Coral reefs are dying at an unprecedented rate due to global warming

Dear EarthTalk: How are the world’s coral reefs doing these days? I haven’t heard much about it lately despite all the recent talk about the adverse effects of climate change.

—Jo. S., Bowie, MD

It’s going to get harder and harder to find Nemo and other clownfish as the world’s living coral reefs become increasingly rare. Credit: George Becker, Pexels

Coral reefs are affected by climate change in every possible way. Wildfires, drought and other land-based climate disasters have grabbed global headlines, but coral reefs have bleached at record levels and as such their future is uncertain. The science of the impact of climate change on coral reefs is simple. As humans pump greenhouse gases into the atmosphere, the ocean acts as a carbon sink, absorbing carbon dioxide (CO₂) and dissolving it into acid. As a result, ocean acidity has increased by around 25% since the start of the 19th century, according to the US Environmental Protection Agency (EPA). This acidity is incredibly harmful to coral reefs. According to the National Oceanic and Atmospheric Association (NOAA), ocean acidification “decreases the growth rates and structural integrity” of coral skeletons, damaging their ability to support the diversity of life that makes up a reef ecosystem.

One of the most immediate threats to coral is rising ocean temperatures. Coral reefs only exist in narrow bands of water that stay within a moderate temperature range, neither too hot nor too cold. Even moderate temperature increases can cause heat stress that contributes to coral bleaching and infectious disease. The ocean has warmed by 1.3 degrees (F) since the Industrial Revolution, meaning many reefs are stuck in dangerously warm water. The stress on the reef creatures has been immense. When coral polyps – small anemone-like animals that form the living base of reefs – are stressed, they push out the symbiotic algae growing there and supply them with nutrients. This is called coral bleaching. Without algae to nourish the coral and give it its color, abandoned coral turns white. This doesn’t necessarily mean it’s dead, but without a nutrient supply its ability to grow and fight off disease is significantly hampered.

Warming water also causes stronger and larger storms, which can destroy entire reef systems as they pass. Hurricane Dorian hit the Bahamas in 2019 and destroyed 30% of the islands’ coral reefs. In 2005, Hurricane Rita caused extensive damage to coral reefs at the Flower Garden Banks National Marine Sanctuary off the coast of Texas. Research suggests that certain storms can sometimes benefit coral reefs by lowering water temperatures. The influx of cool water can reduce heat stress on polyps, according to the Reef Resilience Network. But this temporary relief is not enough to offset the long-term warming.

As surface temperatures rise, scientists hope that coral reefs may slowly shift to cooler waters or that deep-sea reefs already exist undiscovered. Researchers in Tahiti announced in February 2022 that they had found a healthy coral reef nearly two miles long in unusually deep water, suggesting that more deep-sea reefs may exist in unexplored areas.

Yet the rate of human-caused warming far exceeds the rate at which coral reefs can move. Several start-ups and laboratories around the world are developing small artificial coral systems, which could eventually be deposited in the ocean and become complete reefs. But this technology is still a long way off. Until then, reducing emissions by driving less, using energy-efficient appliances and getting out of fossil fuel companies is the best way for people to look to the future of coral reefs.

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Teresa H. Sadler