Which factor poses a major threat to coral reefs worldwide?

The correct option is Ocean warming and acidification.

Coral reefs are extremely sensitive marine ecosystems that rely on a symbiotic relationship with microscopic algae called zooxanthellae. These algae live within the coral tissues, providing them with food through photosynthesis and giving them their vibrant colors.

Here is a step-by-step breakdown of why ocean warming and acidification represent a major threat to coral reefs globally:

1. Ocean Warming and Coral Bleaching:
As global temperatures rise, the oceans absorb a significant amount of excess heat. When water temperatures exceed normal thermal thresholds even by just 1-2 °C, corals experience stress. This causes them to expel the zooxanthellae living in their tissues. Without these algae, the corals lose their primary food source and turn completely white, a phenomenon known as coral bleaching. Prolonged bleaching often leads to coral mortality.

2. Ocean Acidification:
The oceans absorb excess carbon dioxide (CO₂) from the atmosphere. When CO₂ dissolves in seawater, it forms carbonic acid, decreasing the pH of the ocean water. This process reduces the availability of carbonate ions, which corals need to build and maintain their calcium carbonate (CaCO₃) skeletons. Acidic conditions weaken existing reef structures and slow down new coral growth.

While local factors like soil erosion and urban development contribute to coastal degradation, ocean warming and acidification are widespread global threats driven by climate change that affect coral reefs on an unprecedented worldwide scale.