Final answer:
An excess of carbon dioxide in the atmosphere is problematic for Earth's ecosystems because it leads to ocean acidification, which can disrupt the balance of marine ecosystems.
Step-by-step explanation:
An excess of carbon dioxide in the atmosphere is problematic for Earth's ecosystems because it leads to ocean acidification, not promotes photosynthesis or enhances biodiversity. As carbon dioxide levels increase in the atmosphere, they also increase in the oceans, causing the pH of seawater to decrease. This can make it difficult for sea life to produce their hard exoskeletons and shells, which can disrupt the balance of marine ecosystems.