LAUSR

During the Australian summer of 2010–2011, the water off the continent’s west coast got hot—really hot. Sea temperatures soared to 6 °C above normal. Researchers and fisheries managers worried about impacts on valuable species, such as abalone and scallops, and on the vast forests of kelps and other seaweeds that underpin the region’s highly productive coastal ecosystem. But “no one really had any data,” recalls University of Western Australia, Perth, marine botanist Thomas Wernberg. Wernberg had been monitoring coastal plant communities for more than a decade, so he and his colleagues decided to revisit their study sites. First up was a day trip north from Perth to Jurien Bay. The researchers were stunned by what they found. The former lush underwater meadows of strap weed, a relative of kelp, were completely gone. “We said, ‘Jesus Christ, what happened here?’” Wernberg recalls. A year later, Wernberg’s team ventured further north to Kalbarri to assess the productivity of the typically luxuriant kelp forests found in the coastal waters. They had another shock. “There was not a single kelp plant anywhere,” Wernberg says. “That’s when we realized the huge magnitude of what had happened.” Nearly half of the kelp along more than 800 kilometers of coastline had vanished, the researchers showed—and the ecosystem had flipped to an entirely new state dominated by small turf-forming seaweeds. Abalone and scallop populations crashed. Penguin chicks died in massive numbers. The fish communities changed, with influxes of more tropical species that ramped up grazing pressure on seaweeds by 400% (1). Even now, more than a decade later, and long after sea temperatures returned to normal, Wernberg’s data show that the kelp has not recovered, and shellfish populations are way down and only slowly returning to fishable levels (2). The intense 2011 Australian coastal heatwave did more than push an entire ecosystem into uncharted waters. It also spurred the first effort to rigorously define and characterize the concept of marine heatwaves—an interdisciplinary crusade complicated by an underlying relentless uptick in temperatures as a result of climate change. In the last 10 years, “the science on marine heatwaves has exploded,” says physical scientist Dillon Amaya, at the National Oceanic and Atmospheric Administration Earth System Research Laboratories in Boulder, CO. In more than a hundred articles, researchers have chronicled the rise and fall of ocean temperatures in scores of locations around the world, pinpointed the drivers of these heatwaves, documented the often-devastating effects on everything from plants to shellfish, and calculated a multi–billion-dollar annual toll from increasingly extreme heatwaves. Even among researchers expecting big changes owing to climate change, it’s a sobering and somewhat unexpected story; the heatwaves involve temperature jumps far above the slow climb in average ocean temperature. “I think marine heatwave intensification is one of the most pervasive threats to marine ecosystems,” says benthic marine ecologist Dan Smale at the Marine Biological Association of the UK in Plymouth. “We should be really worried.”