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Head to shopify.com/scishow to to  learn more and for a 14 day free trial. [♪ INTRO] “Why is the sea salty?” seems like one  of those questions a preschooler asks pretty early on, right after  “why the sky is blue?”. But here, the answer is  relatively straightforward.

Rain erodes salts out of rocks  and rivers transport this salt to the oceans, where it accumulates. Globally, rivers put four billion  tonnes of salt into the sea every year. But then, when it evaporates  back out of the ocean into rain, it doesn’t take the salt with it.

Most of this is sodium and chlorine,  together as sodium chloride; what we know as table salt. But there are also smaller amounts of  magnesium, sulfate, calcium, and potassium ions, among others, all of which chemists  file under the umbrella of “salts”. If you took a kilogram of seawater  from the ocean today and evaporated it, you would be left with about  35 grams of all of these salts.

But this is one of those  cases where a simple question and a fairly simple answer leads  to a way more interesting question. With rivers adding all this salt every  year, are the oceans getting saltier? At one point in the past,  scientists thought they were.

In 1715, Edmund Halley (yes,  the guy with the comet) proposed using the difference between the salinity  of the ocean and the rivers running into it to calculate how old the Earth is. It’s a smart idea. If the rivers add a bit of  salt to the oceans every year, you could backtrack and figure out  when a fresh ocean first formed.

This calculation says that the  Earth, or at least its oceans, are somewhere between 25  and 150 million years old. Take that with a very big grain of salt, though. With all due respect to the astronomer  who did many amazing things, including naming that comet, we now know  that this figure is very very wrong.

Many other pieces of evidence tell us that the Earth is much older, around 4.5 billion years. The oceans formed by 3.8 billion years ago, and some believe that the early ocean  was actually saltier than today. The issue is that this calculation  did not consider the ways salt can be removed from the oceans.

See, salinity is balanced. Sources like rivers that add more salt are counterbalanced by sinks, which remove it. Sinks like evaporite deposits.

These form when an area of water is  cut off from the rest of the ocean and the water evaporates,  leaving behind salt flats. Around five and a half million years ago, the Mediterranean Sea was cut  off from the rest of the world and almost completely dried up. This took about 10% of the  salt out of the world’s oceans!

The Mediterranean got better, but today  the Dead Sea fills a similar role. Hydrothermal vents also have an impact. They heat up seawater as it flows  through rocks near magma and cause chemical reactions that remove magnesium and sulfate (again, salts) from seawater.

But because the total amount of salt  in the ocean is large compared to what gets added or removed each year, the  balance is remarkably consistent overall. So the ocean stays saltier than rivers,  but doesn’t get much saltier over time. However, salinity can still change  from place to place over time.

That is because salinity is  the amount of salt in water, so if you change the amount of  water, you can change the salinity. Rain and rivers add fresh water. Intense rain like monsoons can have  a big effect on the nearby ocean.

Evaporation can also take water away and  increase the salinity of a particular area. Ice can also be a big factor. When seawater freezes into  ice, that salt stays behind in a process called brine rejection.

So sea ice forming can increase  the salinity of an area, and any ice melting can add  fresh water and decrease it. And all of these processes  happen at the ocean’s surface, so they affect the saltiness of  about the top 200 meters of water.   Add all these up and you see some global patterns. The surface ocean is saltier as  you get closer to the equator, since it’s warmer and evaporation  rates are higher than near the poles.

The exception is right at the equator, where  there’s a lot of rain and lower salinity. For example, the Baltic Sea has  many rivers flowing into it, so its salinity is lower than average. The Mediterranean and Red  Seas have lots of evaporation, so their salinity is higher.

So even though the ocean’s overall  salinity is relatively constant, smaller fluctuations can tell us a lot; about the oceans, and about  the water cycle worldwide. Also, the climate crisis is already  having an impact on salinity. Researchers have noticed that the saltiest  areas of the ocean are getting saltier, and the freshest spots are getting fresher.

This means that overall, both evaporation  and precipitation rates are going up. And this could mean both more  intense droughts and floods on land. Salinity itself can also have  a direct effect on climate.

Salinity and temperature are the primary factors that determine the density of seawater. Denser water will, of course,  sink to the bottom of the ocean, and this movement drives global ocean circulation, which has a significant effect on  how heat flows around our planet. All of this means that understanding  these salinity patterns throughout the oceans and how  they are changing is crucial.

So in 2011, NASA launched the Aquarius satellite, which can measure the ocean’s  surface salinity from space. This probe can survey the salinity  of the entire globe in a week! And that means we should be able to  track how the water cycle is changing, and build better models to try and  predict how that will affect climate.

And so, from asking why  the obvious doesn’t happen, why the ocean doesn’t get saltier  when it seems like it should, we get new ways to understand and monitor  the effects of the climate crisis. Preschoolers ask great questions, sometimes. Thanks for watching this SciShow video  and thanks to Shopify for supporting it!

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