Enough water to go around?
Nature checks out the state of usable water on our planet.
Rachel Courtland
Click to see graphic: Distribution of Earth's water.USGS
How much water is there on the planet?
If our planet were perfectly flat and its water covered all the surface, it would create a layer 2.7 kilometers deep. While this seems like a lot, less than 3% of that is freshwater. Of that, nearly 70% is in ice caps, glaciers, and permanent snow, and 30% sits in ground water. Rivers, lakes, and clouds carry less than 1% of the world's freshwater.
Is there enough water to go around?
It depends on where you live. Every individual requires an estimated minimum of 1,000 cubic metres of water per year, to grow food and meet drinking and hygiene needs (See Water facts). Global water sources vary widely; some areas, such as sub-Saharan Africa, regularly suffer from water scarcity and are unable to meet this requirement.
Why is it an issue?
More than 1 billion people lack access to potable water and 2.6 billion do not have access to adequate sanitation, according to the World Health Organization. A lack of clean water forces people to use unsafe water, which affects health and increases the incidence of diseases like cholera and dysentery. The food supply is also limited by water shortages; a litre of water is required to produce every calorie of food1.
Will there be enough water to go around in the future?
Click to see image: Water scarcity in 2025.World Resources Institute
Many expect a dramatic rise in areas experiencing water scarcity in coming decades. Population and development pressures are creating competing demands for water for basic human needs, agriculture and power (water is used not only in hydroelectric plants and steam turbines, but also to cool fossil fuel and nuclear power plants). In 2000, for example, electricity production and agriculture each accounted for roughly 40% of freshwater withdrawals in the United States. US energy consumption alone is expected to rise by 50% by 2030, putting further pressure on resources (see 'The energy challenge').
How will climate change contribute?
The rise in global temperatures is expected to change the rates of flow of various parts of the water cycle. It will greatly reduce mountain snowpack, for example, a large source of water in many regions including India and the western United States. A recent United Nations report warned that ice loss from glaciers reached record levels in 2006, and many mountain glaciers could disappear completely within decades.
Click to see graphic: Water cycle.US Global Change Research Program
Getting rain instead of snow will also change the variation in water availability at different times of year, and dams set up to capture snowmelt may no longer be optimally placed to capture rainfall. As global temperatures rise, climate scientists generally expect more rain to fall, but less often. Longer dry spells could impact the food supply in a number of areas (see 'A long dry summer').
We can’t drink less water. How do we make sure there’s enough to go around?
For inland areas, one obvious but often controversial idea is to divert water from rivers with heavy flow to areas that are water-deprived. Some countries are planning to do this on a massive scale (see 'Muddy waters').
Making water fit to drink poses other challenges. Advances are being made developing materials that can trap viruses and bacteria, and de-activate them using catalysts triggered by light. The construction of desalination plants is on the rise in coastal areas, but this process has been criticized for being energy-intensive (see 'Purification with a pinch of salt').
It's a good idea too to re-use water whenever possible. Captured water from industry or municiple waste can be lightly treated and re-used on the spot for irrigation or industry, saving the energy needed to fully clean or re-pump water around (see 'Science and technology for water purification in the coming decades').
And agricultural systems can be designed so that crops need less water and are more tolerant to drought, for example. There is some tension, however, on whether to move forward with genetically modified crops for this purpose (see 'More crop per drop').
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One target of the United Nations Millennium Declaration is to see the proportion of people without safe drinking water and basic sanitation halved by 2015. Will we meet that goal?
Perhaps. “There’s a chance that we could meet the Millenium goals for safe water, but almost no chance we could meet it for sanitation, and in fact, we’re falling behind,” says Peter Gleick of the Pacific Institute, a non-profit based in Oakland, California. Gleick says investment in sanitation facilities has been a relatively low priority. But some question the utility of the current indicators (see 'Improving on the haves and have-nots').
译文:
地球上的水够用么？
《自然》杂志测定出地球上可用水资源的状态。
理查·康兰德
点击查看图片：地球水资源分布，美国地质调查局（USGS）
地球上有多少水？
如果地球是完全平整的，并且将地球上的水覆盖到整个表面，那么将会呈现出一个2.7千米深的水层。看上去我们似乎有很多水资源可以利用，但实际上这么多水中只有不到3%是淡水。而这不到3%的淡水中，有将近70%是被冰层覆盖，或是冰川和终年积雪，30%才是可饮用的。江、河、湖、云携带着地球上1% 的淡水。
水够用么？
这个问题取决于你的居住地。据测算，一个人的年所需水量大概至少1000立方米。这些水用于种植、饮用及卫生需要。全球的水资源分布广泛；但在一些地区，例如撒哈拉以南的非洲，却常年受到水资源匮乏之苦，无法满足日常用水需求。
为何用水成为一大问题？
根据世界卫生组织统计，有超过十亿人口存在饮用水困难，26亿人口无法获取所需的卫生设备。由于缺乏卫生干净的水资源，人们不得不使用劣质水源，这会影响他们的健康，同时也会增加类似霍乱、痢疾等传染病的发病率。每生产一卡路里食物需用水量为一升，所以食物供应也因用水短缺而受限。
未来是否有足够的水可用？
点击查看图片： 2025年水资源缺乏分布图，世界资源研究所（World Resources Institute）
许多人预计，在未来几十年中，水资源短缺地区的数量将会出现一个跳跃式的增长。人口和发展压力会产生对水资源的竞争需求，这包括人类生存所需用水，农业、电力用水（水不仅仅用于水力发电厂和蒸汽涡轮机，也被用于冷却矿物燃料和核能发电站）。例如，2000年美国用于发电和农业的水量就各自占据了大约40%的淡水抽取量。据估计，2030年，仅仅美国的能源消耗量就将上升50%，这将会对能源需求产生更大的压力。
环境变化会产生怎样的影响？
全球气温的上升将会改变水循环中各部分流动的速率。在印度和美国西部等许多水资源丰富的地区，这将大大降低山顶积雪量。一项近期的联合国报道发出警告说，2006年冰川的冰层融化程度已达到历史最高值，许多山地冰川可能在未来的几十年中消失。
点击查看图片：水的循环，美国全球变化研究计划（US Global Change Research Program）
出现只下雨而不是雪的情况，将会在一年中的不同时间段改变水的可用性形态，那些用于控制融雪的水坝也将不再是雨水的合适驻地。随着全球气温的上升，气象学家全都希望有更多的降雨，但却事与愿违。同时，更长的旱季可能会影响许多地区的食物供应。
我们不可能少喝水，那如何才能确保有足够的水呢？
对于内陆地区，一个常见却又备受争议的观点是将大流量河水中的水导入缺水地区。一些国家正在计划实施这项巨大的工程。
使水适合饮用引发了更多的问题。在研发可以捕捉病毒和细菌的物质上正不断取得进步，使用光亮诱发的催化剂能让它们失去活性。在沿海地区，脱盐工厂的数量正逐日增多，但这一现象被指责因能源密集而形成浪费。
如果可能的话，循环利用水资源也将是一个好主意。从工厂或是城市废物中回收的水可以经处理并循环使用于灌溉和工业，这样可以节省用于完全净化或输送水的能源。
农业系统也可以调整，使得农作物需水量减少，同时更耐旱。但这里也有一些矛盾存在——是否需要种植转基因作物来达到这一目的。
联合国千年宣言中的一个目标是，截止至2015年，缺乏安全饮用水和基本卫生设备的人口比例将减半。我们是否能够实现这一目标？
或许，“我们有可能会实现千年目标中安全饮用水的问题，但至于卫生设备就难说了。事实上，我们已经落后了。”太平洋研究所（加利福尼亚州奥克兰市的一个非盈利组织）的彼得· 格雷克说。他认为投资卫生设备已成为一个相对较次要的任务。但是一些人仍对这个千年目标的效用产生质疑。