Climate Change's Rising Ocean Temperatures Bleaching and Killing Earth's Vital Coral

Posted July 26, 2017

MP3 Interview with Steve Palumbi, professor in Marine Sciences at Stanford University, director of the Hopkins Marine Station and senior fellow at the Woods Institute for the Environment, conducted by Melinda Tuhus


[This interview was originally broadcast on Jan. 4, 2017.] Coral reefs worldwide have been experiencing some of the most visible signs of climate change, as coral formations struggle to adapt to warmer ocean water. A recent bleaching event devastated the northern area of the Great Barrier Reef off the coast of Australia. Corals are an extremely important part of their ecosystems, as they are home to a great diversity of fish that provide food for other species, including humans. Due to bleaching events and other problems, scientists estimate that coral populations today are only 30 to 50 percent of what they were a century ago.

About two-thirds of the Great Barrier Reef's northern shallow-water coral, a previously pristine 430-mile stretch is dead. Only a cyclone that reduced water temperatures by up to three degrees Celsius in the south saved the lower section of the 1,400-mile reef, a United Nations natural World Heritage Site, from damage. Other global coral bleaching events occurred in 1998, 2005 and 2010.

Between The Lines' Melinda Tuhus spoke with Steve Palumbi, professor of Marine Sciences at Stanford University who serves as director of the university's marine lab in Pacific Grove, California. Palumbi is an expert on the coral reefs around Palau, an island nation in the south Pacific. Here, he explains that corals are both an incredibly old and resilient life form, yet very vulnerable to human activities, and provide many benefits to the ecosystem that most of us don't know.

STEVE PALUMBI: Coral bleaching is where corals spit out their internal colored symbions and they turn stark white – that's why it's called bleaching. And many of them die as a result of that. These bleaching events have pulsed through the global coral population, in some cases reducing it by 90 percent locally, in some cases, 50 percent.

BETWEEN THE LINES: I think I have some idea of the importance of corals, but can you elaborate on that? If they collapse, what is the impact on the rest of their ecosystems?

STEVE PALUMBI: Say you're living in a coastal city and someone comes up to you and says they've just invented a self-repairing sea wall. And this wall will continue to grow, it will continue to fix itself over time, it will continue to get wider, and by the way it will also produce sand for your beaches. You'd look at those people and say, that's ridiculous. There' no way you can sell me a repairable, growable sea wall that makes my beaches for me. But that's what corals do. They produce these walls in front of coastal villages, cities, towns, coastlines. They protect these coastlines from surge waves, from storms, from tsunamis. They produce the sand that beaches are made of in the tropics, and by the way, they also support almost a quarter of the marine species on the planet. They produce such an amazing volume and diversity of fish that hundreds of millions of people get most of their animal protein just from fishing on coral reefs. They're stunningly beautiful; they inspire people all over the world, and they're one of the most productive ecosystems on earth.

BETWEEN THE LINES: Is that all? Now, Steve Palumbi, can you clarify when bleaching kills corals and when it doesn't?

STEVE PALUMBI: So what's happening in this animal – corals are animals – there's a thin film of living tissue over the skeleton that they've been building little by little underneath them. They are powered by internal symbions that are photosynthetic, so they take the profuse sunlight on tropical reefs and convert that to energy that the coral uses to live and grow. So when they bleach, the coral animal is alive, but the symbion is gone, so their ability to use this sunlight as energy is gone, meaning about 90 percent of their food supply, their energy supply, is now unavailable to them. That doesn’t kill them outright, but it starves them, and it doesn’t take very long for the coral to succumb, with all the other dangers that are out there on the reefs – all the other critters eating them, boring into them, growing over them, and all of that. In places where bleaching is really severe, it means the environmental insult that caused it – usually heating – is also severe, and so the death rate from bleaching is even higher – 90 percent, 95 percent of entire bleached reefs are dead right now in the northern Great Barrier Reef due to the last bleaching event that happened there.

The good news about all that is that those bleaching events, so far anyway, are transient. They happen over a week or two, the water cools back down, and the conditions are generally then okay for corals to grow. So, all else being okay and no other problems existing there, even an intensely bleached reef can recover. And we saw that after the 1998 bleaching in Palau and Australia and other places. A decade later, the reefs were beginning to regrow; they were coming back to a great extent; the diversity wasn#x2019;t quite as high. But reefs, like a lot of marine ecosystems, are incredibly productive and capable of regrowing if they have the chance.

BETWEEN THE LINES: Are any countries doing better than others at protecting coral? I suppose since oceans are global, it must be handled globally.

STEVE PALUMBI: There are aspects of it that can only be handled on a global level, you're absolutely right. Practically everything we do as humans in our economy on the coast is detrimental to corals. Yet, they have been around on the planet for a quarter of a billion years and have survived at least two mass extinctions. On one level they seem really robust and survivors and on the other level, everything we do to them hurts them. Locally, cities, towns, villages, coastlines, have a lot they can do to support coral growth. They can prevent erosion; they can keep pesticides and fertilizers from going into the local water; they can limit over-fishing.

BETWEEN THE LINES: What does your research tell you about coral’s chances of survival as water temperatures keep warming?

STEVE PALUMBI: We’ve found that corals really are quite able to deal with high water temperatures, but just some of them are. There are some species that are very resistant to it, but even within species, some individual corals have the right genes and they have the right environmental history – they're living in the right place so that their physiology is tuned to better resist high water temperatures.

Then there are the two creeping problems that come with climate change – that's the ocean getting warmer and also the ocean getting more acidified than now. Those things are not going to be changed unless we fix the global emissions problem we have, and that requires a global solution to climate change and a global solution to throwing fossil fuels into the air.

Learn more about the urgent issue of coral bleaching by visiting Palumbi Lab at Stephen Palumbi at

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