Corals bleach when they are subjected to some form of stress for a prolonged period of time. These stressors can be either anthropogenic or natural in origin, and a bleaching event may be caused by a combination of these stressors. It is thus difficult to attribute bleaching events to a single cause. However, laboratory bleachings produced by higher than average temperatures have algae ejection patterns that are most similar to those of major bleaching events since 1979. This suggests that temperature changes were the chief cause of these bleaching events.

Corals live in zones of relatively stable temperature. As a result, they have evolved to tolerate a narrow range of temperatures. When temperatures exceed the upper limit of the coral tolerance range, the zooxanthellae algae become more sensitive to light. This damages their photosynthetic machinery. The zooxanthellae begin to produce less sugars and the corals, unable to benefit from algal sugar production, expel the algae into the ocean.

Question: Corals evolved millions of years ago and have survived historical temperature variations. Why do we need to worry about temperature effects on corals now?

Hint: Think again about the rate of temperature change millions of years ago compared to the rate of temperature change today. Long periods of change provide time for organisms to adapt or relocate. In relation to the lifespan of long-lived corals, a century is a short period of time.