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Marine biodiversity exposed to prolonged and intense subsurface heatwaves


Marine heatwaves (MHWs) are becoming increasingly common, with devastating ecosystem impacts. However, MHW understanding has almost exclusively relied on sea surface temperature with limited knowledge about their subsurface characteristics. Here we estimate global MHWs from the surface to 2,000 m depth, covering the period 1993–2019, and explore biodiversity exposure to their effects. We find that MHWs are typically more intense in the subsurface at 50–200 m and their duration increases up to twofold with depth, although with large spatial variability linked to different oceanographic conditions. Cumulative intensity (a thermal stress proxy) was highest in the upper 250 m, exposing subsurface biodiversity to MHW effects. This can be particularly concerning for up to 22% of the ocean, where high cumulative intensity overlapped the warm range edge of species distributions, thus being more sensitive to thermal stress. Subsurface MHWs can hence drive biodiversity patterns, with consequent effects on ecological interactions and ecosystem processes.

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Fig. 1: Global MHW metrics from the surface to 2,000 m for the period 1993–2019.
Fig. 2: Spatial distribution of MHW metrics with depth for the period 1993–2019.
Fig. 3: Spatial distribution of biodiversity exposure to MHWs with depth for the period 1993–2019.
Fig. 4: Local MHW and biodiversity patterns in selected regions of distinct oceanographic or climatic conditions.

Data availability

Daily temperature reanalysis data (GLORYS12V1) are openly provided by the EU Copernicus Marine Service ( The biodiversity dataset is openly available from AquaMaps at The datasets used for the validation of the GLORYS12V1 reanalysis are: (1) the NOAA dataset (, available for download at; (2) the GLODAPv2 dataset ( available for download at; and (3) the ‘polar moorings’ dataset, for which sources and detailed information can be found in Supplementary Table 1.

Code availability

R code for data collection and MHW analyses is openly available on Figshare (


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This study received Portuguese national funds from the Foundation for Science and Technology (FCT) through the projects UIDB/04326/2020, UIDP/04326/2020, PTDC/BIA-CBI/6515/2020, LA/P/0101/2020 and DivRestore/013/2020 to E.F., E.A.S. and J.A., the Individual Call to Scientific Employment Stimulus 2022.00861 to J.A. and the fellowship SFRH/BD/144878/2019 to E.F. A Pew Marine Fellowship was awarded to E.A.S., funding from the Australian Research Council (DP200100201) to T.W., an Australian Research Council Future Fellowship (FT220100475) to A.S.G. and funding from BNP PARIBAS Foundation, through the CORESCAM project (‘Coastal biodiversity resilience to increasing extreme events in Central America’) to M.B.A. Lastly, we acknowledge the Copernicus Marine Environment Monitoring Service for proving the reanalysis data (

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Authors and Affiliations



E.F., J.A. and O.D.C. conceived the study. E.F. and J.A. conducted the analyses. E.F. led the writing with the support of all authors. All authors revised the final draft and approved the submitted paper.

Corresponding authors

Correspondence to Eliza Fragkopoulou or Jorge Assis.

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The authors declare no competing interests.

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Nature Climate Change thanks Shijian Hu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Local marine heatwave (MHW) and biodiversity patterns in selected regions of distinct oceanographic or climatic conditions.

Global map depicting the geographic location of (a) boundary current, (b) oceanographic front, (c) subtropical and (d) tropical gyre, and (e) Arctic Ocean. Time-depth maximum MHW intensity, cumulative intensity, species and warm-edge richness are shown for regions of boundary current and oceanographic front (remaining regions are shown in Extended Data Fig. 2). Depth levels and temperature scales are not to scale. Regional characteristics were estimated from a 2 by 2-degree resolution cell. The global map features MHW cumulative intensity at the surface.

Extended Data Fig. 2 Local marine heatwave (MHW) and biodiversity patterns in selected regions of distinct oceanographic or climatic conditions.

(Continuous from Extended Data Fig. 1) Time-depth maximum MHW intensity, cumulative intensity, species and warm-edge richness for regions of (C) subtropical and (D) tropical gyre, and (E) Arctic Ocean. Depth levels and temperature scales are not to scale. Regional characteristics were estimated from a 2 by 2-degree resolution cell. The geographic location of each region is depicted in the global map of Extended Data Fig. 1.

Supplementary information

Supplementary Information

Supplementary methods and results with Supplementary Figs. 1–9 and Supplementary Tables 1 and 2.

Reporting Summary

Supplementary Data 1

Supplementary Tables 3 to 5.

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Fragkopoulou, E., Sen Gupta, A., Costello, M.J. et al. Marine biodiversity exposed to prolonged and intense subsurface heatwaves. Nat. Clim. Chang. (2023).

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