I dived the same reef twice — once in 2014, once in 2018 — and the difference between those two dives is the clearest way I know to explain what coral bleaching actually means in practice.
In 2014, the site was a mid-shelf platform reef in the central Great Barrier Reef. Visibility was good, maybe twenty metres. The shallow section — between three and ten metres — was dominated by branching Acropora corals in greens, blues, and creamy whites, dense enough that navigating through them required care. Fish life was abundant and diverse. It was, by any measure, a healthy reef.
In 2018, I dived the same site. The structural framework was the same — you could still identify the reef by its topography. But the shallow Acropora field was gone. The branches were there, pale grey-brown, their calcium carbonate skeletons intact but the living tissue absent, replaced by a thin film of turf algae. In four years, the reef had gone from full colour to monochrome.
That’s bleaching. Not as an abstraction. As something you see.
The Mechanism
Coral bleaching is a stress response. When coral polyps experience conditions outside their tolerance range — most commonly elevated water temperature, but also extreme cold, disease, pollution, or intense ultraviolet exposure — they expel the zooxanthellae algae living within their tissues.
These algae, which belong to the family Symbiodiniaceae, are the coral’s primary energy source. They photosynthesise using sunlight and provide the coral with up to 90% of its energy in the form of organic compounds. They’re also responsible for most of the coral’s colour — the greens, browns, and yellows of different coral species come almost entirely from their zooxanthellae populations.
When the zooxanthellae are expelled, the coral tissue becomes transparent. The white calcium carbonate skeleton shows through. The coral turns pale — white, or bleached.
A bleached coral is alive. It’s still metabolising, still breathing, still capable of recovery. But it has lost its primary energy source and is subsisting on what it can catch with its tentacles — a fraction of normal intake. If the stressor is removed within a few weeks, the coral can reabsorb zooxanthellae and recover. If not, it begins to starve, its immune system fails, disease moves in, and it dies.
Temperature and Time
The relationship between temperature stress and bleaching is expressed in a unit called Degree Heating Weeks (DHW). A single DHW represents one week of sea surface temperatures one degree Celsius above the Maximum Monthly Mean — the warmest average temperature that particular reef location typically experiences in its warmest month.
Bleaching typically begins around 4 DHW. Severe bleaching and significant mortality typically occur at 8 DHW or above. The 2016 bleaching event on the northern Great Barrier Reef accumulated DHW values above 10 in many locations — levels that drove bleaching from which many corals could not recover.
What makes recent events so damaging isn’t just the peak temperatures but the duration. A reef that experiences two degrees above average for two weeks may recover. A reef that experiences one degree above average for twelve weeks accumulates the same DHW value and suffers the same bleaching — but because the stress period is extended, the corals have no opportunity to begin recovery before the heat continues.
The Bleaching Record on the Great Barrier Reef
Mass bleaching — bleaching affecting large portions of the reef simultaneously rather than isolated patches — is a phenomenon associated with the elevated sea surface temperatures of El Niño events and the longer-term warming trend of climate change.
The GBR’s bleaching history: 1998 (first major mass bleaching, associated with a strong El Niño), 2002, 2016, 2017, 2020, 2022. The intervals are shortening. Between 1998 and 2016 there were eighteen years. Between 2016 and 2022 there were six years containing four events.
The 2016 event was the most severe in the reef’s recorded history. Aerial surveys conducted by researchers at James Cook University found that 93% of the 900 individual reefs surveyed showed some bleaching. In the northern third of the GBR — the section generally considered the most remote and pristine — approximately 50% of shallow-water coral cover died.
The 2022 event was notable for occurring during a La Niña year — a climate pattern that historically suppresses bleaching by lowering sea surface temperatures. The fact that 2022 produced a mass bleaching event despite La Niña conditions indicates that baseline ocean temperatures have risen to the point where bleaching-level heat stress is achievable even in cooler-than-average years.
Recovery: What It Looks Like and What It Requires
Coral recovery after bleaching is real but conditional. The conditions required are: a return to normal water temperatures, absence of subsequent stressors (no repeat bleaching events, no crown-of-thorns outbreaks, no cyclone damage, low sediment and nutrient runoff), and presence of coral larvae to settle on bare substrate left by dead colonies.
Under good conditions, fast-growing branching corals like Acropora can partially recover visible cover within five to seven years. Massive corals like Porites — slow-growing, old — may take decades to reach their pre-bleaching size.
The problem is the interval between events. Recovery requires time without further bleaching. The current trajectory — bleaching events every few years rather than every decade or two — doesn’t provide that time. The reef is attempting to recover between events that are arriving faster than the recovery can happen.
This is not a counsel of despair. It is a description of a problem with a known solution: reducing the rate of ocean warming, which means reducing greenhouse gas emissions. The reef cannot adapt to warming on a timescale of decades. The only meaningful lever is the temperature itself.
What You See When You Dive a Bleached Reef
I want to be precise about this, because there are two kinds of bleached reef, and they look very different.
An actively bleaching reef — during or immediately after a bleaching event — is visually striking in a way that can be mistaken for health by an inexperienced observer. The corals are white and luminous. Acropora tables glow. Staghorn thickets are pale blue-white. In certain light, it’s beautiful in a disturbing way, the way fog makes a landscape beautiful by hiding what’s in it.
A post-bleaching reef — months after the event — is neither white nor colourful. The dead coral framework is brown-green with turf algae. The structural complexity is intact but the cover is gone. Fish diversity typically drops within months of bleaching events as the specialist species that depend on specific coral types — butterflyfish, many wrasse species, coral-dependent damsels — lose their habitat and food sources.
Recovery in progress looks like patchy new coral growth on bare skeleton, in the white and pink colours of juvenile corals before their zooxanthellae populations establish. It’s subtle. You need to know what you’re looking at to see it. But it’s there, and seeing it matters.
Why I Keep Diving the Reef
People sometimes ask me whether diving a degraded reef is depressing. My honest answer is: sometimes, yes. Dives on heavily bleached sections of the GBR’s northern end have left me quiet and unsettled in a way that takes a while to process.
But I keep going back because the reef is not only its worst sections, and because presence matters. Divers who see what’s happening are the ones who talk about it, who vote and advocate and support research. The reef doesn’t need people to stay away because it’s damaged. It needs people who understand what it’s going through.
Go. See what’s there. See what’s not there. Know the difference.
