A marine heatwave is a period of abnormally high sea surface temperatures, driven by short-term weather events, climate patterns like El Niño, and human-caused climate change. These events can devastate marine ecosystems, triggering coral bleaching, sea star wasting disease, harmful algal blooms, and mass mortality of seafloor communities. Major heatwaves have struck the Great Barrier Reef (2002), the Mediterranean (2003), the Northwest Atlantic (2012), and the Northeast Pacific (2013–2016), causing long-term damage. The IPCC's 2022 report confirms they are more frequent and intense since the 1980s, predicting they will become four to eight times more frequent by 2081–2100 depending on emissions. The term was coined after a 2011 event off Western Australia, and scientists now categorize these events on a scale from 1 (moderate) to 4 (extreme) to compare and communicate their severity.