Asked by: Jess Rochefort
A litre of seawater contains around 35g of dissolved salt, so desalinating the entire ocean would involve removing 45 million billion tonnes of salt. The sudden reduction in weight pressing on the seabed would probably trigger earthquakes and volcanoes around the globe.
Because freshwater is less dense, the Arctic icecap would sink an extra 10cm into the water, creating the largest tidal wave the planet has ever seen along northern Europe, Russia and Canada. A few hours later, virtually all marine life would die as their cells swelled and ruptured due to osmosis (water molecules move towards areas of higher salt concentration). They would sink to the ocean floor, but their bodies wouldn’t decompose, because all marine bacteria would be dead too. Marine algae are responsible for at least half of Earth’s oxygen production, so there would be mass extinctions on land as well.
Eventually, the oceans would resalinate because minerals are continually dissolved from the land by rivers and carried to the sea, but this would take tens of thousands of years.
Subscribe to BBC Science Focus Magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.