When people hear “heatwave”, they usually think of a hot spell in the air. The ocean has them too. A marine heatwave is a period — typically weeks to months — when sea temperatures across a region stay far above the seasonal norm. It is not a single hot day at the beach; it is water that stays unusually warm, day after day.
Slow to warm, slow to cool
Marine heatwaves differ from their atmospheric cousins in pace. The sea heats up slowly, and it also cools down slowly. Long after the air has returned to normal, the water can still hold the memory of a hot summer — which means the consequences for sea life are longer-lasting.
Why corals bleach
Corals live close to their thermal limits. Inside every coral polyp live tiny algae that photosynthesize and share their energy with the coral, giving it both food and color. Under prolonged heat stress, corals expel these algae: the coral turns white — this is bleaching — and loses its main source of energy. A single heatwave can be survived. Several in a row, without time to recover, push a reef toward lasting damage.
Why early warning matters
That is where early warning earns its keep. Knowing that heat stress is building gives scientists, divers and coastal communities time to survey reefs, document stress and intervene where it counts. ReefBeacon, our early-warning project for Hong Kong’s waters, exists for exactly this reason.