Tropical coral reefs form the very foundation of marine biodiversity. Sadly, their seemingly inevitable demise may prove to be one of the first irreversible consequences of climate change.
That’s the conclusion of a comprehensive new report on abrupt climate changes from the National Academy of Sciences. Researchers have long hypothesized about climate-induced points of no return, like sudden catastrophic melting of ice caps or a dramatic shift in the Gulf stream, but the Academy report emphasizes that ecosystem collapse as environmental conditions steadily march past livable thresholds is much more likely in the next few decades. And tropical coral reefs are one of the most precarious ecosystems, thanks to increasingly warm and acidic oceans.
Climate change poses a double threat to coral reefs. Warming ocean waters lead to a potentially fatal process known as coral “bleaching,” in which reef-building corals eject algae living inside their tissues that supply them with most of their food. Coral bleaching occurs when water temperatures are just 2-4°F above normal summertime temperatures. Bleached corals are weak and often succumb to disease.
At the same time as warming waters are pushing corals to the brink of what they can tolerate, the oceans are absorbing about one quarter of annual CO2 emissions from human activities. That’s nearly 24 million tons of CO2 every day. CO2 dissolved in seawater increases ocean acidity. More acidic oceans decrease the availability of carbonate ions, which coral use to build their calcium carbonate skeletons. In short, sour oceans spell the end to reef building.
According to a recent report prepared for the Warsaw climate talks by 500 of the world’s leading ocean acidification experts, the oceans are currently acidifying at an “unprecedented rate, faster than at any time in the last 300 million years. Since the start of the industrial revolution, ocean water has become 26 percent more acidic.
Unrelated to either warmer or more acidic oceans, the world’s deep-sea corals are also endangered by climate change as increasingly stratified water disrupts the nutrient cycle in the water column.
One glimmer of hope in an otherwise dismal outlook for corals comes from research in the Palau archipelago in the West Pacific. Here, coral reefs are healthy and diverse, despite the fact that the water is naturally abnormally acidic. This finding suggests that some corals have adapted to be able to calcify in more acidic waters and might offer a clue to saving corals worldwide.