 180});
         }

src/sim/Planet.cpp

@@ -62,6 +62,7 @@ void Planet::buildGeometry() {
     volcanoes.clear(); sVolRng = cfg.seed ? (cfg.seed ^ 0x70C4F12Au) : 0x70C4F12Au;
     settlements.clear(); sCivRng = cfg.seed ? (cfg.seed ^ 0x017B1A2Eu) : 0x017B1A2Eu;
     sCellSettlement.assign(cells.size(), -1); sHabitability.clear();
+    sCivCond.clear(); sCivDrought.clear();
 }
 
 void Planet::clearDerivedState() {

src/sim/Planet.hpp

@@ -183,10 +183,15 @@ public:
     // / is abandoned). Saved (v20); step-back restores populations via WeatherSnapshot.
     void     computeHabitability();
     void     placeSettlements();
-    CivUpdate stepCivilization(double dtHours);
+    CivUpdate stepCivilization(double dtHours, double liveTime);
     bool     settlementsPlaced() const { return !settlements.empty(); }
     const std::vector<int>&    cellSettlement() const { return sCellSettlement; } // settlement index per cell (-1)
     const std::vector<double>& habitability()   const { return sHabitability; }   // 0..1 per cell (derived)
+    // Per-settlement live conditions (derived each stepCivilization; not saved). condition = the combined
+    // environmental multiplier on carrying capacity (1 = normal, <1 = hardship, >1 = boom); drought =
+    // current drought severity 0..1. Parallel to `settlements`. Used by the viewer for tint + events.
+    const std::vector<double>& settlementCondition() const { return sCivCond; }
+    const std::vector<double>& settlementDrought()   const { return sCivDrought; }
 
     // Build a fine-resolution subgrid patch for one macro cell (phase 4/5 hook).
     std::shared_ptr<SubGrid> makeSubGrid(int cellIndex, int res) const;
@@ -311,6 +316,7 @@ private:
     // a derived habitability field, and a separate RNG so placement never perturbs tectonics.
     std::vector<int>           sCellSettlement;
     std::vector<double>        sHabitability;
+    std::vector<double>        sCivCond, sCivDrought;   // per-settlement live conditions (derived)
     uint32_t                   sCivRng = 1;
 
     // Biota: derived density scalars (0..1; recomputed each tick, not saved) and the

src/sim/PlanetCiv.cpp

@@ -4,6 +4,16 @@
 #include <cmath>
 #include <set>
 
+// Small deterministic hashes/noise for the environment drivers (mirrors the subgrid value-noise in
+// Planet.cpp; kept local). A driver is a pure function of (cell/region, year, seed) so it is constant
+// within a year, region-correlated, and recomputed on a step-back -> reversible with no extra state.
+namespace {
+    inline uint32_t civHash(uint32_t a) { a ^= a << 13; a ^= a >> 17; a ^= a << 5; return a ? a : 1u; }
+    inline double   civHashf(uint32_t a) { return (civHash(a) & 0xFFFFFFu) / double(0x1000000); }  // [0,1)
+    inline double   civSigned(uint32_t a) { return civHashf(a) * 2.0 - 1.0; }                       // [-1,1)
+    inline double   civSmooth(double t) { return t * t * (3.0 - 2.0 * t); }
+}
+
 // --- Civilization Step 2: settlements & habitability -------------------------
 // A per-cell habitability/food score, a one-time placement of settlement point-agents on the best
 // (well-spaced) cells, and their population growth/decline on the Live World clock toward a
@@ -103,32 +113,119 @@ void Planet::placeSettlements() {
         settlements[k].id = (uint32_t)(k + 1);
         sCellSettlement[settlements[k].cell] = k;
     }
+    sCivCond.assign(settlements.size(), 1.0);
+    sCivDrought.assign(settlements.size(), 0.0);
 }
 
-// One live-frame civilization step: each settlement's population moves logistically toward its
+// One live-frame civilization step. Each settlement's population moves logistically toward a food-driven
-// food-driven carrying capacity K = civMaxPopulation * habitability (cut transiently where an active
+// carrying capacity K that varies by the LOCAL environment AND over time -- year-to-year harvests,
-// volcano ashes the area). Grows below K, declines above it; floored at 1 so an abandoned site can
+// multi-year droughts (worse in arid regions), rare cold years and river floods, plus volcano ash -- and
-// revive if K recovers. Pure of any string churn; the set never changes here.
+// growth RATE scales with habitability so fertile cells grow far faster than marginal ones. Storms over a
-CivUpdate Planet::stepCivilization(double dtHours) {
+// town kill people directly (hurricanes worst). All drivers are deterministic functions of (cell, year,
+// seed), so a step-back replays them; population is restored from the snapshot. Floored at 1 so an
+// abandoned site can revive. Conditions are recomputed even when paused (dtHours == 0) so the view tints.
+CivUpdate Planet::stepCivilization(double dtHours, double liveTime) {
     CivUpdate up;
-    if (settlements.empty() || dtHours <= 0.0) return up;
+    if (settlements.empty()) return up;
     const int n = (int)cells.size();
     if ((int)sHabitability.size() != n) computeHabitability();
+    if ((int)sCivCond.size() != (int)settlements.size()) { sCivCond.assign(settlements.size(), 1.0); sCivDrought.assign(settlements.size(), 0.0); }
+
     const double yearHours = std::max(1.0, cfg.dayLengthHours * cfg.yearLengthDays);
-    const double dtYears = dtHours / yearHours;
+    const double dtYears = std::max(0.0, dtHours) / yearHours;
-    for (Settlement& st : settlements) {
+    const int    year = (int)std::floor(std::max(0.0, liveTime) / yearHours);
+    const uint32_t base = cfg.seed ? cfg.seed : 1u;
+    const bool haveMoist = (int)sMoist.size() == n, haveSeason = (int)sTempSummer.size() == n && (int)sTempWinter.size() == n;
+    const bool haveWinter = (int)sTempWinter.size() == n, haveDisch = (int)sDischarge.size() == n;
+
+    for (int k = 0; k < (int)settlements.size(); ++k) {
+        Settlement& st = settlements[k];
         if (st.cell < 0 || st.cell >= n) continue;
-        double K = cfg.civMaxPopulation * sHabitability[st.cell];
+        const int c = st.cell;
-        for (const Volcano& v : volcanoes) {            // active ash plume nearby cuts carrying capacity
+        // A coarse spatial bucket (~20deg cells) so droughts / harvests vary REGIONALLY -- different parts
-            if (v.ashTimer <= 0.0 || v.cell < 0 || v.cell >= n) continue;
+        // of a continent have different fortunes, instead of every town drought-ing together.
-            double ang = std::acos(std::clamp(cells[st.cell].unit.dot(cells[v.cell].unit), -1.0, 1.0));
+        double clat = std::asin(std::clamp(cells[c].unit.y, -1.0, 1.0));
-            if (ang < cfg.volcanoBlastRadius * 1.5) { K *= 0.3; break; }
+        double clon = std::atan2(cells[c].unit.z, cells[c].unit.x);
+        int latB = (int)std::floor((clat + 1.5708) / 0.349), lonB = (int)std::floor((clon + 3.14160) / 0.349);
+        const uint32_t region = civHash((uint32_t)(latB * 131 + lonB) * 2654435761u + 0x9E3779B9u);
+        const double hab = sHabitability[c];
+        const double amp = haveSeason ? std::max(0.0, sTempSummer[c] - sTempWinter[c]) : 8.0;  // climate variability
+        const double aridity = haveMoist ? std::clamp(1.0 - sMoist[c], 0.0, 1.0) : 0.4;
+        const double winter = haveWinter ? sTempWinter[c] : (((int)sTemp.size() == n) ? sTemp[c] : 10.0);
+        const bool onRiver = haveDisch && sDischarge[c] > cfg.riverThreshold;
+
+        // Harvest: region-correlated good/bad year, swing scaled by continentality (amp).
+        double varScale = cfg.civHarvestVar * std::clamp(0.3 + 0.7 * (amp / 30.0), 0.3, 1.6);
+        double rNoise = civSigned(base ^ civHash((uint32_t)region * 0x9E3779B9u + (uint32_t)year * 2654435761u));
+        double cNoise = civSigned(base ^ civHash((uint32_t)c * 2654435761u ^ ((uint32_t)year * 0x85EBCA6Bu)));
+        double harvest = std::clamp(1.0 + (0.7 * rNoise + 0.3 * cNoise) * varScale, 0.2, 1.7);
+
+        // Drought: a slow signal interpolated across multi-year epochs; arid regions cross the threshold
+        // more often, so droughts last several years and bite hardest in dry lands.
+        double ft = (double)year / std::max(1.0, cfg.civDroughtPeriod);
+        double fe = std::floor(ft); int e0 = (int)fe; double tf = civSmooth(ft - fe);
+        double d0 = civSigned(base ^ civHash((uint32_t)region * 0x1B873593u + (uint32_t)e0 * 40503u));
+        double d1 = civSigned(base ^ civHash((uint32_t)region * 0x1B873593u + (uint32_t)(e0 + 1) * 40503u));
+        double dn = d0 + (d1 - d0) * tf;
+        double thr = cfg.civDroughtThresh + cfg.civDroughtArid * aridity;
+        double drought = (dn < thr) ? std::clamp(thr - dn, 0.0, 1.0) : 0.0;
+        double droughtFactor = std::clamp(1.0 - cfg.civDroughtStrength * drought, 0.05, 1.0);
+
+        // Cold year: a rare bad year times how cold-marginal the cell is (near-freezing winter).
+        double coldExposure = std::clamp((5.0 - winter) / 30.0, 0.0, 1.0);
+        double cy = civHashf(base ^ civHash((uint32_t)region * 0x55u + (uint32_t)year * 0x9E3779B9u + 7u));
+        double coldEvent = (cy > 0.85) ? (cy - 0.85) / 0.15 : 0.0;
+        double coldFactor = std::clamp(1.0 - cfg.civColdYearStrength * coldEvent * coldExposure, 0.2, 1.0);
+
+        // Floods (river cells): fertile silt most years, a rare destructive flood.
+        double floodFactor = 1.0;
+        if (onRiver) {
+            double f = civHashf(base ^ civHash((uint32_t)c * 0x2545F491u + (uint32_t)year * 19349663u));
+            floodFactor = (f > 0.94) ? 0.5 : 1.0 + cfg.civFloodBonus * (0.3 + 0.7 * f);
         }
+
+        // Volcano ash nearby (existing).
+        double ashFactor = 1.0;
+        for (const Volcano& v : volcanoes) {
+            if (v.ashTimer <= 0.0 || v.cell < 0 || v.cell >= n) continue;
+            double ang = std::acos(std::clamp(cells[c].unit.dot(cells[v.cell].unit), -1.0, 1.0));
+            if (ang < cfg.volcanoBlastRadius * 1.5) { ashFactor = 0.3; break; }
+        }
+
+        // Site quality: a settlement's MAX size varies enormously with its location -- a great river
+        // (discharge spans orders of magnitude) or a coast hosts a metropolis, a dry inland cell a town.
+        // This is what makes final sizes vary widely instead of all reaching the same cap.
+        double disch = haveDisch ? sDischarge[c] : 0.0;
+        bool coast = false;
+        for (int j : cells[c].neighbors) if (cells[j].elevation <= cfg.seaLevel) { coast = true; break; }
+        double riverQ = std::log10(1.0 + disch / 20.0);                 // ~0 tiny .. ~2.5 a continental river
+        double siteQ = std::clamp(0.45 + cfg.civSiteVariety * ((coast ? 1.2 : 0.0) + riverQ), 0.3, 6.0);
+
+        double cond = harvest * droughtFactor * coldFactor * floodFactor * ashFactor;
+        double K = cfg.civMaxPopulation * hab * siteQ * cond;
+        sCivCond[k] = cond; sCivDrought[k] = drought;
+
+        // Storms over the town kill people directly (acute), hurricanes worst. Storms are snapshotted, so
+        // this stays consistent on a step-back.
+        double stormLoss = 0.0;
+        for (const WeatherSystem& ws : sStorms) {
+            double ang = std::acos(std::clamp(cells[c].unit.dot(Vec3{ws.pos.x, ws.pos.y, ws.pos.z}), -1.0, 1.0));
+            if (ang >= ws.radius) continue;
+            double overlap = 1.0 - ang / std::max(1e-6, ws.radius);
+            bool hur = ws.tropical && ws.strength >= cfg.weatherHurricaneStr;
+            stormLoss += cfg.civStormDeathRate * ws.strength * overlap * (hur ? cfg.civHurricaneDeathMult : 1.0);
+        }
+
+        if (dtYears > 0.0) {
             double P = st.population;
-        P += cfg.civGrowthRate * P * (1.0 - P / std::max(1.0, K)) * dtYears;   // logistic (declines when K<P)
+            double r = cfg.civGrowthRate * (cfg.civGrowthMin + (1.0 - cfg.civGrowthMin) * hab);  // env-driven rate
-        P = std::max(1.0, P);                            // keep a seed so an abandoned site can revive
+            P += r * P * (1.0 - P / std::max(1.0, K)) * dtYears;            // logistic toward K
+            if (K < P) P -= cfg.civFamineRate * (1.0 - K / P) * P * dtYears; // accelerated famine
+            P -= stormLoss * P * dtYears;                                   // acute storm deaths
+            P = std::max(1.0, P);
             if (std::fabs(P - st.population) > std::max(1.0, st.population * 0.0005)) up.recolor = true;
             st.population = P;
         }
+    }
     return up;
 }

src/sim/PlanetIO.cpp

@@ -52,6 +52,9 @@
     D(civMinSpacingRadians) D(civMinHabitability) D(civSeedPopulation) D(civGrowthRate) \
     D(civMaxPopulation) D(civTownPop) D(civCityPop) D(civAbandonPop) \
     D(civHabWaterWeight) D(civHabFoodWeight) D(civHabTempOpt) D(civHabElevPenalty) \
+    D(civSiteVariety) D(civGrowthMin) D(civHarvestVar) D(civDroughtStrength) D(civDroughtPeriod) D(civDroughtThresh) \
+    D(civDroughtArid) D(civColdYearStrength) D(civFloodBonus) D(civFamineRate) \
+    D(civStormDeathRate) D(civHurricaneDeathMult) \
     I(subdivisions) I(plateCount) I(beltWidth) I(splitCheckEvery) I(stalemateWindows) \
     I(miniPlateCells) I(fuseMinPlates) I(babyMinCells) I(seaLevelEvery) \
     I(climateWindPasses) I(climateMoistureSmooth) I(seasonContinentRings) I(weatherSystemMax) \
@@ -263,6 +266,18 @@ std::string validateConfig(const PlanetConfig& cfg) {
     E(rng(cfg.civHabFoodWeight, 0.0, 1.0, "civHabFoodWeight"));
     E(rng(cfg.civHabTempOpt, -20.0, 50.0, "civHabTempOpt"));
     E(rng(cfg.civHabElevPenalty, 0.0, 12000.0, "civHabElevPenalty"));
+    E(rng(cfg.civSiteVariety, 0.0, 4.0, "civSiteVariety"));
+    E(rng(cfg.civGrowthMin, 0.0, 1.0, "civGrowthMin"));
+    E(rng(cfg.civHarvestVar, 0.0, 2.0, "civHarvestVar"));
+    E(rng(cfg.civDroughtStrength, 0.0, 1.0, "civDroughtStrength"));
+    E(rng(cfg.civDroughtPeriod, 0.1, 1000.0, "civDroughtPeriod"));
+    E(rng(cfg.civDroughtThresh, -2.0, 2.0, "civDroughtThresh"));
+    E(rng(cfg.civDroughtArid, 0.0, 4.0, "civDroughtArid"));
+    E(rng(cfg.civColdYearStrength, 0.0, 1.0, "civColdYearStrength"));
+    E(rng(cfg.civFloodBonus, 0.0, 2.0, "civFloodBonus"));
+    E(rng(cfg.civFamineRate, 0.0, 10.0, "civFamineRate"));
+    E(rng(cfg.civStormDeathRate, 0.0, 10.0, "civStormDeathRate"));
+    E(rng(cfg.civHurricaneDeathMult, 1.0, 50.0, "civHurricaneDeathMult"));
     E(irng(cfg.subdivisions, 0, 7, "subdivisions"));
     E(irng(cfg.plateCount, 1, 100, "plateCount"));
     E(irng(cfg.beltWidth, 1, 12, "beltWidth"));

src/sim/PlanetTypes.hpp

@@ -395,4 +395,19 @@ struct PlanetConfig {
     double civHabFoodWeight     = 0.40;   // habitability weight of food (flora/fauna + ecoregion)
     double civHabTempOpt        = 18.0;   // C: most comfortable annual-mean temperature
     double civHabElevPenalty    = 2500.0; // m above which high terrain steeply reduces habitability
+    // Dynamic environment (PlanetCiv.cpp): growth differs by local conditions and varies over time
+    // (harvests, droughts, cold years, floods, storms) so settlements aren't static. Deterministic
+    // functions of (cell, year, seed) -> reversible with the live stepper; no save change.
+    double civSiteVariety       = 1.0;    // 0 = flat capacities, 1 = full site-quality spread (big rivers/coasts host large cities)
+    double civGrowthMin         = 0.25;   // growth-rate fraction at habitability 0 (1 = at habitability 1)
+    double civHarvestVar        = 0.25;   // base year-to-year harvest swing amplitude (scaled by climate variability)
+    double civDroughtStrength   = 0.70;   // how hard a full drought cuts a region's carrying capacity
+    double civDroughtPeriod     = 8.0;    // years per drought-noise epoch (drought duration scale)
+    double civDroughtThresh     = -0.15;  // drought-onset threshold on the slow noise (lower = rarer)
+    double civDroughtArid       = 0.50;   // extra drought-proneness in arid regions (× aridity)
+    double civColdYearStrength  = 0.50;   // crop loss in a rare cold year, × the cell's cold exposure
+    double civFloodBonus        = 0.25;   // fertile-silt bonus on river cells most years (rare flood disaster)
+    double civFamineRate        = 0.15;   // /year accelerated population loss when food < population
+    double civStormDeathRate    = 0.50;   // /year population loss for a full-strength storm over a settlement
+    double civHurricaneDeathMult= 3.0;    // extra storm death multiplier for a hurricane/typhoon
 };

test_civ.cpp

@@ -79,19 +79,59 @@ int main() {
     check(capOk, "settlement count within the cap");
     check(uniqueNames, "settlement names are unique + non-empty");
 
-    std::printf("Civ: food-driven growth + decline\n");
+    std::printf("Civ: environment-driven growth (differentiated + dynamic)\n");
     {
-        int gi = 0; for (size_t k = 0; k < S.size(); ++k) if (p.habitability()[S[k].cell] > p.habitability()[S[gi].cell]) gi = (int)k;
+        int gi = 0, lo = 0;
-        double p0 = p.settlements[gi].population;
+        for (size_t k = 0; k < S.size(); ++k) {
-        for (int k = 0; k < 400; ++k) p.stepCivilization(5.0 * yearH);   // ~2000 yr of small steps
+            if (p.habitability()[S[k].cell] > p.habitability()[S[gi].cell]) gi = (int)k;
-        check(p.settlements[gi].population > p0 * 2.0, "a high-habitability settlement grows");
+            if (p.habitability()[S[k].cell] < p.habitability()[S[lo].cell]) lo = (int)k;
-        // Decline: push one well over its carrying capacity, then step -> it shrinks.
+        }
+        double g0 = p.settlements[gi].population;
+        double lt = 0.0; bool anyDecline = false;
+        std::vector<double> prev(p.settlements.size());
+        for (int yr = 0; yr < 600; ++yr) {
+            for (size_t k = 0; k < p.settlements.size(); ++k) prev[k] = p.settlements[k].population;
+            lt += 2.0 * yearH;
+            p.stepCivilization(2.0 * yearH, lt);                 // advance the clock so harvests/droughts vary
+            for (size_t k = 0; k < p.settlements.size(); ++k)
+                if (p.settlements[k].population > p.cfg.civAbandonPop && p.settlements[k].population < prev[k] * 0.999) anyDecline = true;
+        }
+        check(p.settlements[gi].population > g0 * 2.0, "a fertile settlement grows strongly");
+        // The user's complaint was "they grow the same amount everywhere": now sizes must vary widely.
+        std::vector<double> pops;
+        for (const auto& s : p.settlements) if (s.population >= p.cfg.civAbandonPop) pops.push_back(s.population);
+        std::sort(pops.begin(), pops.end());
+        double med = pops.empty() ? 0.0 : pops[pops.size() / 2];
+        double mx  = pops.empty() ? 0.0 : pops.back();
+        std::printf("    alive %d  median %.0f  max %.0f  (max/median %.1f)\n",
+                    (int)pops.size(), med, mx, med > 0 ? mx / med : 0.0);
+        check(!pops.empty() && mx > med * 3.0, "settlement sizes vary widely (env-driven, not uniform growth)");
+        check(p.settlements[gi].population > p.settlements[lo].population, "fertile ends larger than marginal");
+        check(anyDecline, "settlements decline in bad years (harvest/drought dynamics, not monotonic)");
+        // Over-capacity settlement declines toward its food limit.
         p.settlements[gi].population = 5.0e7;
         double over = p.settlements[gi].population;
-        for (int k = 0; k < 400; ++k) p.stepCivilization(5.0 * yearH);
+        for (int yr = 0; yr < 200; ++yr) { lt += 2.0 * yearH; p.stepCivilization(2.0 * yearH, lt); }
         check(p.settlements[gi].population < over, "an over-capacity settlement declines toward its food limit");
     }
 
+    std::printf("Civ: a hurricane over a town kills people\n");
+    {
+        Planet w; w.generate(cfg); settle(w); drift(w, 400); w.placeSettlements();
+        if (!w.settlements.empty()) {
+            int si = 0; for (size_t k = 0; k < w.settlements.size(); ++k) if (w.settlements[k].population > w.settlements[si].population) si = (int)k;
+            w.settlements[si].population = 1.0e5;
+            WeatherSnapshot snap = w.captureWeather();
+            WeatherSystem ws; ws.id = 999; ws.pos = w.cells[w.settlements[si].cell].unit;
+            ws.radius = 0.3; ws.strength = 1.0; ws.tropical = true; ws.life = 1e9;
+            snap.storms.push_back(ws);
+            w.restoreWeather(snap);                              // inject a stationary hurricane over the town
+            double before = w.settlements[si].population;
+            double lt = 0.0; for (int yr = 0; yr < 3; ++yr) { lt += yearH; w.stepCivilization(yearH, lt); }
+            check(w.settlements[si].population < before * 0.9, "a hurricane parked over a town kills its population");
+        }
+    }
+
     std::printf("Civ: tiers\n");
     check(settleTierOf(100.0, p.cfg.civTownPop, p.cfg.civCityPop) == SettleTier::Village
        && settleTierOf(p.cfg.civTownPop, p.cfg.civTownPop, p.cfg.civCityPop) == SettleTier::Town
@@ -111,7 +151,7 @@ int main() {
     for (int k = 0; k < 40; ++k) {
         double dx = x.cflDtMy(); x.advect(dx); x.step(); x.erode(dx);
         double dy = y.cflDtMy(); y.advect(dy); y.step(); y.erode(dy);
-        if (k == 20) { y.placeSettlements(); y.stepCivilization(yearH); }
+        if (k == 20) { y.placeSettlements(); y.stepCivilization(yearH, yearH); }
     }
     bool terrainSame = true;
     for (int i = 0; i < n; ++i) if (std::fabs(x.cells[i].elevation - y.cells[i].elevation) > 1e-9) terrainSame = false;