Analysis drawing on satellite data and 13 climate models concludes that climate change makes marine heatwaves 20 times more likely. Marine heatwaves linked to the deaths of 30,000 fish off the Western Australia coast were up to 100 times more likely to occur because of climate change, new research has found. Waters off WA have been affected by prolonged marine heatwaves since September last year. Regions off the north-west coast were hottest, with ocean temperatures 1.5C higher than average over a five-month period and sometimes 4-5C higher at the surface.

Sign up for Guardian Australia’s breaking news email. Analysis by the non-profit group Climate Central found climate change had made the marine heatwave 20 times more likely to occur – and the most impacted period in November 100 times more likely. Heatwave conditions are triggered when an area is hotter than 90% of recorded temperatures for that time of year over at least five consecutive days. Dr Andrew Pershing, the chief program officer at Climate Central, which has adapted climate attribution methods for studying major weather events on land to the ocean, said the escalating heat seen off the coast of WA was “not a normal event”.

“This is an event that is directly tied to burning fossil fuels,” he said. The group’s Ocean Climate Shift Index drew on satellite data from organisations like the US National Oceanic and Atmospheric Administration, and applied 13 climate models, to isolate the effect of carbon pollution from human activities, Pershing said. Nearly 90% of marine heatwaves were now attributable to human-caused global heating and are expected to increase in frequency, intensity and duration as fossil fuel emissions continue to rise, previous studies have found .

Marine heatwaves caused “prolonged thermal stress” to marine life, which authorities say likely contributed to the mass fish kill observed at beaches along WA’s Pilbara coast. Australia was “on the frontline” of these effects, Pershing said. These events led to fish kills and coral bleaching and had widespread consequences for industries like fishing and tourism. A marine heatwave off the WA coast in 2010-11 damaged more than a third of seagrass meadows in Shark Bay.

Dr Matt Rayson, an oceanographer at the University of Western Australia, who was not involved in the attribution study, said the heat that began building in the north in September had slowly crept down into the western side of Australia, with the ocean getting warmer at the surface. Sign up to Afternoon Update. Our Australian afternoon update breaks down the key stories of the day, telling you what’s happening and why it matters.

after newsletter promotion. Autonomous ocean robots, deployed by the university, had also measured temperatures below the surface, finding unusual levels of heat. Rayson said 90% of the extra energy trapped by greenhouse gases was stored in the ocean, increasing the probability of ocean heatwaves. As well as effects on marine life, this additional energy could translate into severe weather events like tropical cyclones.

WA’s unusual marine heat was continuing to escalate. In January, ocean temperatures were consistently at least 1.6C hotter than average, according to Climate Central. Pershing said marine heatwaves weren’t random. They were connected to climate change, he said, and becoming more intense and frequent as humans continued to pollute the atmosphere. “Humans are affecting the planet in a lot of ways. We’re used to thinking of how it affects us on land, but it’s affecting everything on the planet, and the ocean is such a huge part of our planet.”.