Episode 4: A World Shaped by Water

(sound FX: ocean waves)

[0:03] Narrator: Last June, Astronaut Kathy Sullivan traveled to the deepest part of the ocean.

It’s the opposite direction you’d expect an astronaut to go, but then again, the deep ocean is not unlike outer space - an alien realm full of mysteries, infinitely dark, with hostile conditions, and largely unexplored.

Her roughly 4-hour journey would have gone something like this:

(sound FX: water splash)

(music)

As we sink beneath the surface waves, we travel through the realm where the bulk of ocean life exists. Tiny plants known as phytoplankton harvest the sunlight that can penetrate these shallow waters, and they form the base of the ocean’s food chain. Because the top part of the ocean is in contact with air and wind, oxygen and other gases needed by life can filter down into these turbulent waters.

[1:07] Different parts of sunlight’s spectrum start to fade the deeper we go -- around 100 meters down, we lose the red portion of light, and with it, the Sun’s warmth. By 300 meters down, or 1,000 feet, all that’s left is blue.

(ping)

That ping just marked our first 500 meters, or 1640 feet… the pressure down here is 50 times greater than what it is at the surface. And yet we still can see familiar fishes like salmon, swordfish and sharks.

(ping)

1,000 meters deep. We have now reached the midnight zone, the end of sunlight. From this point on, the blackness will only be pierced by luminescent creatures that are like strange stars in the inky void.

(ping)

[2:01] Now we’re 1,500 meters down. The precipitous decline in water temperature slows around this point – from here on out the temperature only gradually drops from 3 to zero degrees Celsius, or from 37.5 to 32 degrees Fahrenheit. Fresh water freezes at zero C, but depending on the saltiness of the ocean, ice-cold waters keep flowing down to around negative 2 C, or 28 degrees F.

(ping)

2,000 meters down, and the pressure is 200 times greater than at sea level. But our submersible vessel can withstand it thanks to the thick titanium-alloy hull. The sea creatures down here don’t have air spaces like lungs inside their bodies, so the pressure doesn’t trouble them. Other animals that dive to these depths, like whales, are able to temporarily collapse their lungs to prevent any internal damage.

(ping)

[3:03] 2,500 meters. The bottom of the ocean isn’t at the same point everywhere in the world; it varies greatly in depth and terrain.  It was at this depth of 2500 meters that scientists in 1977 first discovered tube worms and other strange life forms that live off the chemistry of underwater volcanic vents.

(ping)

At 3,000 meters down, we should be on the lookout for cosmic jellyfish, vampire squid, and zombie worms.

(ping)

3,500 meters down. We’re now close to the average depth of the ocean, but here in the Challenger Deep, at the southern end of the Mariana Trench in the Pacific, we’re not even halfway to the bottom.

(ping)

4,000 meters down. We have entered the zone known as “the abyss.” Life here feeds mostly on the dead that fall like snow from above.

(ping)

[4:07] 4,500 meters. Calcium seashells that fall to this point dissolve due to the higher carbon dioxide content of these deep waters. 

(ping)

5,000 meters, or 3 miles down. We are now in the vise-like grip of pressures 500 times greater than the atmospheric pressure at sea level.

(ping)

5,500 meters. If we were in the Arctic Ocean, we would have hit the deepest point.

(ping)

6,000 meters. Welcome to the Hadal Zone. If you’ve been feeling claustrophobic, that will only intensify because we’ve now left the open ocean and slipped down into the narrow canyon of an ocean trench -- depressions in the sea floor caused by one tectonic plate diving under another.

(ping)

[5:01] 6,500 meters – More than 98 percent of the ocean is now above us.

(ping)

7,000 meters – Earlier this year, scientists using a robotic submersible were surprised to discover an octopus at this depth, swimming in the Java trench in the Indian Ocean.

(ping)

7,500 meters – this part of the sea is so unexplored, every dive brings new discoveries. Just last year, three new species of snailfish were found at this depth off the coast of Chile.

(ping)

8,000 meters – The Mariana snailfish, the deepest-living fish ever captured, has developed incredible adaptations to survive here, including proteins in their muscles that function best under high pressure.

(ping)

8,500 meters – if we were in the Atlantic Ocean, we would have now reached the deepest point.

(ping)

[6:06] 9,000 meters – we are now deeper down than Mount Everest is tall.

(ping)

9,500 meters – More people have flown the Moon – for the record, that’s 24 - than have explored this far down in the ocean.

(ping)

10,000 meters - we’re almost there. The first people who made this journey were Jacques Piccard and Don Walsh, in 1960. Right before they reached this point, they heard a loud bang…

(sound FX: metal bang and rattles)

…that shook their cramped steel sphere. If something was wrong with their vessel, as Walsh later said, “we’d have been dead before we knew we were dead.” And so they continued on. They later discovered that bang had been a Plexiglas window cracking under the pressure. Luckily, that window was in the flooded entrance tunnel, safely walled off from their cabin.

(ping)

[7:06] 10,500 meters – As we descend, the lights on our vessel start to reveal a muddy-looking, rather featureless plain, with small sea cucumbers and shrimplike amphipods scavenging for food.

(ping- ping-ping)

We have now reached the bottom of the Challenger Deep -- just shy of 11,000 meters, or 7 miles beneath the surface of the ocean. 

This dramatic dip in the ocean floor was discovered in 1875, by the British Naval research ship “HMS Challenger.” During their epic journey around the world to study the ocean, the crew would send down weighted lines every few days to determine depth. When they got to this area above the Mariana Trench, on their first attempt they ran out of rope. They later recorded the depth to be 5 miles, not realizing that an even deeper point – the one named after them today – was right nearby.

[8:10] The Space Shuttle Challenger was named after this scientific research ship, and it was on Space Shuttle Challenger that Kathy Sullivan went on her first space mission in 1984. She was the first American woman to go out on a spacewalk. Her descent into the Challenger Deep echoes that flight of 36 years ago, for she is now the first woman ever to float down into this marine space. Kathy was an oceanographer before she became an astronaut, so in a way, this voyage has brought her full circle.

The Challenger Deep is near the island of Guam, and Kathy was interviewed by NPR while she was still aboard a ship in the area. She compared the voyage to the deepest part of the sea to space flight.

NPR interview with Kathy Sullivan, June 14, 2020
Kathy Sullivan: When you launch off the planet, you feel like you're embedded in a gigantic ball of energy. It's an explosive, turbulent, massive amount of power. And when you go down into the deep of the ocean, it's a comparatively really serene experience. It was like a magic carpet ride, just sort of slowly descending straight down. But the similarities are also there to me, as well. In each case, I'm in a vehicle - I'm in a craft that has been innovated and engineered to keep inside of it the kind of conditions humans need to survive, you know, sea level kind of pressure, oxygen, nitrogen to breathe, a reasonable temperature range, life support, human waste. We humans are clever enough. We can create these vehicles that let us go to places that we otherwise have no business at all being.

(intro music)

[10: 23] Narrator: Welcome to “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory. I’m Leslie Mullen, and in this third season we’re traveling to the ends of the Earth with scientists who explore every aspect of our amazing world. This is episode 4: A World Shaped by Water.

(music and sound FX ocean waves)

Narrator: Earth is the only planet in the solar system with liquid water on the surface. A few other planets and moons have water, but it’s either trapped deep below the surface, or it’s frozen solid.

Earth’s ocean covers more than 70 percent of the globe. In fact, so much of our planet is dominated by ocean water that the science fiction writer Arthur C. Clarke noted, “"How inappropriate to call this planet Earth, when it is clearly Ocean."

The ocean rules and regulates our world, influencing the weather, temperature and climate. Much of our oxygen is produced by ocean plants, and much of the carbon sent into the atmosphere is absorbed by the ocean. Over Earth’s billions of years, the continents have all at some point been submerged under ocean waters.

[11:36] Without our ocean, there likely would be no life today. It’s possible that life only came to be because of the unique conditions of our ocean. Life’s origin also may be related to the fact that water is a universal solvent, a seemingly magical concoction that dissolves more substances than any other known liquid.

Liquid water wants to move, to follow gravity’s pull. Because of this, our ocean is connected to the Cosmos. The gravity of the Moon and the Sun tug on the ocean, causing high tides when the Moon, Sun and Earth are in line. But when the orbit of our tiny but close Moon sets it at odds with the huge but distant Sun, we get low tides.

[12:23] Our planet’s daily rotation also affects the movement of the ocean. Earth spins at a tilt, and so it’s unevenly heated by the Sun. This leads to differences in air pressure that generate winds. The ocean currents -- the flow of water from one area of the ocean to another -- are to a great extent driven by winds. And as Earth turns forever towards the east, the force of that spin also turns the winds and the ocean water – circulating them to the right in the north, and to the left in the south -- as long as no land masses interfere.

Some ocean circulations get stuck in a rut due to the local geography. For instance, the shape of the seabed near a few islands in northern Norway results in some of the largest and strongest whirlpools in the world. This eerie phenomenon has inspired many artworks and writings, including Edgar Allen Poe’s story, “A Descent into the Maelström.”

“A Descent into the Maelström” by Edgar Allen Poe
The edge of the whirl was represented by a broad belt of gleaming spray; but no particle of this slipped into the mouth of the terrific funnel, whose interior, as far as the eye could fathom it, was a smooth, shining, and jet-black wall of water, inclined to the horizon at an angle of some forty-five degrees, speeding dizzily round and round with a swaying and sweltering motion, and sending forth to the winds an appalling voice, half shriek, half roar, such as not even the mighty cataract of Niagara ever lifts up in its agony to Heaven. The mountain trembled to its very base, and the rock rocked. I threw myself upon my face, and clung to the scant herbage in an excess of nervous agitation. "This," said I at length, to the old man -- "this can be nothing else than the great whirlpool of the Maelström!"

[14:21] Narrator: Although greatly exaggerated in such stories, there is an undeniable horror of being pulled down into the whirlpool’s abyss, a nautical black hole, a force from which there seems no escape. The sea has always had an epic presence in our stories and dreams, symbolizing the border between life and death, order and chaos, knowledge and ignorance. The beckoning horizon of an ocean voyage is like the promise of a passage towards self-discovery.

(sound FX: beach sounds)

JPL oceanographer Ben Holt grew up in Los Angeles, California, a city famous for its sun-drenched beaches and surfers riding the waves.

[15:07] Ben Holt: When I was a teenager, I had the opportunity to go to a summer camp on Catalina Island off of Los Angeles. And I really, really liked it. I learned how to skin dive and sail and did everything on a boat that I possibly could. So that got my interest in the ocean piqued, for certain.

Narrator: Many years later, a sailboat trip steered him toward a job at JPL, on a mission to study the ocean.

Ben Holt: A friend who sailed with us knew a guy that needed some help, and didn't require much experience, particularly. So I fell into the opportunity to work at JPL working on Seasat, and I was asked to do something with the SAR instrument.

It's a synthetic aperture radar or SAR, S-A-R. It doesn't require sunlight as optical sensors do, so it's very effective when there's no light, as in the polar regions during the winter. It can see through clouds. And because it's a radar, it sees the roughness of the surface of anything that it's looking at.

[16:22] Narrator: SAR instruments send microwave signals down to Earth, and what bounces back creates a detailed image of the surface. It’s kind of like how bats map their environment by calling out and then listening for the echo of the sound waves.

The “synthetic aperture” aspect of the radar means that a small antenna combines with the spacecraft’s motion to mimic what you’d get with a larger, unwieldy antenna that would be more difficult to fly.

The Seasat mission was the first Earth-orbiting satellite dedicated to studying the ocean. When Ben became part of Seasat’s SAR instrument team, little did he know that the mission was about to sail off into the sunset.

Ben Holt: My job was to do mission planning, to determine when and where to take SAR observations. I did one day on the job and the next day, I came to work and the satellite failed. It wasn’t my fault – I had nothing to do with the failure!

[17:23] Narrator: A short circuit in the electrical system was actually to blame for the end of the Seasat mission in 1978. But the satellite had already orbited Earth for several months and gathered a lot of data.

Ben Holt: So I had one day doing what I was hired to do, and the next day the satellite failed, so the focus changed a little bit to processing the data and sending it out. But it also gave me the opportunity to just start looking at the data. And I got very interested in seeing how rapidly things changed, the details of the ocean. And most of the scientists on that team were involved with oceanography.

Narrator: Ben had studied other fields of science when he was in college, including marine biology, but this job at JPL inspired him to go back and earn a degree in oceanography. He now uses a range of satellites and radar instruments to study various aspects of the sea, like whirlpool eddies or marine oil spills, including the 2010 Deepwater Horizon accident in the Gulf of Mexico.

[18:31] 60 Minutes news report: “The Blow Out” (Deepwater Horizon)
Reporter: The gusher unleased in the Gulf of Mexico continues to spew crude oil. There are no reliable estimates of how much oil is pouring into the Gulf, but it comes to many millions of gallons since the catastrophic blow-out. Eleven men were killed in the explosions that sank one of the most sophisticated drilling rigs in the world, the Deepwater Horizon.

Ben Holt: We had an aircraft flight with our imaging radar that we did over the Deepwater Horizon in 2010, as did everybody with an observation system. The whole world imaged that thing. And we had some really great results that were unique, because we’d been trying to measure oil thickness.

You know, if you see a little diesel or gasoline coming off an engine in the ocean, it's a rainbow color. So you can always see extent of the oil, but when the oil gets a little thicker, it becomes emulsified. The classic example is like a chocolate mousse. It gets a little puffier mixed in with the seawater. And that has a different response on the SAR than just a smooth sheen of oil.

[19:42] So the thicker oil is where the agencies that have to clean this stuff up want to clean up first, because it's the most damaging to the environment.

Narrator: Ben doesn’t just rely on remote satellites and airplanes to explore the ocean; he’s often out on scientific expeditions. His first field trip was to the Beaufort Sea, north of Alaska and Canada’s Northwestern Territories.

Ben Holt: It was 1982; we were based at a weather station in the Canadian archipelago called Prince Patrick Island. It's about 76 degrees North, and it’s adjacent to the Arctic Ocean. We had a couple helicopters and flew out to the sea ice, to different locations. I was so excited, I couldn't sleep. It was in June and July, and so there was 24 hours of light. And you're standing on ice, and it was pretty cool to walk on the ocean, literally. And the ice was just really pretty with different colors, hues of blue and white, the changes it goes through every day. That was the thing that really got me was what a beautiful landscape it was.

[20:50] Narrator: That journey was the start of a series of Arctic expeditions over several decades to study how the ocean and sea ice interact.

Ben Holt: There's been an extensive amount of summer melt in the Arctic. So at the end of the summer, which is technically the end of September, there's been a significant retreat or decrease in the amount of ice that remains through the summer. This happened in the last 13 years, and that's left exposed a lot of open water, particularly North of Alaska.

(sound FX: ocean storm)

And that has led to an increase in ocean wave generation. So surface waves like we have along our beaches, there's an increase in the wave height that can occur in this extensive open water. Because the winds have more presence on the ocean, and are able to blow over a longer distance and generate higher waves than have been ever seen in the Arctic before.

[21:56] Narrator: Waves are mainly created by wind stirring the surface of the ocean. Ben has had to cope with some monster waves during his Arctic adventures, when storm winds whipped the water into a frenzy.

Ben Holt: There was one episode where we departed from Nome, Alaska, but we came back to Dutch Harbor down in the Aleutians. And while were we in the Bering sea on the way back, we encountered a weather bomb, which is when the pressure decreases rapidly -- I think 24 millibars over 24 hours. And you have this sudden cyclonic pressure system, and along with it came some really steep waves.

So we had to endure two days of pounding through these really high waves at a very slow speed, because the waves were coming from where we were going. So we had to take this zigzag track to get back. We rolled a lot. Nobody was hurt, but there was some damage to tables and chairs and stuff. It was really uncomfortable.

[23:07] Narrator: To avoid being knocked around by gigantic waves, scientists often send in robot troops. For instance, to help forecast the behavior of a hurricane, the National Oceanic and Atmospheric Administration, or NOAA, drops drift buoys in the storm’s path to gather vital information.

Ben Holt: They have different kinds of drift buoys now that go around the world, and are measuring ocean currents and other properties. I think there's hundreds if not thousands of these ocean drifters. But then there's other ocean instruments called gliders -- it might look like a little airplane, a couple of meters long -- and they work autonomously. You can send those out, cycling up and down through the upper ocean for a few months at a time. They're slow moving, but they go back and forth and collect ocean data. Those are great.

[24:08] Narrator: NOAA also collaborates with other groups, such as for the Argo project, a cooperation of 30 countries that has sent out thousands of floaters to monitor ocean salinity and temperature.

Boats, buoys, floaters and gliders stay at or relatively near the ocean’s surface. The deeper ocean is more difficult to monitor, due to the extreme conditions. And radar on airplanes and satellites can only see, at best, a few millimeters down beneath the waves. Sound waves can travel farther down in water than light waves or radar, so boats equipped with sonar instruments can map the sea floor, and locate sunken objects like shipwrecks. Sonar also can be used to map ocean currents, and indicate water properties like temperature and salinity.

Another technique, used by the NASA twin GRACE satellites and their successor mission, GRACE-Follow-On, provides unique perspectives of the ocean, and all of Earth’s fresh water as well.

JPL scientist Cedric David has worked with data from the GRACE missions.

[25:16] Cedric David: We think of satellite missions as basically cameras that are flying around the planet, but the measurement system for GRACE is very different. It's a gravity system. And so it's not taking pictures per se. It's measuring the distance between two satellites, flying one behind the other.

And so by measuring very accurately the distance between these two satellites, we're able to infer mass on Earth. But what is measured by the mission is not actually the mass, it’s the changes in mass. So it's kind of like if you were to hop on the scale at home and your scale doesn't tell you how much you weigh, but basically how much you've gained or how much you've lost, compared to an average. Because water is so heavy, when there are places where water has been added or removed, it shows very nicely on the measurements of the changes in gravity.

Now, the beauty of the measurement is it includes everything, including the water in the trees, the water stored as soil moisture, the water in lakes and rivers. And perhaps most importantly, the water underground.

[26:21] Now that said, the gravity measurements are not perfect because they're very coarse. And so the images that we're able to create from the gravity measurements are blurry in a sense. We're talking about pixel sizes on the order of a hundred, 300 kilometers. So you can't see your house, you can't see the field where you're growing crops, you can barely see cities.

But in the past 10 years, the gravity measurements that we've made have really put the spotlight on water availability and water variability. And so we've been able to see where groundwater is overly pumped in a lot of places around the world, including the boundary between India and Pakistan, places with high conflicts in the middle East, but also right here at home in California, where we grow a lot of the produce that's shipped around the nation and elsewhere.

[27:16] Narrator: While the GRACE Follow-on mission keeps tabs on water’s movement around the world, many other NASA satellites and instruments are examining different aspects of Earth’s water, like how much rain is falling, or how much is frozen as mountain snow. Cedric is a terrestrial hydrologist – that’s someone who studies the flow of freshwater on land. He says there will be even more missions to study our plant’s freshwater over the next few years.

Cedric David: I mean, the 2020s, a lot of us are calling it the Golden Era for hydrologic observations from space, because we'll be able to observe all components of the terrestrial water cycle with Earth-orbiting satellites. So there is no telling what we're going to discover when we look at it all together.

So the observations that we do are of course important for scientific discovery. But they're also tremendously important for what we generally call applied sciences, which is basically the field of study where you use what you've learned, and you try to help people with that information.

(music)

[28:25] Narrator: Cedric was part of a three-year project to translate NASA satellite data into a tool to help one area of the world that has a lot of issues with their fresh water.

Cedric David: I've been fortunate to work with a team of people in the developing world, through a project that was jointly supported by NASA and by the US Agency for International Development. So I spent some time in South Asia, working with countries like Nepal and Bangladesh. All in all, South Asia is about a billion people. So it's a good seventh or an eighth of the human population that is within this geographic area.

(sound FX: river)

Nepal is home to nine of the 10 biggest mountains in the world. I spent some time up in the mountains and it's just beautiful. I mean, you see the streams, they're basically sparkling. The water is so fresh and so nice and I've swam in some of those; it’s just outstanding. Now you go a couple hundred kilometers south to the capital city, and it's just not the same story. You close your windows when you cross a river in a car, because the smell is not great. And so how does the water become so dirty, starting at the most pristine water sources on Earth, going downstream to the next city? And if you go further south to Dhaka, I mean, that's a mega metropolis. It’s densely populated; traffic is incredible there. And so there is still a lot of work to be done there, and frankly across the world, in managing the quality of our surface waters.

[29:57] And so we put together a flood forecasting system that is being used now in Bangladesh. Bangladesh is at the outlet of two of the world's largest rivers, the Ganges and the Brahmaputra rivers. And these river basins are so big that basically they cover many countries. Now, water being as important as it is for society, information related to water is sensitive, and so a lot of places around the world do not share measurements. So what we ended up putting together is a system that takes the continental-scale picture of how water moves down the river systems. And we put together a forecasting approach to basically knowing how much water would enter the country of Bangladesh across all of its borders.

So they’ve been using it for about a year now. When we delivered the tool, it was right before the monsoon season, which is the big floods happening in South Asia.

[30:56] CNN news story – Bangladesh 2020 monsoon flood
Reporter: Houses made out of corrugated metal and thatch were no match for this deluge in Bangladesh. Heavy monsoon rains that have been falling for several weeks have engulfed homes and farms, and marooned hundreds of thousands of people across the country. Local officials say nearly a third of the country is underwater, with one calling the floods the worst in a decade. There’s little choice for the stranded except to endure the conditions and wait for the waters to recede…”

Narrator: Flooding is expected to get worse and more frequent as the climate warms. To understand why, let’s take a look at the water cycle.

(music)

[31:34] Pretend you’re a tiny drop of water on the surface of the ocean. Sunlight heats you up and sets you free, high into the air, as water vapor. You latch onto a particle of dust or salt and join your fellow condensed water drops in clouds. There you float along until a change of conditions causes you to parachute down together as rain or snow, depending on the temperature. You may fall onto a tree leaf, a mountain top, or the roof of a house, but eventually you’ll join a stream that’s racing back to the ocean. And then the cycle begins again.

But actually, it’s more complicated than that. Some water drops circulate many times between the land and sky without ever heading back to the ocean. “Evapotranspiration” is a large word to describe water evaporating off the land and being released from plants. As we heard in episode three, plants sweat when they get hot, just like we do. And the warmer it gets, the higher the rate of evapotranspiration. In many areas, that’s the largest factor determining how much rain will fall.

[32:48] Urbanization complicates this even more. Normally, rainfall is mainly taken up by plants or soaks down into the soil, while a smaller fraction is diverted into streams. Most groundwater is held in tiny pore spaces in the soil, and that water slowly makes its way to underground aquifers or gradually seeps toward stream beds. It’s a finely balanced arrangement that helps reduce flooding while also acting as a water filtration system.

But concrete jungles don’t allow the landscape to act like a natural sponge. Instead the water runs off the hard surfaces directly into rivers and reservoirs, which quickly become overwhelmed.

A warmer atmosphere can hold more water than a cooler one – consider the humid air of summer versus the dry air of winter. So not only is the warming world increasing the amount and rate of ocean evaporation, evapotranspiration, and glacier melting, but because the warmer air can hold more water, storms can dump more water over a brief time.

Film trailer: Waterworld
Announcer: The future… the polar ice caps have melted, and the Earth lies beneath a watery grave. Those who survived have adapted to this new world…

[34:15] Narrator: Scientists say we aren’t heading for a situation like in the movie “Waterworld,” with the entire planet underwater. But sea levels are projected to rise between half a meter to 2.5 meters, or 1 to 8 feet, in the next hundred years, changing coastlines and river systems.

And yet, on the other side of that coin, some areas of the world could be steadily marching toward extreme drought.

In traditionally dry areas, soils are expected to dry out even more in hotter weather, altering the soil chemistry so much that plants may struggle to get the nutrients they need. Increased heat also can parch plants beyond their capacity to survive. This could then break down the evapotranspiration cycle that had previously brought moisture raining back down. And climate change is also expected to alter weather patterns, migrating water-laden clouds farther north or south from where they’re needed most.

[35:17] There’s a saying among scientists who study climate change: expect wet areas to get wetter, and dry areas to get drier. For a planet so dominated by water, it seems incredible that drought would ever be a major concern.

Cedric David: It's the blue planet. We have water everywhere. So what's the deal with, “We don't have enough?” And the answer there is that salty water in the ocean doesn't do a whole lot of good when it comes to drinking it. And filtering, basically removing the salt from ocean water, is very hard. It's very energy intensive. I've worked with some people who did membrane-based desalination studies and, yeah, it's hard. A lot of great work is being done, but it's also very expensive. So only a handful of countries are really able to afford that.

Also, once you've extracted the salt from the water, you got to put the salt somewhere. And a lot of times you just put it back in the ocean, and that's not great for the ocean systems. So the fact that the planet is covered by water is not enough to support human consumption of water, because the vast majority of the water is salty.

[36:29] Narrator:  Only about two-and-a-half percent of Earth’s water is fresh. And most of that fresh water is difficult to access, because it’s either deep underground or in the form of ice and snow.

Cedric David: I've spent so much of my time looking at rivers, because that's the easiest accessible fresh water that we have. One might really wonder, what's the point in studying rivers? The volume of water that is stored in the rivers and lakes around the world is minuscule compared to what's underground and what's in the ice caps and the snow cover on the mountains. And the answer there is because looking at volume is really not the right way to look at renewable water resources. Basically, it's a story of fluxes. So it's the water that flows, more so than the water that's stored.

[37:16] It turns out that the actual flow of water is very large. When you look at all the flows in all the rivers of the world, it's about 10 times as big as all of human consumption for agriculture, industry and domestic use. So rivers are very small as far as storage is concerned, but they move so much that it's actually a very big component of the renewable water management.

(sound FX: river)

Rivers are very much alive, not only in the sense that they flow, but also because they move and the meanders move. We've seen some very nice pictures of how the rivers have migrated over the past 30 years, and that's really fascinating. So how they move is definitely one question, and also how much water they carry is really fascinating to understand.

So for the past decade or so, I've been developing a system that allows us to simulate how water flows in the rivers of the world. And it's kind of cool cause you're able to display the water moving on the continents in a way that's really reminding of how the blood flows in the veins of the human body.

[38:23] Narrator: The resemblance of river networks to biological veins and vessels is a striking reminder that water is our planet’s lifeblood. We may be creatures of the land, but we are bound to rivers and lakes; our earliest settlements and cities were built next to their banks and shores.

(music)

Going farther back in time, our early ancestors came from the ocean, and its waters still live in us. How many of us feel renewed after contemplating the rhythm of waves, smelling the sea spray, or swimming in the swell of the tides? As the writer Isak Dinesen once put it, “I know of a cure for everything: salt water. Sweat, or tears, or the salt sea.”

For Cedric, ever since he can remember, the sea was a place of solace and joy.

[39:19] Cedric David: I grew up in a city called Marseille. It's a relatively big city by French standards. It's the second biggest city by population. It’s also at the gates of the Mediterranean for France, and so we basically used to go swim every weekend.

I was born with a handicap. When I was born, my parents were told that I would never walk. My right foot was messed up, and the chances of walking were very limited. I was fortunate to undergo surgery when a very little kid -- I was about one -- and after that things turned around and I've been able to walk, but walking is still something that is uncomfortable. And basically growing up, water was the only place that I was really comfortable, you know, that I was like everyone else. And so, I have an emotional attachment to water, because that's a place where I'm happy.

And so, I ended up spending a big portion of my life studying it. You know, there are always two versions of a story. The scientific story is I like science and I like the environment and I really had a whole lot of fun looking into that. But there is a little bit of a love story with water too, which led me to where I am today.

(music continues)

Narrator: If you like this podcast, please subscribe, rate us on your podcast platform, and share us on social media. We’re “On a Mission,” a podcast of NASA’s Jet Propulsion Laboratory.

[run time = 40:53]