Population and urbanisation：全部卡片

Urbanisation

提示The countryside empties as the built-up areas swell.

为什么It is measured as the percentage of people living in urban areas, and worldwide that share passed half around 2007. Cities grow both by people arriving and by the residents already there having children.

Flat lowland

提示Easier to farm, easier to build on, easier to cross.

为什么Gentle relief, deep soil, a mild climate and fresh water draw people in; deserts, high mountains, dense rainforest and frozen tundra push them away. Half the world lives on a small fraction of the land surface.

A fording point

提示Every road funnels towards the one shallow place travellers could wade over.

为什么Oxford grew at exactly such a crossing, because all the traffic converged there. Other classic site factors are a wet point spring line, a dry point above the flood, and a defensive hilltop or meander loop.

Natural change

提示It ignores anyone moving in or out; only births and deaths count.

为什么If births outnumber deaths the figure is positive and the population grows with nobody arriving at all. Migration is counted separately, and the two added together give the total.

Stage 2

提示Clean water and medicine arrive well before family sizes shrink.

为什么Better sanitation, food and medicine cut deaths sharply while families are still large, so the population booms. Births only fall at the next step, as children survive, women work and education spreads.

push

提示A reason to leave, not a reason to arrive.

为什么War, drought, unemployment and persecution drive people away. Jobs, safety, schools and relatives already settled draw them somewhere else. Most journeys are a mixture of both halves.

Rural-urban migration

提示The one-way journey from field to factory.

为什么It is the single biggest driver of city growth in lower-income countries. Poor harvests and low wages push people out, while jobs, schools and hospitals pull them in.

Ten million

提示Roughly the population of Portugal or Greece, packed into one place.

为什么The United Nations sets the bar there. Tokyo, Delhi and Shanghai lead the list, and almost every new one is appearing in Asia or Africa rather than Europe.

An informal settlement

提示With no legal deeds, the city will not lay on pipes or power.

为什么Dharavi in Mumbai and Rocinha in Rio grew where the city gained people faster than it built homes. What residents lack is secure land title, clean water and sanitation - not skill or effort.

The Human Development Index

提示The UN publishes it each year, scored from zero to one.

为什么Income on its own can hide huge inequality, so this score adds life expectancy and years of schooling. A figure near 1 means long, educated, prosperous lives; near 0 the opposite.

The development gap

提示The distance between the two ends of the world's wealth ladder.

为什么It is not random: colonial history, debt, climate, being landlocked and the low prices paid for raw exports all feed it. The same divide exists inside countries, between a capital city and its rural regions.

Emergency aid

提示Blankets, clean water and medics within days, not a school built over years.

为什么It saves lives but changes nothing once it stops. Long-term projects build wells, clinics and training, while fairer trade prices and debt relief move far more money than donations ever do.