Coral reefs off WA have been hit hard by a 2024-25 marine heatwave that was the longest, largest and most intense on record for the state, scientists and managers say.
And temperature monitoring on site is revealing a hotter difference with satellite-recorded readings, they say.
Their findings came ahead of a one-day marine heatwave symposium held yesterday (Tuesday) in Perth which attracted researchers, marine park managers, state politicians and traditional custodians.
The symposium was organised by The Australian Institute of Marine Science (AIMS) and state government.
AIMS senior research scientist Dr James Gilmour said the intensity of the heatwave was relentless as it hit reefs that had previously escaped bleaching.
“There has been little reprieve this time for any of our northwest reefs,” he says.
“Areas which had given us hope because they’d rarely or not bleached before like the Rowley Shoals, north Kimberley and Ningaloo have been hit hard this time. Finally, climate heating has caught up with these reefs.”
Dr Gilmour says they have never before seen the length and intensity of heat stress that they saw on most reefs.
He says AIMS is still crunching the numbers for individual reefs for Degree Heating Weeks (DHW), a measure of heat stress on corals.
“Early estimates are over 15 DHWs on all reefs and up to 30 DHWs at some Pilbara reefs; eight DHWs is the level of heat stress we consider severe enough to cause coral mortality,” he explains.
AIMS coral scientist Nicole Ryan, who co-ordinates the WA Coral Bleaching Group, says the heat stress will beat records set during the 2011 La Nina and 2016 El Nino on all but the southernmost reefs.
“It will take several months to understand how much this year’s event will have impacted coral cover,” she says.
“A bleached coral is not a dead coral – it can recover. For some coral species, it can take time after bleaching for it to recover or die. WA reefs are also spread across thousands of kilometres with (many of them) remote, making them difficult to monitor.”
DIFFERENCES IN TEMPERATURES
AIMS oceanographer Dr Camille Grimaldi, who recorded water temperatures at coral reefs along the northwest during this period, found that in-situ (on site) readings often exceeded satellite-recorded sea surface temperatures (which are used to monitor reef conditions globally).
“At Ashmore Reef, for instance, in-situ temperatures recorded in the lagoon were up to 1.4 degrees Celsius degrees higher than satellite estimates,” she says.
“Temperatures measured at 20m depth exceeded satellite readings by nearly 0.5°C, indicating that ocean warming extended not just at the surface but throughout the water column, spanning the depth range for most corals.”
Dr Grimaldi adds: “These differences highlight how local oceanographic processes, such as shallow water depth, upwelling or mixing, can amplify or dampen the effects of regional heatwaves, making extreme events either more severe or more tolerable at specific reef locations.”
Dr Claire Spillman is the Seasonal and Marine Applications Team Leader from The Bureau of Meteorology.
“2024 was the warmest year on record for global oceans. For Australian waters, sea surface temperatures during the summer of 2024-25 were the warmest since official records began in 1900,” she says.
“The northwest waters of WA also experienced a warm winter in 2024, with persistent warm water at depth, and this background warmth increased the chances of marine heatwave conditions.
“While the 2010-11 WA marine heatwave was associated with a strong La Nina event, this was not the case this summer. The increased frequency of extreme and record-breaking ocean temperatures is associated with global warming.”
GOVERNMENT RESPONSE
Environment Minister Matthew Swinbourn detailed the government’s support for long-term marine ecosystem monitoring and management programs at the symposium.
He claims his government is “deeply invested” in the WA marine environment, from conservation to ecotourism to recreation.
“The information shared at the symposium will improve overall understanding of the marine heatwave event and help prioritise the next steps in monitoring, research and future management,” he says.
“Long-term marine park monitoring programs can provide important and early insights into the impacts and recovery of marine ecosystems from environmental events, as well as human-induced activity.”
MASS CORAL BLEACHING
“This huge WA bleaching event comes at a concerning time for coral reefs in Australia,” Dr Gilmour says.
“It synchronised with another mass bleaching event on the Great Barrier Reef and is part of the ongoing fourth Global Coral Bleaching Event, which began in 2023 and circumnavigated our oceans in a wave of coral mortality.”
EARLIER STORY: Great Barrier Reef bleaching worsening
He adds: “Climate change is driving these events, which are becoming more frequent, more intense and more widespread, giving our amazing, valuable coral reefs little time to recover. They need 10 to 15 years to recover fully.”
Dr Gilmour says the key to helping coral reefs survive under climate change is to reduce greenhouse gas emissions.
“Good management of local issues like water quality and overfishing and using interventions developed by marine science to help reefs will also assist.”
