The world's oceans have reached their highest recorded temperatures in 2023, and it is forecasted that the intensity, duration, and frequency of marine heatwaves - defined as periods of elevated sea temperatures lasting at least five days - will continue to increase throughout the century. Until now, the impacts and projections of these events have focused on the sea surface, however, localized observations suggest that marine heatwaves can occur below the surface and persist for years. The rising of ocean temperatures has profound consequences for the planet's geochemical and biological systems, directly affecting marine flora and fauna, and therefore the resources humans depend on.

Using global-level sea temperature observations and reanalysis data - past short-range weather forecasts rerun with modern weather forecasting models - Eliza Fragkopoulou of the Centre of Marine Sciences (CCMAR, University of Algarve, Portugal) and her colleagues estimated the duration and intensity of marine heatwaves from 1993 to 2019 at depths down to 2,000 meters. In research published today in the journal Nature Climate Change, the authors found that the highest intensity of marine heatwaves is not at the surface but in the subsurface, specifically between 50 and 250 meters deep. "Against our initial expectations, we found that marine heat waves are more intense below the surface, and their duration can last up to double, when compared to the surface," notes Fragkopoulou.

The authors combined these findings with data on marine species ranges and predicted that subsurface biodiversity could be at risk due to higher cumulative intensity (an indicator of thermal stress)  in the first 250 meters. Jorge Assis (CCMAR), who led the research explains: "Across the globe, we identified regions of higher risk for marine biodiversity, including significant portions of the Indian and North Atlantic Oceans, where high cumulative heatwave  intensity coincides with areas of predicted high species sensitivity to thermal stress. What we've discovered is that beyond readily detectable surface effects, such as losses of ecosystem structuring species of mangrove and kelp forests and coral reefs, marine heatwaves can have profound effects on deeper biodiversity, down to 250 meters."

This study was the first to use daily temperature data from the last 30 years to analyse heat waves from the surface to greater depths (2000 metres). As the frequency of marine heatwaves is expected to increase due to climate change, the authors emphasise the need for more research to explore the impact these events will have on subsurface marine ecosystems. "Extreme temperature events are expected to redistribute marine species from the surface to the deep ocean. However, there are other factors that can prevent this redistribution, with unpredictable consequences," explains Miguel Bastos Araújo, a researcher at the National Museum of Natural Sciences (MNCN-CSIC), who contributed to this study. Additional monitoring of marine biodiversity in extreme climatic conditions is therefore necessary in order to better understand and predict its effects.

Direct URL for the article: https://www.nature.com/articles/s41558-023-01790-6