By now you’ve probably heard that there’s been a major coral bleaching event in Australia’s Great Barrier Reef. What you might not know is why it’s such a big deal, so let’s talk about coral bleaching: how it happens, why it happens, and what it means to the reef, to people, and to the planet.
Although there are two types of coral reefs – shallow tropical and deeper cool-water reefs – coral bleaching is only an issue for the tropic kind, so when I talk about reefs I’m talking about the coral communities found in calm, warm tropical waters in the shallow regions around islands or along edges of larger land masses.
Coral reefs (dark pink dots) are found in calm, shallow tropical waters around the world. 
Coral reefs are formed over long periods of time in places where weather and ocean conditions are generally pretty stable. The hard mounds and ridges that form the structure of the reef are made of calcium carbonate: the remains of corals and other animals lived there . Large reefs like Australia’s World Heritage-listed Great Barrier Reef take thousands of years to form, with living corals growing on top of their ancestors: a colourful living layer atop the reefs of time past.
Corals themselves are tiny little animals that are related to jellyfish (phylum cnidaria). What we think of as a single coral – the single structure – is actually a colony of teeny individuals all of the same species, living together as some kind of super-organism. They’re not on their own though: the corals take on boarders called zooxanthellae, wee single-celled algae that live within the coral animal’s body tissues. The corals and their algae have a mutually-beneficial relationship, with the corals keeping the algae safe from other critters who would eat them, and the algae supplementing the coral’s food source by sharing some of the sugars they may through photosynthesis . It’s like having your own personal power plant just under your skin.
The wondrous bright colours of corals are a result of this happy cohabitation arrangement: pigments produced by the algae that get trapped beneath the coral’s skin. Without their algae boarders, corals lose their colour. Worse, without the sugars the algae provide through photosynthesis, the corals can’t catch enough food to meet their energy needs. Corals without algae turn white. This is called bleaching . A bleached coral is a very stressed coral indeed, and there’s a pretty good chance it’ll die.
So what makes corals lose their algae pals?
Prolonged warm water temperatures cause corals to kick out their zooxanthellae algae and turn white, and if conditions persist, die. 
Corals lose their zooxanthellae in response to stress, and things that stress corals out include extremes of temperatures, pollution, and too much sunlight. While temperatures that are just too hot or too cold for corals to be happy will cause sudden, large bleaching events, pollution tends to be a chronic stress that makes corals less able to cope with temperature shifts and is rarely fatal on a massive scale .
Bleaching across large areas of reef all at once only happens when water temperatures get too hot for too long, causing corals enough stress that they expel the little algae . In the summer of 2016 conditions in the Great Barrier Reef got very hot indeed.
You know about El Niño, right? Ocean current patterns cause big changes in rainfall and temperatures in countries around the Pacific Ocean: Australia is hotter and dryer in El Niño years, and cooler and wetter under the opposite pattern, La Niña. Temperatures in the oceans change too, and in El Niño years heat builds up in waters around northern Australia and up into South-East Asia.
The summer of 2015-2016 was in a very strong El Niño year, with much warmer than normal ocean temperatures around much of the word, including record heat in the Great Barrier Reef area in February and March. This long exposure to hot water caused the recent bleaching event, and a combination of particularly big temperature increases plus heat that stuck around for a really long time made it the worst coral bleaching event the Great Barrier Reef has ever experienced .
Ocean temperature data for the Great Barrier Reef shows that maximum surface water temperatures in 2016 have been between 1 and 4 oC warmer than average. 
A section of the Great Barrier Reef showing moderate to severe levels of bleaching (photo credit: Mia Hoogenboom for ARC Centre of Excellence for Coral Reef Studies).
The northernmost section of the Great Barrier Reef – the part previously in the best condition – has been hit the hardest as that’s where the water was hottest. In this section over 99 % of all the individual reefs showed some sign of bleaching. Down at the southern end, where temperature increases were smaller, one quarter of the reef area shows no sign of bleaching at all, and the bleaching is mostly mild enough that the corals will largely recover .
When a reef is recorded as bleached, it doesn’t mean that all the corals in that reef have lost their zooxanthellae and will die, but that some percentage of the corals over the reef area have done so. The bleaching may be mild (1 to 10 % of corals), moderate (10 to 50 % of corals, or severe (> 50 % of corals)  and the bleaching may be evenly distributed or restricted to bad pockets. When you hear people say that they can’t see bleaching and that their dive reefs look as good as ever, it’s because they’re looking at small sections and not the Great Barrier Reef as a whole.
Official data from the Great Barrier Reef Marine Park Authority indicates that only 7 % of the total reef area has escaped some degree of bleaching. As says Professor Terry Hughes, a coral reef expert based at James Cook University, “the will change the Great Barrier Reef forever”. 
Data from aerial surveys of 911 reefs within the Great Barrier Reef show extensive bleaching in the north, with the severity and extent of bleaching reducing towards the south.  (Credit: ARC Centre of Excellence for Coral Reef Studies / Tom Bridge and James Kerry)
It’s not just the Great Barrier Reef though. Warmer temperatures have been occurring across all the coral reef regions around the world, with widespread reports of bleaching, including form areas that have never been known to bleach before, like the coral reefs of Western Australia, New Caledonia, and the Caribbean . It’s the third, and biggest, global mass coral bleaching event. So we’ve got the worst coral bleaching we’ve ever seen and it has happened on unprecedented scales. What’s going on?
The data tells us that this is climate change. 
As greenhouse gasses build up in the atmosphere, our world gets hotter. Lots of that heat gets absorbed into the ocean and stored deep down. The ocean currents of an El Niño event drag all that heat up into surface waters and spread it around, where it also warms up our weather conditions on land and changes rainfall patterns. Although El Niño events are natural, climate change is making them hotter, last for longer, and happen more frequently.
All the evidence about the 2015-2016 severe El Niño and associated mass coral bleaching points to human-induced climate change as the root cause. Put bluntly, our use of coal and oil is costing us the world’s coral reefs.
Sea surface temperature record for the year to date (top) show that the 2016 warm water event has been global in extent, with conditions for large-scale coral bleaching events (bottom) reached in most of the world’s coral reef regions.
What does it matter if we lose our reefs? For those who value nature and biodiversity for its own sake any loss is a cause for grief. We are destroying a natural treasure: an ecosystem of complexity and astonishing beauty that our children and grandchildren will not get to enjoy, robbing the future to live unsustainably now.
Not everyone values nature for just existing though. For other people it’s about monetary values, so we try to quantify the impact in financial terms. There’s the tourism losses as visitors stop coming – valued at $5 billion a year  – plus impacts on fisheries of losing important fish habitats and food sources. Modelling from 2009 puts the cost of permanent bleaching at $37.7 billion total, or $1.08 billion per year over the next century, or 3.5 % of national GDP .
Losing the Reef will cost us.
It’s also a worrying indicator of the bigger picture, of the losses still to come as climate change becomes here and now. This global coral bleaching event represents and ecological shift: a big enough change in environmental conditions that coral reef ecosystems may disappear forever. One of many recent signs that climate change is now out of control and that our planet is rapidly and permanently changing (like the burning of Tasmania’s highland peat forests, and the loss of California’s giant sequoia forests). Whole ecosystems are being lost, and we don’t know what will replace them.
For people who study the environment and who understand how utterly dependent we all are on it, the bleaching of the Reef looks very bleak. There’s a deep sense of grief for what is being lost, coupled with anger at the political and economic systems that have brought us here and still refuse to change. Climate change is altering our planet before us while politicians and lobbyists still argue it’s not real. The situation is grim: disempowering, demoralising and downright depressing.
What can we do?
Responding meaningfully to such bad news is seriously challenging. It’s natural to get depressed and to disengage from bad things you feel you can’t change. Facts are, using fabric shopping backs, doing your recycling, and taking the train to work isn’t going to do much; certainly not on the kind of political and economic scale we need to address climate change.
That doesn’t mean that people like us can’t do anything though. It just means we need to be a little more strategic, to focus our efforts on ways to shift energy and production systems away from fossil fuels that have impacts at economically-significant scales. Individuals acting collectively can and do change markets and alter economies.
So how can you and I respond to the bleaching of the Reef, and to the urgency of climate change action in general? This is something I want to explore more in-depth in the future, but for now here’s a few things that we know are having an impact:
Contact your local candidates and quiz them on their party’s policies. Ask how we’re going to meet our international commitments. Talk to your family, your friends and your colleagues about it. Click on newspaper articles to keep the media reporting on it (you don’t even have to read them). Don’t argue over whether climate change is real, just demand action.
The mass coral bleaching event you’ve heard about is seriously bad news. It means climate change is here and now, reshaping the planetary systems we’re all dependent on. The Great Barrier Reef may never recover, and it seems likely that coral reef ecosystems in general will be heavily reduced or perhaps utterly lost. From here on in our world starts to look quite different, with unknown impacts on our ability to produce food, run our cities, supply water, and maintain our present ways of life. Watching this happening, a sense of despair is natural, but we are not without agency.
In response, it’s critical that we focus our energies and concentrate our efforts on where they’re most likely to have meaningful effects. The problems are systemic, so our actions must be too. If we want a future that includes coral reefs we need to change the power structures and short-term outlooks that are keeping us entangled with fossil fuels. That’s hard to do, but not impossible.
Stressed coral at risk of bleaching, Lizard Island (Credit: Dorothea Bender-Champ for ARC Centre of Excellence for Coral Reef Studies).
 Burke L, Reytar K, Spalding M & Perry A 2011; Reefs at risk revisited, World Resources institute; Washington DC, USA.
 Anon 2014; What are coral reefs?; GRID-Arendal (United Nations Environment Program); Arendal, Norway.
 CoRIS 2014; What are coral reefs; National Oceanic and Atmospheric Administration, United States Department of Commerce; Washington DC, USA.
 Marshall P & Schuttenberg H 2006; A reef manager’s guide to coral bleaching; Great Barrier Reef Marine Park Authority; Townsville, Australia.
 BOM 2016; Coral bleaching in the Great Barrier Reef; Bureau of Meteorology, Commonwealth of Australia; Canberra, Australia.
 GBRMPA 2016; Coral bleaching (media release); Great Barrier Reef Marine Park Authority; Townsville, Australia.
 NOAA Satellite and Information Service 2016; Coral reef watch satellite monitoring (geospatial database); National Oceanic and Atmospheric Administration, United States Department of Commerce; Washing DC, USA.
 ARC Centre of Excellence for Coral Reef Studies 2016; Only 7% of the Great Barrier Reef has avoided coral bleaching (media release); James Cook University; Townsville, Australia.
 GBRMPA 2006; Great Barrier Reef coral bleaching response plan, summer 2006 – 2007; Great Barrier Reef Marine Park Authority; Townsville, Australia.
 McCutcheon P 2016; Great Barrier Reef coral bleaching at 95 per cent in northern section, aerial survey reveals; ABC News (published 28 March 2016); Australian Broadcasting Corporation.
 NOAA 2015; NOAA declares third ever global coral bleaching event; National Oceanic and Atmospheric Administration, United States Department of Commerce; Washing DC, USA.
 Slezak M 2016; Great Barrier Reef bleaching made 175 times likelier by human-caused climate change, say scientists; The Guardian Online (published 29 April 2016); Guardian News and Media Ltd.
 Oxford Economics 2009; Valuing the effects of Great Barrier Reef Bleaching; Great Barrier Reef Foundation; Newstead, Australia.