Key points
- The long-range marine heatwave outlook will now be a regular fixture over the Australian summer with forecasts made available publicly leading into and throughout the 2025-26 summer season.
- These forecasts will provide up to three months’ advance warning of ocean temperature extremes and the likelihood and location of marine heatwaves.
- Marine heatwave forecasting provide the opportunity to develop rapid responses in future.
Just as the 2019-20 summer is remembered for devastating bushfires, the summer of 2024-25 is likely to be remembered for Australia-wide marine biological disruption associated with marine heatwaves.
Marine heatwaves are a well-established cause of coral bleaching.
But the marine heatwaves that characterised Australia’s 2024-25 summer contributed to multiple biological disruptions affecting marine species, ecosystems and coastal communities, extending their impact far beyond the reefs.
It led to fish kills, out-of-range species, species outbreaks and algal blooms – including the large-scale bloom off South Australia’s coast.
The 2024-25 summer was also the second year in a row that scientists from CSIRO and the Bureau of Meteorology delivered trial seasonal forecasts at national marine climate briefings as part of the Marine Heatwave Prediction Project.
The briefings are designed to help prepare researchers and governments for possible marine heatwave (MHW) emergence and impacts, supported by Fisheries Research and Development Corporation and the Department of Climate Change, Energy, the Environment and Water.
The long-range marine heatwave outlook will now be a regular fixture over the summer with forecasts made available publicly leading into and throughout the 2025-26 summer season. These forecasts will provide up to three months’ advance warning of ocean temperature extremes and the likelihood and location of marine heatwaves.
CSIRO Chief Research Scientist Dr Alistair Hobday co-presents the briefings and stressed that forecasts are only one part of managing risk.
“Information about the future can support decision making,” he said.
“Marine heatwave forecasting tells you how you might be loading the dice for a range of ecological impacts.”
Marine heatwaves might not be the sole contributing factor to these marine disruptions, but the events of summer 2024-25 indicate the range of potential MHW impacts expected around the world and serve as a stress test of the environment under a changing climate.
What marine heatwave forecasting can provide is the opportunity to develop rapid responses in future.
“I really hope Australia is able to take advantage of these early warnings,” said Dr Hobday.
Summer of ’24-25: tracking MHWs around the Australian coast
The national marine briefings provided by CSIRO and the Bureau of Meteorology started in November 2024 and quickly confirmed that October/November had been the hottest on record for Australian waters as a whole – priming the system for the summer.
They tracked the progress of marine heatwave conditions around the coast and tested their forecasts against the outcome in historical reanalyses. The results showed that the forecasts could be a trusted tool for predicting and managing conditions.
The forecast model predicted the MHW conditions down the Western Australian (WA) coast and across the Great Australian Bight. The extreme conditions were recorded at depth as well as the surface with unpredictable long-term stress impacts.
In the Pilbara, it was a fish kill in early January, days after the World Meteorological Organization confirmed in January that 2024 had been the warmest year on record.
The WA Coral Bleaching Working Group soon reported observations of mass bleaching along inshore Kimberley coral reefs, fishers were reporting warm conditions and aquaculture farmers were reassessing their summer operational plans.
In South Australia, there has been an extensive harmful algal bloom of the dinoflagellate Karenia, which has been blamed for fish kills, human health effects, losses for ocean-dependent industries and unquantified effects on the broader marine ecosystem.
In Tasmania, a range of impacts were linked to warm water including blooms of salps, Noctiluca and jellyfish.
“It’s unusual to see the west coast and the south coast with such strong heating this year,” Dr Hobday told the briefing.
“The Great Australian Bight has escaped these conditions since we’ve been doing this work in the last 4-5 years.”
Infographic showing map of Australia with icons around the coast showing the marine events which occurred over the 2024-25 summer. A key on the left explains the icons.
Deeper ocean observations for better forecasting
There are four types of marine heatwaves:
- Shallow MHW
- Sub-surface MHW
- Subsurface-intensifies MHW
- Deep MHW.
Shallow and sub-surface marine heatwaves can be broken down by strong wind events or by storms that pass over them.
MHWs where heat is extended deep below the surface into the water column are the most concerning as they can retain heat even when winds blow over the surface.
MHW forecasting has relied on satellite-based information to describe extreme heating events or extreme cooling events with surface temperature well resolved from space.
“The suite of satellites from Japan, the European Union and the US provide the basis for much of what we observe in the ocean at the moment,” said Dr Hobday.
“But we know that’s only a very shallow surface picture and it’s really important to understand how heat is distributed in the ocean. It’s that amount of heat that does things like supercharge cyclones, or slow ocean mixing which can lead to heatwaves lasting for an extended period of time.”
Australia is investing in deeper ocean observations:
- Moorings around Australia from the IMOS national reference stations
- Gliders deployed at different locations around Australia through IMOS which can provide information on the structure of marine heatwaves in real time
- Deploying instruments on fishing gear in an IMOS/FRDC program called FishSOOP which is helping provide real time measurements of what’s happening in the ocean
- Profiling Argo floats that drop down to about 2000m in depth, drift around for about 7 days and take a profile as they come back up to the surface to transmit that data.
“We’re starting to get our hands on a pretty good set of data around Australia for understanding subsurface structure and I hope over the next few years you will see the value of that data collection paying off in terms of what we understand about the heat content that’s in it,” he said.
“It would be great to have more information about what’s happening below the surface because that would help us with prediction and also understanding the impacts.”
Understanding compound events and preparing for the impacts
Dr Hobday emphasises that the biological disruptions observed in the marine environment are frequently caused by multiple factors, not just marine heatwaves.
“Compound events where two things are happening at the same time such as a MHW with a cyclone, or with an atmospheric heatwave or coastal flooding.
“That’s going to be a problem for the future when we have two of these things going on at once.”
However, marine heatwaves are a strong indicator of impact.
The public marine briefings bring together experts, researchers and frontline responders and provide a forum for information sharing.
In the same way Australia responded to the Black Summer bushfires with increased coordination and collaboration across states and territories, the same response will be needed to marine heatwaves and their impacts.
Developing rapid responses
But how can we respond to marine heatwaves?
Responses to MHWs around the globe vary. CSIRO and the Bureau of Meteorology identified seven different interventions:
- Warnings issued
- Coral conservation
- Commercial and recreational fisheries closed
- Government subsidies or fee relief provided
- New/increased commercial and recreational fisheries opportunities
- Increase tourism opportunities for new species in novel areas
- Species and habitats monitored for conservation.
Almost all these interventions are reactive.
Dr Hobday said the off season was the time to consider proactive responses.
“Can you manipulate your system if you had early warning as a conservation manager, as a restoration practitioner, as a commercial fisher, as a recreational fisher, as an aquaculture business. If you get information about the future, what can you do ahead of time to reduce your impact?
“As we approach summer, it’s really important to look at baseline data, where your upwelling areas are and are they likely to provide any refuge and look at your vulnerable industries and populations.
“If you are able to use this time to prepare for taking action, you will find yourself less on the receiving end and more on the proactive end.”
The harmful algal bloom in South Australia, along with other events around Australia in the 2024-25 summer, are a sign of what Australia will face in future.
“Marine heatwaves are a stress test for the future as well because what we see today in a heatwave is what we'll experience every day in 20 years’ time.”