GWEN IFILL: Next: the decline of coral reefs and the connections with rising levels of carbon dioxide.
New reports this week show there were nearly 38 billion tons of carbon dioxide emitted around the globe last year. That, among other things, is having a very real effect in places like the Florida Keys.
Hari Sreenivasan traveled there recently and filed this report for our series Coping With Climate Change.
HARI SREENIVASAN: Ken Nedimyer has been diving in the Florida Keys almost his entire life, but the teeming coral reefs that he remembers from his early days of diving are now long gone.
KEN NEDIMYER, Coral Restoration Foundation: It's maybe 20 percent of what it was, in some places 5 percent, from what I remember seeing when I was young.
You know, I remember seeing fields of elkhorn coral that you couldn't see through it, you couldn't see beyond it, and those same areas are all dead, you know, 99 percent dead.
HARI SREENIVASAN: His memory is backed up by the facts. According to data from more than 100 monitoring stations in the Florida Keys, there has been a 44 percent decline in coral reefs over the past 20 years. On many Caribbean reefs, it's even worse. The decline is up to 80 percent over the past three decades.
KEN NEDIMYER: So, there is a lot of things working against coral reefs right now.
HARI SREENIVASAN: The 358 miles of reef along the Florida coast have been struggling to adapt to a wide range of problems, including overfishing and pollution stemming largely from increased human activity.
That's why Ken Nedimyer is mobilizing small armies to help him restore the reefs.
I joined Nedimyer along with a group of ecotourists. These volunteers donate their vacation time, dive in and help reconstruct the corals that support the underwater ecosystems they come to see.
Nedimyer has built underwater nurseries, where young endangered staghorn and elkhorn coral are allowed to mature for more than a year before being replanted on existing reefs.
MAN: This is one of the corals that we planted a couple years ago.
HARI SREENIVASAN: The nurseries have been successful, growing a small forest of elkhorn coral that Nedimyer and his volunteers almost can't replant fast enough.
But there's a bigger challenge these corals face: the impact of increased carbon dioxide in the water. It's what scientists like Chris Langdon of the University of Miami study.
CHRIS LANGDON, University of Miami: And it's enough railroad cars stacked end to end to wrap around the earth seven times. That's how much carbon is going into the ocean every single year.
HARI SREENIVASAN: All that carbon has caused global sea surface temperatures to rise by about 1.5 degrees Fahrenheit over the last century, according to the National Oceanic and Atmospheric Administration.
Periods of high water temperature cause corals to bleach or expel the colorful algae that live in their tissue, exposing their skeletons.
But the CO2 doesn't just warm the ocean. It also changes the pH levels, which measure the amount of acid in the water and shows they are becoming acidic.
CHRIS LANGDON: What's really and completely unique about what's going on now is the rate of change. And that's what is so difficult for organisms.
Things are incredibly adaptable, but the adaptation rate -- evolution takes time.
HARI SREENIVASAN: Acidification acts a lot like osteoporosis does in humans. But in marine animals, it makes their shells and skeletons brittle. The more acidic the water, the harder it is for corals to grow their skeletons. That leaves them more susceptible throughout their lives to other stressors like disease.
But there is also evidence that it's harder for corals to reproduce when the ocean is more acidic.
CHRIS LANGDON: So, that means, if a coral dies, there is less likelihood that a new baby coral is going to be able to replace it in the future.
HARI SREENIVASAN: Scientists have been running lab experiments to see how coral reefs will react to the dual impacts of increased warming and acidification.
What do we have in these tanks? What is happening?
RENEE CARLTON, National Oceanic and Atmospheric Administration Coral Health and Monitoring Program: We are controlling lights. And then we're also controlling the amount of CO2 that's in the tanks.
So we are pumping in CO2 to elevate it to conditions that are predicted for 2100. We're looking at the growth rates of these corals and we're looking to see how these corals are able to grow under these different conditions.
HARI SREENIVASAN: So these corals are in a way living in the future?
RENEE CARLTON: Yes, that's a good way to think about it. They are essentially living in the future.
HARI SREENIVASAN: It will be a tough future. Langdon finds that when corals are exposed to the expected CO2 levels for 2100, bleaching occurs 50 percent more often.
CHRIS LANGDON: Elevated CO2 can aggravate the sensitivity to temperature. It actually lowers their resistance to elevated temperatures. What that means, in effect, is that they can actually -- they will start to show signs of bleaching at a lower temperature than they would have before.
So the combination of elevated temperature and CO2 is worse than just high temperature alone.
HARI SREENIVASAN: But he adds that there are some coral species that may be able to adapt to high CO2 and that corals can recover if pH levels can be raised, although that's a challenge requiring balancing the acidity locally while also reducing global CO2.
The decline of coral reefs has ecological and economic consequences. Coral reefs are the most biodiverse ecosystem in the world. With more than 500 species of fish living on Florida's reefs, less coral has a ripple effect up the food chain, says Chris Bergh, director of the Florida Nature Conservancy.
CHRIS BERGH, Florida Nature Conservancy: The fish and the lobster and all these other animals that are so important to our economy and for the environment, they depend upon that coral growth and coral reef. And if you have that much of a loss, it really has a cascading impact.
HARI SREENIVASAN: Florida is no stranger to storms, and healthy reefs buffer up to 90 percent of the force of incoming waves, providing shoreline protection to people and property from storm surge and erosion.
Then there's the dollars and cents. More than 33,000 jobs in the Florida Keys alone are supported by ocean recreation and tourism industry, which accounts for 58 percent of the local economy and an average of $2.3 billion a year.
CHRIS BERGH: It is the lifeblood of our economy in the keys. We get millions of visitors a year who spend millions of hours out on the ocean diving and fishing on our coral reefs.
HARI SREENIVASAN: Amy Slate's dive resort depends on coral and the divers who come to see them.
AMY SLATE, Amoray Dive Resort: Because we deal so much with nature and with diving, it's probably life or death for my business, I hate to say.
But if the coral reefs thrive and grow, the more wildlife you have and the nicer it will be for everyone, and the more the divers will want to come here.
HARI SREENIVASAN: Divers like the volunteers helping Ken Nedimyer rebuild reefs.
KEN NEDIMYER: I think our effort is certainly not the answer. It's a part of the solution. It's doing something. It's buying us time.
HARI SREENIVASAN: Time that maybe is running out for coral reefs in Florida and elsewhere.
JUDY WOODRUFF: Hari's next story is about the impact warmer and more acidic waters have on shellfish. You can preview that and previous reports on our Coping With Climate Change page on our website.