Extreme marine heatwave threat to Caribbean coral reefs

Scientists say extreme marine heatwaves in 2023 have engulfed much of the eastern tropical Pacific Ocean and Caribbean, threatening reef wildlife and economies.

“Corals are literally dying before they even have a chance to bleach,” says Dr Sophie Dove, a coral reef ecologist at The University of Queensland who contributed to the report published in Science overnight.

“Of course, this amplifies the seriousness of the escalating change on our precious coral reefs.”

At the beginning of November, the CSIRO’s research vessel RV Investigator tracked a severe subsurface ocean heatwave off the Sydney coast in the western Pacific.

That heatwave, extending deep beneath the surface is, according to voyage leader Professor Moninya Roughan, “enormous and hot” and more than 3°C above average for the area.

Professor Ove Hoegh-Guldberg is the lead author of the report on the Caribbean heatwave in Science. The University of Queensland based coral reef scientist and inaugural director of the Global Change Institute at the University of Queensland, and who is also speaking at COP28 currently being held in Dubai, says information about the extent of this year’s marine heatwave indicates: “we are well off the track when it comes to keeping global surface temperatures from attaining a very dangerous condition by mid to late century”.

“We are on the brink of losing coral reefs. Surely our world leaders won’t let the fate of the world’s coral reefs simply slip through our fingers! We must increase our ambition.”

Hoegh-Guldberg, a member of the IPCC, says heat stress puts immense pressure on fragile tropical ecosystems such as coral reefs, mangrove forests and seagrass meadows.

“That heat stress is driven by marine heatwaves (MHW), which are strongly correlated with rising sea surface temperatures and climate cycles such as El Niño–Southern Oscillation,” they write.

“Extreme MHWs engulfing much of the eastern tropical Pacific and wider Caribbean have caused unusual spikes in sea surface temperatures this year. Many Caribbean reef areas experienced historically high heat stress that started 1-2 months earlier than usual and was sustained for longer than the usual recorded seasonal changes.”

The authors say patterns of SST from the past 40 years indicate unprecedented mass coral bleaching and mortality will likely occur across the Indo-Pacific throughout 2024.

Watch: Video provided by Underwater Earth of a bleached coral reef at Cairns, North Queensland, Australia.

Read more: Marine heatwave in western Pacific.

In July Dr Derek Manzello, head of the U.S. National Oceanic and Atmospheric Administration’s Coral Reef Watch in Washington DC, said in a NOAA blog: “We were shocked to see such an unexpectedly early onset of heat stress in Florida and the wider Caribbean.

“If ocean temperatures are higher than the maximum monthly average, for a month or more, especially during the warmest part of the year – even by as little as 1-2 degrees Celsius (2-3 degrees Fahrenheit), corals will experience bleaching,” he said on the blog.

“A bleached coral is essentially starving to death because it has lost its main source of nutrition — the algae that live symbiotically within its tissues. The damage corals experience from marine heatwaves is a function of the duration, or how long the heat stress occurs, plus the magnitude of the heat stress. 

“Corals can recover from bleaching if the heat stress subsides, but the corals that are able to recover frequently have impaired growth and reproduction and are susceptible to disease for two to four years after recovery.”

NOAA has a near-real time marine heat map.

Underwater Earth, a group of communication and media specialists focussing on the marine environment, says coral reefs are among the most diverse ecosystems on earth, supporting more species per unit than any other marine environemnt. It says reefs are home to more than 4,000 species of fish, 800 species of hard corals and hundreds of other species.

It says the first coral reefs formed on Earth 240 milion years ago, and most coral reefs today are between 5,000 and 10,000 years old.