A changing climate for coral reefs (excerpts)

Tropical coral reefs are magical places, yet if all the world’s reefs were placed together they would occupy a relatively small area—less than half the size of France. These most diverse of marine ecosystems punch above their size, however, supporting up to a third of all marine species and acting as home to between hundreds of thousands or possibly millions of reef-associated organisms—even marine biologists just don’t know how many. (…)

People are responsible for both the local degradation of reefs due to over-exploitation and pollution, and the global-scale consequences of interfering with the Earth’s climate system. And it is people who will be affected by these consequences. And it is people who could protect these reefs. (…)

As well as being vulnerable to warmer oceans, coral reefs are also threatened by climate change’s “evil twin”: ocean acidification. About a third of the extra carbon dioxide that humans have injected into the atmosphere has been absorbed by the oceans; if this had not happened, the world would have warmed even more than it has. But this extra carbon dioxide is changing the chemistry of water of the world’s seas, making them less alkaline and more acidic (that is, lower pH) with fewer carbonate ions necessary for coral calcification.

A site in Papua New Guinea where carbon dioxide naturally bubbles up from the sea floor has acted as a sort of natural laboratory, allowing us to travel from reefs with normal pH to those with the lower pH levels expected by the end of the 21st century. There are clear and stark differences between these reefs. Most dramatic is the loss of biodiversity, from a structurally complex reef containing many coral and other species to a reef dominated by a single massive coral species. Progressive ocean acidification will also increasingly slow the growth of corals and shell-forming reef organisms.

Humans have hit the world’s tropical coral reefs with a multiple whammy: over-exploitation of reef resources, pollution of reef waters combined with warmer and more acidic oceans. In addition, extreme weather events (exacerbated by global warming) such as freshwater low salinity flood waters and more intense tropical cyclones will further compromise reefs. (…)

Janice Lough, Bulletin of the Atomic Scientists, 11 October 2016.

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