【经济学人】Thirsty planet

AS IT SCOURS the universe for signs of extraterrestrial life, NASA has a motto-cum-mission-statement: “Follow the water”. About 70% of the human body is made up of water, it says, and 70% of Earth’s surface is covered in the stuff. “Water creates an environment that sustains and nurtures plants, animals and humans, making Earth a perfect match for life in general.”

美国国家航空航天局（NASA）在宇宙中搜寻外星生命的迹象时有这样一句座右铭兼使命宣言：“追踪水”。人体约70%由水构成，它说，而地表的70%被水覆盖。“水创造了一个维持和供养动植物及人类的环境，使地球完美地匹配一般意义上的生命。”

If water is a proxy for life itself, it is perhaps not surprising that worries about the health and availability of supplies here on Earth can take on apocalyptic overtones. A scorching, arid future marked by a fierce, bloody struggle for a few drops of water is a standard theme of dystopian fiction and film-making. This report will examine how close such nightmares are to reality. It will look at the state of the world’s freshwater and at the increasing demands on it, and consider the ways they can be met.

如果水是生命本身的代名词，那么对地球上水质和水供应量的担忧会带着世界末日的意味或许也就不足为奇了。未来，人们将在酷热、干旱的气候里为了几滴水展开激烈、血腥的争斗——这是反乌托邦小说和电影的标准题材。本专题报道将审视这样的噩梦距离现实有多近。它将考察世界上淡水资源的现状以及对它不断增长的需求，并思索满足这些需求的方式。

The first thing to recognise is that the 70% figure is largely irrelevant to the debate. The sea it represents is salty, accounting for 97.5% of all the water on Earth. A further 1.75% is frozen, at the poles, in glaciers or in permafrost. So the world has to rely on just 0.75% of the planet’s available water, almost all of which is subterranean groundwater, though it is from the 0.3% on the surface that it draws 59% of its needs (see chart). This report will argue that misuse of water may indeed lead to a series of catastrophes. But the means to dodge them are already known, and new technologies are constantly evolving to help.

首先要认识到的是，70%这个数字与这场辩论无甚关系。它所代表的海洋是咸水，占地球上水总量的97.5%。另外还有1.75%冻结在地球两极、冰川或永冻层中。如此，全世界只能依靠其余的0.75%的水。这一部分几乎全是地下水，虽然位于地表的0.3%满足了59%的需求（见图表）。本报道将论述滥用水资源可能确实会引致一系列灾难。但是，避免它们的方法已经众所周知，而新技术也在不断演进以改善局面。

The fundamental problems, however, are neither the resource itself, since water is likely to remain abundant enough even for a more populous Earth, nor technical. They are managerial, or, more precisely, how to withstand economic, cultural and political pressures to mismanage water. In the harsh words of Asit Biswas, a water expert at the Lee Kuan Yew School of Public Policy in Singapore: “Lack of money, scarcity, and so on—they’re all excuses. The problem everywhere is bad management.” Or, as Jean-Claude Juncker, president of the European Commission, put it in an entirely different context: “We all know what to do, we just don’t know how to get re-elected after we’ve done it.”

然而，根本问题并不在于水资源本身，因为即使地球人口增加，水可能仍然足够丰富。问题也不在技术上。它们是管理上的，或者更确切地说，是如何抵御因经济、文化和政治方面的压力而错误地管理水资源。新加坡李光耀公共政策学院（Lee Kuan Yew School of Public Policy）的水资源专家阿西特·比斯瓦斯（Asit Biswas）言辞犀利地说道：“缺钱、水荒，诸如此类的，都是借口。所有地方的问题都是管理不善。”欧盟委员会主席让-克洛德·容克（Jean-Claude Juncker）则用了一种全然不同的表达：“我们都知道该做什么，我们只是不知道做完后还怎么能再当选。”

Even governments not facing the vexatious business of winning over voters struggle to institute sensible water policies. People regard access to water as a fundamental human right and hence as something that should be available on the basis of need, rather than the ability or willingness to pay. That makes it hard to charge a proper price for it, which in turn encourages profligate use. Even those who would be willing to curb their consumption for the benefit of generations to come may not be aware how much they are using. They consume it mostly not through drinking or washing, but through the water that has gone into the food they eat and the clothes they wear.

即使那些不用为争取选票伤脑筋的政府也难以制订出明智合理的水政策。民众把使用水视为一项基本人权，因而认为它应按需供应，而不是基于购买力或购买意愿。这就使得政府很难恰当地给水标价，而这继而又助长了浪费水的习惯。有些人愿意为了子孙后代的福祉节约用水，但他们仍然可能不清楚自己到底用了多少。他们消耗水主要不是饮用或洗涤，而是用来生产他们吃的食物和穿的衣服。

In any event, water seems an infinitely renewable resource. Used in a bath, it can be reused—to water plants, for example. Rainwater can be “harvested” or may seep into the ground to replenish an aquifer. Water that evaporates from lakes, swimming pools and reservoirs, or “transpires” in the photosynthetic process whereby water passes into the leaves of plants, joins the atmosphere and will eventually be recycled. Over 60% of the rain and snow that falls is returned in this way through “evapotranspiration”. But, like water that has run into the sea, it cannot be used again until nature has recycled it.

无论如何，水看起来是一种无限再生的资源。比如，洗澡用过的水可以用来浇花。雨水可以被“收集”或渗入地下而补充地下含水层。从湖泊、泳池和水库中蒸发出来的水，或者在水进入植物叶子的光合作用过程中“蒸腾”出来的水，都会进入大气中，最终构成循环。超过60%的降雨和降雪通过这种“蒸发蒸腾作用”返回大气中。但是，和那些流入了海洋中的水一样，在大自然完成循环之前，你无法再次使用它。

The present-day world provides ample examples of environmental devastation that serve as a warning that water usage has its natural limits. Boats are stranded aground in the middle of nowhere, amid the vanished waters of what was once the world’s fourth-largest saline lake, the Aral Sea, between Uzbekistan and Kazakhstan. Last year Cape Town in South Africa averted only narrowly the unwanted prize for being the first of the world’s big cities to run out of water. By the time rain finally broke a three-year drought, water levels in the reservoirs supplying the city had fallen to below 20%, and officials were discussing the feasibility of towing an iceberg from Antarctica to provide meltwater to drink. Four years earlier, it had been São Paulo in Brazil that had teetered on the brink, with reservoirs reduced to 5% of capacity.

当今世界提供了足够多的环境恶化实例，警告我们水的使用是有其自然限制的。在乌兹别克斯坦和哈萨克斯坦之间的咸海（Aral Sea）曾是世界第四大咸水湖，如今那里大片水域干涸消失，船只搁浅在无人之地。去年，南非开普敦差一点就摘得了一个它不想要的荣誉——成为世界上第一个无水可用的大城市。当雨水终于打破那里持续了三年的干旱天气时，为该市供水的水库的水位已经降到20%以下，当时官员们已经在讨论是否可以从南极拖一座冰山来，把冰融化来做饮用水。再往前四年，当时濒临同样绝境的城市是巴西的圣保罗，当地水库里的水已经只剩下5%。

Even the sober assessment of the UN’s latest annual world “water development report” smacks of a kind of desperation. Already, it notes, more than a quarter of humanity—1.9bn people, with 73% of them in Asia—live in areas where water is potentially severely scarce (up, other studies suggest, from 240m, or 14% of the world’s population, a century ago). The number facing shortages almost doubles if you count those at risk at least one month a year. Meanwhile, global water use is six times greater than it was a century ago—and is expected to increase by another 20-50% by 2050. The volume of water used—about 4,600 cubic kilometres a year—is already near the maximum that can be sustained without supplies shrinking dangerously. A third of the world’s biggest groundwater systems are in danger of drying out. So the numbers living under severe water stress are expected to climb to as many as 3.2bn by 2050, or 5.7bn taking seasonal variation into account. And they will not just be in poor countries (see map). Australia, Italy, Spain and even America will endure severe water shortage.

连联合国最新的年度《世界水发展报告》中的冷静评估也传递出一种绝望的气息。它指出，超过四分之一的人类——19亿人，其中73%在亚洲——已经生活在水资源面临严重稀缺的地区（其他研究表明一个世纪前这一数字为2.4亿，占世界人口的14%）。如果把那些一年中至少有一个月面临这种危险的地区也计入在内，那么这个数字几乎要翻一番。与此同时，全球用水量已达到一个世纪前的六倍，预计到2050年还将增加20%到50%。目前的用水量为每年约4600立方千米，已经接近引发水供应危险萎缩的阈值。世界上最大的地下水系统中有三分之一面临干涸的危险。因此，预计到2050年，生活在严重缺水压力下的人数将增至32亿，如果计入季节性变化则为57亿。而他们不只生活在贫穷国家（见地图）。澳大利亚、意大利、西班牙，甚至美国都将遭遇严重水荒。

Three main factors will drive the continued growth in demand: population, prosperity and climate change. In 2050 the number of people in the world is expected to increase to between 9.4bn and 10.2bn, from just under 8bn now. Most of the increase will come in parts of the world, in Africa and Asia, that are already short of water. People will be leading more water-intensive lifestyles and move into cities, many of them in places at great risk of water shortage.

三个主要因素将推动需求的持续增长：人口、经济繁荣和气候变化。到2050年，世界总人口预计将从目前的不到80亿增加到94亿至102亿。增加的大部分将来自非洲和亚洲目前已经水供应不足的地区。人们将过着更密集耗水的生活方式并移居城市，其中许多人生活在有严重缺水风险的地方。

The biggest uncertainty in projecting future demand lies in estimating how much will be needed for agriculture, which currently accounts for about 70% of water withdrawals, mostly for irrigation. Some forecast a big increase in demand, as food production has to rise to feed a growing population. Others, such as the OECD, have predicted a small decline in water use in irrigation thanks to a reduction in wastage and a rise in productivity.

预测未来需求的最大不确定性在于估算农业用水——目前占总用水量的70%左右，主要用于灌溉。一些人预测需求将大幅增加，因为粮食产量势必要上升才能满足不断增长的人口的需求。经合组织等其他机构则预测，由于减少浪费和生产率提高，灌溉用水量将小幅下降。

Still less predictable is the impact of climate change. The scientific consensus is that, in the words of Henk Ovink, the Dutch government’s special envoy on water matters, the process will be “like a giant magnifying glass, making all our challenges more extreme”. Wet places will become wetter and dry places drier. The world’s water endowment is already highly unequal—just nine countries account for 60% of all available fresh supplies. China and India have about 36% of the world’s people, but only about 11% of its freshwater. Climate change will exacerbate this inequity. And rainfall, such as the South Asian monsoons, on which much of subcontinental economic life hinges, will become more erratic.

更不可预测的是气候变化的影响。科学界有一个共识，用荷兰政府水务特使亨克·沃温卡（Henk Ovink）的话来说就是气候变化的过程将“像一个巨大的放大镜，让所有的挑战都变得更为极端”。潮湿的地方会更潮湿，干燥的地方更干燥。世界各地的水资源分布已经高度不平等——仅九个国家拥有占全球60%的淡水资源。中国和印度的人口占世界的36%，但淡水拥有量却只占约11%。气候变化将加剧这种不平等。而降雨会变得更不稳定，比如与南亚经济生活息息相关的南亚季风。

The most dramatic short-term effects have been the increasing number of extreme weather events. Over the past two decades these have affected on average about 300m people every year. Last September’s almost simultaneous storms—Hurricane Florence in the east of America, and super-Typhoon Mangkhut in East Asia—were linked by scientists to rising levels of greenhouse gases, warming oceans and changing climate. Measurements of sea temperatures down to 2,000 metres show a steady rise since the 1950s, to new records. Climate models have long forecast that warmer oceans will lead to more intense, longer-lasting storms. The rising temperatures are accompanied by rising sea levels—at a rate of about 3mm a year—as the warmer water expands, and as ice at both poles melts. Higher seas bring storm surges that can reach farther inland. And warmer air temperatures mean the atmosphere can hold more moisture that eventually falls as precipitation.

最具戏剧性的短期影响是极端天气事件的频率增加。过去20年中，这类事件平均每年影响约三亿人。科学家们认为去年9月几乎同时发生的两场风暴——美国东部的佛罗伦萨飓风和东亚的超强台风山竹——与温室气体浓度升高、海洋变暖及气候变化相关。在深海2000米处的测量显示，自上世纪50年代以来水温稳步上升至新高。长期以来，各种气候模型都预测更温暖的海洋将导致更强烈、持久的风暴。随着更温暖的海水膨胀，以及两极的冰融化，海平面也将随之上升——每年约3毫米。更高的海平面带来的风暴潮可以冲击更远的内陆地带。而更温暖的气温意味着大气中会含有更多水分，最终变成降水。

In the long run, however, the bigger problem from climate change will not be too much water but too little. As a report by the World Bank puts it: “The impacts of water scarcity and drought may be even greater, causing long-term harm in ways that are poorly understood and inadequately documented.” Of course, a lot depends on how much the climate changes and how fast.

然而，长远来看，气候变化带来的更大问题不是水太多而是水太少。正如世界银行的一份报告所说：“水荒和干旱的影响可能更大，而人们对它们如何造成长期损害知之甚少，也未能充分记录。”当然，这很大程度上取决于气候变化的幅度和速度。

Last October the Intergovernmental Panel on Climate Change published a report comparing the consequences of restraining global temperature rises to 1.5°C above pre-industrial levels as opposed to 2°C. It concluded “with medium confidence” that, with a 2°C rise, an additional 8% of the world’s population in 2000 will be exposed to new or aggravated water scarcity by 2050. With a 1.5°C rise, that falls to 4%. There would be considerable regional variation. For example, it cited research showing that, in the Mediterranean region, a 1.5°C rise in temperatures would lead to statistically insignificant changes in the mean annual flow in its rivers and streams. A 2°C rise, however, would bring decreases of 10-30%.

去年10月，政府间气候变化专门委员会（IPCC）发布了一份报告，比较了将全球气温升高控制在比工业化前高1.5°C和高2°C的后果。它以“中等置信度”得出结论称，如果温度上升2°C，到2050年，会有相当于2000年全球人口数8%的人面临新的或加剧的水资源短缺；如果温度上升1.5°C，则这一比例跌至4%。地区间差异会相当大。例如，它引用的研究表明，在地中海地区，气温上升1.5°C对其河流和溪流年平均水流量带来的变化在统计并不显著，但上升2°C却会令水量下降10%到30%。

Decreasing streamflow is a worldwide phenomenon. Some of it results from declining rainfall. But much is the direct result of human intervention—the damming and diversion of rivers for flood control, water-storage and irrigation. And, where rivers still flow, the water in them is often unsafe to drink or even bathe in. In surveying the Earth, surface water is an obvious place to start. As throughout this report, examples will be drawn worldwide, but especially from two countries with very contrasting experiences: Israel, which is sometimes held up as a model of sensible water management; and India, which almost never is.

河川径流量减少是一个全球现象。其中一些因降雨量减少造成。但很大一部分是人为干预的直接后果——为防洪、蓄水和灌溉而筑坝及让河流改道。此外，在依然水流潺潺的河道，水却往往不能饮用，甚至连洗澡都不安全。在勘察整个地球时，地表水是一个显而易见的起点。本报道将援引世界各地的实例，但会重点探讨两个经验截然相反的国家：有时被誉为合理管理水资源范本的以色列，以及几乎从来不曾如此的印度。