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REGINA BARBER, HOST:
It was 1944. World War II had been going on for almost five years.
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BARBER: And the allies were planning the largest sea-to-land invasion in history.
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BARBER: First, they needed to assemble tens of thousands of troops off the coast of France, which you might remember from that famous scene in "Saving Private Ryan."
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UNIDENTIFIED ACTOR: (As Soldier) God be with you.
TOM HANKS: (As Captain John Miller) Port side stick. Starboard side stick. Move fast and clear those murder holes.
BARBER: Then they needed to send in soldiers overnight, crossing the English Channel to Normandy as swiftly and as silently as possible.
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HANKS: (As Captain John Miller) Keep those actions clear. We'll see you on the beach.
BARBER: But in order to make all of that happen, they needed to make sure the tides were right.
GREGORY DUSEK: So the first thing to think about is that the tide range in Normandy is about 20 feet. So that means from low tide to high tide, you know, that's 20 feet of water, which is a lot. In most places in the U.S., we have tide ranges around, say, 3 to 6 feet or something. And so that means that your beach can change in width dramatically, right? So at low tide, you might have another three, four hundred yards of beach that you would have to traverse so you're not exposed.
BARBER: That's Gregory Dusek, an oceanographer from the National Oceanic and Atmospheric Association or NOAA. And in operations like D-Day, changing tides had the potential to put the entire invasion at risk.
DUSEK: If you got that wrong, you could have had all of your landing craft stranded on the beach and potentially completely messed up the D-Day invasion.
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BARBER: D-Day is just one example of how important tides are for humans. High and low tides affect everything from shipping and fishing to floods. But what are tides? Imagine a huge water droplet the size of a planet, and the Earth is rotating inside of that. When the sun and the Earth pull on each side of this huge droplet, it makes it bulge. That's why we have high and low tides.
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DUSEK: So end up with these - kind of these two bulges on either side of the Earth of increased water level. And then away from those, as you go further away, you get lower and lower water level. You know, if I'm standing on this water-covered Earth at a point somewhere, the Earth is rotating underneath this water. And so as I pass one of these bulges, I'm experiencing a high tide. And then as I get past it, I experience a low tide.
BARBER: Understanding these tidal bulges is crucial to protecting coastlines and infrastructure from natural disasters like flooding. NOAA has just put out a report that predicts flooding up to a year in advance. This is all done by charting tides and taking into account climate change.
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BARBER: Today on the show, we're talking about the science of ocean tides and how predicting those tides have affected everything from World War II battles to where we build our homes, to what might be a green energy for the future. I'm Regina Barber, and you're listening to SHORT WAVE, the science podcast from NPR.
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BARBER: OK. So Greg, before we go back to D-Day, we're going to need to backtrack a little. And let's do, like, tides 101, OK? You used the image of Earth completely covered in water. But can you tell me first, like, why do we have low and high tides?
DUSEK: So the moon and the sun are, you know, pulling on this water-filled Earth. And the greatest amount of pull happens to be when, you know, you have the Earth, moon and sun in alignment, and that happens during full or new moons. And it can be either a full or a new moon because, you know, they don't have to be all pulling in exactly the same direction because the Earth is rotating on its axis. So even if the sun and the moon are on opposing sides of the Earth, you're still generating that tidal force in the same kind of direction.
BARBER: And every moon in between, you're getting those high and low tides that are, like, kind of, you know, average or less than that.
DUSEK: Yeah. That's right. Yeah. So you won't see as high or as low tides when, you know, you have a half-moon or a quarter-moon, for instance.
BARBER: Right. And you know people have written about tides since at least 1100 B.C. in India. So why are humans so interested in tides?
DUSEK: So, I mean, the big thing is that, I think, you know, civilization starts near the water, right?
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DUSEK: And often that's the coast because coasts provide food sources, and they provide means for transportation. And the tide is - you know, is one of the primary ways that water moves up and down along the coastlines. And so it dictates, you know, where you're going to build.
When you're talking about boating and shipping and marine transportation, you know, you need to know what the tides are so that you don't run your vessels aground. And then, of course, for food - the tides play a role in what types of food is going to be available, where fish might be, what types of plants are growing. You know, every part of civilization really is going to be touched by just that regular rise and fall of the water alongshore.
BARBER: And charting tides was crucial for that D-Day effort, right?
DUSEK: Yeah.
BARBER: So how did the scientists in World War II figure out the exact date and time to storm the beach?
DUSEK: By this time in World War II, we had a lot of water level observations across the globe.
BARBER: Cool.
DUSEK: Once you have that water level, you can predict what the tides are going to do potentially decades into the future.
BARBER: Wow.
DUSEK: And so in D-Day, they had water level data in that area, and so they had some tide predictions relatively nearby the landing beaches. And then they take all this information, and they put it into these giant machines. They're, like - probably, like, 7, 8 feet long and about as high as a person. And there are these brass machines - metal machines made up of a bunch of gears, and the gears would represent different parts of the tide, basically. And so for Normandy, they ran this information, you know, collected the data they had, ran it through this machine to get the predictions and were able to put that together at a very - very accurately. After the fact, they've gone back and looked at the predictions generated for D-Day using modern methods and found that they were actually very close.
BARBER: Wow. That's so cool. And it's super important because without that, everything gets beached.
DUSEK: Yes. And that was really important because even, you know, 30 minutes of error could have been too much.
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DUSEK: And then on top of all that, you know, it wasn't just one location along the beach. It was over tens of miles of beachfront at several different landing sites. And the tide was a little bit different at each of those locations, so the timing had to be staggered, depending on what beach people were arriving on. So a lot of complexity for such a really important and momentous human occasion.
BARBER: So they were doing predictions for different spots along tens of miles.
DUSEK: Yes. Yeah. I think it's - I think - I want to say, like, 100 kilometers or so of beach. But the night before, they were actually sending in paratroopers, you know, to get on the ground behind enemy lines. So they wanted to be able to see so that, you know, they could parachute down and see where they're going. So they actually wanted a full moon the night before, which meant that it was a pretty larger-than-normal tide range.
And then they also wanted the low tide at first - close to first light as possible because you're crossing the English Channel. They wanted to do that in darkness. It all also depended on the weather because they could have set this all up, and if there was bad weather, those landing craft would have had trouble crossing the Channel.
BARBER: And this is a great transition because we're going to talk about weather because tides aren't just historically important, obviously, but, like, there's other things. Like, tides can affect flooding. And because sea level is rising due to climate change, like, there could be other things happening. So, like, how does climate change complicate our relationship with tides?
DUSEK: You know, I think when you think about the tide and how it's impacting people, you know, we built a lot of our infrastructure along the coastlines in many parts of the world, you know, hundreds of years ago, when the sea level was much lower and you didn't have to worry about storms and tides flooding because you were removed from the coastline. So back a hundred years ago, as a good example, you know, we've had a station in Charleston, S.C., since the early 1900s.
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DUSEK: And for the first, you know, 20 or so years that that station was in, we had, like, one day that it hit the flood threshold where we would typically see flooding. And it was, like, a pretty significant storm. And, you know, sea level has risen a lot in the last hundred years in Charleston. And so, you know, you look back - we average, I think, about 10 floods per year in Charleston.
BARBER: And it's more now.
DUSEK: And so it's just moving that baseline up closer so that you don't need a storm anymore, or you just need a small storm, or you just need some subtle winds. And when you have those high tides, you start seeing flooding.
BARBER: OK. So what you're saying is that even if tides weren't super high to begin with in some given place, overall sea level is rising. So people who live there might have to worry about flooding more often. Like, high tides themselves are getting higher. And NOAA just released a new model that helps predict coastal flooding a year in advance. So maybe there's some positive news here.
DUSEK: Yeah. So I guess the one positive takeaway is that because the tide is such an important role in this, and we can predict the tide really well, that gives us a leg up to be able to tell people when they might experience these impacts, even without being able to predict the weather, because that, you know, we can't predict until much closer, you know, a week or so before. And so we predict the likelihood of flooding up to a year in advance on a daily basis at many of our tide gauges across the U.S.
BARBER: You mentioned tide gauges, and I realized we, like, didn't talk about how NOAA measures tides now. I'm assuming they don't use, like, those big, brass tide machines anymore.
DUSEK: You have this instrument that's about the size of a football, and it's called the microwave radar. And it sits up above the water surface a ways, and it emits microwaves down at the water, which bounce off the water surface back to the sensor. And you're able to tell then how high the water is relative to the sensor. So NOAA operates over 200 tide gauges or water level stations across the U.S., and these stations measure the tide in real time.
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DUSEK: And so we're measuring the water surface constantly, and then we average those to provide kind of a standard water level value every six minutes. And then those data are used, you know, for a whole host of things in real time, including making sure that, you know, people can pull into port safely and not run their ship aground, as well as monitoring things like large storms and storm surge and flooding.
BARBER: Yeah. And let's actually, like, end on hope here, right? Like, so another hopeful thing about tides is generating green energy. So, like, one way scientists are doing this is by using the power of tidal currents, right? Can you tell me more about that?
DUSEK: Yeah. So in some places, the tidal currents can be quite strong, especially, for instance, the Bay of Fundy, which is where they have the largest tides in the world. In places where you have kind of constricted coastal areas but that also have these large tidal ranges, you can have really strong currents. And so because of that, you're able to potentially leverage those currents to generate power.
And so there's been ongoing testing and research around putting turbines in the water in some of these places and generating a significant amount of tidal power potentially to power communities that might not otherwise have easy access to power in their location.
BARBER: So they would basically - I looked this up - they basically look like little windmills - just, like, instead of on land, they're underwater.
DUSEK: Yeah. So the water's pushing these turbines and generating electricity. And it's pretty amazing. It's hard to keep things in the ocean for a long time 'cause things tend to foul, grow on them and salt water is trouble. But so far, is, you know, some that has shown some success and, again, especially in places where otherwise they might not have access to power or not easy access to power.
BARBER: Greg, thank you so much for talking to us today about tides.
DUSEK: Thank you for having me.
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BARBER: This episode was produced by Rachel Carlson and Hannah Chin. It was edited by our showrunner Rebecca Ramirez, and it was fact-checked by Rachel, Rebecca and me. Robert Rodriguez was the audio engineer. I'm Regina Barber. Thank you for listening to SHORT WAVE, the science podcast from NPR.
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