| Summary: | Abstract Since the early 1980ies, the shallow and brackish Baltic Sea, located in northern Europe, has warmed fastest of all the world’s coastal seas. Hence, the total heat exposure during marine heatwaves increased in frequency and duration, which may have a major impact on the marine ecosystem. We found that marine heatwaves, using two different, commonly used definitions, occurred everywhere on the sea surface since the early 1990s. Periods with sea surface temperatures larger than 20 °C lasting longer than 10 days were particularly numerous in the shallow coastal zone of the southern and eastern Baltic Sea. Furthermore, summer marine heatwaves that reached the seabed mainly occurred in water depths of less than 20 m, where they led to a decrease in oxygen concentration. Since the early 1990s, a positive trend in the expansion of marine heatwaves has been observed on the seabed. The increasing number of marine heatwaves increases the risk of hypoxia events in the coastal zone.
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