July 17, 2023 — On 4 July 2023, the World Meteorological Organization declared the beginning of an El Niño phase, a climate pattern that drives up temperatures across land and sea.
Past weather events provide clues about what extreme temperatures could mean for the ocean. For instance, in December 2010, a wave of unusually warm water swept across the luxuriant and biodiverse seagrass meadows of Australia’s Shark Bay. In a matter of days, it destroyed a third of the habitat, unthreading the delicate seagrass quilt and, over the next three years, releasing between 2 and 9 billion tonnes of carbon into the atmosphere. “The losses there were phenomenal,” says Kathryn Smith, a researcher with the UK’s Marine Biological Association.
Scientists call the event a “marine heatwave”, meaning a period of exceptionally high water temperature that starts suddenly and continues for days to months, distinguishing it from long-term warming trends. Like heatwaves on land that threaten terrestrial ecosystems, heatwaves at sea harm marine life, posing “a clear and present threat to the systems we depend on,” says Sarah Cooley, director of climate science at the Ocean Conservancy.
These impacts are expected to grow. The UN’s climate science body, the IPCC, projects that by 2100 marine heatwaves will be up to 50-fold more frequent, and 10-fold more intense compared to pre-industrial times. Scientists are now developing ways to forecast these events. Their research can feed into measures that mitigate the threats for vulnerable habitats, species, and the coastal communities that depend on them.