Why can't we drink seawater?

When we observe our planet from space, one color dominates, earning Earth the moniker "the Blue Planet." Approximately three-quarters of our world's surface is covered with water. However, there's a catch: a staggering 96.5 percent of this water is locked in the oceans. And if you recall the first time your parents took you to the seaside, you'll remember that drinking ocean water is a big no-no. So, why is ocean water salty and unsuitable for consumption? There are two primary reasons.

The first reason is runoff water from the land. Rainwater, which is slightly acidic with a pH ranging from five to five and a half (pure water has a pH of seven, while battery acid is even lower), plays a crucial role. When rainwater falls on the ground, it erodes rocks, releasing ions like sodium and chloride. These ions find their way into rivers and streams, ultimately flowing into the ocean. Although some of these beneficial ions are removed by living organisms, the remainder accumulates over time, increasing their concentration in the water. Oceans have their own salt sources too. Vents on the seafloor release hydrothermal fluid, a process where water seeps into gaps on the ocean floor and is heated by the Earth's core, freeing seawater from oxygen and picking up metals like iron and zinc. During underwater volcanic eruptions, these vents release metallic water back into the ocean, contributing to the salt and mineral content. Over time, salt accumulates on the seafloor, forming domes.

These salt deposits can also be found beneath dry land, with some regions having numerous salt domes. These underwater and underground deposits influence the salinity of ocean water. Other factors that impact the saltiness of bodies of water include evaporation, air temperature, and precipitation. As a general rule, salinity is lower near the equator and the poles, with higher salinity in the oceans and seas in between. Scientists estimate that dissolved salts constitute approximately three and a half percent of the weight of the world's seawater.

Now, why can't we drink seawater? To answer this question, we must delve into our planet's history. Researchers believe that ancient seas were not as salty as they are today. Over time, rainfall washed away rocks on land, transporting vast amounts of salt into the oceans. This process has continued for more than 3.8 billion years, with roughly 4 billion tons of dissolved salts entering Earth's oceans each year. The input and output of salt remain fairly balanced, maintaining the stability of seawater's salinity.

The reason we cannot drink seawater lies in its salt content. The percentage of salt in our blood is nearly four times lower than the percentage of salt in seawater. Our bodies are not equipped to process such high salt concentrations. When we consume salt as part of our diet, we also drink liquids to balance it out. In contrast, drinking seawater exacerbates thirst because our bodies absorb both water and salt, leading to dehydration. The kidneys play a vital role in removing excess salt from our blood, but they require water to do so. The higher the salt content, the more water the kidneys need to wash it away. This cycle ultimately leaves us thirstier than when we began drinking seawater.

Marine mammals such as whales and seals, along with some birds like seagulls, can drink seawater because of their highly efficient kidney function or specialized glands. However, among land animals, only camels can safely consume seawater. Fish, on the other hand, do drink seawater, but their gills and kidneys help eliminate the excess salt.

For humans to make seawater potable, it must undergo desalination, a process that removes salt. However, desalination poses significant challenges due to its substantial energy requirements and carbon footprint. It currently accounts for less than half a percent of the world's drinking water production. As global demand for freshwater continues to rise, finding more energy-efficient desalination methods is essential.

Imagine for a moment that we removed all the salt from Earth's oceans, yielding an endless supply of drinking water. However, this would come at a considerable cost. Millions of animal and plant species adapted to saltwater would struggle to adapt to freshwater conditions. The abrupt shift would have profound effects, including altering the density of the Arctic ice cap and triggering enormous tidal waves. While global-scale desalination seems appealing, it should be approached cautiously, considering the ecological consequences.