## 3 Case Study Results
### Historical Climate, Sea-Level Change, and Hydomorphology
#### 3.1.2 Mean Sea-Level Change  
The rising seawater level is mostly due to a combination of meltwater from glaciers and ice sheets and thermal expansion of seawater as it warms as a result of climate change and rising air temperatures. These are extremely slow changes in relation to climate change, which alters the river system and position of the coast, river mouth, and delta environment. It affects the river geopotential energy, energy, and mass balance of the river system--that is, the system entropy.  
During the Ice Age, the sea level was approximately 125 m lower than it is today; hence, the mouth of the Neretva was in the middle of today's Adriatic Sea (Figure 3), and the area of today's delta was a river valley. Therefore, the basin and length of the river were considerably greater than today [22; 23]. The warming and melting of the ice masss during the Pliocene, Pleistocene, and Holocene caused the sea level to rise, varying from as low as about 6.0-9.9 mm/yr to as high as 30-60 mm/yr during brief periods of accelerated rise. This caused the movement of the river mouth and delta toward the interior of the continent (top transgressive coast; Figure 3). In the early Neolithic, the sea was approximately 35 m above today's level (10,000 years ago), and 5000 years ago it was 4 m above the current MSL (Figure 4a) [23].  
Figure 2: Central Greenland temperature: 4000 BP to present [21].  
With the cooling that occurred after the Roman Warm Period, the MSL rise slowed down, and the sediment filled the available area in the delta and expanded and moved the land, coasts, and estuaries deeper into the sea (bottom) regressive coast. How warm and cold periods throughout distant history impacted MSL changes in shorter periods is unknown; however, the general trend is known (Figure 4a). However, data from recent history (the Roman period) are more reliable because they can be detected based on archaeological data (Figure 4b).  
The sea-level rise after the Roman Warm Period was slower (about 0.6 mm/yr); hence, the mouth of the Neretva River moved from the interior to its present position (Figure 3) [23]. This could be the period when the Neretva Delta that we know now formed. In the Adriatic Sea, the MSL 2000 years ago was lower, at \(-\)1.26 \(\pm\) 0.06 m (Figure 4b), and the water depth in the delta area was several meters higher than today. Presently, the Roman port on the Adriatic coast is at a depth approximately 1.5 m below MSL; therefore, the bottom next to the port was probably several meters lower. The rise in sea level in the first 900 years AD was even slower; the river valley and delta were increasingly filled with sediment and spread into the sea area. Thereafter, the MSL rise accelerated, reaching 1.8 mm/yr in  
Figure 4: Relative rise in mean sea level in the world, Mediterranean, and Adriatic Sea. (**a**) Post-glacial sea-level rise. RSM: World sea level in the Holocene; RB: Sea level derived from the Bakar mareograph data; SKB: Land subsidence derived from the Bakar mareograph data [24] (CC BY 4.0). (**b**) Relative rise in mean sea level in the Mediterranean [25].  
Figure 3: Deposition area of the Neretva River during the Pliocene, Pleistocene, and Holocene; A, B, and C are interpreted 2D seismic lines [22] (CC BY 4.0).  
the last 100 years, amounting to about 20.32 cm [24]. Based on satellite data, from 1993 to 2020, the average MSL rise was 3.25 \(\pm\) 0.3 mm/yr, while acceleration was estimated at 0.1 mm/yr\({}^{2}\), making the delta susceptible to faster transgression [26]. Sea-level rise is a reliable indicator of climate change.