Short answers, pulled from the story.
Scientists coined the phrase marine heatwave to describe periods when water temperature is abnormally warm for that time of year. A 2016 study defined such an event as lasting five or more days with temperatures warmer than the 90th percentile based on a 30-year historical baseline period.
Researchers developed a four-tier classification system where events are rated on a scale from one to four depending on severity. Category one represents a moderate event while category two indicates strong conditions, category three describes severe situations, and category four marks extreme occurrences.
Human-caused global warming has doubled the frequency of marine heatwaves since the 1980s. The IPCC Sixth Assessment Report stated that these events are very likely attributable to anthropogenic climate change since 2006.
A massive event struck Western Australia again in 2011 covering an area of 0.95 million square kilometres. That specific incident reached an intensity of 4.9 degrees Celsius above normal during this unprecedented heat event.
Mass mortality events occurred when 25 benthic species died off in the Mediterranean during 2003. Coral bleaching became a widespread consequence of rising sea surface temperatures across multiple regions.