Reef monitoring around the world has revealed that heat-stress-related coral bleaching is now happening so frequently that there is insufficient time to recover in between events. A sharp rise in ocean acidification will further aggravate the situation. Parallels from the geological past show that this combination will likely cause widespread extinction of coral reefs, with severe to complete biodiversity collapse in the oceans. It is time to act.

Early in the 1980s, bleaching events happened once every three decades or so. Today, they are happening every six years or so, and more frequently in some places. The events occur because the corals are stressed by high water temperatures, which cause their polyps to lose their symbionts. The symbionts are algae that photosynthesise by trapping energy from sunlight, using pigments.

Ejection of the algae thus causes the coral polyp to lose its colour and become transparent. This in turn allows the bright, white calcium-carbonate skeleton to be seen through the polyp — the coral appears bright white, or “bleached”.

Recovery is possible. If the bleaching events don’t follow each other too rapidly, the coral polyps can take up new symbionts, and regain their colourful appearance. However, recovery typically takes a couple of years, and may be incomplete — some sections of the reef may die. If the stressful bleaching events follow each other too rapidly, then most of the reef will perish.

Examples have been well documented for the famous Great Barrier Reef complex off north-eastern Australia. In March 2016, the Australian Research Council’s Centre of Excellence for Coral Reef Studies reported that 95% of its northern half, from Papua New Guinea to the northern Australian coastal city of Cairns, had been severely bleached. Only four out of 520 surveyed reefs in this sector were spared.

In April 2017, the Centre reported a similarly long section of bleaching, albeit a bit further south toward the centre of the reef. Especially in the overlapping region of these events, around Cairns, the corals have had preciously little time for recovery.

Warm sea-surface waters, and increased coral bleaching, used to be restricted to severe El Niño years. El Niño is part of ENSO, the El Niño Southern Oscillation, a quasi-regular atmosphere-ocean interaction in the low latitudes. El Niño years are the warmer ones in this cycle. This is still true, but the underlying trend of global warming has caused cold years in today’s ENSO cycle to be warmer than the cycle’s warmest years 30 years ago. In consequence, coral bleaching now occurs every few years, or even — as on the Great Barrier Reef — every year.

It is clear that global warming has made heat stress, and eventually heat death, a firm part of our imminent future. This is true both on land, where billions will face repeated deadly heatwaves, and in the sea where it will severely affect coral reefs. It is easy to think that widespread heat death of the reefs would be less alarming than deadly heatwaves on land. But this would be a mistake: the impacts on Earth’s ecosystems will be tremendous, because coral reefs are home to about a third of all biodiversity in the oceans, and the oceans cover more than two thirds of Earth’s surface area.

Exacerbating the stress resulting from heat, we are further stressing reefs through ocean acidification (caused by uptake of part of our CO 2 emissions in ocean water), pollution, overfishing, and physical damage. Thus, we are fast approaching a crunch-point regarding the very survival of coral reefs. To understand what we might expect if we let this continue, it is useful to consider times in Earth’s geological past when marine biodiversity crashed.

The first massive, wave-resistant reef systems appeared during the Silurian Period, 444 to 416 million years ago, followed by their peak development during the Devonian Period, 416 to 359 million years ago. Devonian reefs and their biodiversity rivalled those of today.

Later, climatic instability caused reefs to become less dominant, but they continued until the world’s largest mass extinction, 252 million years ago: the end-Permian event. This wiped out 70% of all land-based and 96% of all marine life, including all reef builders of the time. The corals we know today all evolved more recently. The end-Permian event really was a total ecosystem collapse in the oceans, and something alarmingly close to that on land.

The end-Permian event is quite well understood. About 60,000 years before it, volcanic activity increased massively, especially in Siberia. This pushed CO 2 levels up, and there was about 8 degrees Celsius of global warming. One consequence was widespread de-oxygenation of the world oceans (something that is also occurring today), which kicked off the extinction sequence 10,000 years after the volcanism increased. Then, a second, much more rapid carbon release took place. Within 10,000 years, ocean pH (a measure of acidity) dropped by up to 0.7 pH units. The acidification finished off the process that had been started with the warming and ocean deoxygenation: almost all life was wiped from the oceans.

For scale, the dramatic end-Permian pH change still is 30 times slower than the rate of ocean acidification today, which is projected to reach 0.5 pH units by the year 2100. And the underlying fastest rate of end-Permian warming is thought to have been possibly similar, but more likely 10 times slower than that of today.

A drop of only 0.2 pH units can result in seizures, coma, and death in humans. Fish have a similar sensitivity. In addition, marine organisms that form calcium-carbonate skeletons, such as corals and shells, will run into great difficulties in forming their skeletal framework under strong acidification. And it makes it difficult for larvae to find new places to settle.

We saw already that the end-Permian acidification succeeded in wiping out all reef builders; they did not have enough time to adapt or evolve against that massive change. Given that the end-Permian acidification was similar in size, but about 30 times slower than the current change, things are looking grim for today’s reefs. They find themselves under a barrage of extremely fast warming and unprecedentedly fast acidification, as well as the other human impacts on reefs.{%recommended 6582%}

It would be another grave mistake to pin our hopes on recovery after first making no improvements to our behaviour, and then magically cleaning up our act by waving a wand of some sorts. Dead is dead. Although recovery happened after all of Earth’s past mass extinctions, including the end-Permian one, this typically took many hundreds of thousands to millions of years. And the recovering ecosystem was completely different from what existed before the event. We humans — complex, multi-cellular eukaryotes with high dependence on other complex, multi-cellular eukaryotes for our survival — won’t stand a chance. Much better to use our intelligence right now and prevent things from going completely off the rails.

One final warning: viewed on human timescales, an extinction would creep up on you almost unnoticed. They are not Hollywood-style “one day to the next” events. Instead, they are periods with a rate of extinction that is enhanced relative to normal background rates, but not dramatically so. But biologists are helping us by monitoring extinction. And today’s extinction rates are, alarmingly, 1000 to 10,000 times higher than normal background rates. This may even be 10 to 100 times higher than the rate during the end-Permian event! So, a mass extinction is already upon us — we’re on the top of the slippery slope, and must ensure that we don’t ignore it until it’s too late.

What should we do? We need stop pretending that there is an argument about what’s being observed all around the world. Instead, we need to turn the tide of both global warming and ocean acidification. This means that we must make a very rapid transition toward a zero-carbon-emissions society, and that we urgently need to develop ways of removing excess, already emitted, carbon from the coupled atmosphere-ocean system. By the way, these measures will also reduce pollution, improve public health, boost research and development into new technologies, increase new business opportunities, and so on. Why not?