A Symbiosis Under Threat
Coral reefs are built on a partnership between coral animals and microscopic algae called zooxanthellae. These algae live within the coral's tissue, photosynthesizing sugars that provide up to 90% of the coral's energy. In return, the coral provides shelter and nutrients. This symbiosis has built the world's most biodiverse marine ecosystems — but it has a critical vulnerability: it breaks down when water gets too warm.
The Bleaching Threshold
Each coral species has evolved to thrive within a narrow temperature range. When sea surface temperature exceeds the local summer maximum by just 1°C for several weeks, the coral's stress response kicks in. Zooxanthellae begin producing toxic reactive oxygen species, and the coral expels them in self-defense. Without their algae, the coral's white calcium carbonate skeleton becomes visible — hence "bleaching." The coral is alive but starving.
Degree Heating Weeks
NOAA's Degree Heating Weeks (DHW) metric captures the cumulative thermal stress that triggers bleaching. It sums how many degrees above the bleaching threshold the water has been, and for how long. A DHW of 4 signals possible bleaching. A DHW of 8 — equivalent to 2°C above threshold for 4 weeks, or 1°C for 8 weeks — predicts mass bleaching. Above 16 DHW, widespread mortality is expected.
Recovery and the Frequency Trap
After a bleaching event, surviving corals can recover if temperatures return to normal — slowly reabsorbing zooxanthellae over months. But full reef recovery takes 10-15 years. As climate change increases the frequency of bleaching events, reefs are being hit again before they can recover. This "frequency trap" threatens to convert vibrant coral reefs into algae-dominated rubble fields, devastating the quarter of all marine species that depend on them.