A marine heat wave has been cooking the North Pacific Ocean over the last several months, endangering underwater ecosystems.
When the heat wave formed earlier this year, experts worried that it may pose a threat to Hawaiʻi's reefs.
"As the event was first building, the predictions were that it was going to have severe coral bleaching impacts in Hawaiʻi," said Rob Toonen with the Hawaiʻi Institute of Marine Biology.
Cloudy weather saved the day, sparing corals from damaging irradiation. Temperatures are now dropping, and Toonen said that if impacts occur, they will likely be minor.
But as marine heat waves occur more frequently, it may not be long before the next major disaster strikes Hawaiʻi's reefs.
What causes coral bleaching?
Corals depend on microscopic algae that live within their tissues. These algae are what give corals their vibrant colors. When the algae photosynthesize, they provide the corals with energy.
Toonen said that a spike in ocean temperatures, combined with exposure to intense sunlight, can cause the symbiotic relationship between corals and their algae to break down.
When stressed, corals may expel their algae, a phenomenon called coral bleaching in which corals turn bright white.
Without the algae, corals are unable to produce food for themselves.
"That basically sets a countdown to starvation," Toonen said.
Corals may retake algae if environmental stressors are reduced, but long-lasting marine heat waves often make that recovery impossible.
"[With] things like the marine heat waves, where the temperatures get very high for an extended period of time, that starvation leads to large-scale death across our coral reefs," Toonen said.
Return of the Blob
This year's heat wave has drawn comparisons to another oceanic event. In 2013, a mass of warm water settled in the North Pacific and lingered until 2016. At its largest, it was roughly the size of the continental U.S.
That heat wave was nicknamed the “Blob,” because of its distinctive shape on ocean temperature charts — a huge blob of red-hot water spanning the North Pacific.
In some parts of the Blob, sea surface temperatures were 7 degrees Fahrenheit above average.
When the Blob arrived in Hawaiʻi in 2014, it had a devastating impact on local reefs, particularly off the west coast of Hawaiʻi Island.
"We lost somewhere between 25% and 50% of our corals in west Hawaiʻi Island alone," said Greg Asner, the director of Arizona State University's Center for Global Discovery and Conservation Science in Hilo.
Amy Olsen grew up on Hawaiʻi Island and now works as a research scientist with the Seattle Aquarium. She's part of a team that has been conducting annual reef health and fish population surveys at eight sites in the waters along the Kona coast since 2009.
"It wasn't until the Blob hit in 2014 that we started to notice degradation in the corals," she said.
More than a decade later, Olsen said the recovery of those reef systems has been "mixed."
"What we're trying to tease apart is how can we bring these systems back to pre-marine heat wave levels? Or if we monitor them for long enough, are they able to recover to those predisturbance levels?" she said.
While Hawaiʻi’s coral systems have likely avoided similar impacts this year, that respite may be short-lived, as marine heat waves are becoming more common and more intense.
In addition to the Blob and this year's near-miss, a marine heat wave in 2019 also threatened Hawaiʻi's reefs.
Moreover, the effects of recurrent marine heat waves are compounded by the increase in baseline ocean temperatures.
Climate physicist Malte Stuecker said that sea surface temperatures around Hawaiʻi have risen 2 degrees since the mid-19th century.
"The impacts of marine heat waves will become much more severe, because those swings will occur in a generally warmer ocean," he said.
Toonen said that making sure Hawaiʻi's coral reefs are in good condition will help them endure the next marine heat wave.
"One of the most important things that we can do as individuals is to make sure that we are doing everything we can to minimize the effects of land-based sources of pollution getting into our near-shore waters, so that our corals have the best chance of surviving," he said.
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