Coral reefs harbour the highest biodiversity of any ecosystem globally. Despite covering less than 0.1% of the ocean floor, reefs host more than one quarter of all marine fish species, in addition to many other marine animals. Additionally, reefs provide a wide variety of ecosystem services such as subsistence food, protection from flooding and sustaining the fishing and tourism industries. Their disappearance will therefore have economic, social and health consequences.
Coral reefs are estimated to directly support over 500 million people worldwide, who rely on them for daily subsistence, mostly in poor countries. A 2014 assessment published in the journal Global Environmental Change estimated the social, cultural and economic value of coral reefs at US$1 trillion. A 2015 study by WWF projects that the climate-related loss of reef ecosystem services will cost US$500 billion per year or more by 2100.
Coral reefs are also key indicators of global ecosystem health. They serve as an early warning sign of what may happen to other less sensitive systems, such as river deltas, if climate change is not urgently addressed. Once the tipping point for the survival of coral reefs is passed, the deterioration of other systems may cascade more quickly and irreversibly.