Coral Bleaching: When a Reef's Algae Leave Home

A partnership that turns coral white when it breaks

Reef-building corals are animals that farm their own food: each coral polyp hosts millions of single-celled algae called zooxanthellae (genus Symbiodinium and relatives) living inside its tissue, which photosynthesise and pass most of the sugars they produce directly to the coral, covering up to 90% of its energy needs, while the coral in turn gives the algae shelter and a steady supply of CO₂ and nutrients. The algae also give coral its colour. Bleaching is what happens when that partnership breaks down: heat-stressed polyps expel their algae en masse, exposing the coral's naturally translucent white skeleton underneath — the coral is not dead yet, but it has lost its main energy source and its resilience.

Degree Heating Weeks: measuring accumulated heat stress

Corals do not bleach from a single hot day — they bleach from sustained heat above their normal seasonal maximum. NOAA's Coral Reef Watch program quantifies this with Degree Heating Weeks (DHW), which accumulates how far sea-surface temperature exceeds the local maximum monthly mean (MMM, the historical average of the warmest month) over a rolling 12-week window, in units of °C-weeks.

HotSpot(day) = max(0, SST(day) - MMM)
DHW = sum of HotSpot over the trailing 12 weeks, in °C-weeks
  DHW >= 4  ->  significant bleaching likely
  DHW >= 8  ->  severe bleaching and coral mortality likely

A DHW value of 4 °C-weeks is the standard alert threshold for significant bleaching risk; 8 °C-weeks or more predicts severe bleaching with substantial coral mortality. Because DHW integrates both how hot and how long, a reef can bleach from a moderate but prolonged marine heatwave just as readily as from a brief, extreme spike — the total accumulated stress is what matters.

El Niño and the global bleaching events

El Niño events warm the tropical Pacific and shift ocean circulation patterns in ways that push regional sea temperatures well above the seasonal maximum across huge swathes of reef simultaneously, which is why the worst global mass bleaching events — 1998, 2010, and the prolonged 2014-2017 global event, followed by another global event beginning in 2023 — have all coincided with strong El Niño phases layered on top of a steadily rising baseline from long-term ocean warming.

From stress to death: the bleaching-to-mortality pathway

Bleached coral is not necessarily dead. If the thermal stress subsides quickly enough, surviving polyps can reacquire zooxanthellae (sometimes a different, more heat-tolerant symbiont strain) over weeks to months and recover close to full colour and function. But a bleached coral is starving and immunocompromised for as long as it stays pale, making it far more vulnerable to disease and less able to compete for space or reproduce; prolonged or repeated bleaching without recovery time between events leads to the polyps' actual death, leaving behind a bare calcium-carbonate skeleton that algae quickly overgrow.

Ocean acidification: a second, compounding stressor

Roughly a third of anthropogenic CO₂ emissions dissolve into the ocean, forming carbonic acid and lowering seawater pH — a process separate from warming but happening at the same time. Lower pH reduces the availability of carbonate ions corals need to build their aragonite skeletons, slowing growth and skeletal density even in colonies that never bleach, which compounds the damage from heat stress and slows reef recovery after a bleaching event.

What the simulation shows

This simulation runs a synthetic sea-surface-temperature record against a reef's baseline maximum, accumulating a live Degree Heating Weeks value and stepping coral colonies through the healthy-stressed-bleached-dead progression as DHW crosses the 4 and 8 °C-week thresholds, showing directly why sustained moderate heat is just as dangerous as a short, extreme spike.

Frequently asked questions

Try it live

Everything above runs in your browser — open Reef Bleaching and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.