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Malti🪸 Coral Bleaching Simulation
Interactive coral reef simulation: sea-surface-temperature anomaly and heat-stress duration drive algae expulsion, colour fade, bleaching and recovery.
A living reef cross-section responds to rising sea-surface temperature: as heat stress accumulates, coral polyps expel their colour-giving algae, fade from vibrant to pale to bleached white, and — if the stress persists too long — die. Turn stress down in time and recovery mode lets living coral slowly regrow its colour.
🔬 What It Demonstrates
A simplified thermal-stress expulsion model: algae loss scales with how far the SST anomaly exceeds each coral's tolerance, and with a Degree-Heating-Weeks-style accumulated heat metric — the same concept reef scientists use to forecast real bleaching events.
🎮 How to Use
Drag the SST anomaly slider up to add heat stress, widen the stress window to accumulate more Degree Heating Weeks, and flip on recovery mode after cooling things down to watch surviving coral regrow its colour.
💡 Did You Know?
The 1998, 2010, and 2014–2017 global bleaching events, driven largely by El Niño-amplified ocean warming, damaged coral reefs across every ocean basin — and back-to-back heat waves now strike some reefs before they've had time to fully recover.
About the Coral Bleaching Simulation
This canvas-based model recreates why warming seas turn colourful reefs pale and white. Reef corals host symbiotic algae (zooxanthellae) that give them their colour and much of their food energy. When the sea-surface-temperature anomaly climbs above a coral's tolerance for a sustained period, the algae's photosynthesis breaks down and the coral expels them to protect itself, exposing the white skeleton beneath — this is bleaching.
The simulation tracks a simplified Degree Heating Weeks metric, the same accumulated-heat concept reef scientists use to forecast real bleaching risk, and applies it to dozens of individual coral colonies with slightly different heat tolerances. Push the SST anomaly and stress window high enough for long enough and colonies bleach, and eventually die, turning permanently grey. Ease the stress and switch on recovery mode, and surviving corals slowly regrow their colour — but dead coral never comes back. Adjust the sliders to explore how intensity and duration of a marine heat wave interact to determine a reef's fate.
Frequently Asked Questions
What is coral bleaching?
Coral bleaching is the loss of the symbiotic algae (zooxanthellae) that live inside coral tissue and give the coral its colour. When corals are stressed, most commonly by water that is too warm for too long, they expel these algae, exposing their white calcium-carbonate skeleton through their now-transparent tissue.
What are zooxanthellae and why do corals need them?
Zooxanthellae are microscopic photosynthetic algae living inside coral cells in a mutually beneficial symbiosis. The algae photosynthesise and share sugars and nutrients with the coral, supplying up to 90% of its energy needs, while the coral provides shelter and compounds the algae need to photosynthesise. This partnership is what lets coral reefs thrive in nutrient-poor tropical waters.
What is Degree Heating Weeks (DHW)?
Degree Heating Weeks is a metric used by coral reef monitoring programmes that accumulates how far sea-surface temperature exceeds a warm-season baseline threshold, summed over a trailing multi-week window. Roughly 4 DHW typically signals a likelihood of bleaching, and around 8 DHW signals a high risk of coral mortality — thresholds this simulation mirrors in a simplified form.
Does bleached coral always die?
No. Bleached coral is stressed and starving but still alive, surviving for a time on stored energy reserves. If the water cools back within its tolerance soon enough, the coral can reacquire zooxanthellae and regain its colour over weeks to months. Death only follows when severe bleaching persists for too long, exhausting the coral's reserves.
What do the SST anomaly and stress window controls do?
The SST anomaly slider sets how many degrees above the baseline the water currently is, driving the rate at which corals lose algae. The stress window sets how many trailing simulated weeks of excess heat are summed into the Degree-Heating-Weeks readout, matching how real reef forecasts accumulate heat stress over time rather than judging a single day in isolation.
What does recovery mode change?
With recovery mode on, any coral that is bleached but not yet dead will slowly regrow its algae density once the SST anomaly drops back below that colony's individual tolerance, fading back toward its original colour. Dead coral, shown in permanent grey, cannot recover under any setting — coral death is irreversible.
Why do some corals in the simulation bleach before others?
Each coral colony is given a slightly different heat tolerance, just as real coral species and even individual colonies vary in their thermal thresholds. This produces a realistic mix of healthy, bleaching, and recovering coral across the reef even though every colony experiences the same water temperature.
Is this simulation scientifically accurate?
It is a simplified, stylised model built for clarity rather than a research-grade thermal-stress simulator. The relationships — algae loss scaling with excess heat, an accumulated Degree-Heating-Weeks metric, bleaching and mortality thresholds, and reversible-until-death recovery — all reflect real coral biology and the metrics reef managers actually use, but the exact numbers are tuned for an illustrative, interactive experience rather than precise prediction.
Thermal stress builds as Degree Heating Weeks pass the bleaching threshold — trigger a marine heatwave and watch coral pale, bleach white and recover.
3D · Three.js / WebGL renderer · 60 FPS target · runs fully client-side, no install