Ocean acidification

Ocean acidification refers to the ongoing decrease in the pH of the Earth's oceans. This process is driven by the absorption of carbon dioxide (CO2) from the atmosphere. As the ocean takes in this gas, its chemical balance shifts. This change is significant because it alters the fundamental chemistry of seawater. The ocean acts as a massive carbon sink for human activities. It has absorbed about 26% of all anthropogenic CO2 emissions since 1850.

This absorption helps manage atmospheric levels, but it changes the water in the process.

The mechanism of acidification begins when CO2 dissolves in seawater. Once dissolved, the CO2 reacts with water to form carbonic acid (H2CO3). This carbonic acid then dissociates, or breaks apart, into bicarbonate ions (HCO3-) and hydrogen ions (H+). The increase in the concentration of these free hydrogen ions is what lowers the pH. A lower pH value indicates that the water is becoming more acidic.

Although the ocean is still alkaline, with a pH above 8, the rising hydrogen ion concentration is a major shift. Because the pH scale is logarithmic, a small change of 0.1 represents a 26% increase in hydrogen ions.

This chemical shift directly impacts the availability of carbonate ions (CO32-). To maintain chemical equilibrium, the extra hydrogen ions combine with existing carbonate ions to create more bicarbonate. This reaction reduces the concentration of carbonate ions available in the water. This is a critical problem for marine calcifiers. These are organisms that use calcification to build their bodies. Calcification is the process of turning dissolved ions into solid calcium carbonate (CaCO3) structures.

Without enough carbonate ions, these organisms struggle to build and maintain their shells and skeletons.

Many different types of marine life are vulnerable to these changes. Mollusks, such as clams and snails, rely on stable carbonate levels. Corals are also highly sensitive because they build massive reef structures from calcium carbonate. Microscopic organisms like coccolithophores and foraminifera are also part of this group.

Even small creatures like pteropods can be affected. In some cases, the seawater can become so low in carbonate saturation that existing shells actually begin to dissolve.

This puts the very foundation of many marine habitats at risk.

Scientists have documented a rapid change in ocean chemistry over recent history. In the pre-industrial era, atmospheric CO2 levels were approximately 280 parts per million (ppm). By 2023, these levels had risen to over 410 ppm. This increase is primarily due to fossil fuel combustion and deforestation. Between 1950 and 2020, the average surface pH fell from about 8.15 to 8.05.

These rapid growth rates in CO2 are unprecedented in the last 55 million years. The ocean sink has increased in pace alongside the rise in human emissions.

The consequences of acidification extend to entire ecosystems and human societies. Reduced calcification and lowered immune responses have been observed in marine life. These changes can disrupt the food web, as many species depend on calcifying organisms for food. This could eventually lead to a decline in fish stocks. This is a major concern for the one billion people who depend on coral reefs. They rely on these reefs for fishing, tourism, and coastal protection.

A collapse in reef ecosystems would threaten these livelihoods and food chains.

Addressing ocean acidification requires looking at the broader climate system. One primary solution is the reduction of carbon dioxide emissions through climate change mitigation. Removing CO2 from the atmosphere would help reverse the acidification process. Scientists are also investigating ocean-based methods like ocean alkalinity enhancement. This would involve adding substances to the water to increase its capacity to resist pH changes. However, these methods currently have a low technology readiness level and carry many risks. Understanding the connection between the atmosphere and the ocean is vital for protecting marine life.