This simulation models coral bleaching on a 20×12 reef grid using the Degree Heating Weeks (DHW) thermal-stress metric that reef managers use in the real world. Each coral accumulates heat stress from sea-surface temperature (SST) anomaly, optionally boosted by an El Niño toggle, while a Marine Protected Area (MPA) slider and ocean pH control how much stress corals absorb and how fast they recover. Watch individual coral colonies pass through healthy, stressed, bleached, recovering and dead states over a 24-week run as the DHW curve crosses the bleaching and mortality thresholds.
A grid of 240 coral colonies (5 shape types: Branching, Massive, Plate, Fan, Encrusting) each tracking their own accumulated DHW value with random per-colony noise. Every simulated week, DHW increases when SST anomaly is above 1°C; once a colony's DHW exceeds 4 it turns white (bleached), and past 8 it dies and grows dark algae. A second chart plots the reef-wide average DHW against the bleaching (DHW>4) and mortality (DHW>8) threshold lines alongside the % bleached and % dead curves over time.
Drag SST anomaly (0–5°C) to set background ocean warming, and toggle El Niño to add a further +1.5°C spike. Ocean pH (7.7–8.35) controls acidification: lower pH slows post-bleaching recovery. MPA protection (0–100%) cuts incoming thermal stress by up to 60%, simulating how shading, reduced pollution and fishing bans buffer reefs. Press Run to play out a 24-week episode, or Reset to start over, and watch the DHW, % Stressed, % Bleached, % Recovering, % Dead and estimated Recovery time readouts update live. To see recovery, pull the SST anomaly back below 1°C once the reef has bleached but before DHW passes 8: the hot weeks roll out of the DHW window, stress falls away, and the bleached survivors slowly regain their colour — the dead ones never do.
Degree Heating Weeks is the actual metric NOAA Coral Reef Watch uses to forecast real bleaching events worldwide: DHW of 4°C-weeks reliably predicts significant coral bleaching, and DHW of 8°C-weeks or more predicts widespread coral mortality. In the model, recovery is far from guaranteed — if SST drops back to baseline, bleached corals only recover if their accumulated stress resets in time, and that recovery rate is throttled by ocean acidification (lower pH), just as in real reef ecosystems.
Degree Heating Weeks (DHW) is a measure of accumulated thermal stress: each simulated week the model computes a "HotSpot" of max(0, SST anomaly − 1), scaled by a per-colony noise factor and reduced by MPA protection, and a colony's DHW is the sum of its HotSpots over a rolling 12-week window — exactly how NOAA Coral Reef Watch computes real-world DHW from satellite sea-surface temperature. Because the window rolls, stress fades once the water cools: hot weeks drop out of the window and DHW falls back towards zero, which is what allows a reef that survived the heatwave to recover.
The code switches a coral's state to "bleached" once its accumulated DHW passes 4, turning it white, and to "dead" once DHW passes 8, at which point dark algae growth is drawn over it. Below DHW 1 a coral stays healthy; between 1 and 4 it is marked "stressed" (shown orange) but not yet bleached. These exact thresholds (4 and 8) are drawn as dashed reference lines on the DHW history chart.
Checking the El Niño checkbox adds a flat +1.5°C to whatever SST anomaly value the slider is set to before any other calculation runs, so it directly raises the weekly stress accumulation for every coral. It models how a real El Niño event pushes ocean temperatures well above the seasonal maximum, which is exactly what triggers major global bleaching years such as 1998, 2010, and 2015-2016.
The MPA slider (0–100%) is converted to a factor that multiplies incoming weekly stress by (1 − MPA% × 0.6), so a reef at 100% MPA protection only accumulates 40% of the thermal stress an unprotected reef would face. This reflects real research suggesting that reduced local stressors (pollution, sedimentation, overfishing) inside protected areas can improve coral resilience and recovery, though MPAs cannot fully block ocean-wide heat waves.
The simulation computes a pH-recovery factor as (pH − 7.7) / (8.35 − 7.7), so a healthier, more alkaline ocean (closer to 8.35, roughly pre-industrial pH) speeds up how quickly bleached corals regain healthy status, while a more acidic ocean (closer to 7.7) slows recovery toward a floor of 10% of the normal rate. This mirrors real ocean-acidification science: lower pH impairs coral calcification and stress recovery, compounding the damage from heat alone.