Ocean Currents and Global Heat

Water has a surprising trick. Most liquids get denser as they get colder, and fresh water does too, but only down to about 4 degrees Celsius. Colder than that, fresh water gets lighter again. Salty seawater is different: it keeps getting denser right down to the temperature where it freezes. Either way, ice is lighter than the liquid water around it, which is why ice floats on top of the ocean instead of sinking to the bottom.

This matters a lot for how the ocean moves. Cold water is dense, so it sinks. Salty water is also denser than fresh water, so it sinks too. When ocean water near the poles gets very cold and very salty, it becomes heavy and sinks down deep. Warmer water flows in to take its place. This slow, giant loop of sinking cold water and flowing warm water is called thermohaline circulation. The word comes from "thermo" (heat) and "haline" (salt), because temperature and saltiness together drive the whole thing.

The ocean plays a big part in the story of climate change. When people burn fuels like coal, oil, and gas, extra carbon dioxide goes into the air and traps extra heat around the Earth, like a blanket getting thicker. So where does all that extra heat go?

Most of it goes into the ocean. Scientists estimate that the ocean has soaked up more than 90 percent of the extra heat trapped by our atmosphere. It has also absorbed about a quarter of the extra carbon dioxide that humans have added to the air.

That means the ocean has slowed down how fast the air warms up. But it comes at a cost. Warmer ocean water can change currents, harm sea creatures, and even change the weather on land. Marine biologist Dr. Sylvia Earle has spent her life studying the sea, and she says that taking care of the ocean is one of the most important things we can do for the whole planet.

You can see how ocean currents work with a simple experiment. Take two containers of water and make one saltier than the other. The salty water is denser, which means the same amount of it weighs more. When the two waters meet, the heavier salty water slides underneath and the lighter fresh water floats on top.

Scientists do experiments just like this to understand the real ocean. They measure carefully, put their data into spreadsheets, and make graphs to compare what they see with what math predicts.

In the real ocean, the same thing happens on a giant scale. Where water gets saltier, like where seawater freezes into ice and leaves the salt behind, the water gets denser and sinks. Where water is fresher and warmer, it stays near the surface. These density differences, caused by saltiness and temperature, push ocean water around the globe in a slow loop called thermohaline circulation. It is one of the main engines that moves heat from the warm tropics toward the cold poles.

Heat can travel in a few different ways, and one of the most important is called convection. Convection happens when warm material itself moves from one place to another and carries heat along with it.

Here is how it works. When air or water gets warm, it expands and becomes lighter, so it rises. When it cools down, it gets denser and sinks. That rising and sinking creates a loop of moving material called a convection current.

Earth's atmosphere works this way. Hot air flows from the tropics toward the poles high up in the sky, and cold air flows back from the poles toward the tropics near the ground. (The spinning of the Earth bends these flows, which is why winds do not blow in a straight line.)

The ocean works the same way. Warm water flows away from the equator, and cold, dense water sinks near the poles and creeps along the deep sea. You use convection every day too: a car engine stays cool because water pumps carry heat away from it, just like ocean currents carry heat around the planet.