The heat is on Anthropogenic climate change is causing not only more episodes of historically high air temperatures but also more frequent spells of unusually increased ocean temperatures. Marine heatwaves, defined as periods of anomalously high regional surface ocean temperatures, have also become common in recent decades. Laufkötter et al. show that the frequency of these events has already increased more than 20-fold because of anthropogenic global warming, making marine heatwaves, which typically occurred once in hundreds to thousands of years in preindustrial times, likely to occur on an annual to decadal basis if the global average air temperature rises by 3°C. Science , this issue p. 1621