The change in CO2 levels has significantly affected marine life primarily through ocean acidification, which occurs when excess CO2 dissolves in seawater forming carbonic acid. This acidification decreases the ocean's pH, making it more acidic and impacting marine organisms, especially those that rely on calcium carbonate to form shells and skeletons, such as corals, mollusks, and some plankton. This results in slower shell growth, weaker structures, and increased dissolution, threatening biodiversity and ecosystem stability. Coral reefs, vital habitats for 25% of marine life, face degradation, which affects the entire marine food web including fish stocks. Furthermore, marine life plays a critical role in carbon sequestration, and damage to these ecosystems threatens the ocean's capacity to absorb CO2, exacerbating climate change impacts. Overall, rising CO2 levels and the resulting acidification pose multifaceted risks to marine biodiversity, ecosystem services, and fisheries.

Impacts on Marine Organisms

• Calcifying organisms such as corals, clams, snails, starfish, and sea urchins experience difficulty building and maintaining their calcium carbonate shells and skeletons due to reduced carbonate availability and increased dissolution rates.
• Marine plankton like pteropods, crucial in marine food chains and as food for commercial fish, are similarly threatened.
• Larval and juvenile stages of marine animals are particularly vulnerable, often suffering higher mortality rates in acidified conditions.

Ecosystem and Biodiversity Consequences

• Coral reefs are damaged, leading to habitat loss for a diverse range of species, reduced nursery grounds, and compromised coastal protection.
• The disruption of the food chain caused by the decline in shell-forming organisms cascades through marine ecosystems, potentially reducing fish stocks and biodiversity.
• Acidification combined with warming and deoxygenation creates "deadly" synergistic effects, increasing risks of species decline and ecosystem disruption.

Wider Climate Feedbacks

• Marine life contributes significantly to the biological carbon pump, sequestering billions of tons of CO2 annually.
• Damage to marine ecosystems reduces this natural sequestration capacity, potentially increasing atmospheric CO2 levels and global warming.

In summary, increased atmospheric CO2 leads to ocean acidification, which harms marine organisms and ecosystems with cascading effects on biodiversity, fisheries, and climate regulation.