Voyager: How is animal life being affected by the increasing levels of CO2?

Increases in carbon dioxide in the atmosphere affect marine animals in several ways. To understand how, we must first look at chemistry. The ocean absorbs a great deal of CO₂ in the atmosphere. As it does, the ocean becomes more acidic.

In general, the increase of CO₂  in the ocean probably has little direct impact on large marine animals like fish. Plants need CO₂ for photosynthesis and there is evidence that some marine phytoplankton could multiply with more CO₂ in the ocean. More phytoplankton could help support larger animal populations.

The major downside is the impact a more acidic ocean would have on animals that produce calcium carbonate shells, such as corals, sea urchins, and mollusks. Acidification not only makes it more difficult for these organisms to successfully produce shells, but it would ultimately tip the chemical balance and cause the shells to actually dissolve! A fall in the numbers of shelled animals could cause a chain reaction since they make up the basic food for a wide range of organisms. Whole ecosystems could be impacted.

Corals face an especially complicated fate. Much of their nutrition comes from photosynthesis by algae that live within the coral tissues. In experiments, corals bathed in seawater with amounts of CO₂ doubled showed greater levels of photosynthesis, but a significant drop in the growth of their calcium carbonate skeletons. Such an imbalance would most likely severely hamper reef building.

The scientific consensus is that we should avoid further acidification of our oceans. The consequences are mostly unpredictable, and what we do understand looks bad for the oceans and for us.

— Tony Haymet, director, Scripps Institution of Oceanography