Did you know that around ninety-seven percent of the water on Earth's surface is saline, with the oceans and seas doing most of the heavy lifting? It's a number that has probably prompted a question in nearly everyone at some point: why does sea water taste salty when the rivers and streams feeding into it taste sweet?
Before we dig in, here's a striking thought. It's said that if you could remove all the salt from the oceans and seas and pile it up on land, you'd end up with a layer around one hundred and fifty metres high. Imagine walking past a wall of pure salt as tall as a skyscraper's worth of stacked apartments, just to get a sense of how much of it sits dissolved in the water.
Following the path of the water cycle
To understand the salt, we can turn to the water cycle we all studied in school. Take a look at the diagram below, which traces the endless loop of evaporation, clouds and rain.
Rain water, as it comes into contact with the atmosphere, becomes slightly acidic in nature. That mild acidity matters, because it helps the water do its slow work on the land. As it travels down through the mountains by way of little streams and rivers before reaching the sea, it picks up salts and particles through slight erosion along the way, wherever the water grazes the rocks.
Those particles get carried into the seas and oceans. As more salt arrives, the salinity of the water body keeps rising, and the salt content in it steadily grows. It's like taking a glass of fresh water and adding salt at regular intervals: eventually, the water becomes far too salty to drink.
So why aren't rivers salty?
You might now be thinking: if rivers carry all those particles, why don't they taste salty themselves? The answer lies back in the water cycle. From evaporation to condensation to precipitation, rain replenishes river water again and again, washing fresh water back into the channels. Returning to the glass example, it's as if the river keeps emptying half the glass and refreshing it with clean water before the salt has a chance to build up.
So why doesn't the same thing happen to the seas and oceans? It comes down to how much salt comes in versus how much goes out. Once salt and minerals reach the sea or ocean, the salt stays behind while the water evaporates as part of the cycle. Even if a little salt does get carried away, the difference is negligible, so over the ages the seas simply keep their seasoning.
A nod to climate shifts
There are also dramatic climate shifts to consider, like the huge salt deposits in the Mediterranean Sea, which once dried up and was later refilled through the Atlantic stream. So there may well have been a time when some oceans or seas were fresh, or held only a little salinity. But as time moved on and the water cycle kept doing its work, salinity gradually climbed, drop by drop, century by century.
So the next time you taste that unmistakable salt on a sea breeze, you'll know it's the product of rocks, rain and the endless recycling of the planet's water. The ocean has simply been collecting its seasoning for a very, very long time.