It’s a cruel irony of the blue planet: Most of the earth is awash in oceans, yet seawater is undrinkable. Large-scale efforts to remove salt from seawater -- the process known as desalination -- trace back to the 1950s, and today almost 20,000 facilities from China to Mexico are making salt water drinkable to sustain burgeoning populations. But this modern-day alchemy is under scrutiny as critics question whether the benefits of desalination justify its potential harm to marine environments and contribution to global warming.
Ancient Greeks understood that boiling seawater produces a drinkable vapor; 18th century sailors experimented with capturing this vapor in sponges to ease their thirst on long voyages. Modern desalination began similarly: boiling seawater (by burning oil or natural gas) and recovering droplets of fresh water in a process known as thermal desalination. Cleaner, more energy-efficient technologies such as reverse osmosis -- passing seawater through plastic membranes to remove salt and other impurities -- have become more prevalent. The big exception is the Middle East, where countries rely on fossil fuel-based thermal plants for two-thirds of their desalination needs. The region accounts for roughly 90 percent of thermal treatment of seawater worldwide, according to the International Energy Agency.