Global water distribution and scarcity
FoundationAbout 97.5% of Earth's water is salty and only 2.5% fresh; of that, nearly 70% is locked in glaciers and about 30% underground, so under 1% is readily available. Some 2.2 billion people lack safe drinking water, with the Middle East and North Africa under the greatest stress.
- 97.5% of Earth's water is salty; only 2.5% is fresh
- About 69% of fresh water is locked in glaciers and 30% underground; under 1% is directly usable
- Scarcity is physical or economic — different causes, different remedies
文體小知識：數據解讀課：先記住「97.5%鹹水、2.5%淡水」這組關鍵數字，再學會用「水資源壓力」的三條線衡量一個地區缺不缺水。
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1Earth is the 'blue planet', with water covering about 70% of its surface — but most of it is undrinkable: about 97.5% of the world's water is salty and only 2.5% is fresh. Of that fresh water, about 68.7% is locked in glaciers and ice sheets at the poles and on high mountains, about 30.1% lies underground, and less than 1% sits in rivers and lakes. The water we can actually reach is a tiny fraction of the whole.
2There are two kinds of scarcity. Physical scarcity means a place has little rain and few rivers, as in the Sahara and the semi-arid Middle East; economic scarcity means water is not scarce in total, but population, pollution and waste put heavy pressure on it, as in fast-growing cities and industrial regions. Both raise 'water stress'. The UN measures it by renewable water per person per year: below 1,700 cubic metres is stress, below 1,000 is scarcity, and below 500 is absolute scarcity.
3About 2.2 billion people lack safely managed drinking water, worst in sub-Saharan Africa and South Asia; the Middle East and North Africa face the world's highest water stress — Egypt's per-person resources sit near the absolute-scarcity line, and Saudi Arabia has almost no permanent rivers. Shortage ripples outward: failed harvests, sickness (unsafe water makes diarrhoea a leading killer of children), and hours spent each day fetching water that keeps women and children out of school and work.
4Demand keeps rising: population growth, urban sprawl, changing diets (meat takes far more water than grain) and pollution all squeeze fresh water, and climate change makes rainfall less reliable. This lesson maps the problem; the next looks at how people compete for it.
1Summary 重點摘要
About 97.5% of Earth's water is salty and only 2.5% fresh; of that, nearly 70% is locked in glaciers and about 30% underground, so under 1% is readily available. Some 2.2 billion people lack safe drinking water, with the Middle East and North Africa under the greatest stress.
2Vocabulary cards Vocabulary
Water of the oceans and salt lakes, about 97.5% of all water — undrinkable and unfit for irrigation.
Water with very little salt — just 2.5% of the total, held in glaciers, groundwater, rivers and lakes.
Water stored in rock and soil, about 30% of fresh water and a major source of drinking and irrigation supply.
When demand approaches or exceeds available supply; the UN marks stress below 1,700 cubic metres of renewable water per person a year.
Natural shortage caused by dry climates and few rivers, as in the Middle East and the Sahara.
Water exists, but poverty, pollution or poor infrastructure keeps people from safe supply.
Water replenished yearly by the water cycle — the measure of each region's supply potential.
A region's total water divided by its population — the more people, the less each has.
The direst level of shortage: under 500 cubic metres of renewable water per person a year.
3Step by step Steps
- Para 1: the global water ledgerEarth looks watery, yet usable fresh water is tiny: about 97.5% of it is seawater and only 2.5% fresh; of that fresh water, about 68.7% is locked in glaciers, 30.1% lies as deep groundwater, and rivers and lakes together hold under 1%. If all the world's water were 100 cups, the river and lake water we could scoop would fill less than 0.03 of a cup. Read this ledger and the value of fresh water — and the urgency of protecting it — becomes clear.
- Para 2: two kinds of scarcityScarcity comes in two kinds. Physical scarcity means nature provides little — the Middle East is the classic case; economic scarcity means water exists but pollution, waste or weak infrastructure keep it out of reach — the Congo basin is drenched, yet some cities lack treatment plants and pipes, and residents still buy water by the container. Classifying the type is the first step in any answer, because different causes demand entirely different remedies.
- Para 2: the water-stress rulerThree internationally used lines measure water stress: renewable water below 1,700 cubic metres per person a year means stress; below 1,000, scarcity; below 500, absolute scarcity. Egypt sits near 500-plus cubic metres, close to the absolute line, while the Middle East and North Africa top the world stress table. With this ruler, any country's water position can be quantified and compared, giving answers a firmer footing.
- Para 3: who suffers mostAbout 2.2 billion people lack safely managed drinking water. In some rural parts of sub-Saharan Africa, women and children spend hours a day fetching water, crowding out school and work; unsafe sources make diarrhoea a leading killer of children under five. Scarcity is therefore not only a resource issue but a development one, touching health, education and gender equality in an unbroken chain.
- Para 4: why demand keeps climbingDemand climbs on four forces: population growth widens the base; urbanisation concentrates water use; diet shifts run deep — a kilogram of beef takes about 15,000 litres of water against about 1,500 for wheat, so the spread of meat-heavy diets multiplies the pressure; pollution shrinks usable supply. Climate change adds a wild card, scrambling rainfall patterns and making supply less stable, so the gap between supply and demand keeps widening.
- Method: answering scarcity questions in three stepsAnswer scarcity questions in three steps: distribution (which regions run short), type (physical or economic scarcity) and impacts (farming, health and education). Take the Middle East: physical scarcity limits farming, deepens reliance on food imports, and makes water a sensitive geopolitical issue. With all three steps present, the answer is complete and three-dimensional.
4Diagrams Diagrams
5Worked example Example
Example: compare why the Sahara and an Indian megacity such as Delhi run short of water.
- Type: the Sahara faces physical scarcity — under 100 mm of rain a year across much of it, and almost no permanent rivers
- Delhi is mainly economic scarcity — monsoon totals are decent, but 20 million-plus people, over-pumped groundwater, leaking pipes and pollution strain supply
- Compare impacts: both ration or buy water, but the Sahara lacks water itself while Delhi lacks management and fair distribution
- Different remedies: the Sahara needs transfers and desalination (new supply); Delhi needs pipe repair, sewage treatment and pricing (saving and governance)
- Answer: The Sahara is physically short (rain and rivers are naturally scant); Delhi is mostly economically short (population, pollution, over-pumping, management). Different causes, different cures — new supply for one, saving and governance for the other. Tip: classify the scarcity first, then match remedies.
6Common mistakes Mistakes
7Quick check Quick check
Q1What share of Earth's water is salty and what share is fresh?
Q2Where is fresh water mainly stored?
Q3How does the UN define water scarcity?
Q4Distinguish physical and economic scarcity, with one example each.
Q5Roughly how many people lack safely managed drinking water?
8Recap Recap
- 97.5% of Earth's water is salty; only 2.5% is fresh
- About 69% of fresh water is locked in glaciers and 30% underground; under 1% is directly usable
- Scarcity is physical or economic — different causes, different remedies
- The UN's three lines: under 1,700 cubic metres per person a year is stress, 1,000 scarcity, 500 absolute scarcity
- About 2.2 billion people lack safe drinking water; the Middle East and North Africa face the greatest stress