WILLIAM BRANGHAM: As heat waves roll across so much of the world, oceans are also heating up.

Surface temperatures in many oceans are right now breaking historical records, including in the Atlantic, where these warmer waters threaten ocean life and coral reefs.

For example, off the coast of Southern Florida, surface water temperatures have topped a shocking 100 degrees Fahrenheit, triggering a sudden massive bleaching event on some coral reefs.

To help us understand more about what's happening and what can be done, we're joined by Katey Lesneski.

She studies the coral reefs at the Florida Keys National Marine Sanctuary for the National Oceanic and Atmospheric Administration.

Katey Lesneski, it is so good to have you on the "NewsHour."

I know you have been diving in some of the reefs recently.

I can't imagine what it's like swimming in water that is that warm.

Can you just tell us a little bit about what you have been seeing on those reefs?

KATEY LESNESKI, Florida Keys National Marine Sanctuary: Yes, thanks, William, for having me on today to talk about this important and timely issue.

So I work on the reefs as many days as we can get out there.

And I have been out over the last several days.

And, immediately, when you jump into the water, it feels like you are in a hot tub.

Fortunately, the waters on the reefs here are not at 100.

They're in the low 90s.

That 100-degree temperature was taken in a shallow landlocked area.

But we are still seeing temperature records being shattered.

And that has direct impact on all of the amazing life on the reefs, including the coral reefs.

And what I'm seeing right now are stark white corals that are undergoing what we call bleaching, which, if corals can't recover, can lead to their pretty immediate death.

WILLIAM BRANGHAM: If corals experience that die-off or that bleaching, does that mean that the corals cannot recover?

KATEY LESNESKI: So, corals can indeed recover from periods of bleaching.

Bleaching can actually be caused by a number of different factors where environmental conditions aren't quite right for the corals.

Of course, the most well-known one at this point, the most well-studied perhaps, is increasing temperatures.

And if conditions don't return to a preferable state, so if the waters don't cool off, these corals can actually die from that bleached state in a number of days to a number of weeks.

WILLIAM BRANGHAM: And, at that point, then, they're not coming back.

KATEY LESNESKI: Exactly.

Once they die, there are other reef organisms that will actually settle on that skeleton, take up space, and the coral tissue can't grow back, unfortunately.

WILLIAM BRANGHAM: I mean, for people who may have only seen coral reefs in videos or photographs, I mean, we know how extraordinary they are to look at.

But can you remind us why they're so valuable within the ocean ecosystem?

KATEY LESNESKI: So, corals and coral reefs provide a really wide number of ecological and economic benefits.

We can basically bin those into two categories.

So, just from an ecological standpoint, over 25 percent of all of the world's marine species depend on coral reefs and the structure of corals at some point in their lifetime.

So that's everything from sea turtles and sharks that people love to see to lobster and conch, other commercially fish species, game fish.

Florida alone depends on reefs for somewhere between $2 billion and $4 billion dollars a year from tourism and all the other valuable parts of the reef.

So, the ecology and the economics are very closely intertwined.

WILLIAM BRANGHAM: Is there anything that you have been seeing that gives you hope that there might be some solution, some way out of this?

KATEY LESNESKI: Mm-hmm.

So I have been diving on some of the deeper reefs that still show signs of what we call resistance to bleaching, where corals are not undergoing any of those signs associated with bleaching.

And so that -- these depth pockets can almost serve as a refuge.

There are other individuals at other sites, these very shallow reefs, that still look quite healthy.

So that tells us something about potentially the genetic diversity and the unique genetics that could -- in these corals could contribute to their ongoing resistance to this bleaching.

So we will be looking at those deeper reefs and tracking those unique individuals as much as we can into the future.

WILLIAM BRANGHAM: Can I ask you what it is like for you personally, as someone who studies these, to see the temperatures go up like they are and then to go in and to see these direct impacts?

I mean, that's got to be a difficult thing to witness.

KATEY LESNESKI: Yes, it's been very difficult for me and many of our colleagues who not only have the reefs part of our day-to-day job, but it's something that many of us enjoy spending time on, on the weekends and what we have really made our career out of to study.

We did not expect to see this amount of bleaching occurring this early in the season.

These predictions, they seem to be changing by the day, based on just how rampant this widespread heat wave is here, as well as the prolonged ocean temperatures.

And we're already seeing hundreds of corals dying all across the Florida Keys.

So, every day, we're holding our breath, and we will see what tomorrow brings.

WILLIAM BRANGHAM: All right, Katey Lesneski at the Florida Keys National Marine Sanctuary, thank you so much for being here.

KATEY LESNESKI: Thank you, William.