Which of the following is the most significant factor in coral bleaching?

The correct answer is D) Global warming. Coral bleaching happens when corals expel the symbiotic algae (zooxanthellae) that live in their tissues and supply most of their food and colour. The main trigger is thermal stress: when sea surface temperatures stay even 1 to 2 degrees above the local normal for several weeks, the coral-algae partnership breaks down. Mass bleaching events in the Great Barrier Reef (1998, 2016, 2017) and elsewhere are linked to marine heatwaves, which is why the question asks for the most significant factor.

• A) Mining of coral rocks is a serious local threat that destroys reef structure, but it does not cause bleaching on a large scale.
• B) Outbreak of coral diseases causes tissue loss and mortality. It is a separate problem from bleaching, though the two can occur together.
• C) Siltation of seawater cuts light and smothers polyps, which stresses corals. Its effect is local and secondary, not the dominant driver.

This links to the GS3 environment and climate change syllabus: ocean warming, acidification and reef loss are standard Mains points on the impacts of anthropogenic climate change on marine ecosystems and fisheries.

Trap: aspirants pick siltation or pollution because they are visible local stressors. Ask whether the factor works at reef-wide scale; for bleaching, that is temperature.