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q-bio.PE · Populations and Evolutionphysics.ao-ph · Atmospheric and Oceanic Physics

Coral reef pixels with low 1986–2013 heat stress kept lower heat-stress ranks in 2014–2025, but only 25% of them stayed below 4 DHW in 2023–2025 (NOAA CRW 5-km data)

Question: do coral reefs with low past heat stress (thermal refugia) stay low-stress during global bleaching events? We used NOAA Coral Reef Watch CoralTemp v3.1 5-km annual maximum Degree Heating Weeks (DHW) for 54,765 reef pixels, 1986–2025, with a plan written before the results. A refugium is a pixel in the lowest 20% of 1986–2013 mean DHW inside its 10° block. Within blocks, 1986–2013 exposure predicted the 2023–2025 maximum DHW (Spearman 0.44, block-bootstrap 95% CI 0.34–0.54) and the 2014–2017 maximum (0.49). This skill was not lower than in earlier decades (0.33–0.38 for 1986–1999 to 2000–2013). Pixels that stayed below 8 DHW in 2014–2017 had 3.8 times the odds of staying below 8 DHW in 2023–2025 (CI 2.7–5.9). But the absolute protection decreased: 88% of refugia stayed below 8 DHW in 2014–2017 (others 69%), but only 60% in 2023–2025 (others 44%). Only 25% of refugia stayed below the 4 DHW bleaching level in 2023–2025. A logit trend fitted to 1986–2013 predicted 3% of reef pixels per year with DHW of 8 or more in 2014–2025; the observed mean was 13.5%, and 24–39% in 2023–2025. The mean return interval of severe stress per pixel decreased from about 300 years (1986–1995) to 6.4 years (2016–2025). In 5,452 Global Coral-Bleaching Database surveys (2014–2019), refugium surveys had lower odds of recorded bleaching within block-year strata (OR 0.56, CI 0.43–0.79). Results did not change with 5° blocks or after removal of January carry-over. Limitation: an independent SST product (CoRTAD) supported the CRW refugium ranks only weakly, so fixed climatology errors can contribute to the persistence. Refugia selected from historical DHW identify reefs with relatively less stress, not reefs that avoid bleaching-level stress.

coral reefs · thermal refugia · degree heating weeks · marine heatwaves · coral bleaching · Coral Reef Watch · global bleaching event · out-of-sample test · conservation planning · climate change

Admitted 1 citing publication

References

10
  1. usesdoi:10.3390/rs12233856

    Describes the NOAA CRW CoralTemp v3.1 DHW product that is the main input.

  2. usesdoi:10.26008/1912/bco-dmo.773466.2

    Global Coral-Bleaching Database v2 surveys and CoRTAD TSA_DHW used for the field check.

  3. doi:10.1038/s41597-022-01121-y

    Data paper of the Global Coral-Bleaching Database.

  4. doi:10.1038/srep38402

    Earlier CRW analysis of reef warming and bleaching stress for 1985–2012.

  5. supportsdoi:10.1126/science.aan8048

    Reported a decrease of severe bleaching return time to about 6 years; our satellite pixel estimate for 2016–2025 is 6.4 years.

  6. doi:10.1111/conl.12587

    50 Reefs portfolio selected partly from historical thermal stress; we test that selection principle out of sample.

  7. doi:10.1111/cobi.14108

    Critique of DHW-based refugia selection.

  8. doi:10.1007/s00338-024-02504-w

    Describes the fourth global coral bleaching event.

  9. doi:10.1038/srep39666

    Projections of annual severe bleaching onset.

  10. Mean bleaching was 6.8 percentage points higher with high surface heat at 61 Reef Check sites matched by depth and month in 2003–2019 sci:sha256:262d317d99a66bf44e1bb284f545d17130ce6d4434b7fa30f217aae2cb57276f

    Earlier SCI analysis of Reef Check bleaching and heat; its retained GCBD file is our input.

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