## 5 Discussion
### Reliability of the Susceptibility Assessment  
In areas where hydrate evidence and indicators have been reported, the assumption of a salinity of 36 psi for the theoretical GHSZ calculated by [PERSON] [61] along the study area involves an error in the thickness calculation of \(\pm\)2-10 m in the Arctic region (33-35 psi), \(\pm\)5 m in the Mediterranean Sea (38 psi) and \(\pm\)30 m in the Black Sea (17 psi) [64]. Nevertheless, considering the regional scale of the susceptibility assessment (cell size of 5 \(\times\) 5 km), these errors are acceptable in the thickness calculation because they lay inside the vertical precision of the assessment. The error made by the difference in salinity in the theoretical GHSZ is lower than the vertical precision of this calculation due to the variation of the bathymetry each 5 km along the continental slope. In particular, \(\pm\)15-120 m in the Arctic region, and \(\pm\)100-200 m in both the Mediterranean and Black Sea. Regarding the Black Sea, although errors coming from salinity are still acceptable, the susceptibility resulting should be taken with caution, as values could be higher.  
In order to assess the reliability of the susceptibility inference, a qualitative value of uncertainty (very high, high, middle, low and very low) was established as a function of the data density taken into account in the susceptibility calculation (Figure 9). The reliability (\(u\)) is thus equal to the sum of the density maps of geothermal gradient (\(\rho_{gr}\)), seafloor temperature (\(\rho_{st}\)) and hydrate evidence and indicators (\(\rho_{hy}\)):  
\[u=\rho_{gr}+\rho_{st}+\rho_{hy} \tag{2}\]  
Five levels of reliability were established. The reliability is considered \"very low\" with values from 0 to 36 data per 100,000 km\({}^{2}\), approximately less than ca. 1 datum per 50 km in mean; and \"low\", \"middle\", \"high\" and \"very high\" from 36 to 144, from 144 to 648, from 648 to 3149, and from 3149 to 15,218 data per 100,000 km\({}^{2}\), respectively. These levels were defined by the geometrical segmentation of \(u\)-value, except \"very low\" and \"low\", which were defined by expert criteria. Very low reliability areas were catalogued as global knowledge gaps (KG in Figure 9) that are critical (CKG) in the vicinity of the up-dip limit of the GHSZ and hydrocarbon seabed fluid flow structures.  
Areas located in the proximity of the continental shelf and intensively surveyed by oceanographic cruises show the most reliable results in the susceptibility assessment. By contrast, areas distant from the coastline (e.g., the mid-Atlantic Ocean) and areas that are inaccessible because of the presence of icebergs (e.g., east Greenland) or political issues (e.g., north of Libya) have very high uncertainty.  
Figure 8: Knowledge gaps of bathymetry data. (**a**) EMODnet Bathymetry mosaic in the study area (cell size ca. 200 \(\times\) 200 m). (**b**) Detail of knowledge gap on the Algerian margin. (**c**) Detail of knowledge gap on the Libyan margin. Details downloaded from the EMODnet Bathymetry web portal ([[https://portal.emodnet-bathymetry.eu/](https://portal.emodnet-bathymetry.eu/)]([https://portal.emodnet-bathymetry.eu/](https://portal.emodnet-bathymetry.eu/)); accessed on 1 January 2021).  
Figure 9: Reliability of the susceptibility assessment of hydrate presence on the European continental margins. (**a**) Reliability prediction in the inference of the susceptibility of the seafloor to the presence of hydrates. KG, global knowledge gap; CKG, global critical knowledge gap. (**b**) Reliability and susceptibility assessments overposted.