Resources and environment

Non-renewable

HintIt took millions of years to form, so once it is burnt there is no more.

The whyCoal, oil, gas and metal ores form far slower than we use them. Wind, sunlight and flowing water replace themselves constantly, and timber does too - but only while it is replanted as fast as it is cut.

The flow of energy

HintIt points towards whoever gains from the meal.

The whyArrows run from the eaten to the eater, tracing where the meal goes. Producers such as grass capture sunlight, and every consumer above them is spending what the sun supplied.

Freeze-thaw weathering

HintIce takes up more room than the water it came from, so it prises the crack wider.

The whyWater expands by about a tenth as it turns to ice, wedging cracks open a fraction each night until a fragment splits away. It is why British uplands are littered with scree.

Humus

HintThe dark, crumbly matter that makes good topsoil rich.

The whySoil is a mixture of weathered rock fragments, rotted remains, water, air and living organisms. The rotted part stores nutrients and holds moisture, which is why losing topsoil is so serious.

The canopy

HintA continuous green roof about thirty metres above the ground.

The whyIt intercepts most of the light and rain, leaving the forest floor dark and still. A few emergents push through above it, and the shrub layer sits below.

Poor

HintAlmost everything the forest owns is locked up in its trees, not the ground.

The whyHeavy rain washes nutrients out, and what remains is recycled straight from fallen leaves back into the roots. Clear the trees and the fertility leaves with them after a few harvests.

Cattle ranching

HintThe trees are burnt to open up grazing for herds destined to be beef.

The whyRoughly two-thirds of cleared Amazon land ends up as pasture, with soya, logging, mining and road building behind it. Because the soil is poor, ranchers move on and clear more after a few years.

Carbon dioxide

HintThe same gas that fizzes in lemonade and that every animal breathes out.

The whyLiving trees hold huge amounts of carbon, and burning them releases it as this gas, so deforestation causes roughly a tenth of global emissions. The cleared land also stops recycling rainfall, and the region dries out.

fossil fuels

HintBuried sunlight from millions of years ago, dug up and burnt.

The whyThey are the compressed remains of ancient plants and plankton, so burning them releases carbon locked away for hundreds of millions of years. Wind, solar and hydro harvest energy arriving today instead.

Economic water scarcity

HintThe rain falls; the money to store and pipe it does not exist.

The whyPhysical shortage means the supply simply is not there, as across the Sahel. The other kind means it is there but unreachable, which is common in central Africa - a poverty problem rather than a rainfall problem.

Intensive farming

HintSqueezing a bigger harvest from every hectare rather than clearing more hectares.

The whyThese methods fed the population boom of the last century, but they cost energy, water and soil health, and fertiliser run-off pollutes rivers. Cutting waste is cheaper - about a third of the food grown is never eaten.

The primary sector

HintThe first stage: taking raw materials straight from the land or sea.

The whyThis work extracts raw materials, the secondary sector manufactures, the tertiary provides services and the quaternary handles research and information. As a country develops its workers shift towards the last two.