【经济学人 双语】为什么海水是咸的？

Why the sea is salty

【经济学人 双语】为什么海水是咸的？

Jan 23rd 2017, 23:27 BY T.S.

2017年1月23日 BY T.S.

　翻译：巍峨群山    微信公众号：Yeetii    个人博客：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?

从太空俯视，地球是一个淡蓝色的小不点。三分之二的面积被水覆盖。但其中绝大部分——大约97%——是咸水。剩下大约3%是淡水，这是人类需要的，日常饮用，清洗物品，生产制造和（最多用处）制作食物，但这其中又有三分之二淡水锁在冰川，冰盖和永久冻土层里。这样，这个星球上只有不到1%的水资源是人类容易获取的淡水，出现在河流，湖泊或者土壤含水层里。简而言之，虽然海水很多，但因为海水是咸的，人类可以使用的水资源是稀缺的。那么，为什么海水是咸的呢？

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. 

海水中的盐大部分来自于风化作用，这种地质现象把陆地上存在于岩石中的盐传送到海洋。雨水不是纯净水，雨水穿过空气时，会吸收少量二氧化碳，这使得雨水有一点点弱酸性。当这种弱酸性的雨水降落到地面，岩石中一丁点矿物质会分离成为带电的离子，溶解在水里，这些离子被雨水携带着进入溪水，河流，最后到达海洋。这些矿物质离子又持续不断的被海洋植物和海洋动物从海水中吸收走，但还有不少留在海水中，然后经历了数百万年的浓缩，海水不断变咸。现在，海水中的矿物离子大约占海水总重量的3%，其中90%是钠离子和氯离子，而这两者正是合成盐的化学物质。别的地质活动也会使海水变咸。例如水底的火山和热液喷口会把矿物盐排入海水中。而没有通道与其它水体连接的单独水体，水份不断蒸发，留下矿物质，变得更咸。死海（海水以重量计其中30%是矿物盐）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. 

海水变咸的自然过程，可以通过脱盐技术来逆转。这个技术包括煮沸海水然后冷凝淡水，或者给海水施加高压,让它通过反渗透薄膜，水分子可以顺利通过薄膜，而大一些的矿物质分子不能通过，从而生成淡水。这些技术都是耗能大户，不过，在过去几年，反渗透膜技术的能效提高很快。因此，脱盐工厂通常建设在淡水很珍贵而能源不稀奇的地区，例如中东地区。

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.”

由于全球气候变化带来“全球干燥”现象——使地球上潮湿的地区更潮湿，干燥地区更干燥的气候现象——导致今后对淡水的需求会更加紧迫，到2050年，全球将有一半的人口居住在缺水地区。我们需要更好的水资源管理政策，和更多节水型的农业技术（例如滴灌技术）。脱盐技术的提高也会有很大的帮助，它使人类可以使用原来无法使用的海水。在1961年，美国总统肯尼迪曾说过：“如果我们能全力以赴的从盐水中分离出淡水，并且成本低廉，这将垂范百世，使其它科学成就相形见绌。”

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