Dear EarthTalk:

How are the world’s coral reefs doing these days? I haven’t heard much about them lately despite all the recent talk about climate change’s ill effects.

Jo. S., Bowie, MD

Coral reefs are being hit by climate change in just about every way possible. Wildfire, drought and other land-based climate disasters have captured global headlines, but coral reefs have been bleaching at record levels, and as such their future is uncertain. The science of climate change’s impact on coral reefs is simple. As humans pump greenhouse gasses 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 about 25 percent since the early 19th century, according to the U.S. Environmental Protection Agency (EPA). That acidity is incredibly harmful to coral reefs. According to the National Oceanic and Atmospheric Association (NOAA), ocean acidification “decreases 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 ocean temperature increases. Coral reefs exist only in narrow bands of water that stay within a moderate temperature range, not too hot or cold. Even moderate temperature increases can cause thermal stress that contributes to coral bleaching and infectious disease. The ocean has warmed 1.3 degrees (F) since the Industrial Revolution, meaning many reefs are stuck in dangerously hot water. The stress on reef creatures has been immense. When coral polyps—small, anemone-like animals that form the living base of reefs—get stressed, they expel the symbiotic algae that grows on them and provides them with nutrients. This is what’s called coral bleaching. With no algae to feed coral and give it its color, the abandoned coral turns white. That doesn’t necessarily mean it’s dead, but with no nutrient supply its ability to grow and fight off diseases is significantly hampered.

Warming water also causes stronger and bigger storms, which can destroy entire reef systems as they pass. Hurricane Dorian hit the Bahamas in 2019 and destroyed 30 percent of the islands’ coral reefs. In 2005, Hurricane Rita caused extensive damage to coral reefs in the Flower Garden Banks National Marine Sanctuary off the coast of Texas. Research suggests some storms may at times be beneficial for coral reefs by lowering water temperature. The influx of cool water can reduce heat stress on polyps, according to the Reef Resilience Network. But that temporary relief isn’t enough to make up for long-term warming. 

As surface temperatures increase, scientists hope that coral reefs might be able to slowly move themselves into cooler water—or that deep-water reefs already exist undiscovered. Researchers in Tahiti announced in February 2022 that they had found a nearly two-mile-long healthy coral reef in uncharacteristically deep water, leading to speculation that more deep-water reefs might exist in unexplored areas.

Still, the rate of human-caused warming far outpaces the speed at which coral reefs can move. Several start-ups and labs around the world are developing small, human-made coral systems, which could eventually be deposited in the ocean and grow into full reefs. But that technology is still a long way away. Until then, cutting emissions by driving less, using energy-efficient appliances and divesting from fossil fuel companies is the best way individuals can look out for the future of coral reefs.

CONTACTS

• EPA on ocean acidification, epa.gov/ocean-acidification/understanding-science-ocean-and-coastal-acidification#ocean
• NOAA, oceanservice.noaa.gov/facts/coralreef-climate.html
• Tahiti deep-water reef, washingtonpost.com/world/2022/01/20/tahiti-coral-reef-discovery/.