Jan 23rd 2017, 23:27 BY T.S.

2017年1月23日 BY T.S.

　翻譯：巍峨群山    微信公眾號：Yeetii    個人博客：Yeetii.com

SEEN from space, the Earth is a pale blue dot. Two-thirds of its surface is covered by water. But the vast majority of that water—around 97%—is salty. Of the 3% that is fresh water—which is the kind humanity needs to drink, wash, make things and (most of all) produce food—about two-thirds is locked up in glaciers, ice caps and permafrost. That leaves less than 1% of the planet’s water easily accessible in rivers, lakes or aquifers. In short, the salinity of the oceans means useful water is scarce, while the less useful kind is abundant. So why is the sea salty?

從太空俯視，地球是一個淡藍色的小不點。三分之二的面積被水覆蓋。但其中絕大部分——大約97%——是咸水。剩下大約3%是淡水，這是人類需要的，日常飲用，清洗物品，生產制造和（最多用處）制作食物，但這其中又有三分之二淡水鎖在冰川，冰蓋和永久凍土層里。這樣，這個星球上只有不到1%的水資源是人類容易獲取的淡水，出現(xiàn)在河流，湖泊或者土壤含水層里。簡而言之，雖然海水很多，但因為海水是咸的，人類可以使用的水資源是稀缺的。那么，為什么海水是咸的呢？

The salt in the ocean mostly got there as the result of a process called weathering, which transfers mineral salts from rocks on land into the sea. Rain is not pure water, but contains small amounts of carbon dioxide absorbed from the air; this makes rainwater very slightly acidic. When this weak acid falls on land, tiny traces of minerals dissolve from rocks into the water, separating into charged particles called ions. These ions then travel along with the water into streams, rivers and eventually into the ocean. Many of these mineral ions are subsequently removed from the sea water by marine plants and animals, but others remain in the water, and their concentration builds up over millions of years. Over 90% of the ions in sea water, accounting for about 3% of the ocean by weight, are sodium and chlorine ions, which are the chemical constituents of common salt. Other processes also play a role. Underwater volcanoes and hydrothermal vents discharge mineral salts into sea water. And isolated bodies of water, with insufficient drainage, may become increasingly salty through evaporation, which carries water away while leaving dissolved minerals behind. The Dead Sea (which contains about 30% mineral salts by weight) is the best-known example. 

海水中的鹽大部分來自于風化作用，這種地質現(xiàn)象把陸地上存在于巖石中的鹽傳送到海洋。雨水不是純凈水，雨水穿過空氣時，會吸收少量二氧化碳，這使得雨水有一點點弱酸性。當這種弱酸性的雨水降落到地面，巖石中一丁點礦物質會分離成為帶電的離子，溶解在水里，這些離子被雨水攜帶著進入溪水，河流，最后到達海洋。這些礦物質離子又持續(xù)不斷的被海洋植物和海洋動物從海水中吸收走，但還有不少留在海水中，然后經歷了數(shù)百萬年的濃縮，海水不斷變咸。現(xiàn)在，海水中的礦物離子大約占海水總重量的3%，其中90%是鈉離子和氯離子，而這兩者正是合成鹽的化學物質。別的地質活動也會使海水變咸。例如水底的火山和熱液噴口會把礦物鹽排入海水中。而沒有通道與其它水體連接的單獨水體，水份不斷蒸發(fā)，留下礦物質，變得更咸。死海（海水以重量計其中30%是礦物鹽）就是最著名的例子。

The natural processes that make the seas salty can be reversed by desalination technologies that turn sea water into fresh water. This involves either boiling and then recondensing water, or pumping it at high pressure through reverse-osmosis membranes that allow water molecules to pass but are impermeable to larger mineral ions. Both processes are energy-intensive, however, though reverse osmosis has become far more energy-efficient in recent years. Accordingly, desalination plants are generally found in places where water is scarce but energy is cheap, such as the Middle East. 

海水變咸的自然過程，可以通過脫鹽技術來逆轉。這個技術包括煮沸海水然后冷凝淡水，或者給海水施加高壓,讓它通過反滲透薄膜，水分子可以順利通過薄膜，而大一些的礦物質分子不能通過，從而生成淡水。這些技術都是耗能大戶，不過，在過去幾年，反滲透膜技術的能效提高很快。因此，脫鹽工廠通常建設在淡水很珍貴而能源不稀奇的地區(qū)，例如中東地區(qū)。

As climate change causes “global drying”—making some wet parts of the world wetter, and dry parts drier—demand for fresh water will intensify in the coming years; half the world’s population is expected to live in water-stressed areas by 2050. Better water-management policies and more water-efficient agricultural practices (such as drip irrigation) are needed. Improvements to desalination technology would help too, by allowing mankind to tap the oceans’ inconveniently salty water. “If we could ever competitively—at a cheap rate—get fresh water from salt water,” observed President John F. Kennedy in 1961, “that would be in the long-range interests of humanity, which would really dwarf any other scientific accomplishment.”

由于全球氣候變化帶來“全球干燥”現(xiàn)象——使地球上潮濕的地區(qū)更潮濕，干燥地區(qū)更干燥的氣候現(xiàn)象——導致今后對淡水的需求會更加緊迫，到2050年，全球將有一半的人口居住在缺水地區(qū)。我們需要更好的水資源管理政策，和更多節(jié)水型的農業(yè)技術（例如滴灌技術）。脫鹽技術的提高也會有很大的幫助，它使人類可以使用原來無法使用的海水。在1961年，美國總統(tǒng)肯尼迪曾說過：“如果我們能全力以赴的從鹽水中分離出淡水，并且成本低廉，這將垂范百世，使其它科學成就相形見絀。”