The widespread bleaching of corals in reef communities due to climate change is a well-documented phenomenon. When reef-building corals are exposed to above-average water temperatures for long periods, they expel their algal symbionts and “bleach”, turning white. Repeated heat stress can eventually kill off entire colonies of coral, turning once vibrant ecosystems into comparatively barren ones. 

Global projections of sea-surface temperatures paint a grim picture for reefs in the future. At the current rate of global warming, in 30-50 years most of the world’s reef-building corals will experience annual bleaching events from higher sea-surface temperatures, threatening their survival. Healthy coral reefs are more than just hotspots of marine biodiversity—they’re also invaluable to long-term resilience against coastal storms. 

Thankfully, rising sea-surface temperature isn’t the only factor to consider when assessing coral reef resiliency. A 2020 study by researchers at the USGS Pacific Coastal and Marine Science Center and partners published in the journal Scientific Reports finds that some reefs are regularly flushed with cooler water from the depths, creating “thermal refugia” that buffer corals from warmer surface temperatures. 

Global sea-surface temperatures are generally measured using satellite observations. Using satellite data to project how rising temperatures will impact coral reefs in the future can lead to the assumption that water temperatures over coral reefs are the same as sea-surface temperature data. 

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“These data have long been used to monitor the temperature of the global ocean and identify current and future coral-bleaching threats. The problem with using sea-surface temperatures to characterize threats to corals is that corals live beneath the surface, where temperatures can fluctuate with internal waves, currents, and tides,” said Curt Storlazzi, USGS Research Geologist at the Pacific Coastal and Marine Science Center and lead author of the study. “Thus, predictions of coral bleaching using sea-surface temperatures, which overlook those subsurface processes that may act to mitigate thermally-induced bleaching, may overestimate the threat in some places where such processes are dominant.” 

Internal tides and waves occur throughout the global ocean and are not specific to coral reefs. They’re formed by differences in water density, such as at the boundary between warmer, less dense surface water and cooler, deeper, denser water. 

“It’s important to note that internal tides alone won’t mitigate potential impacts to coral reefs, particularly under the higher greenhouse gas-emission scenarios that are projected for the future. But for particular places, they create important thermal refugia that may buy more time for mitigation and adaptation measures” said Olivia Cheriton, USGS Oceanographer at PCMSC and a co-author of the study. 

Understanding where these refugia exist now and into the future is critical for the long-term management of coral reefs, the study authors note. Healthy reefs buffer storm waves and reduce flood risk, among much else; restoring and protecting them offers a way to build coastal resiliency in a rapidly changing world. 

The study is featured in Nature’s Scientific Reports 2021 Editor's Choice: Corals and Reefs collection, under the “Anthropogenic Pressures” section.