Ocean acidification: what it really means for the sea

The ocean is gradually becoming more acidic, a phenomenon driven by the absorption of excess carbon dioxide (CO₂) from the atmosphere. This CO₂ originates primarily from the burning of fossil fuels, industrial activities, and land-use changes, acting as a major driver of climate change.

As CO₂ dissolves in seawater, it triggers a series of chemical reactions. Initially, it forms carbonic acid, which then dissociates, releasing hydrogen ions (H⁺). These ions lower the ocean's pH, making it more acidic. Critically, these hydrogen ions also combine with carbonate ions (CO₃²⁻), converting them into bicarbonate. This reduction in available carbonate ions is a major concern because many marine organisms, including corals, mollusks, and certain types of plankton, rely on carbonate to build their calcium carbonate structures, such as shells and skeletons.

While the term "acidification" might suggest a drastic shift to a highly corrosive environment, the reality is a subtle but significant change in pH. A decrease of approximately 0.1 pH units since the pre-industrial era represents a marked increase in acidity due to the logarithmic nature of the pH scale. This change, though seemingly small, has profound implications for marine life and the economies that depend on healthy ocean ecosystems. Organisms that struggle to build or maintain their shells and skeletons face increased vulnerability, potentially disrupting entire food webs that support vital fisheries.

The impact of ocean acidification is not uniform across the globe. Colder regions, such as the Southern Ocean, and areas with intense vertical mixing in the Atlantic tend to absorb more CO₂. In these zones, carbonate saturation levels can decrease more rapidly, potentially leading to earlier and more severe impacts on marine communities, particularly those of mollusks and crustaceans. Scientists monitor this phenomenon through direct measurements of pH, dissolved CO₂, and carbonate saturation states, recognizing that the chemical processes involved mean recovery from current emission rates is a slow and challenging prospect.