Western Australia marine heatwave

Much of the ocean off the west coast of Australia has been in a marine heatwave (MHW) since September 2024. The Climate Shift Index: Ocean shows that during this MHW, daily ocean surface temperatures were made on average at least 20 times more likely to occur as a result of climate change.

• Over the full scope of the region that Climate Central analyzed (see figure 1 below), the percentage of area in a heatwave state increased in mid-September to above 20% (see figure 2 below) and continued growing over time (despite a short dip in early December) before a sharp increase in early January, when more than 60 percent of the region met the criteria for a  MHW.

• The hottest region is just off the northwest coast of Australia, where average temperature anomalies over a five-month period have exceeded 1.5 degrees Celsius (see figure 1, top). Additionally, much of this region has been in the MHW condition for more than 120 days since August 2024 (figure 1, bottom).

• The Climate Shift Index: Ocean (Ocean CSI) shows that during this MHW, daily ocean surface temperatures were made on average at least 20 times more likely to occur as a result of climate change throughout the event (see figure 3 below). Meanwhile, during the most climate-impacted period in Late November, temperatures were made, on average, more than 100 times more likely to occur across affected regions.

• The severity of this MHW is increasing. A growing portion of the ocean in this region is in the MHW, and the unusual heat associated with the MHW continues to escalate. In September, average temperature anomalies stayed at around 1.2 degrees Celsius. Since then, average temperature anomalies have steadily increased. In January, mean temperature anomalies never dipped below 1.6 degrees Celsius, and on three separate occasions exceeded 2 degrees Celsius. 

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What do experts say?

Dr. Andrew Pershing, chief program officer at Climate Central said, "This prolonged marine heat wave off Western Australia is a clear sign of a warming climate, with extreme ocean temperatures causing severe ecosystem impacts. It highlights the profound effect of human-caused warming on our oceans."

Methodology

We followed the methodology established by Hobday et. al (2016) to establish which regions analyzed were part of the MHW. This methodology suggests that areas above the 90th percentile temperature for that time of year for more than 5 days in a row are in a MHW. Using NOAA’s Optimum Interpolation Sea Surface Temperature (OISST), we first found the 90th percentile sea surface temperature (SST) threshold across the region for each location for each calendar day (using an 11 calendar day rolling window) from 1991-2020 and computed the 90th percentile from those values. A location was in the MHW if its SST was above these thresholds for 5 days in a row (we call each period of  5 days or more above the 90th percentile a persistent event), or if it was in a period of two days or less between subsequent 5 day persistent events. In other words, if a location experienced 5 days above the 90th percentile, then 2 days below the 90th percentile, then 7 days above the 90th percentile, all 14 days were deemed to be in the MHW. We used this determination to measure the coverage of the MHW during the time period as well as the temperature anomaly associated with this coverage. Finally, we used the Ocean CSI to measure the impact of climate change on this MHW. The Ocean CSI is grounded in peer-reviewed methodology and high-quality data. It quantifies the influence of climate change on sea surface temperatures. The Ocean CSI indicates how human-caused climate change has influenced the likelihood of daily sea surface temperatures occurring at nearly any location around the world’s oceans.