Our oceans are becoming more acidic because they are absorbing too much carbon dioxide from the air, which harms marine life.

Key idea: Human activities, especially burning fossil fuels, release large amounts of carbon dioxide, and the ocean absorbs a significant portion of it.

So, where does all this extra carbon dioxide come from? Mostly, it's from human activities, especially burning fossil fuels like coal, oil, and natural gas for energy, and also from cutting down forests. Since the start of the Industrial Revolution, we've been releasing huge amounts of CO2 into the air. The ocean has been a hero, absorbing about a quarter of all the CO2 we've put out there.

For example, in the time before industries really took off, around the 1750s, the amount of CO2 in the air was about 280 parts per million (ppm). Today, it's over 422 ppm. This rapid increase is something Earth hasn't seen in at least 55 million years. While the ocean helps by taking in this CO2, it comes at a cost to its own health.

This process is part of the natural carbon cycle, where carbon moves between the air, land, and oceans. But our actions are pushing this cycle out of balance, forcing the ocean to take on more carbon than it can handle without changing its own chemistry.

“The ocean has been a hero, absorbing about a quarter of all the CO2 we've put out there.”

Key idea: When the ocean absorbs carbon dioxide, it forms carbonic acid, which releases hydrogen ions, lowering the ocean's pH and making it more acidic.

When carbon dioxide from the air dissolves in seawater, it doesn't just disappear. It reacts with the water to form a weak acid called carbonic acid. This carbonic acid then breaks down into other substances, including hydrogen ions. It's these extra hydrogen ions that make the water more acidic.

The more hydrogen ions there are, the lower the pH of the water becomes. The pH scale measures how acidic or basic something is. A lower pH means more acidic, and a higher pH means more basic. Even though the ocean is becoming more acidic, it's important to remember it's still generally basic (its pH is still above 7). It's just less basic than it used to be.

Between 1950 and 2020, the average pH of the ocean surface dropped from about 8.15 to 8.05. This small change in pH actually means there's about 26% more hydrogen ions in the water. That's because the pH scale is logarithmic, meaning each whole number change represents a tenfold difference in acidity. So, a small drop in pH means a much larger increase in acidity.

This change in chemistry also reduces the amount of carbonate ions available in the water. Carbonate ions are super important because many marine animals use them to build their shells and skeletons.

Key idea: Ocean acidification makes it harder for marine organisms to build and maintain their shells and skeletons, impacting entire food webs.

This change in ocean chemistry has big consequences for marine life. Many creatures, like corals, oysters, clams, and tiny snails called pteropods, rely on calcium carbonate to build their shells and skeletons. Think of calcium carbonate as the building blocks for their homes.

With fewer carbonate ions available and more acidic water, it becomes much harder for these animals to build and maintain their shells. It's like trying to build a brick wall when someone keeps taking away the bricks and trying to dissolve the mortar. Some animals might have to use more energy just to build their shells, leaving less energy for other important things like growing or reproducing.

For example, pteropods, tiny sea snails that are a key food source for many Arctic animals, have shells that can start to dissolve in more acidic water. If these creatures struggle, it can affect the entire food web, from small fish to whales, potentially threatening fisheries and the livelihoods of people who depend on them.

This problem is often called the "evil twin of global warming" because it's another major issue caused by CO2 emissions, working alongside rising ocean temperatures and reduced oxygen levels to create a "deadly trio" of threats to marine environments.

“Ocean acidification has been called the 'evil twin of global warming'.”

Key idea: The rate and impact of ocean acidification vary by region and depth, with colder waters and deeper ocean areas often experiencing more severe effects.

Scientists have seen that the effects of ocean acidification are not the same everywhere. Colder waters, like those in the Arctic, can absorb more CO2, which means they can become more acidic faster. This is a big concern for the Arctic, where many animals, like the pteropods and brittle stars, are already at risk.

Also, the depth of the ocean matters. In deeper waters, where pressure is higher and temperatures are lower, calcium carbonate shells naturally start to dissolve. Ocean acidification is making this depth, called the carbonate compensation depth, shallower, meaning shells dissolve at shallower levels than before. This can affect creatures living in those deeper parts of the ocean.

Other factors like ocean currents, nearby rivers, and even how much sea ice there is can also influence how much CO2 the ocean absorbs and how quickly acidification happens in different areas.

Key idea: Reducing carbon dioxide emissions is the most effective way to address ocean acidification, though other solutions are being explored.

The most direct way to tackle ocean acidification is to reduce the amount of carbon dioxide we release into the atmosphere. This means finding cleaner ways to produce energy and reducing our overall carbon footprint. Many global efforts to fight climate change also help with ocean acidification, as both problems share the same root cause: too much CO2.

Scientists are also exploring other ideas, like removing CO2 directly from the atmosphere or even adding substances to the ocean to make it less acidic. These methods, like ocean alkalinity enhancement, are still being studied and have their own challenges and potential risks. For example, adding alkaline minerals to the ocean could help buffer the acidity, but we need to understand the full impact on marine life and ecosystems.

While these solutions are being developed, the most effective and immediate step is to cut down on CO2 emissions globally. This is a big task that requires cooperation from countries and individuals around the world.

Key idea: Research into ocean acidification has evolved over decades, from recognizing the ocean's CO2 absorption to understanding its harmful impacts and becoming a global policy concern.

Scientists have been studying ocean acidification for decades, but it's only more recently that the full scale of the problem has become clear. The pH scale itself was invented by a Danish chemist in 1909, laying the groundwork for understanding acidity.

By the 1950s, experts knew the ocean was absorbing a lot of CO2, but the wider scientific community didn't fully grasp the potential negative consequences. For a long time, the ocean's ability to soak up CO2 was seen as purely beneficial, helping to slow down climate change. It took key events and more research to realize that "too much of a good thing" could be harmful.

In the early 1970s, debates started about the long-term effects of CO2 accumulation in the sea. By the mid-1990s, scientists studying coral reefs became very concerned about the rising CO2 levels and the inevitable changes in pH. By 2003, enough new research had emerged that the first major international symposium on "The Ocean in a High-CO2 World" was planned.

Today, ocean acidification is recognized as a critical global issue, included as one of the seven Global Climate Indicators by the World Meteorological Organization. It's also a specific target (14.3) under the United Nations' Sustainable Development Goal 14, which aims to conserve and sustainably use the oceans.

“The ocean has been minimally considered at previous climate negotiations. Our study provides compelling arguments for a radical change.”