The study examines the changes in marine heatwaves (MHWs) over the past century, using a range of ocean temperature data including global records of daily satellite observations, daily in situ measurements, and gridded monthly in situ-based data sets. The findings reveal significant increases in MHW frequency and duration, with a 34% and 17% increase, respectively, from 1925 to 2016, leading to a 54% increase in annual MHW days globally. These trends are primarily driven by increases in mean ocean temperatures, suggesting that further increases in MHW days are expected under continued global warming. The study also highlights the severe ecological and socioeconomic impacts of MHWs, such as loss of kelp forests, coral bleaching, and reduced fishery yields. The analysis uses a standardized MHW definition and a quantitative framework to compare MHW characteristics across regions and events, revealing significant centennial-scale increases in MHW properties. The results emphasize the pressing need to quantify trends and patterns of MHWs to better understand and mitigate their impacts.The study examines the changes in marine heatwaves (MHWs) over the past century, using a range of ocean temperature data including global records of daily satellite observations, daily in situ measurements, and gridded monthly in situ-based data sets. The findings reveal significant increases in MHW frequency and duration, with a 34% and 17% increase, respectively, from 1925 to 2016, leading to a 54% increase in annual MHW days globally. These trends are primarily driven by increases in mean ocean temperatures, suggesting that further increases in MHW days are expected under continued global warming. The study also highlights the severe ecological and socioeconomic impacts of MHWs, such as loss of kelp forests, coral bleaching, and reduced fishery yields. The analysis uses a standardized MHW definition and a quantitative framework to compare MHW characteristics across regions and events, revealing significant centennial-scale increases in MHW properties. The results emphasize the pressing need to quantify trends and patterns of MHWs to better understand and mitigate their impacts.