Coral Thermometry Shows Exceptional 3˚C warming in the Subtropical North Pacific Over the Past Century

Coral Thermometry Shows Exceptional 3˚C warming in the Subtropical North Pacific Over the Past Century | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Physical Sciences - Article Coral Thermometry Shows Exceptional 3˚C warming in the Subtropical North Pacific Over the Past Century Tsuyoshi Watanabe, Ryohei Uchiyama, Taro Nojiri, Atsuko Yamazaki This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7414589/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Tropical and subtropical oceans have warmed substantially over the past century, but model study reconstructions suggest that the rate of warming may differ greatly across regions. Sr/Ca ratios in massive Porites corals provide a reliable archive of past sea surface temperatures (SST), enabling high-resolution reconstructions of long-term climate trends. Yet many parts of the ocean remain underrepresented in such records, hindering a spatially resolved understanding of ocean warming. Here we present SST warming rates over the past century based on coral Sr/Ca records from Hawaii, which show an increase of 2.7 to 3.0 °C per century–far exceeding warming rates reported from other coral Sr/Ca records worldwide, most of which remain below 1.5 °C per century. This finding suggests that coral reef in the subtropical North Pacific may be warming more rapidly than previously recognized based on sparse reanalysis dataset. Earth and environmental sciences/Climate sciences/Climate change Physical sciences/Astronomy and planetary science/Planetary science/Geochemistry Figures Figure 1 Main Text Global sea surface temperatures (SST) have increased steadily over the past century, yet this warming has not occurred uniformly across the world’s oceans 1–3 . Some model study indicates that subtropical North Pacific have experienced largest waring for the past century 3 . Yet, there are limited observational SST records or proxy-based reconstructions, hindering examine the models 4,5 . Hawaii is located near the center of the world’s largest subtropical gyre, far from the ocean basin margins, making it a strategic location with minimal anthropogenic influence from continental sources. To extend SST records beyond the instrumental era and resolve centennial-scale regional trends, geochemical proxies in coral skeletons are widely used 2,6 . Sr/Ca ratios in massive Porites corals are particularly robust thermometer, enabling high-resolution reconstructions of past SST variability 2,7,8 . Such SST reconstructions have been applied across many tropical and subtropical regions–including the western Pacific, Indian Ocean, and Caribbean—revealing diverse patterns of long-term SST warming 2,8 (Extended Data Table 1). However, the subtropical North Pacific remains underrepresented in the coral archive despite its climatic importance. Consistent century-scale SST warming in contrasting sites We reconstructed century-scale SST variability using Sr/Ca ratios measured in Porites sp. coral skeletons from Makai Pier and KoʻOlina, located on the windward and leeward coasts of Oahu, respectively. Sr/Ca data were converted to SST using site-specific calibration equations derived from in situ observations 9 , and were resampled to monthly resolution. Linear regression of annual mean SSTs revealed significant warming of 3.00°C per century at Makai Pier for the period 1909–2019 and 2.79°C per century at KoʻOlina for the period 1957–2019 (Fig. 1 ). Although the two sites differ in hydrographic setting–Makai Pier being a shallow, semi-enclosed lagoon and KoʻOlina a deeper offshore fore-reef–the reconstructed SST records exhibit remarkably similar long-term warming rates. On seasonal timescales, the two corals exhibited non-synchronous patterns. At Makai Pier, SST warming was more pronounced in seasonal maximum temperatures (SST max ) than in seasonal minima (SST min ), whereas at KoʻOlina, warming is more pronounced in SST min than in SST max (Extended Data Table 2). These differences likely reflect site-specific thermal regimes, such as variations in diel heating and water circulation due to the wind stress 10 . However, the centennial-scale trends of 3.00°C and 2.79°C per century are consistent across both sites, indicating that the observed SST warming is not an artifact of local variability but rather reflects a robust regional signal. Also, even when accounting for potential issues related to the sampling transects 11 , high SST warming rates based on SST mean are maintained (2.4°C per century at Makai Pier and 3.6°C per century at KoʻOlina). Furthermore, even when considering potential breakdowns in the Sr/Ca–SST relationship due to biological stressors such as thermal stress 12–17 , the SST mean warming rates still remain high (2.4°C per century at Makai Pier and 3.7°C per century at KoʻOlina). These results indicate that elevated SST warming rates very likely persist at both locations. Underestimated reef warming The reconstructed SST trends from coral Sr/Ca records are substantially higher than those indicated by long-term gridded SST products (Extended Data Table 3). HadISST and ERSST data for the same region and period (1909–2019) yield warming rates of only 0.49°C and 0.55°C per century, respectively–three to six times lower than our reconstructed SST. In contrast, higher-resolution datasets show better agreement with the reconstructed SST. Limited AVHRR satellite SSTs (1981–2019) indicate a trend of 1.7°C per century, while in situ weekly observations at Koko Head (1955–1992) yield comparable values (1.9˚C per century) 10 . Our results suggest that coarse-resolution reanalysis datasets may significantly underestimate warming in coastal subtropical regions. During the first half of the 21st century, the available historical SST data in the central North Pacific on which these analyses are based, are relatively low spatial and temporal resolution due to their dependence on ship-based measurements 18 . In reef environments, high solar energy input and low winds elevated SST more than in offshore water due to restricted water circulation 10 , making coral reef area more prone to warming and cooling than the broader grid SST datasets. In particular, the Sr/Ca-SST relationship used in this study is expected to function as a robust in situ thermometer 9 , suggesting that coral reefs have recorded long-term warming trends more sensitively than open-ocean environments. The KoʻOlina site, relatively a deeper offshore fore-reef location, also shows a high rate of SST warming, indicating that elevated SST warming has occurred not only in semi-enclosed lagoonal areas such as Makai Pier, but also across broader coastal regions of Oahu. Exceptional SST warming rate in Hawaii Compared with other regions, the warming rates observed in the Hawaiian coral records are exceptionally high (Extended Data Table 1). A synthesis of 41 century-scale coral Sr/Ca-based SST reconstructions from the Pacific, Indian, and Atlantic Oceans, and Red Sea, shows that most trends fall below 1.0°C per century, with only a few exceeding 1.5°C. Even the most rapidly warming sites, such as Vanuatu in the central South Pacific (1.42°C per century) 19 and Western Australia in the Indian Ocean (1.57°C per century) 20 , remain well below our Hawaii SST. This places the Oahu records as a potential outlier in global ocean warming trends. Although in situ SST observations in Hawaii are limited, previous studies of sea level air temperature support the high rates of coastal warming observed in the region. During a similar monitoring period as our study (1905–2017), average daily air temperatures at sea level in Hawaii have warmed 1.2°C per century with the nighttime low temperature increasing 1.7°C per century 21,22 . A warming trend of the nightly low temperature is consistent with an SST warming trend relative to air temperatures. At night coastal air temperature cooling is moderated by sea to air heat flux in conjunction with onshore northeastern trade winds which continue to blow at night despite the land-sea heat gradient. The present study reveals that coral reefs in Hawaii is experiencing one of the most rapid SST warming documented in the world’s tropical or subtropical oceans over the past century. Our findings support previous studies that highlighted uncertainties in global SST reanalysis datasets 23 , and suggest that the actual range of warming rates may be broader than previously estimated when considering SST variability at coral reefs in subtropical North Pacific. Declarations Acknowledgements The corals were collected under the State of Hawaii DLNR Special Activity Permits 2016-81, 2016-07, and 2019-61. We thank S. Kahng. for assistance with coral sample identification and collection in Hawaii. Author Contributions R.U. performed the experiments and data analyses based on the initial research design by T.W., and wrote the manuscript through discussions with T.W. and A.Y. T.N. contributed to the Sr/Ca ratio analyses. Competing interests The authors declare no competing interests. Supplementary Information Supplementary information is available for this paper. References Latif, M. et al. Strengthening atmospheric circulation and trade winds slowed tropical Pacific surface warming. Commun Earth Environ 4, (2023). Tierney, J. E. et al. Tropical sea surface temperatures for the past four centuries reconstructed from coral archives. Paleoceanography 30, 226–252 (2015). Xu, Z. et al. Long-term evolution of global sea surface temperature trend. International Journal of Climatology 41, 4494–4508 (2021). Toda, M., Kosaka, Y., Miyamoto, A. & Watanabe, M. Walker circulation strengthening driven by sea surface temperature changes outside the tropics. Nat Geosci 17, 858–865 (2024). Watanabe, M. et al. Possible shift in controls of the tropical Pacific surface warming pattern. Nature 630, 315–324 (2024). Hu, W. et al. Reconstructing tropical monthly sea surface temperature variability by assimilating coral proxy datasets. NPJ Clim Atmos Sci 7, (2024). Beck, J. W. et al. Sea-Surface Temperature from Coral Skeletal Strontium/Calcium Ratios. Source: Science, New Series vol. 257 (1992). Abram, N. J. et al. Early onset of industrial-era warming across the oceans and continents. Nature 536, 411–418 (2016). Uchiyama, R., Watanabe, T., Kahng, S. E. & Yamazaki, A. Calibration of Sr/Ca Ratio and In Situ Temperature Using Hawaiian Corals. Geochemistry, Geophysics, Geosystems 24, (2023). Jokiel, P. L. & Brown, E. K. Global warming, regional trends and inshore environmental conditions influence coral bleaching in Hawaii. Glob Chang Biol 10, 1627–1641 (2004). DeLong, K. L., Quinn, T. M., Taylor, F. W., Shen, C. C. & Lin, K. Improving coral-base paleoclimate reconstructions by replicating 350years of coral Sr/Ca variations. Palaeogeogr Palaeoclimatol Palaeoecol 373, 6–24 (2013). Marshall, J. F. & McCulloch, M. T. An assessment of the Sr/Ca ratio in shallow water hermatypic corals as a proxy for sea surface temperature. Geochim Cosmochim Acta 66, 3263–3280 (2002). Hayashi, E. et al. Growth-rate influences on coral climate proxies tested by a multiple colony culture experiment. Earth Planet Sci Lett 362, 198–206 (2013). Clarke, H. et al. Differential response of corals to regional mass-warming events as evident from skeletal Sr/Ca and Mg/Ca ratios. Geochemistry, Geophysics, Geosystems 18, 1794–1809 (2017). D’Olivo, J. P. & McCulloch, M. T. Response of coral calcification and calcifying fluid composition to thermally induced bleaching stress. Sci Rep 7, (2017). Leupold, M. et al. El Niño-Southern Oscillation and internal sea surface temperature variability in the tropical Indian Ocean since 1675. Climate of the Past 17, 151–170 (2021). Standish, C. D. et al. Geochemical responses of scleractinian corals to nutrient stress. Geochim Cosmochim Acta 351, 108–124 (2023). Deser, C., Alexander, M. A., Xie, S. P. & Phillips, A. S. Sea surface temperature variability: Patterns and mechanisms. Ann Rev Mar Sci 2, 115–143 (2010). Lawman, A. E. et al. A Century of Reduced ENSO Variability During the Medieval Climate Anomaly. Paleoceanogr Paleoclimatol 35, (2020). Zinke, J. et al. Corals record long-term Leeuwin current variability including Ningaloo Niño/Niña since 1795. Nat Commun 5, (2014). Kagawa-Viviani, A. K. & Giambelluca, T. W. Spatial Patterns and Trends in Surface Air Temperatures and Implied Changes in Atmospheric Moisture Across the Hawaiian Islands, 1905–2017. Journal of Geophysical Research: Atmospheres 125, (2020). McKenzie, M. M., Giambelluca, T. W. & Diaz, H. F. Temperature trends in Hawaiʻi: A century of change, 1917–2016. International Journal of Climatology 39, 3987–4001 (2019). McCulloch, M. T., Winter, A., Sherman, C. E. & Trotter, J. A. 300 years of sclerosponge thermometry shows global warming has exceeded 1.5 °C. Nat Clim Chang 14, 171–177 (2024). Methods Sample collection and preparation Two massive Porites coral cores were collected from Oahu, Hawaii (Extended Fig. 1 ). A 150 cm-long and an additional small core from Makai Pier (21.320°N, 157.669°W; depth ~ 3 m, depending on the tidal height) on 22 December 2015, and on 16 March 2019, respectively. Also, a 60 cm-long core from KoʻOlina (21.327°N, 158.129°W; depth 7m) on 14 September 2019 at the depth of 7 m. Detail reef settings of Makai Pier and KoʻOlina are described in a previous study 9 . Briefly, Makai Pier site is a shallow inshore lagoon buffered from the open ocean by a fringing reef on the windward side of Oahu but located in an area with very minimal terrigenous influence due to the watershed characteristics. KoʻOlina site is a deeper offshore forereef site representative of the leeward side of the island. Coral slabs (5 mm thick) were cut along the major vertical growth axis identified by X-ray imaging. Skeletal transects were cleaned by ultrasonic bath (NR-50M, Microtec) and sampled at 0.2–0.4 mm intervals using a low-speed micro-mill to obtain powder samples with approximately monthly resolution. Sr/Ca analysis Analytical protocols of trace elemental were based on previous report 24 . Coral powder samples were dissolved in 6% HNO3 and analyzed for Sr/Ca ratios using an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP6200, Thermo Fisher Scientific) at Hokkaido University. Elemental concentrations were determined using external calibration with matrix-matched standards. The long-term analytical precision, based on replicate analyses of JCp-1 carbonate standard, was better than 0.07% relative standard deviation (RSD). Supplementary Table 1 lists all Sr/Ca values and the results of screening based on the diagenesis check described later. SST reconstruction, chronology, and trend analysis Sr/Ca ratios were converted to SST using site-specific linear calibrations derived from in situ temperature data: Sr/Ca(mmol/mol) = -0.061 × SST(˚C) + 10.59 (Makai Pier), and Sr/Ca(mmol/mol) = -0.054 × SST(˚C) + 10.43 (KoʻOlina). The accuracy (1 sigma) of reconstructed temperature by using these thermometers were 0.63 ˚C and 0.68 ˚C for Makai Pier and KoʻOlina, respectively. Because there is a notable difference in seasonal temperature cycle between satellite and in-situ measurements at shallow lagoon site, for Makai Pier coral Sr/Ca, the highest (lowest) Sr/Ca value were assigned to 1st February (15th August), the average dates of the minimum (maximum) seawater temperature from four years of in situ temperature data. Only one year (2018–2019) from coral nubbin, the highest (lowest) Sr/Ca values were assigned to the dates of the minimum (maximum) in situ temperature. For KoʻOlina coral, the highest (lowest) Sr/Ca values were assigned to the dates of the minimum (maximum) seawater temperature from AVHRR. For the period before 1981, when the satellite data is not available, the average dates of the minimum temperature (9th March) and maximum temperature (23rd September) during the period with available observational data were used. SST minima and maxima were assigned based on the climatological seasonal timing from AVHRR and in situ records. Annual mean SST (SST mean ) was calculated as the average of monthly interpolated values from April to the following March. Linear regression was applied to derive century-scale trends in SST mean , seasonal maxima (SST max ), minima (SST min ). Supplementary Table 2 lists monthly resampled reconstructed SST for Makai Pier and KoʻOlina. Screening for diagenesis Diagenetic alterations such as marine secondary aragonite precipitation and secondary calcite precipitation can also cause anomalously high and low Sr/Ca values respectively 25–27 . Portions of the Makai Pier coral skeleton showing anomalously high or low Sr/Ca values (> 2σ from the mean) were examined by scanning electron microscopy (SEM; TM3000, Hitachi) to identify evidence of secondary aragonite precipitation or dissolution. Skeletal regions with visible diagenetic features were excluded from the final reconstruction (Extended Fig. 2; Scenario A in Supplementary Table 1). Additional screening of anomalous coral Sr/Ca values Sampling path can also cause Sr/Ca artifacts with disorganized corallites (merging or terminating corallite fans) and corallites perpendicular to the slab can cause anomalously high and low Sr/Ca values respectively 11 . Decoupling of the temperature and Sr/Ca relationship has also been associated with thermal, mechanical, and nutrient stress (i.e., night nitrate, low phosphate) causing anomalously high Sr/Ca values 12–17 . To further evaluate the influence of problematic Sr/Ca data, we examined not only the removal of Sr/Ca values affected by diagenesis (scenario A in Supplementary Table 1), but also the exclusion of data obtained along sub-optimal or disorganized sampling paths (scenario B in Supplementary Table 1), and the additional removal of Sr/Ca values with disrupted Sr/Ca–temperature relationships due to thermal stress, indicated by high Sr/Ca anomalies (scenario C in Supplementary Table 1). Data availability Sr/Ca data that support the findings of this study will be deposited in data repository at KIKAI Institute for coral reef sciences (https://coralogy.kikaireefs.org/C-1%20Scientific%20data.html), after acceptance. The coral data used in Extended Table 1 were sourced from the NOAA/World Data Service for Paleoclimatology (https://www.ncei.noaa.gov/products/paleoclimatology). SST data used in Extended Data Table 3 were taken from ERSST v5 28 (available at https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.ersst.v5.html), HadISST 29 (available at https://www.metoffice.gov.uk/hadobs/hadisst/), Kaplan Extended v2 30 (available at https://www.esrl.noaa.gov/psd/data/gridded/data.kaplan_sst.html), and OISST v2 version2p1 AVHRR Daily SST 31 (available at https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.oisst.v2.html). Reference for Methods 9. Uchiyama, R., Watanabe, T., Kahng, S. E. & Yamazaki, A. Calibration of Sr/Ca Ratio and In Situ Temperature Using Hawaiian Corals. Geochemistry, Geophysics, Geosystems 24, (2023). 11. DeLong, K. L., Quinn, T. M., Taylor, F. W., Shen, C. C. & Lin, K. Improving coral-base paleoclimate reconstructions by replicating 350years of coral Sr/Ca variations. Palaeogeogr Palaeoclimatol Palaeoecol 373, 6–24 (2013). 12. Marshall, J. F. & McCulloch, M. T. An assessment of the Sr/Ca ratio in shallow water hermatypic corals as a proxy for sea surface temperature. Geochim Cosmochim Acta 66, 3263–3280 (2002). 13. Hayashi, E. et al. Growth-rate influences on coral climate proxies tested by a multiple colony culture experiment. Earth Planet Sci Lett 362, 198–206 (2013). 14. Clarke, H. et al. Differential response of corals to regional mass-warming events as evident from skeletal Sr/Ca and Mg/Ca ratios. Geochemistry, Geophysics, Geosystems 18, 1794–1809 (2017). 15. D’Olivo, J. P. & McCulloch, M. T. Response of coral calcification and calcifying fluid composition to thermally induced bleaching stress. Sci Rep 7, (2017). 16. Leupold, M. et al. El Niño-Southern Oscillation and internal sea surface temperature variability in the tropical Indian Ocean since 1675. Climate of the Past 17, 151–170 (2021). 17. Standish, C. D. et al. Geochemical responses of scleractinian corals to nutrient stress. Geochim Cosmochim Acta 351, 108–124 (2023). 24. Watanabe, T. K., Watanabe, T., Ohmori, K. & Yamazaki, A. Improving analytical method of Sr/Ca ratios in coral skeletons for paleo-SST reconstructions using ICP-OES. Limnol Oceanogr Methods 18, 297–310 (2020). 25. Quinn, T. M. & Taylor, F. W. SST artifacts in coral proxy records produced by early marine diagenesis in a modern coral from Rabaul, Papua New Guinea. Geophys Res Lett 33, (2006). 26. Hendy, E. J., Gagan, M. K., Lough, J. M., McCulloch, M. & deMenocal, P. B. Impact of skeletal dissolution and secondary aragonite on trace element and isotopic climate proxies in Porites corals. Paleoceanography 22, (2007). 27. Mcgregor, H. V & Gagan, M. K. Diagenesis and Geochemistry of Porites Corals from Papua New Guinea: Implications for Paleoclimate Reconstruction. (2003). 28. Huang, B. et al. Extended reconstructed Sea surface temperature, Version 5 (ERSSTv5): Upgrades, validations, and intercomparisons. J Clim 30, 8179–8205 (2017). 29. Rayner, N. A. et al. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research: Atmospheres 108, (2003). 30. Kaplan, A. et al. Analyses of global sea surface temperature 1856-1991. J Geophys Res Oceans 103, 18567–18589 (1998). 31. Reynolds, R. W. et al. Daily high-resolution-blended analyses for sea surface temperature. J Clim 20, 5473–5496 (2007). Additional Declarations There is NO Competing Interest. Supplementary Files SupplementaryTable1.xlsx Supplementary Table 1 SupplemetaryTable2.xlsx Supplementary Table 2 SupplementaryInformation.docx Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7414589","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Physical Sciences - Article","associatedPublications":[],"authors":[{"id":507532450,"identity":"5d76d4c4-a31b-40da-8fc2-4d78bfd5586a","order_by":0,"name":"Tsuyoshi Watanabe","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5klEQVRIiWNgGAWjYFACHgaGDyBSgoGZgYENxAUCCQJaGGeQrIWZB6IKrIUwMGc/e/CxzR87GQbp5sPGPGVABvvhBwyWO3BrsezJSzbObUvmYZA5lpzMcw7I4EkzYJA8g1uLwYEcM+nchgNAv+QYH+ZtAzkyh4FBsg2PlvNvzH9b/IFrqedh4H9DQMuNHDNmBjaIlmTetsMgBiEt75Ile4F+YZNISzacc+44kPHM4ABev5zPPfjhxx87e36J5MMSb8qq7fn5kx8+lsQTYnDAhsw4LNlAhBYUwPiRZC2jYBSMglEwjAEAGTJCipOl4AYAAAAASUVORK5CYII=","orcid":"","institution":"Hokkaido University","correspondingAuthor":true,"prefix":"","firstName":"Tsuyoshi","middleName":"","lastName":"Watanabe","suffix":""},{"id":507532451,"identity":"0b58a8b8-86a9-4f2d-a397-ee2facef9639","order_by":1,"name":"Ryohei Uchiyama","email":"","orcid":"","institution":"Hokkaido University","correspondingAuthor":false,"prefix":"","firstName":"Ryohei","middleName":"","lastName":"Uchiyama","suffix":""},{"id":507532452,"identity":"a5e5521e-325a-4bda-a4bd-e381a0bb88be","order_by":2,"name":"Taro Nojiri","email":"","orcid":"","institution":"Hokkaido University","correspondingAuthor":false,"prefix":"","firstName":"Taro","middleName":"","lastName":"Nojiri","suffix":""},{"id":507532453,"identity":"6e1c3005-99b2-4dfe-a3d5-ea0390ae5140","order_by":3,"name":"Atsuko Yamazaki","email":"","orcid":"","institution":"Nagoya University","correspondingAuthor":false,"prefix":"","firstName":"Atsuko","middleName":"","lastName":"Yamazaki","suffix":""}],"badges":[],"createdAt":"2025-08-20 07:36:00","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7414589/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7414589/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":93334555,"identity":"3edc52e8-07e9-4d86-bfb1-de13abcfd867","added_by":"auto","created_at":"2025-10-12 13:44:39","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":802124,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMonthly SST records at Makai Pier and KoʻOlina.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTime series of SST reconstructed from Sr/Ca ratios at Makai Pier (red), KoʻOlina (blue). Solid lines represent monthly data, while thick linear trend lines indicate long-term SST changes over the observational period. Makai Pier and KoʻOlina exhibit substantial warming trends, with rates of 3.00 ˚C per century at Makai Pier and 2.79 ˚C per century at KoʻOlina.\u003c/p\u003e","description":"","filename":"Figure1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7414589/v1/1da838205f13d6b0d93a68c3.jpg"},{"id":93335729,"identity":"ab0ccb0b-294f-4492-917a-ba927e19d01a","added_by":"auto","created_at":"2025-10-12 14:00:43","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1307158,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7414589/v1/a599672a-7187-4f79-9b45-55d7df4a139c.pdf"},{"id":93334585,"identity":"00ffa698-f984-492d-beaf-9f2f90cd5ceb","added_by":"auto","created_at":"2025-10-12 13:52:39","extension":"xlsx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2123673,"visible":true,"origin":"","legend":"\u003cp\u003eSupplementary Table 1\u003c/p\u003e","description":"","filename":"SupplementaryTable1.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7414589/v1/5a144b39ef383f0cb4e5101d.xlsx"},{"id":93334558,"identity":"b7080a83-0593-4d28-a42c-96af95942471","added_by":"auto","created_at":"2025-10-12 13:44:39","extension":"xlsx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":192740,"visible":true,"origin":"","legend":"Supplementary Table 2","description":"","filename":"SupplemetaryTable2.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-7414589/v1/57e77cb1bc4df5eae0e2b734.xlsx"},{"id":93334559,"identity":"ab33bc6a-edf1-42a9-944a-98fc03002ea8","added_by":"auto","created_at":"2025-10-12 13:44:39","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":2949950,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-7414589/v1/b4ebace1fd69d56f45119edc.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Coral Thermometry Shows Exceptional 3˚C warming in the Subtropical North Pacific Over the Past Century","fulltext":[{"header":"Main Text","content":"\u003cp\u003eGlobal sea surface temperatures (SST) have increased steadily over the past century, yet this warming has not occurred uniformly across the world’s oceans\u003csup\u003e1–3\u003c/sup\u003e. Some model study indicates that subtropical North Pacific have experienced largest waring for the past century\u003csup\u003e3\u003c/sup\u003e. Yet, there are limited observational SST records or proxy-based reconstructions, hindering examine the models\u003csup\u003e4,5\u003c/sup\u003e. Hawaii is located near the center of the world’s largest subtropical gyre, far from the ocean basin margins, making it a strategic location with minimal anthropogenic influence from continental sources.\u003c/p\u003e\u003cp\u003eTo extend SST records beyond the instrumental era and resolve centennial-scale regional trends, geochemical proxies in coral skeletons are widely used\u003csup\u003e2,6\u003c/sup\u003e. Sr/Ca ratios in massive \u003cem\u003ePorites\u003c/em\u003e corals are particularly robust thermometer, enabling high-resolution reconstructions of past SST variability\u003csup\u003e2,7,8\u003c/sup\u003e. Such SST reconstructions have been applied across many tropical and subtropical regions–including the western Pacific, Indian Ocean, and Caribbean—revealing diverse patterns of long-term SST warming\u003csup\u003e2,8\u003c/sup\u003e (Extended Data Table\u0026nbsp;1). However, the subtropical North Pacific remains underrepresented in the coral archive despite its climatic importance.\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eConsistent century-scale SST warming in contrasting sites\u003c/h2\u003e\u003cp\u003eWe reconstructed century-scale SST variability using Sr/Ca ratios measured in \u003cem\u003ePorites\u003c/em\u003e sp. coral skeletons from Makai Pier and KoʻOlina, located on the windward and leeward coasts of Oahu, respectively. Sr/Ca data were converted to SST using site-specific calibration equations derived from in situ observations\u003csup\u003e9\u003c/sup\u003e, and were resampled to monthly resolution. Linear regression of annual mean SSTs revealed significant warming of 3.00°C per century at Makai Pier for the period 1909–2019 and 2.79°C per century at KoʻOlina for the period 1957–2019 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Although the two sites differ in hydrographic setting–Makai Pier being a shallow, semi-enclosed lagoon and KoʻOlina a deeper offshore fore-reef–the reconstructed SST records exhibit remarkably similar long-term warming rates. On seasonal timescales, the two corals exhibited non-synchronous patterns. At Makai Pier, SST warming was more pronounced in seasonal maximum temperatures (SST\u003csub\u003emax\u003c/sub\u003e) than in seasonal minima (SST\u003csub\u003emin\u003c/sub\u003e), whereas at KoʻOlina, warming is more pronounced in SST\u003csub\u003emin\u003c/sub\u003e than in SST\u003csub\u003emax\u003c/sub\u003e (Extended Data Table\u0026nbsp;2). These differences likely reflect site-specific thermal regimes, such as variations in diel heating and water circulation due to the wind stress\u003csup\u003e10\u003c/sup\u003e. However, the centennial-scale trends of 3.00°C and 2.79°C per century are consistent across both sites, indicating that the observed SST warming is not an artifact of local variability but rather reflects a robust regional signal. Also, even when accounting for potential issues related to the sampling transects \u003csup\u003e11\u003c/sup\u003e, high SST warming rates based on SST\u003csub\u003emean\u003c/sub\u003e are maintained (2.4°C per century at Makai Pier and 3.6°C per century at KoʻOlina). Furthermore, even when considering potential breakdowns in the Sr/Ca–SST relationship due to biological stressors such as thermal stress\u003csup\u003e12–17\u003c/sup\u003e, the SST\u003csub\u003emean\u003c/sub\u003e warming rates still remain high (2.4°C per century at Makai Pier and 3.7°C per century at KoʻOlina). These results indicate that elevated SST warming rates very likely persist at both locations.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eUnderestimated reef warming\u003c/h3\u003e\n\u003cp\u003eThe reconstructed SST trends from coral Sr/Ca records are substantially higher than those indicated by long-term gridded SST products (Extended Data Table\u0026nbsp;3). HadISST and ERSST data for the same region and period (1909–2019) yield warming rates of only 0.49°C and 0.55°C per century, respectively–three to six times lower than our reconstructed SST. In contrast, higher-resolution datasets show better agreement with the reconstructed SST. Limited AVHRR satellite SSTs (1981–2019) indicate a trend of 1.7°C per century, while in situ weekly observations at Koko Head (1955–1992) yield comparable values (1.9˚C per century)\u003csup\u003e10\u003c/sup\u003e. Our results suggest that coarse-resolution reanalysis datasets may significantly underestimate warming in coastal subtropical regions. During the first half of the 21st century, the available historical SST data in the central North Pacific on which these analyses are based, are relatively low spatial and temporal resolution due to their dependence on ship-based measurements\u003csup\u003e18\u003c/sup\u003e. In reef environments, high solar energy input and low winds elevated SST more than in offshore water due to restricted water circulation\u003csup\u003e10\u003c/sup\u003e, making coral reef area more prone to warming and cooling than the broader grid SST datasets. In particular, the Sr/Ca-SST relationship used in this study is expected to function as a robust in situ thermometer\u003csup\u003e9\u003c/sup\u003e, suggesting that coral reefs have recorded long-term warming trends more sensitively than open-ocean environments. The KoʻOlina site, relatively a deeper offshore fore-reef location, also shows a high rate of SST warming, indicating that elevated SST warming has occurred not only in semi-enclosed lagoonal areas such as Makai Pier, but also across broader coastal regions of Oahu.\u003c/p\u003e\n\u003ch3\u003eExceptional SST warming rate in Hawaii\u003c/h3\u003e\n\u003cp\u003eCompared with other regions, the warming rates observed in the Hawaiian coral records are exceptionally high (Extended Data Table\u0026nbsp;1). A synthesis of 41 century-scale coral Sr/Ca-based SST reconstructions from the Pacific, Indian, and Atlantic Oceans, and Red Sea, shows that most trends fall below 1.0°C per century, with only a few exceeding 1.5°C. Even the most rapidly warming sites, such as Vanuatu in the central South Pacific (1.42°C per century)\u003csup\u003e19\u003c/sup\u003e and Western Australia in the Indian Ocean (1.57°C per century)\u003csup\u003e20\u003c/sup\u003e, remain well below our Hawaii SST. This places the Oahu records as a potential outlier in global ocean warming trends.\u003c/p\u003e\u003cp\u003eAlthough in situ SST observations in Hawaii are limited, previous studies of sea level air temperature support the high rates of coastal warming observed in the region. During a similar monitoring period as our study (1905–2017), average daily air temperatures at sea level in Hawaii have warmed 1.2°C per century with the nighttime low temperature increasing 1.7°C per century\u003csup\u003e21,22\u003c/sup\u003e. A warming trend of the nightly low temperature is consistent with an SST warming trend relative to air temperatures. At night coastal air temperature cooling is moderated by sea to air heat flux in conjunction with onshore northeastern trade winds which continue to blow at night despite the land-sea heat gradient.\u003c/p\u003e\u003cp\u003eThe present study reveals that coral reefs in Hawaii is experiencing one of the most rapid SST warming documented in the world’s tropical or subtropical oceans over the past century. Our findings support previous studies that highlighted uncertainties in global SST reanalysis datasets\u003csup\u003e23\u003c/sup\u003e, and suggest that the actual range of warming rates may be broader than previously estimated when considering SST variability at coral reefs in subtropical North Pacific.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe corals were collected under the State of Hawaii DLNR Special Activity Permits 2016-81, 2016-07, and 2019-61. We thank S. Kahng. for assistance with coral sample identification and collection in Hawaii.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eR.U. performed the experiments and data analyses based on the initial research design by T.W., and wrote the manuscript through discussions with T.W. and A.Y. T.N. contributed to the Sr/Ca ratio analyses.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupplementary Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary information is available for this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLatif, M. et al. Strengthening atmospheric circulation and trade winds slowed tropical Pacific surface warming. Commun Earth Environ 4, (2023).\u003c/li\u003e\n\u003cli\u003eTierney, J. E. et al. Tropical sea surface temperatures for the past four centuries reconstructed from coral archives. Paleoceanography 30, 226\u0026ndash;252 (2015).\u003c/li\u003e\n\u003cli\u003eXu, Z. et al. Long-term evolution of global sea surface temperature trend. International Journal of Climatology 41, 4494\u0026ndash;4508 (2021).\u003c/li\u003e\n\u003cli\u003eToda, M., Kosaka, Y., Miyamoto, A. \u0026amp; Watanabe, M. Walker circulation strengthening driven by sea surface temperature changes outside the tropics. Nat Geosci 17, 858\u0026ndash;865 (2024).\u003c/li\u003e\n\u003cli\u003eWatanabe, M. et al. Possible shift in controls of the tropical Pacific surface warming pattern. Nature 630, 315\u0026ndash;324 (2024).\u003c/li\u003e\n\u003cli\u003eHu, W. et al. Reconstructing tropical monthly sea surface temperature variability by assimilating coral proxy datasets. NPJ Clim Atmos Sci 7, (2024).\u003c/li\u003e\n\u003cli\u003eBeck, J. W. et al. Sea-Surface Temperature from Coral Skeletal Strontium/Calcium Ratios. Source: Science, New Series vol. 257 (1992).\u003c/li\u003e\n\u003cli\u003eAbram, N. J. et al. Early onset of industrial-era warming across the oceans and continents. Nature 536, 411\u0026ndash;418 (2016).\u003c/li\u003e\n\u003cli\u003eUchiyama, R., Watanabe, T., Kahng, S. E. \u0026amp; Yamazaki, A. Calibration of Sr/Ca Ratio and In Situ Temperature Using Hawaiian Corals. Geochemistry, Geophysics, Geosystems 24, (2023).\u003c/li\u003e\n\u003cli\u003eJokiel, P. L. \u0026amp; Brown, E. K. Global warming, regional trends and inshore environmental conditions influence coral bleaching in Hawaii. Glob Chang Biol 10, 1627\u0026ndash;1641 (2004).\u003c/li\u003e\n\u003cli\u003eDeLong, K. L., Quinn, T. M., Taylor, F. W., Shen, C. C. \u0026amp; Lin, K. Improving coral-base paleoclimate reconstructions by replicating 350years of coral Sr/Ca variations. Palaeogeogr Palaeoclimatol Palaeoecol 373, 6\u0026ndash;24 (2013).\u003c/li\u003e\n\u003cli\u003eMarshall, J. F. \u0026amp; McCulloch, M. T. An assessment of the Sr/Ca ratio in shallow water hermatypic corals as a proxy for sea surface temperature. Geochim Cosmochim Acta 66, 3263\u0026ndash;3280 (2002).\u003c/li\u003e\n\u003cli\u003eHayashi, E. et al. Growth-rate influences on coral climate proxies tested by a multiple colony culture experiment. Earth Planet Sci Lett 362, 198\u0026ndash;206 (2013).\u003c/li\u003e\n\u003cli\u003eClarke, H. et al. Differential response of corals to regional mass-warming events as evident from skeletal Sr/Ca and Mg/Ca ratios. Geochemistry, Geophysics, Geosystems 18, 1794\u0026ndash;1809 (2017).\u003c/li\u003e\n\u003cli\u003eD\u0026rsquo;Olivo, J. P. \u0026amp; McCulloch, M. T. Response of coral calcification and calcifying fluid composition to thermally induced bleaching stress. Sci Rep 7, (2017).\u003c/li\u003e\n\u003cli\u003eLeupold, M. et al. El Ni\u0026ntilde;o-Southern Oscillation and internal sea surface temperature variability in the tropical Indian Ocean since 1675. Climate of the Past 17, 151\u0026ndash;170 (2021).\u003c/li\u003e\n\u003cli\u003eStandish, C. D. et al. Geochemical responses of scleractinian corals to nutrient stress. Geochim Cosmochim Acta 351, 108\u0026ndash;124 (2023).\u003c/li\u003e\n\u003cli\u003eDeser, C., Alexander, M. A., Xie, S. P. \u0026amp; Phillips, A. S. Sea surface temperature variability: Patterns and mechanisms. Ann Rev Mar Sci 2, 115\u0026ndash;143 (2010).\u003c/li\u003e\n\u003cli\u003eLawman, A. E. et al. A Century of Reduced ENSO Variability During the Medieval Climate Anomaly. Paleoceanogr Paleoclimatol 35, (2020).\u003c/li\u003e\n\u003cli\u003eZinke, J. et al. Corals record long-term Leeuwin current variability including Ningaloo Ni\u0026ntilde;o/Ni\u0026ntilde;a since 1795. Nat Commun 5, (2014).\u003c/li\u003e\n\u003cli\u003eKagawa-Viviani, A. K. \u0026amp; Giambelluca, T. W. Spatial Patterns and Trends in Surface Air Temperatures and Implied Changes in Atmospheric Moisture Across the Hawaiian Islands, 1905\u0026ndash;2017. Journal of Geophysical Research: Atmospheres 125, (2020).\u003c/li\u003e\n\u003cli\u003eMcKenzie, M. M., Giambelluca, T. W. \u0026amp; Diaz, H. F. Temperature trends in Hawaiʻi: A century of change, 1917\u0026ndash;2016. International Journal of Climatology 39, 3987\u0026ndash;4001 (2019).\u003c/li\u003e\n\u003cli\u003eMcCulloch, M. T., Winter, A., Sherman, C. E. \u0026amp; Trotter, J. A. 300 years of sclerosponge thermometry shows global warming has exceeded 1.5 \u0026deg;C. Nat Clim Chang 14, 171\u0026ndash;177 (2024).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003eSample collection and preparation\u003c/h2\u003e\u003cp\u003eTwo massive \u003cem\u003ePorites\u003c/em\u003e coral cores were collected from Oahu, Hawaii (Extended Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). A 150 cm-long and an additional small core from Makai Pier (21.320\u0026deg;N, 157.669\u0026deg;W; depth\u0026thinsp;~\u0026thinsp;3 m, depending on the tidal height) on 22 December 2015, and on 16 March 2019, respectively. Also, a 60 cm-long core from KoʻOlina (21.327\u0026deg;N, 158.129\u0026deg;W; depth 7m) on 14 September 2019 at the depth of 7 m. Detail reef settings of Makai Pier and KoʻOlina are described in a previous study\u003csup\u003e9\u003c/sup\u003e. Briefly, Makai Pier site is a shallow inshore lagoon buffered from the open ocean by a fringing reef on the windward side of Oahu but located in an area with very minimal terrigenous influence due to the watershed characteristics. KoʻOlina site is a deeper offshore forereef site representative of the leeward side of the island.\u003c/p\u003e\u003cp\u003eCoral slabs (5 mm thick) were cut along the major vertical growth axis identified by X-ray imaging. Skeletal transects were cleaned by ultrasonic bath (NR-50M, Microtec) and sampled at 0.2\u0026ndash;0.4 mm intervals using a low-speed micro-mill to obtain powder samples with approximately monthly resolution.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003eSr/Ca analysis\u003c/h2\u003e\u003cp\u003eAnalytical protocols of trace elemental were based on previous report\u003csup\u003e24\u003c/sup\u003e. Coral powder samples were dissolved in 6% HNO3 and analyzed for Sr/Ca ratios using an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP6200, Thermo Fisher Scientific) at Hokkaido University. Elemental concentrations were determined using external calibration with matrix-matched standards. The long-term analytical precision, based on replicate analyses of JCp-1 carbonate standard, was better than 0.07% relative standard deviation (RSD). Supplementary Table\u0026nbsp;1 lists all Sr/Ca values and the results of screening based on the diagenesis check described later.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eSST reconstruction, chronology, and trend analysis\u003c/h3\u003e\n\u003cp\u003eSr/Ca ratios were converted to SST using site-specific linear calibrations derived from in situ temperature data: Sr/Ca(mmol/mol) = -0.061 \u0026times; SST(˚C)\u0026thinsp;+\u0026thinsp;10.59 (Makai Pier), and Sr/Ca(mmol/mol) = -0.054 \u0026times; SST(˚C)\u0026thinsp;+\u0026thinsp;10.43 (KoʻOlina). The accuracy (1 sigma) of reconstructed temperature by using these thermometers were 0.63 ˚C and 0.68 ˚C for Makai Pier and KoʻOlina, respectively.\u003c/p\u003e\u003cp\u003eBecause there is a notable difference in seasonal temperature cycle between satellite and in-situ measurements at shallow lagoon site, for Makai Pier coral Sr/Ca, the highest (lowest) Sr/Ca value were assigned to 1st February (15th August), the average dates of the minimum (maximum) seawater temperature from four years of in situ temperature data. Only one year (2018\u0026ndash;2019) from coral nubbin, the highest (lowest) Sr/Ca values were assigned to the dates of the minimum (maximum) in situ temperature.\u003c/p\u003e\u003cp\u003eFor KoʻOlina coral, the highest (lowest) Sr/Ca values were assigned to the dates of the minimum (maximum) seawater temperature from AVHRR. For the period before 1981, when the satellite data is not available, the average dates of the minimum temperature (9th March) and maximum temperature (23rd September) during the period with available observational data were used. SST minima and maxima were assigned based on the climatological seasonal timing from AVHRR and in situ records.\u003c/p\u003e\u003cp\u003eAnnual mean SST (SST\u003csub\u003emean\u003c/sub\u003e) was calculated as the average of monthly interpolated values from April to the following March. Linear regression was applied to derive century-scale trends in SST\u003csub\u003emean\u003c/sub\u003e, seasonal maxima (SST\u003csub\u003emax\u003c/sub\u003e), minima (SST\u003csub\u003emin\u003c/sub\u003e). Supplementary Table\u0026nbsp;2 lists monthly resampled reconstructed SST for Makai Pier and KoʻOlina.\u003c/p\u003e\n\u003ch3\u003eScreening for diagenesis\u003c/h3\u003e\n\u003cp\u003eDiagenetic alterations such as marine secondary aragonite precipitation and secondary calcite precipitation can also cause anomalously high and low Sr/Ca values respectively\u003csup\u003e25\u0026ndash;27\u003c/sup\u003e. Portions of the Makai Pier coral skeleton showing anomalously high or low Sr/Ca values (\u0026gt;\u0026thinsp;2σ from the mean) were examined by scanning electron microscopy (SEM; TM3000, Hitachi) to identify evidence of secondary aragonite precipitation or dissolution. Skeletal regions with visible diagenetic features were excluded from the final reconstruction (Extended Fig.\u0026nbsp;2; Scenario A in Supplementary Table\u0026nbsp;1).\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003eAdditional screening of anomalous coral Sr/Ca values\u003c/h2\u003e\u003cp\u003eSampling path can also cause Sr/Ca artifacts with disorganized corallites (merging or terminating corallite fans) and corallites perpendicular to the slab can cause anomalously high and low Sr/Ca values respectively\u003csup\u003e11\u003c/sup\u003e. Decoupling of the temperature and Sr/Ca relationship has also been associated with thermal, mechanical, and nutrient stress (i.e., night nitrate, low phosphate) causing anomalously high Sr/Ca values\u003csup\u003e12\u0026ndash;17\u003c/sup\u003e. To further evaluate the influence of problematic Sr/Ca data, we examined not only the removal of Sr/Ca values affected by diagenesis (scenario A in Supplementary Table\u0026nbsp;1), but also the exclusion of data obtained along sub-optimal or disorganized sampling paths (scenario B in Supplementary Table\u0026nbsp;1), and the additional removal of Sr/Ca values with disrupted Sr/Ca\u0026ndash;temperature relationships due to thermal stress, indicated by high Sr/Ca anomalies (scenario C in Supplementary Table\u0026nbsp;1).\u003c/p\u003e\u003c/div\u003e\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSr/Ca data that support the findings of this study will be deposited in data repository at KIKAI Institute for coral reef sciences (https://coralogy.kikaireefs.org/C-1%20Scientific%20data.html), after acceptance.\u003c/p\u003e\n\u003cp\u003eThe coral data used in Extended Table 1 were sourced from the NOAA/World Data Service for Paleoclimatology (https://www.ncei.noaa.gov/products/paleoclimatology).\u003c/p\u003e\n\u003cp\u003eSST data used in Extended Data Table 3 were taken from ERSST v5\u003csup\u003e28\u003c/sup\u003e (available at https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.ersst.v5.html), HadISST\u003csup\u003e\u0026nbsp;29\u003c/sup\u003e (available at https://www.metoffice.gov.uk/hadobs/hadisst/), Kaplan Extended v2\u003csup\u003e30\u003c/sup\u003e (available at https://www.esrl.noaa.gov/psd/data/gridded/data.kaplan_sst.html), and OISST v2 version2p1 AVHRR Daily SST\u003csup\u003e31\u003c/sup\u003e (available at https://www.esrl.noaa.gov/psd/data/gridded/data.noaa.oisst.v2.html).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReference for Methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e9.\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Uchiyama, R., Watanabe, T., Kahng, S. E. \u0026amp; Yamazaki, A. Calibration of Sr/Ca Ratio and In Situ Temperature Using Hawaiian Corals. Geochemistry, Geophysics, Geosystems 24, (2023).\u003c/p\u003e\n\u003cp\u003e11.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;DeLong, K. L., Quinn, T. M., Taylor, F. W., Shen, C. C. \u0026amp; Lin, K. Improving coral-base paleoclimate reconstructions by replicating 350years of coral Sr/Ca variations. Palaeogeogr Palaeoclimatol Palaeoecol 373, 6\u0026ndash;24 (2013).\u003c/p\u003e\n\u003cp\u003e12.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Marshall, J. F. \u0026amp; McCulloch, M. T. An assessment of the Sr/Ca ratio in shallow water hermatypic corals as a proxy for sea surface temperature. Geochim Cosmochim Acta 66, 3263\u0026ndash;3280 (2002).\u003c/p\u003e\n\u003cp\u003e13.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Hayashi, E. et al. Growth-rate influences on coral climate proxies tested by a multiple colony culture experiment. Earth Planet Sci Lett 362, 198\u0026ndash;206 (2013).\u003c/p\u003e\n\u003cp\u003e14.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Clarke, H. et al. Differential response of corals to regional mass-warming events as evident from skeletal Sr/Ca and Mg/Ca ratios. Geochemistry, Geophysics, Geosystems 18, 1794\u0026ndash;1809 (2017).\u003c/p\u003e\n\u003cp\u003e15.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;D\u0026rsquo;Olivo, J. P. \u0026amp; McCulloch, M. T. Response of coral calcification and calcifying fluid composition to thermally induced bleaching stress. Sci Rep 7, (2017).\u003c/p\u003e\n\u003cp\u003e16.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Leupold, M. et al. El Ni\u0026ntilde;o-Southern Oscillation and internal sea surface temperature variability in the tropical Indian Ocean since 1675. Climate of the Past 17, 151\u0026ndash;170 (2021).\u003c/p\u003e\n\u003cp\u003e17.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Standish, C. D. et al. Geochemical responses of scleractinian corals to nutrient stress. Geochim Cosmochim Acta 351, 108\u0026ndash;124 (2023).\u003c/p\u003e\n\u003cp\u003e24.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Watanabe, T. K., Watanabe, T., Ohmori, K. \u0026amp; Yamazaki, A. Improving analytical method of Sr/Ca ratios in coral skeletons for paleo-SST reconstructions using ICP-OES. Limnol Oceanogr Methods 18, 297\u0026ndash;310 (2020).\u003c/p\u003e\n\u003cp\u003e25.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Quinn, T. M. \u0026amp; Taylor, F. W. SST artifacts in coral proxy records produced by early marine diagenesis in a modern coral from Rabaul, Papua New Guinea. Geophys Res Lett 33, (2006).\u003c/p\u003e\n\u003cp\u003e26.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Hendy, E. J., Gagan, M. K., Lough, J. M., McCulloch, M. \u0026amp; deMenocal, P. B. Impact of skeletal dissolution and secondary aragonite on trace element and isotopic climate proxies in Porites corals. Paleoceanography 22, (2007).\u003c/p\u003e\n\u003cp\u003e27.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Mcgregor, H. V \u0026amp; Gagan, M. K. Diagenesis and Geochemistry of Porites Corals from Papua New Guinea: Implications for Paleoclimate Reconstruction. (2003).\u003c/p\u003e\n\u003cp\u003e28.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Huang, B. et al. Extended reconstructed Sea surface temperature, Version 5 (ERSSTv5): Upgrades, validations, and intercomparisons. J Clim 30, 8179\u0026ndash;8205 (2017).\u003c/p\u003e\n\u003cp\u003e29.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Rayner, N. A. et al. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research: Atmospheres 108, (2003).\u003c/p\u003e\n\u003cp\u003e30.\u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026nbsp;Kaplan, A. et al. Analyses of global sea surface temperature 1856-1991. J Geophys Res Oceans 103, 18567\u0026ndash;18589 (1998).\u003c/p\u003e\n\u003cp\u003e31. \u0026nbsp; \u0026nbsp; \u0026nbsp;Reynolds, R. W. et al. Daily high-resolution-blended analyses for sea surface temperature. J Clim 20, 5473\u0026ndash;5496 (2007).\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-7414589/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7414589/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Tropical and subtropical oceans have warmed substantially over the past century, but model study reconstructions suggest that the rate of warming may differ greatly across regions. Sr/Ca ratios in massive Porites corals provide a reliable archive of past sea surface temperatures (SST), enabling high-resolution reconstructions of long-term climate trends. Yet many parts of the ocean remain underrepresented in such records, hindering a spatially resolved understanding of ocean warming. Here we present SST warming rates over the past century based on coral Sr/Ca records from Hawaii, which show an increase of 2.7 to 3.0 °C per century–far exceeding warming rates reported from other coral Sr/Ca records worldwide, most of which remain below 1.5 °C per century. This finding suggests that coral reef in the subtropical North Pacific may be warming more rapidly than previously recognized based on sparse reanalysis dataset.","manuscriptTitle":"Coral Thermometry Shows Exceptional 3˚C warming in the Subtropical North Pacific Over the Past Century","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-12 13:44:34","doi":"10.21203/rs.3.rs-7414589/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"communications-earth-and-environment","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"commsenv","sideBox":"Learn more about [Communications Earth and Environment](https://www.nature.com/commsenv/)","snPcode":"","submissionUrl":"","title":"Communications Earth \u0026 Environment","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Communications Series","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a51544b7-fbfe-4451-b258-b74bc2825d34","owner":[],"postedDate":"October 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":53913769,"name":"Earth and environmental sciences/Climate sciences/Climate change"},{"id":53913770,"name":"Physical sciences/Astronomy and planetary science/Planetary science/Geochemistry"}],"tags":[],"updatedAt":"2025-10-12T13:44:34+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-12 13:44:34","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7414589","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7414589","identity":"rs-7414589","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}