Short answers, pulled from the story.
A marine heatwave is a period when sea surface temperatures stay abnormally high for a given location and season. A 2016 study set the threshold at temperatures warmer than the 90th percentile based on a 30-year historical baseline, lasting five or more days. Events are classified on a scale from 1 to 4, ranging from moderate to extreme.
The term was coined following an unprecedented warming event off the west coast of Australia in summer 2011. That event caused rapid dieback of kelp forests along hundreds of kilometers of coastline, prompting scientists to name the phenomenon.
The IPCC's 2022 report projects marine heatwaves will become four times more frequent in 2081-2100 compared to 1995-2014 under the lower greenhouse gas emissions scenario, and eight times more frequent under high emissions. These projections rely on the CMIP6 mathematical model.
Marine heatwaves are estimated to have caused USD 6.6 billion in fisheries losses between 1985 and 2022, affecting an industry that supports livelihoods for 500 million people. They trigger habitat degradation in seagrass beds, coral reefs, and kelp forests, and contribute to mass mortality events such as the death of 25 benthic species in the Mediterranean in 2003. Sea star wasting disease, coral bleaching, and toxic algal blooms are among the documented consequences.
Marine heatwaves form through a combination of local processes including ocean advection, air-sea heat exchange, thermocline stability, and wind stress. Teleconnection processes such as atmospheric blocking and jet-stream positioning link distant weather systems to localized ocean warming. At the planetary scale, decadal oscillations like the Pacific Decadal Oscillation and El Nino Southern Oscillation have driven major named events.
The tropical Indian Ocean is projected to experience 220-250 heatwave days per year by the end of the 21st century, up from roughly 20 days per year based on the 1970-2000 baseline. In the western North Pacific, mean event duration is projected to grow from about 11 days to roughly 138 days per event, with annual heatwave days approaching 270 by 2100 under high emissions.