Rapid warming off the western coast of Australia has transformed large stretches of kelp forests into tropical and subtropical marine ecosystems, a new study reports. The results reveal a dramatic transition over a short period of time, where a marine heatwave resulted in the loss of 963 km2 of kelp forests. Since the 1970s, kelp forests along the mid-west section of the Australian coast have experienced steadily increasing ocean temperatures, recently punctuated by three of the warmest summers in the past 215 years. To better understand how these temperature increases may impact ecosystems in the region, Thomas Wernberg et al. surveyed kelp forests, seaweeds, fish, mobile invertebrates, and corals at 65 reefs between 2001 and 2015, during which time an extreme marine heat wave affected the region. Before the extreme heat wave in December of 2010, kelp forests covered more than 70% of shallow rocky reefs in the midwest, Yet, two years later, 43% of these kelp forests were lost, the survey data reveal. The region north of 29°S was severely impacted, with previously dense kelp forests having disappeared completely, or been reduced by more than 90%. Five years after the heatwave, the researchers observed no signs of kelp forest recovery. In contrast, they note dramatic increases in turf-forming seaweeds and a large shift in species characteristic of subtropical and tropical waters. For example, they found a 400% increase in the biomass of scraping and grazing fishes, organisms typically found in coral reefs.