Water: Thirsty Planet

水：干渴的星球

As it sours 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.”

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.

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. 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.

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.”

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.考生如果怕自己錯過考試報名時間和考試時間的話，可以 免費預約短信提醒，屆時會以短信的方式提醒大家報名和考試時間。

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.

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.

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. Australia, Italy, Spain and even America will endure severe water shortage.

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.

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.

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.

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.

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.

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%.

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.

美國國家航空航天局(NASA)在宇宙中搜尋外星生命的跡象時有這樣一句座右銘兼使命宣言：“追蹤水。”人體約70%由水構成，它說，而地表的70%被水覆蓋。“水創(chuàng)造了一個維持和供養(yǎng)動植物及人類的環(huán)境，使地球完美地匹配一般意義上的生命。”

如果水是生命本身的代名詞，那么對地球上水質(zhì)和水供應量的擔憂會帶著世界末日的意味或許也就不足為奇了。未來，人們將在酷熱、干旱的氣候里為了幾滴水展開激烈、血腥的爭斗——這是反烏托邦小說和電影的標準題材。本專題報道將審視這樣的噩夢距離現(xiàn)實有多近。它將考察世界上淡水資源的現(xiàn)狀以及對它不斷增長的需求，并思索滿足這些需求的方式。

首先要認識到的是，70%這個數(shù)字與這場辯論無甚關系。它所代表的海洋是咸水，占地球上水總量的97.5%。另外還有1.75%凍結在地球兩極、冰川或永凍層中。如此，全世界只能依靠其余的0.75%的水。這一部分幾乎全是地下水，雖然位于地表的0.3%滿足了59%的需求。本報道將論述濫用水資源可能確實會引致一系列災難。但是，避免它們的方法已經(jīng)眾所周知，而新技術也在不斷演進以改善局面。

然而，根本問題并不在于水資源本身，因為即使地球人口增加，水可能仍然足夠豐富。問題也不在技術上。它們是管理上的，或者更確切地說，是如何抵御因經(jīng)濟、文化和政治方面的壓力而錯誤地管理水資源。新加坡李光耀公共政策學院(Lee Kuan Yew School of Public Policy)的水資源老師阿西特·比斯瓦斯(Asit Biswas)言辭犀利地說道：“缺錢、水荒，諸如此類的，都是借口。所有地方的問題都是管理不善。”歐盟委員會主席讓-克洛德·容克(Jean-Claude Juncker)則用了一種全然不同的表達：“我們都知道該做什么，我們只是不知道做完后還怎么能再當選。”

即使那些不用為爭取選票傷腦筋的政府也難以制訂出明智合理的水政策。民眾把使用水視為一項基本人權，因而認為它應按需供應，而不是基于購買力或購買意愿。這就使得政府很難恰當?shù)亟o水標價，而這繼而又助長了浪費水的習慣。有些人愿意為了子孫后代的福祉節(jié)約用水，但他們?nèi)匀豢赡懿磺宄约旱降子昧硕嗌佟Ｋ麄兿乃饕皇秋嬘没蛳礈?，而是用來生產(chǎn)他們吃的食物和穿的衣服。

無論如何，水看起來是一種無限再生的資源。比如，洗澡用過的水可以用來澆花。雨水可以被“收集”或滲入地下而補充地下含水層。從湖泊、泳池和水庫中蒸發(fā)出來的水，或者在水進入植物葉子的光合作用過程中“蒸騰”出來的水，都會進入大氣中，最終構成循環(huán)。超過60%的降雨和降雪通過這種“蒸發(fā)蒸騰作用”返回大氣中。但是，和那些流入了海洋中的水一樣，在大自然完成循環(huán)之前，你無法再次使用它。

當今世界提供了足夠多的環(huán)境惡化實例，警告我們水的使用是有其自然限制的。在烏茲別克斯坦和哈薩克斯坦之間的咸海(Aral Sea)曾是世界第四大咸水湖，如今那里大片水域干涸消失，船只擱淺在無人之地。去年，南非開普敦差一點就摘得了一個它不想要的榮譽——成為世界上第一個無水可用的大城市。當雨水終于打破那里持續(xù)了三年的干旱天氣時，為該市供水的水庫的水位已經(jīng)降到20%以下，當時官員們已經(jīng)在討論是否可以從南極拖一座冰山來，把冰融化來做飲用水。再往前四年，當時瀕臨同樣絕境的城市是巴西的圣保羅，當?shù)厮畮炖锏乃呀?jīng)只剩下5%。

連聯(lián)合國最新的年度《世界水發(fā)展報告》中的冷靜評估也傳遞出一種絕望的氣息。它指出，超過四分之一的人類——19億人，其中73%在亞洲——已經(jīng)生活在水資源面臨嚴重稀缺的地區(qū)(其他研究表明一個世紀前這一數(shù)字為2.4億，占世界人口的14%)。如果把那些一年中至少有一個月面臨這種危險的地區(qū)也計入在內(nèi)，那么這個數(shù)字幾乎要翻一番。與此同時，全球用水量已達到一個世紀前的六倍，預計到2050年還將增加20%到50%。目前的用水量為每年約4600立方千米，已經(jīng)接近引發(fā)水供應危險萎縮的閾值。世界上最大的地下水系統(tǒng)中有三分之一面臨干涸的危險。因此，預計到2050年，生活在嚴重缺水壓力下的人數(shù)將增至32億，如果計入季節(jié)性變化則為57億。而他們不只生活在貧窮國家。澳大利亞、意大利、西班牙，甚至美國都將遭遇嚴重水荒。

三個主要因素將推動需求的持續(xù)增長：人口、經(jīng)濟繁榮和氣候變化。到2050年，世界總人口預計將從目前的不到80億增加到94億至102億。增加的大部分將來自非洲和亞洲目前已經(jīng)水供應不足的地區(qū)。人們將過著更密集耗水的生活方式并移居城市，其中許多人生活在有嚴重缺水風險的地方。

預測未來需求的最大不確定性在于估算農(nóng)業(yè)用水——目前占總用水量的70%左右，主要用于灌溉。一些人預測需求將大幅增加，因為糧食產(chǎn)量勢必要上升才能滿足不斷增長的人口的需求。經(jīng)合組織等其他機構則預測，由于減少浪費和生產(chǎn)率提高，灌溉用水量將小幅下降。

更不可預測的是氣候變化的影響?？茖W界有一個共識，用荷蘭政府水務特使亨克·沃溫卡(Henk Ovink)的話來說就是氣候變化的過程將“像一個巨大的放大鏡，讓所有的挑戰(zhàn)都變得更為極端”。潮濕的地方會更潮濕，干燥的地方更干燥。世界各地的水資源分布已經(jīng)高度不平等——僅九個國家擁有占全球60%的淡水資源。中國和印度的人口占世界的36%，但淡水擁有量卻只占約11%。氣候變化將加劇這種不平等。而降雨會變得更不穩(wěn)定，比如與南亞經(jīng)濟生活息息相關的南亞季風。

最具戲劇性的短期影響是極端天氣事件的頻率增加。過去20年中，這類事件平均每年影響約三億人?？茖W家們認為去年9月幾乎同時發(fā)生的兩場風暴——美國東部的佛羅倫薩颶風和東亞的超強臺風山竹——與溫室氣體濃度升高、海洋變暖及氣候變化相關。在深海2000米處的測量顯示，自上世紀50年代以來水溫穩(wěn)步上升至新高。長期以來，各種氣候模型都預測更溫暖的海洋將導致更強烈、持久的風暴。隨著更溫暖的海水膨脹，以及兩極的冰融化，海平面也將隨之上升——每年約3毫米。更高的海平面帶來的風暴潮可以沖擊更遠的內(nèi)陸地帶。而更溫暖的氣溫意味著大氣中會含有更多水分，最終變成降水。

然而，長遠來看，氣候變化帶來的更大問題不是水太多而是水太少。正如世界銀行的一份報告所說：“水荒和干旱的影響可能更大，而人們對它們?nèi)绾卧斐砷L期損害知之甚少，也未能充分記錄。”當然，這很大程度上取決于氣候變化的幅度和速度。

去年10月，政府間氣候變化專門委員會(IPCC)發(fā)布了一份報告，比較了將全球氣溫升高控制在比工業(yè)化前高1.5°C和高2°C的后果。它以“中等置信度”得出結論稱，如果溫度上升2°C，到2050年，會有相當于2000年全球人口數(shù)8%的人面臨新的或加劇的水資源短缺;如果溫度上升1.5°C，則這一比例跌至4%。地區(qū)間差異會相當大。例如，它引用的研究表明，在地中海地區(qū)，氣溫上升1.5°C對其河流和溪流年平均水流量帶來的變化在統(tǒng)計并不顯著，但上升2°C卻會令水量下降10%到30%。

河川徑流量減少是一個全球現(xiàn)象。其中一些因降雨量減少造成。但很大一部分是人為干預的直接后果——為防洪、蓄水和灌溉而筑壩及讓河流改道。此外，在依然水流潺潺的河道，水卻往往不能飲用，甚至連洗澡都不安全。在勘察整個地球時，地表水是一個顯而易見的起點。本報道將援引世界各地的實例，但會重點探討兩個經(jīng)驗截然相反的國家：有時被譽為合理管理水資源范本的以色列，以及幾乎從來不曾如此的印度。

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