Table_1_Appraisal of coral bleaching thresholds and thermal projections for the northern Red Sea refugia.xlsx

Corals in the northern Red Sea exhibit high thermal tolerance despite the increasing heat stress. It is assumed that corals throughout the Red Sea have similar bleaching thresholds (32°C or higher), and hence greater bleaching tolerance of corals in the northern Red Sea region is likely due to lower ambient water temperatures (25–28°C) that remain well below the corals’ physiological maxima. Whether bleaching patterns across the Red Sea are independent of the local maximum monthly mean of seawater temperature and aligned with an assumed 32°C threshold has yet to be determined. Here, we used remotely sensed surface sea temperature data spanning 1982–2020 to model spatial distributions of Degree Heat Weeks across the Red Sea in relation to assumed coral thermal threshold values of 30, 31, and 32°C. We also used the Coupled Model Intercomparison Project Phase 5 model outputs to predict warming trends in the Red Sea under different greenhouse gas representative concentration pathways (RCPs). We show that applying 32°C thresholds dramatically reduces effective Degree Heat Weeks in the north, but not in central or southern Red Sea regions, a finding that is consistent with historical bleaching observations (1998–2020) throughout the Red Sea. Further, model predictions under the most extreme RCP8.5 scenario exhibited ~3°C warming by the end of the 21st century throughout the Red Sea with less pronounced warming for the northern Red Sea (2–2.5°C) compared to the central and southern regions (2.7–3.1°C).This warming rate will remain below the assumed thermal threshold for the northern Red Sea which should help this region to serve as refugia (i.e., maintaining favorable temperatures) for corals to persist for decades ahead. Together, our results support the notion that corals have similar thresholds throughout the Red Sea; hence, coral bleaching thresholds are independent of the local maximum monthly mean. Consequently, where regional warming projections suggest the northern Red Sea will not reach assumed bleaching thresholds (32°C) before the end of the 21st century, coral reefs in the northern region may be among the last standing against climate change.

红海北部珊瑚表现出极高的热耐受性，即便面临不断加剧的热应激。据推测，红海全域的珊瑚均具有相似的褪色阈值（32°C或更高），因此红海北部珊瑚的较高褪色耐受性很可能源于较低的环境水温（25–28°C），这一水温远低于珊瑚的生理极限。红海地区的珊瑚褪色模式是否独立于当地最高月平均海水温度，并与假设的32°C阈值相一致，尚待确定。在本研究中，我们利用1982年至2020年间远程感知的表层海水温度数据，建模了红海区域内热量周数分布的空间模式，并关联于假设的珊瑚热阈值值，分别为30°C、31°C和32°C。此外，我们还利用耦合模型比较计划第五阶段（CMIP5）的模型输出，预测了红海在不同温室气体代表性浓度路径（RCPs）下的升温趋势。我们发现，应用32°C阈值在北部显著降低了有效热量周数，而在红海中部或南部地区则没有这种效果，这一发现与红海全域历史上的褪色观测结果（1998–2020年）相吻合。进一步地，在最极端的RCP8.5情景下，模型预测到21世纪末红海全域将升温约3°C，而北部红海（2–2.5°C）的升温程度相对于中部和南部地区（2.7–3.1°C）则较为缓和。这种升温速率将低于北部红海假设的热阈值，有助于该地区成为珊瑚的庇护所（即保持适宜的温度），从而在未来数十年内维持珊瑚的生存。综上所述，我们的研究结果支持了红海全域珊瑚具有相似阈值这一观点；因此，珊瑚的褪色阈值与当地最高月平均温度无关。据此，当区域升温预测表明北部红海在21世纪末之前不会达到假设的褪色阈值（32°C）时，北部地区的珊瑚礁可能成为抵御气候变化中的最后防线。