Description:   Extreme events, i.e., the normally rare occurrences when a system is far outside its norm, severely impact organisms and ecosystem on land. Yet, in comparison, our understanding of such extreme events in the ocean is generally poor. This is especially the case for ocean biogeochemical extremes, while the knowledge on marine heatwaves has grown rapidly in recent years. With trends in ocean warming, acidification, deoxygenation, and surface nutrient concentrations projected to continue for decades, marine heatwaves and ocean biogeochemical extreme events are likely to intensify, occur more often, persist for longer, and extend over larger regions. Of particular concern are compound events, i.e., when conditions are extreme concurrently for multiple properties. Compound extremes can lead to especially severe impacts, since the individual properties may interact synergistically. Here we combine observations with large ensemble simulations of Earth system models to assess the spatial characteristics, underlying drivers and trends in ocean extremes and compound events and their impacts on marine ecosystems, with a focus on compound marine heatwaves and ocean acidity extremes. Since the conditions exhibited by today’s extreme events are a harbinger of what may become ‘normal’ in the future, our results may help to better understand the response of marine organisms and ecosystems to future climate change.

About the author:  Thomas Frölicher is currently an Assistant Professor at the Climate and Environmental Physics Division of the University of Bern and the head of the ocean modelling group. He is interested in marine ecosystem-carbon-climate interactions with focus on ocean extreme events and their impacts on climate and on marine organisms and ecosystem services. He studied environmental sciences at ETH Zürich and graduated in Physics at the University of Bern. He worked as a postdoctoral fellow at Princeton University and as a senior researcher at ETH Zürich.

 He is the recipient of the 2019 Theodor Kocher Prize of the University of Bern. Thomas is strongly involved in the European H2020 projects COMFORT (ocean extremes and tipping points), 4C (global carbon cycle), AtlantECO (Atlantic ecosystems) and PROVIDE (overshoot scenarios). He was the lead author of chapter six on Extremes, Abrupt Changes and Managing Risks of the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate, including the summary for policy makers, and contributed to the fifth and sixth assessment report of working group I and II of the IPCC and to the second World Ocean Assessment.