The evidence that shows oceans are becoming more acidic is primarily tied to the increasing difficulty for some species to make shells. This phenomenon occurs because higher levels of carbon dioxide (CO2) in the atmosphere lead to more CO2 being dissolved in ocean water, resulting in the formation of carbonic acid, which lowers the pH of seawater. This increased acidity makes it harder for marine organisms, such as shellfish and coral, to create calcium carbonate shells and structures.
The other options listed, while related to climate change and the effects on oceans, do not directly indicate ocean acidification in the same way as the challenges faced by shell-forming species.
In summary, the correct response is:
- Difficult for some species to make shells.