1. What Is the Average Salinity of the World's Oceans?
When we talk about seawater, we often think of a standard taste. In the world's oceans, the average salt concentration is about 35 parts per thousand (‰), which equals 3.5% by weight [1]. This is the baseline for comparison. But this average hides a wide range of conditions. The balance between water leaving (evaporation) and water entering (rain and rivers) changes the salinity significantly. If evaporation is high and freshwater input is low, the sea becomes saltier. If freshwater input is high and evaporation is low, the sea becomes fresher. The connection to other oceans also matters, as it allows salt to move in or out [1].
2. Why Is the Red Sea So Salty? Evaporation and No Rivers
The Red Sea, located between the Middle East and Africa, is one of the saltiest seas in the world. Its average salinity is about 40‰, with the northern part reaching 41‰ and the southern part 36‰ [2][3]. While the numbers might not look drastically different from the global average at first glance, the reasons behind them reveal a sea defined by subtraction. In the Red Sea region, annual rainfall is only about 60 mm. In contrast, evaporation removes about 2,050 mm of water each year [2]. This means evaporation is roughly 34 times greater than rainfall. Furthermore, there are no major rivers flowing into the Red Sea [2]. The sea loses water through evaporation, but almost no fresh water returns to dilute it.
The only way to reduce this extreme salinity is through water exchange with the Indian Ocean via the Bab-el-Mandeb Strait at the southern end [2]. Even with this connection, the Red Sea remains much saltier than the global average. It is a place where evaporation works continuously, while rain and rivers do almost no work.
3. How Does the Mediterranean Sea Exchange Water with the Atlantic?
The Mediterranean Sea, surrounded by Europe and Africa, also has evaporation rates that exceed rainfall and river inflow. Its average salinity is about 38 PSU (Practical Salinity Units, which are similar to ‰) at a depth of 5 m [4]. Evaporation is particularly strong in the eastern part, so salinity increases as you move east [4]. However, the Mediterranean does not become as extreme as the Red Sea because of its connection to the Atlantic Ocean. The Strait of Gibraltar, which is only 14 km (about 9 miles) wide at its narrowest point, allows for a constant exchange of water [5].
This exchange works like a two-story building. Less salty Atlantic water flows into the Mediterranean at the surface, while the saltier, denser water from the Mediterranean flows out into the Atlantic at deeper levels [5]. This net inflow of Atlantic water is estimated at about 70,000 cubic meters per second [4]. If this inflow were to stop, scientists estimate that the Mediterranean's water level would drop by about 1 m (about 3 feet) per year [4]. The Mediterranean stays balanced because the Atlantic provides a steady supply of lower-salinity water to counteract the high evaporation.
4. Why Is the Baltic Sea Fresher? Many Rivers and Narrow Exits
The Baltic Sea in northern Europe is the opposite of the Red Sea. It is much fresher than the global average. The area of land that drains into the Baltic Sea is about four times larger than the sea itself. More than 250 rivers pour fresh water into it, and annual freshwater inflow is estimated at roughly 500 to 660 cubic km, depending on the calculation method [6]. However, the exit to the open ocean is limited. The only path to the North Sea is through shallow, narrow straits between Denmark and Sweden. These straits have shallow thresholds, such as depths of 7 m (about 23 feet) and 18 m (about 59 feet), which restrict the flow of heavy, salty water from the North Sea [6].
Because so much fresh water enters and so little salty water can enter, the surface salinity of the Baltic Sea is only 0.3% to 0.9% (3 to 9‰) [6]. In some places, this is less than one-tenth of the global average. The saltiest parts are in the southwest near the straits (0.7–0.9%), while the freshest part is the Gulf of Bothnia in the north, which drops to 0.3% [6]. The Baltic Sea is defined by high freshwater input and restricted mixing.
5. Why Is the Dead Sea, a Lake with No Exit, Much Saltier Than the Seas Above?
In the Middle East, there is a body of water that is technically a lake, not a sea, yet it is far saltier than the seas discussed so far. This is the Dead Sea. It has an inflow from the Jordan River, but no outflow. It is a terminal lake with no exit to the ocean [7][8]. Because there is no outflow, the only way water leaves is through evaporation. In this dry climate, evaporation continues relentlessly, and the dissolved salt remains in the lake [7][8].
In 2011, measurements showed the Dead Sea's salinity was 342 g of dissolved salts per kg of Dead Sea water, or about 34.2% by weight. This is approximately 9.6 times saltier than average seawater [7]. Since the 1960s, large-scale irrigation projects in Israel and other countries have reduced the amount of water flowing from the Jordan River into the Dead Sea [7][8]. With less water flowing in, no river flowing out, and evaporation continuing, the Dead Sea is an extreme example of a body of water where the diluting factors have almost disappeared.
6. How Do Evaporation, Rainfall and Rivers, and Mixing Compare Across These Waters?
We can summarize these four bodies of water using the three factors: evaporation (water out), rainfall and rivers (water in), and mixing with other seas. The Red Sea has low water input and high evaporation, with only a narrow strait for mixing, resulting in high salinity [2]. The Mediterranean also has high evaporation, but the large inflow from the Atlantic prevents it from becoming as salty as the Red Sea [4]. The Baltic Sea has high freshwater input from rivers, low evaporation due to its cool climate, and restricted mixing with the North Sea, resulting in very low salinity [6]. The Dead Sea is the extreme case: inflow is decreasing, there is no outflow, and evaporation is the only process removing water, leading to extreme salinity [7].
Although maps show all these areas in blue, their climates and geography create completely different balances of water flow. This explains why "seawater" can taste so different depending on where you are.
7. How Can You Observe Evaporation at Home with Saltwater?
You can observe evaporation yourself using table salt and water. Prepare two identical flat containers with the same amount of saltwater at the same concentration. Place one in a sunny window and the other in a shaded, poorly ventilated spot. Mark the water level every day at the same time. Over a few days to a week, observe if there is a difference in how much the water evaporates in each location. This simple test shows how location affects evaporation, just like in the Red Sea or the Dead Sea where evaporation works year-round.
You can also look for satellite maps of global ocean salinity online. Check what color the Red Sea and the Baltic Sea are on these maps to see how scientists visualize these differences.