This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Once regarded as potential safe havens for corals in a warming climate, deep reefs, which lie at depths between 90 and 150 meters (295 and 490 feet), are no longer sheltered from increasingly intense marine heat waves, according to a new study in Geophysical Research Letters. Marine heat waves increase the chances of disrupting the tiny algae that live within corals, leading to coral bleaching and even mass die-off events.
As human activities continue to raise global temperatures, the growing intensity of marine heat waves at depth may leave tropical corals without cooler waters to retreat to, threatening biodiversity, fisheries and tourism throughout reef regions. "Ocean warming is not just a surface problem. What happens deep beneath the surface can also have serious consequences for coral reef survival," said Chunzai Wang, an oceanographer at the Chinese Academy of Sciences and co-author of the study.
Scientists have proposed using deep reefs as "seed banks"—transplant sites for reproduction-ready corals from suffering shallow reefs. But "going deeper does not necessarily mean escaping the heat," Wang said. To understand how marine heat waves affect reefs beyond the top 30 meters (98 feet) of the ocean, researchers used global maps of warm-water coral reefs to filter ocean-wide simulations of daily water temperatures down to 150 meters (490 feet) from 1993 to 2023.
These warm reefs occur around the world within 35 degrees of the equator, at latitudes roughly between Santa Barbara, California, and Adelaide, Australia. The simulated water temperatures use ocean circulation patterns and physics to fill the gaps between real measurements, creating a more complete record of reef heat waves than is available from satellite measurements, which can only see surface temperatures. With this deeper view of reef heat exposure, researchers tracked the number, durations and temperatures of marine heat waves, which oceanographers define as a stretch of at least five days with seawater warmer than 90% of the historical temperatures recorded for that time of year.
The team also calculated heat wave "intensity," a metric that accounts for both temperature and duration. According to NOAA, corals risk bleaching once they've received four "degree-weeks" of heat—the equivalent of four weeks at 1 C above normal temperatures, or a commensurately shorter time at higher temperatures (for example, two weeks at 2 C hotter, or just one week at 4 C hotter). Deep reefs may still have been plausible coral refuges prior to 2010, the researchers suggested.
Before this date, marine heat waves typically impacted less than 50% of global reef areas per year, largely sparing deeper reefs from sustained high temperatures. Between 1993 and 2010, the average deep reef heat wave lasted less than 15 days except during the 1997–1998 El Niño. From 2010 onward, however, heat waves hit over half of the world's tropical reefs in eight of the 13 years studied—roughly four times more often than in the preceding 17 years.
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