How does climate change contribute to coral reef decline?

Coral reefs make up a globally dispersed ecosystem that is home to millions of species, and scientists fear that human activity could destroy them within our lifetimes. The activity threatening them is the emission of greenhouse gases into Earth’s atmosphere, which is warming the oceans along with the rest of the planet. As water temperatures rise, corals begin to bleach—the colorful algae that feed the corals through photosynthesis are shed, making them weak. When bleaching isn’t severe, reefs can recover. But in recent decades, bleaching has occurred too frequently for them to do so.

The first global bleaching event happened in 1998, then in 2010 and most recently from 2014 to 2017. In 2016, 30 percent of Australia’s Great Barrier Reef lost at least two-thirds of its coral cover. (The Great Barrier Reef accounts for 10 percent of total global reef area.)

Global warming affects coral reefs in another way, too. As the planet warms and the Arctic melts, the further corals seek beneath the water’s surface. The deeper the corals are, the less sunlight they receive, and the slower they grow.

There are several direct effects from the loss of coral reefs. When they are abundant and healthy, they protect shorelines from waves. They also contain up to 25 percent of all marine species. And roughly 275 million people depend on them for their livelihood and the food sources they nourish.

The Intergovernmental Panel on Climate Change (IPCC) has tried to blow the horn on this issue, concluding that even if the Earth’s temperature stays within the limit preferred by the Paris Agreement—2.7 degrees Fahrenheit of warming above pre-industrial levels—up to 90 percent of coral reefs could be lost. If the planet warms by 5.4 degrees Fahrenheit, which is the outer boundary of the agreement, the reefs would be virtually wiped out.

Although it’s possible that generations in the near future will be unable to see coral reefs for themselves, their loss is not likely to be permanent. Coral reefs, which first appeared during the Ordovician period more than 400 million years ago, have disappeared during extreme climate periods and returned when oceans became habitable again.

—Agya K. Aning

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