A Changing Arctic

Have you ever made lemonade that was too sour, and then tried to fix it by adding more water? Believe it or not, something similar is happening to the Arctic Ocean. But instead of lemon juice getting diluted, it is the salty seawater itself becoming less concentrated.

The Arctic Ocean is the northernmost ocean of the planet. While it is the smallest of the world’s oceans, it is still as large as all of Europe. The Arctic Ocean is cold and covered by ice for most of the year. This makes it important to the whole globe because it keeps Earth’s climate balanced. In recent years, scientists have discovered that the Arctic Ocean is freshening [1]. This means there is less salt in the water. But why is that happening? And why does it matter?

What Does “Freshening” Mean?

Everyone knows the ocean is salty. But did you know that some parts of the ocean are actually saltier than others? How salty seawater is depends on the climate and on how much freshwater is being added.

When the climate is hot, more seawater evaporates and turns into vapor. As the water leaves the ocean, the salt stays behind, making the ocean saltier. In other places, lots of freshwater is added by rain, rivers, or melting ice. This makes the water less salty. You can think of each ocean like a big bathtub. Some bathtubs are hot and steamy, which leaves more salt behind. Other bathtubs have giant faucets pouring in freshwater that dilutes the salt.

When scientists talk about the freshening of the Arctic Ocean, they mean that more and more freshwater is mixing into the salty seawater—just like if you were to open up the faucet and add more freshwater to a salty bathtub.

Where is All This Freshwater Coming From?

More freshwater is being added to the Arctic Ocean now than there was in the past. But where is all this water coming from?

One source is the melting of sea ice and glaciers [1]. As the Arctic gets warmer, ice melts faster than it used to. All that melted ice turns into liquid freshwater that flows into the ocean (Figure 1). Both sea ice and glaciers act as sources of freshwater when they melt, but only melting glaciers cause sea levels to rise. That is because glaciers sit on land, so when they melt, they add new water to the ocean. Since sea ice is already floating in the water and taking up space, when it melts it does not change sea level. But melting sea ice still makes the top layer of the ocean fresher.

Another source of freshwater is the huge rivers that flow northward and drain into the Arctic Ocean (Figure 1). These include the Mackenzie River in Canada, the Lena River in Russia, and the Yukon River in Alaska. These rivers carry a lot of freshwater from the land into the ocean, and this amount has been increasing [2]. The Arctic is also getting more rain and snow as the climate changes. When rain falls or snow melts, it adds even more freshwater to the ocean.

Not Every Part of the Arctic Feels the Same

In reality, the Earth’s oceans are not like separate bathtubs. Instead, they are all connected, like big swimming pools joined together in a water park. The water moves around between these pools in pathways called currents. Currents carry salt and heat to different places.

The Arctic Ocean is connected to two other oceans: the Pacific Ocean and the Atlantic Ocean. Water comes into the Arctic from both of these oceans, bringing extra salt and heat (Figure 1). At the same time, cold, fresher water from the Arctic flows back out into the Atlantic. Because of these currents, the water in the Arctic Ocean is not the same everywhere. Some parts are warmer or saltier, and other parts are colder or fresher.

One important region of the Arctic Ocean is called the Beaufort Gyre. It is a large, spinning circle of water in the western Arctic that collects and holds a lot of freshwater from melting ice and rivers (Figure 1). Over time, this freshwater builds up, making the water there less salty [3]. In the eastern Arctic, warm and salty water flows in from the Atlantic Ocean. This makes that part of the ocean warmer and saltier than other areas [3]. Scientists study these different ocean areas and how they are connected, to learn how the Arctic Ocean is changing.

Freshening Changes with the Seasons

The Arctic Ocean does not stay the same all year round. It has some of the most extreme seasons on Earth! In winter, the Arctic is very cold and dark. Most of the ocean is covered by thick sea ice. Then, in summer, the sun shines all day and night, and much of that ice melts. This freezing and melting cycle changes how much freshwater is in the ocean. In winter when the water freezes, the salt is left behind, making the remaining water saltier. Then when ice melts in summer, it adds a lot of freshwater, making the water less salty (Figure 2).

Rivers also change with the seasons. In spring, when the snow and ice on land start to melt, rivers rush more freshwater to the Arctic Ocean. But because the Arctic is warming, this rush of river water is starting earlier. Sometimes this is starting in May instead of June and July [1]. All these changes mean that Arctic Ocean freshening changes with the seasons too. Scientists watch these seasonal shifts closely to understand how the Arctic is changing over time.

Why Does Freshening Matter?

Freshening might sound harmless, but it can cause some big changes in the ocean. When freshwater pours into the Arctic Ocean, it forms layers because freshwater is lighter than salty water (Figure 2). Scientists call this layering stratification. This means the freshwater stays on top and does not mix well with the saltier water below. Have you ever been swimming and felt warm water near your neck but cold water near your toes? That is one way you can feel stratification.

This layering stops the ocean from mixing properly, keeping the surface water separated from deeper water. Deeper waters often have more nutrients because dead plants and animals sink downward, where they slowly break down and release nutrients. Normally, ocean mixing brings these nutrients back up toward the surface. However, when the water does not mix, important nutrients cannot rise to the surface [4]. Tiny plants in the ocean, called phytoplankton, need these nutrients to grow. Phytoplankton are super important because they are the base of the ocean food web [5]. If they do not grow, animals that eat them will struggle. This includes everything from tiny shrimp, to fish, and even larger seals and whales higher up the food chain.

Freshwater from rivers can also carry mud and dirt that block sunlight. Like plants on land, phytoplankton need sunlight to grow. So, muddy water can make it harder for them to get enough light.

Freshening might even affect global ocean currents. These are slow-moving rivers of water that flow through the oceans all around the Earth. These currents help keep our planet’s climate balanced by moving heat and moisture between the equator and the poles. Some scientists worry that too much freshwater in the Arctic could slow down or even stop these currents [6]. That would change weather and climate worldwide.

The Future Arctic Ocean

In the future, the Earth’s climate is likely to get even warmer. This means that more freshwater from rivers, ice melt, and precipitation will enter the Arctic Ocean, making it less salty than it is today. A fresher Arctic Ocean will be more stratified. This can make it more difficult for deeper, nutrient-rich ocean waters to mix up to the surface where they feed the tiny plants at the base of the food web. This means there may be changes in who is living in the Arctic Ocean ecosystem. Not only does Arctic Ocean freshening impact the food chain, but it might also slow down the global ocean currents that move heat around our planet. This makes it important for scientists to study how freshwater in the Arctic Ocean is changing so we can better prepare for the future.

Glossary

Arctic Ocean: ↑ The smallest and coldest of Earth’s oceans, located at the North Pole.

Freshening: ↑ The process of ocean water becoming less salty.

Freshwater: ↑ Natural water without any salt.

Sea Ice: ↑ Ice floating on the ocean’s surface, made from frozen seawater.

Glacier: ↑ A giant sheet of ice that moves slowly across the land.

Currents: ↑ Slow-moving rivers of water that flow through the ocean and move heat around the Earth.

Stratification: ↑ When layers of water form and do not mix well.

Phytoplankton: ↑ Tiny ocean plants that are the base of the food chain.

Conflict of Interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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