• “Over last 30 years 50% of coral has disappeared”

• “Based on current trends, within the next 30 years annual bleaching will kill most of the world’s coral”

Earlier this month a new climate impact documentary was released, called Chasing Coral. As the name suggests, it’s all about coral reefs – and was made by the same director as Chasing Ice (another highly recommended documentary film about the effects of climate change – on big glaciers).

We watched Chasing Coral yesterday evening and – as we are a science website with a special interest in the ecological effects of climate change, thought it might be good to publish a short fact check – to see if the statements that are made in the documentary are in line with scientific consensus.

• “In about 25 years (average projection) all across the planet the oceans become too warm for coral reefs to survive – they will bleach every year, and will not be able to recover” (Ove Hoegh-Guldberg)

• “We will see the eradication of an entire ecosystem in our lifespan” (Ruth Gates)

Well – as a specialised science journalist you usually know if a documentary (if it’s in your field) is good or bad after watching let’s say the first 15 minutes.

In the case of Chasing Ice we were hooked and honestly forgot about our intention to focus just on the facts & figures. The film is simply too beautiful, and the people who made it (personal favourite: Zack Rago, ‘coral nerd’) too heart-warmingly passionate about this class [Anthozoa – if you allow us to pile stony corals (Scleractinia), soft corals (Alcyonacea), gorgonians and sea anemones together] of colony-building marine invertebrates with surprisingly complex biology and indeed, as the movie shows, sometimes breathtakingly beautiful expressions of biodiversity…

Coral is beautiful on the macro (aerial footage), meso (divers’ perspective) and micro scale, the latter shown in these close-up images of living (and moving – go watch that movie to enjoy!) coral from Chasing Coral.

But today is the next day and we just had our coffee, so let’s get back to those facts and say – sadly – the movie also has a very solid scientific foundation. That’s sad, because the main message it contains, one that sometimes it shouts in a way that only science can shout, is that Earth’s tropical coral reefs are dying – due to the direct effects of anthropogenic climate change.

Chasing Coral lets 9 leading coral experts explain the science

Let’s however not start with the statements, but with the sources. Chasing Coral is a documentary that lets the actual global experts speak – no less than 9. In order of appearance they are:

• Phil Dustan (coral reef researcher at University of Charleston)
• James Porter (marine biologist at University of Georgia)
• Ruth Gates (coral reef biologist at University of Hawai’i)
• Ove Hoegh-Guldberg (coral reef biologist at University of Queensland)
• Mark Eakin (coral reef oceanographer at NOAA Coral Reef Watch)
• John (Charlie) Veron (coral communication pioneer and retired chief scientist of Australian Institute of Marine Science)
• Justin Marshall (marine biologist at University of Queensland)
• Joanie Kleypas (marine ecologist at National Center for Atmospheric Research)
• Neal Cantin (coral core archivist at Australian Institute of Marine Science)

Distractingly beautiful images in climate impact documentary Chasing Coral – yet with a very disturbing underlying message: coral bleaching due to heat-induced die-off of symbiotic algue is increasing across the globe – and with ever rising temperatures, reefs have less and less time to recover, leading to very dire projections as global climate change continues… Healthy coral reef on left image – on right side same reef after heat-induced coral bleaching event. Image from Chasing Coral documentary.

Another strong point: Chasing Coral has excellent geographical coverage

You can’t make a very convincing statement about the state of the world’s forests by looking at a dying tree. [Nor the opposite – in fact we have a very healthy tree in our own backyard, while the Amazon is drying out (climate change), the boreal forests are plague-infested (climate change) and Asia’s remaining low-land rainforests are turned into shampoo and other consumer articles (humans).]

The same applies to coral reefs. Depending on size, latitude and ocean or sea different tropical reefs consist of very different coral species. In the Caribbean 65 different species of reef-building corals can be found; around Hawaii science has described about 70 reef-building species, some 200 have been documented around the Pacific island of Guam, while Australia’s Great Barrier Reef consists of no less than 600 different coral species. Including the equally beautiful (horn-like) gorgonians and sea anemones the class of Anthozoa consists of thousands of different species. So no doubt it’s possible to capture one that’s not feeling too well, healthy scepticism would dictate.

Even from the air a coral reef ecosystem is beautiful. There are very many different coral reefs across the world, each of them unique ecosystems with unique species – and vital to life in the wider oceans as spawning grounds for very many species of fish.

But that’s not how the picture is drawn in Chasing Coral. The film crew don’t focus on one single hotspot of coral bleaching but in fact go through great lengths to develop global coverage, creating an overview of the state of tropical reef systems around the world. In order of appearance, the documentary makers try to capture (time-lapsed) bleaching at various reefs and atols: Carysfort Reef (Florida Keys), Discover Bay (Jamaica), Hawaii, Airport Reef (American Samoa), Bermuda, Bahamas, Keppel & Heron Island (Southern Great Barrier Reef), Lizzard Island (Northern Great Barrier Reef) and New Caledonia.

Image from latest IPCC report, illustrating expected (simplified) marine ecological effects of climate change. Caribbean and Indo-Pacific coral reef systems are especially vulnerable. See our special article for broader summary of climate-induced extinction risks according to IPCC AR4 & AR5.

Later in the documentary (within the global diving community) they ask other coral bleaching witnesses to join their effort and testimonies are included from Mexico, St Thomas, Bonaire, Costa Rica, El Salvador, East Texas Flower Gardens, French Polynesia, Kiribati, Tuvalu, Vanuatu, Coral Sea, Papua New Guinea, West Papua, Republic of Palau, Okinawa, Philippines, Bali, Scott Reef & Rowley Shoales (West Australia), Indonesia, Christmas Island, Malaysia, Sri Lanka, Thailand, the Maldives, Swio, Réunion, Mayotte, the Seychelles, Tanzania and Egypt – wonderful, not just because it’s such excellent coverage of the Caribbean, Pacific and Indian Ocean, but also because it shows these people are able to mobilise the help of so many others across the globe(!)

Now to the actual science – yes, coral reefs are the canary in the coalmine, starting the ecological collapse of anthropogenic climate change

It really is hard to over-dramatise the gravity of a mass extinction, and sadly there is very broad scientific consensus that the world is entering one on our present watch – the Holocene-Anthropocene Mass Extinction, ‘one of six’ such events in Earth’s ±540 million year long history of complex life if you express the Anthropocene Extinction as a relative decline in biodiversity and possibly the worst one in Earth’s full geological history if you express it in the potential absolute number of extinctions.

Anthropogenic climate change is just one of the (synergistic) drivers of this mass extinction – and it is the aim of this journalistic series to investigate the extent of its role. For coral reefs specifically, that role is exceptionally large – as ‘it’ (our CO2 emissions) cuts both ways through the reefs: climate change (coral bleaching as a result from ocean warming) and ocean acidification (calcium carbonate disturbances from ever-increasing ocean CO2 concentrations).

Chasing Coral plays it safe and chooses not to focus on ocean acidification (which is complex and uncertain, yet still an extra risk to coral reefs)

The directors of Chasing Coral choose to fully ignore the latter though [to our knowledge – our attention could have slipped somewhere(?)], and from the perspective of creating a criticism-free documentary that’s probably a wise move. Not that ocean acidification is not a potentially grave problem (to entire marine ecosystems, including coral reefs) but it’s very complex science* that is sometimes very worrying (hampered carbonate formation! – possibly extremely damaging to many species of shellfish, including oysters – and cold water snails) prompting a forecast that Indo-Pacific coral reefs might die from ocean acidification below a pH of 7.7 with occasionally more hopeful updates – for instance regarding biochemically adaptive capabilities of individual marine life forms, including some more acidity-resilient coral species that may profit at the expense of others, or the alkalinity producing help of those friendly compensating sea cucumbers in the Great Barrier Reef.

[*) Ecological disturbances of coral ecosystems can of course manifest itself in many other forms, including algae and seaweed plagues. Science also teaches us that ecological damage to reef ecosystems can be irreversible, requiring millions of years of evolution to get reefs back in place.]

…as coral bleaching is ‘bad enough’ to illustrate the impacts of anthropogenic climate change on coral reefs – an impact that is already clearly measurable

So how about that coral bleaching? Coral bleaching – as neatly explained in the documentary – is the result of a die-off of the symbiotic algae that live inside the soft tissue of the coral animals, and that the coral depends on as a photosynthesizing (carbon sequestering!) food source.

Coral bleaching in response to sea water temperature rise, as captured by the documentary makers in Chasing Coral.

When the algae die, the coral animals actively expell them from their bodies, leaving colourless, transparant tissue that shows the carbonate coral skeleton beneath – which is white, hence coral bleaching – the reefs literally turn from diversely coloured to white, as the algae die.

At first that is: a bright white-bleached coral reef is still alive. Sick – but if conditions improve quickly enough for the algae it depends on to return, it could recover. When it starves and dies, plague algae show up and cover the dead coral in a haze, after which the carbonate reefs slowly start to discolour and erode.

When the coral dies, even the bright white of coral bleaching disappears – after which the carbonate reefs will eventually erode completely, leaving nothing but sand, completely wiping away an ecosystem and all the species that depend on it – including many fish.

• “29% of (the corals on) the Great Barrier Reef have died in 2016”

(fact check: according to the ARC Centre of Excellence for Coral Reef Studies in the northern part of the Great Barrier Reef coral bleaching in 2016 reached 95% – in 2017 the mass bleaching continues, now hitting the central part of the reef system…)

This coral bleaching is caused by something so banal, it’s almost hard to believe: temperature rise – and a really rather subtle one: 1 degree, 2 degrees – that order of magnitude. When we think of tropical coral reefs we imagine a lukewarm-to-warm bath (possibly a supplementary cocktail) because that is what tropical seas are, right? ‘So how big can a difference be?’

But then we tend to overlook that ocean water has both a large mass and high conductivity, so it tends to also be quite stable in temperature – a rather specific temperature. And coral species, as goes for Earth’s biodiversity in general, evolved over very long periods of time – to very specific conditions. Specific biochemistry, specific ecosystem dependence (symbiotic algae!) and also specific climate conditions. There’s an acceptable margin for natural fluctuations, but for many species (especially in the long run) – it’s not much larger than that: when a stressor builds up beyond a tipping point positive feedbacks become dominant and ecosystems collapse. This goes for the Arctic, this goes for the Amazon rainforest, and this also very much goes for tropical coral reef ecosystems.

Now where is that threshold? Well, lower than for many other climate change tipping points in Earth’s biosphere – and possibly even lower (on average, there are different oceans, there are different reefs) than that really ambitious Paris climate target of limiting global warming to no more than 1.5 degrees (‘1.5ish’ – 1.5-2 degrees, see margin (‘Paris range’) in graph below).

Coral reefs are most sensitive to anthropogenic climate change, this 2016 comparison of climate tipping points shows. Only the very ambitious RCP2.6 emissions scenario offers hope for the future of the world’s coral reefs.

The above graph is not from Chasing Coral, but comes from a 2016 publication in Nature by Hans Joachim Schellnhuber, Stefan Rahmstorf and Ricarda Winkemann. (You may remember the first two names as kind participants in our expert survey on (temperature) climate sensitivity – hotshots from the scientific climate community.) To explain the graph: it shows various climate impact tipping points. For instance the West Antarctic Ice Sheet (WAIS) that we addressed in our special series about sea level rise has a tipping point for partial to complete (feedback-induced) melting from 1 (partial melting) to 5.6 (full melting) degrees of global average temperature rise, above pre-industrial climate.

Coral reefs are really the most urgent climate impact showcase that science can think of – they are even more sensitive to anthropogenic climate change than the Arctic sea ice, that we actually see rapidly melting before our eyes. Tropical coral reefs have a (partial) extinction threshold that could be lower than 1.5 degrees – and is definitely lower than 2 degrees global average temperature rise.

The below graph comes from Chasing Coral and shows due to anthropogenic climate change the steady rise of atmospheric temperatures is mirrored in the oceans, as a steady rise in water temperatures. Please take a look at our concluding global temperature graph – or our underlying 25-part series about the global temperature trend if you want any further confirmation on these figures. (Yes, the trend goes up – everywhere.)

Now those dire coral extinction forecasts: even if we ignore coral-damaging impacts of CO2 ocean acidification the critical temperature rise that no longer allows for coral reef recovery will indeed be reached within a few decades. The really depressing thing is: if you add climate inertia to that equation, that warming may already be inevitable…

During a bleaching event – to the surprise of the film crew – some corals around New Caledonia first turn to glowing fluorescent colours as the coral produced a chemical sunscreen to try to protect itself from the heat.

“Incredibly beautiful face of death – as if the corals are saying: please notice” (Chasing Coral, New Caledonia fluorescent bleaching)

The documentary is really, really good, but the story is really, really bad. Now go watch the film, make sure your friends watch it too (and if they care perhaps also share this supplementary article?) and then please if you haven’t already join the global community of coral protectors and the wider global climate movement. There is so much to fight for, in the oceans and on land. And we literally have a mass extinction to prevent. The challenge of a lifetime.

Links:

• Chasing Coral official documentary & community website
• Watch Chasing Coral full documentary on Netflix

© Rolf Schuttenhelm | www.bitsofscience.org