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6 pages. -- Supplementary Figure 1. Current mean maximum summer temperature (average 𝑇!"# """""" for the period 1980-2005) across potential seagrass distribution. -- Supplementary Figure 2. Difference between current mean maximum summer temperature ( 𝑇!"# """""" ) and the Tlimit as a function of latitude. Negative and positive latitude values for southern and northern hemispheres, respectively. -- Supplementary Figure 3. Uncertainty associated to the time (in years) for mean maximum summer temperature to reach seagrass upper thermal limit (Tlim) at the warming rates projected under the RCP8.5 scenario around potential seagrass sites. -- Supplementary Figure 4. Time (in years) for mean maximum summer temperature to reach the upper thermal limits (Tlim) of temperate and tropical affinity seagrass flora at the warming rates projected under the RCP8.5 scenario around potential seagrass sites in the Mediterranean Sea and Queensland (Australia) coastal areas. -- Supplementary Figure 5. The time (in years) to reach Tlimit at the warming rates predicted under the RCP4.5 scenario around potential seagrass sites. -- Supplementary Figure 6. Time (in years) for mean maximum summer temperature to reach the upper thermal limits (Tlim) of temperate and tropical affinity seagrass flora at the warming rates projected under the RCP4.5 scenario around potential seagrass sites in the Mediterranean Sea and Queensland (Australia) coastal areas., Seagrasses have experienced major losses globally mostly attributed to human impacts. Recently they are also associated with marine heat waves. The paucity of information on seagrass mortality thermal thresholds prevents the assessment of the risk of seagrass loss under marine heat waves. We conducted a synthesis of reported empirically- or experimentally-determined seagrass upper thermal limits (Tlimit) and tested the hypothesis that they increase with increasing local annual temperature. We found that Tlimit increases 0.42± 0.07°C per°C increase in in situ annual temperature (R2 = 0.52). By combining modelled seagrass Tlimit across global coastal areas with current and projected thermal regimes derived from an ocean reanalysis and global climate models (GCMs), we assessed the proximity of extant seagrass meadows to their Tlimit and the time required for Tlimit to be met under high (RCP8.5) and moderate (RCP4.5) emission scenarios of greenhouse gases. Seagrass meadows worldwide showed a modal difference of 5°C between present Tmax and seagrass Tlimit. This difference was lower than 3°C at the southern Red Sea, the Arabian Gulf, the Gulf of Mexico, revealing these are the areas most in risk of warming-derived seagrass die-off, and up to 24°C at high latitude regions. Seagrasses could meet their Tlimit regularly in summer within 50-60 years or 100 years under, respectively, RCP8.5 or RCP4.5 scenarios for the areas most at risk, to more than 200 years for the Arctic under both scenarios. This study shows that implementation of the goals under the Paris Agreement would safeguard much of global seagrass from heat-derived mass mortality and identifies regions where actions to remove local anthropogenic stresses would be particularly relevant to meet the Target 10 of the Aichi Targets of the Convention of the Biological Diversity., Peer reviewed

13 pages. -- The PDF file includes: Supporting text. -- Figs. S1 to S1. -- Legends for Movies S1 to S4., Self_organized_appendix.pdf, pnas.2216024120.sm01.mp4, pnas.2216024120.sm02.mp4, pnas.2216024120.sm03.mp4, pnas.2216024120.sm04.mp4, Peer reviewed

Open access. Please, contact corresponding author in case of doubts, Description: The Malaspina2010_chlorophyll.xlsx file contains the fluorimetric chlorophyll a data of the circunnavigation expedition Malaspina 2010, which took place between 14/12/2010 and 14/07/2011 on board the BIO Hespérides. The data presented here were obtained between 16/12/2010 and 11/07/2011. The date and position of the sampling stations are listed in the third sheet of the file. Methods: Water samples for fluorimetric chlorophyll a (Chl a) determination were collected from 3 m depth with a 30-liter Niskin bottle and from selected depths between 10 and 200 m with a Rosette of 24 10-liter Niskin bottles attached to a CTD probe. Chl a determination was carried out as described in Estrada et al. (2012). Briefly, between 200 and 500 cm3 of seawater were filtered through GF/F glass fiber filters that were subsequently frozen at -20°C and, after a minimum of 6 hours, introduced in vials with acetone 90% and left for 24 hours in the dark, at 4ºC. The Chl a concentration in the acetonic extracts was measured with a Turner Designs fluorimeter calibrated with a pure Chl a standard (Sigma-Aldrich); no phaeopigment correction was applied. Size-fractionated analyses of Chl a were performed for samples from surface, the 20% surface PAR and the deep chlorophyll maximum. The analyses were carried out by sequential filtration of a 500 cm3 of seawater through Poretics (polycarbonate) membrane filters of pore sizes 20 μm, 2 μm and 0.2 μm, which were subsequently treated as the GF/F ones (Estrada, 2012), This work was supported by Consolider-Ingenio 2010, CSD2008-00077 of the former Spanish Ministerio de Ciencia e Innovación (ES) (now www.ciencia.gob.es) and the Consejo Superior de Investigaciones Científicas (CSIC) (ES), Authors; Methods; Station positions; leg 1; leg 2; Legs 3-4; leg 5; leg 6; leg 7; References, Peer reviewed

Description: The Malaspina2010_chlorophyll.xlsx file contains the inorganic nutrient concentration data of the circunnavigation expedition Malaspina 2010, which took place between 14/12/2010 and 14/07/2011 on board the BIO Hespérides. The date and position of the sampling stations are listed in the “Data” sheet of the file. Methods: Water samples for measurement of the concentration of nitrate, nitrite (nitrate + nitrite in leg 1), phosphate and silicate were collected from 3 m depth with a 30-liter Niskin bottle and from selected depths between 10 and 200 m with a Rosette of 24 10-liter Niskin bottles attached to a CTD probe. Inorganic nutrient concentrations were determined by means of a Skalar autoanalyzer, following the standard spectrophotometric procedures described in Grasshoff et al. (1999) and Blasco et al. (2012); in leg 1, nitrite was not determined separately and phosphate was measured using a manual method (Vidal et al. 2012), This work was supported by Consolider-Ingenio 2010, CSD2008-00077 of the former Spanish Ministerio de Ciencia e Innovación (ES) (now www.ciencia.gob.es) and the Consejo Superior de Investigaciones Científicas (CSIC) (ES), Peer reviewed

Abundance table, auxiliary data, phylogeny and code for the particle-attached and free-living prokaryotes detected in 30 globally distributed stations from the Malaspina 2010 Expedition. Corresponds to 16SrDNA amplicons Illumina-based sequencing., Peer reviewed