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Equatorial refuge amid tropical warming

Abstract

Upwelling across the tropical Pacific Ocean is projected to weaken in accordance with a reduction of the atmospheric overturning circulation1, enhancing the increase in sea surface temperature relative to other regions in response to greenhouse-gas forcing. In the central Pacific, home to one of the largest marine protected areas and fishery regions in the global tropics, sea surface temperatures are projected to increase by 2.8 °C by the end of this century2,3,4. Of critical concern is that marine protected areas may not provide refuge from the anticipated rate of large-scale warming, which could exceed the evolutionary capacity of coral and their symbionts to adapt5. Combining high-resolution satellite measurements6,7, an ensemble of global climate models4 and an eddy-resolving regional ocean circulation model8, we show that warming and productivity decline around select Pacific islands will be mitigated by enhanced upwelling associated with a strengthening of the equatorial undercurrent. Enhanced topographic upwelling will act as a negative feedback, locally mitigating the surface warming. At the Gilbert Islands, the rate of warming will be reduced by 0.7±0.3 °C or 25 ± 9% per century, or an overall cooling effect comparable to the local anomaly for a typical El Niño, by the end of this century. As the equatorial undercurrent is dynamically constrained to the Equator, only a handful of coral reefs stand to benefit from this equatorial island effect. Nevertheless, those that do face a lower rate of warming, conferring a significant advantage over neighbouring reef systems. If realized, these predictions help to identify potential refuges for coral reef communities from anticipated climate changes of the twenty-first century.

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Figure 1: Chl. across the equatorial Pacific.
Figure 2: SST near the Gilbert Islands.
Figure 3: Global climate model projections.
Figure 4: An eddy-resolving regional model simulation.
Figure 5: Projected changes in local SST at the Gilbert Islands.

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Acknowledgements

We thank the US National Science Foundation for support and D. Oppo and R. Murtugudde for reading and providing comments on a draft of the manuscript. Ocean colour data were acquired through the National Aeronautics and Space Administration ocean colour Giovanni web site. IPCC climate model outputs were acquired from the World Climate Research Program CMIP3 archive, hosted by the Lawrence Livermore National Laboratory. Optimal interpolation SST data sets were acquired from the NOAA National Climate Data Center FTP portal. The ROMS code, including the ROMSTOOLS MATLAB toolbox, was generously provided by L’Institut de recherche pour le développement on their web site (http://roms.mpl.ird.fr). This study was supported by NSF OCE #1031971.

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K.B.K. and A.L.C. designed the study, K.B.K. conducted the high-resolution ocean model experiments, and K.B.K. and A.L.C. analyzed the results and wrote the paper together.

Corresponding author

Correspondence to Kristopher B. Karnauskas.

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The authors declare no competing financial interests.

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Karnauskas, K., Cohen, A. Equatorial refuge amid tropical warming. Nature Clim Change 2, 530–534 (2012). https://doi.org/10.1038/nclimate1499

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