Climate Zones and Biomes of the Earth

Weather, Climate and the Patterns of Earth

Why is the Amazon hot and dripping wet, while the Sahara just across the Atlantic is baking and bone dry? Why do penguins live in one part of the world and palm trees in another? The answers lie in the great patterns of climate that wrap around our planet — and in this book we are going to map them.

First, an important difference. Weather is what the air is doing right now: sunny today, rainy tomorrow, windy next week. Climate is the bigger picture — the usual pattern of weather a place has over many, many years. One cold day does not make a cold climate, and one hot afternoon does not make a desert. Climate is weather's long-term character.

When geographers look at the whole Earth, they see that climate falls into broad zones, and that each zone grows its own community of plants and animals, called a biome. By the end of this book you will understand why each biome sits where it does, and why a polar bear could never live in a rainforest, nor a parrot in the tundra.

Why Some Places Are Hot and Others Cold

The single biggest reason climates differ is the angle of the Sun, and it all comes down to where you are between the Equator and the poles — your latitude.

Near the Equator, the Sun is high overhead and its rays strike the ground almost straight down. That concentrated sunlight delivers a lot of heat to a small patch of land, so these regions are warm all year. Near the poles, the Sun is always low in the sky, so the same rays arrive at a slant and spread thin across a much larger area, like a torch shone sideways. That is why the poles are cold and the tropics are hot.

This gives the Earth three broad temperature bands:

• The tropical zone, hot all year, near the Equator.
• The temperate zones, with warm summers and cool winters, between the tropics and the poles.
• The polar zones, cold all year, at the very top and bottom of the world.

Latitude is the master switch of climate. But it is not the only thing that matters.

The Other Climate-Makers: Oceans, Mountains and Wind

If latitude were the whole story, every place at the same distance from the Equator would have the same climate. They do not, and three other forces explain why.

Oceans act like giant storage heaters. Water warms up and cools down far more slowly than land, so places near the sea have milder, steadier climates — cooler summers and warmer winters — than places deep inside a continent, which roast in summer and freeze in winter.

Mountains change climate too. As air is forced to rise over a mountain range, it cools and drops its rain on the windward side, leaving the far side dry. This is why one slope of a mountain can be green and forested while the other is a parched rain shadow desert.

Wind and ocean currents carry warmth around the globe. Warm currents can give a chilly region a surprisingly mild climate, while cold currents can keep a coast cool and foggy. Together, latitude, oceans, mountains and winds decide the climate of every spot on Earth — and that climate decides the biome.

What Is a Biome?

A biome is a large region of the world defined by its climate and the whole community of living things that has grown up to suit it. The plants and animals of a biome are not there by accident — over countless generations they have become adapted, beautifully suited to the conditions, whether that means surviving drought, cold, or constant rain.

The wonderful thing about biomes is that they appear in similar places all around the globe. Wherever the climate is hot and wet, you find rainforest. Wherever it is dry, you find desert. The animals may differ from continent to continent, but the type of biome repeats. Let's travel through the great biomes of the Earth, from the steamy Equator to the frozen poles.

Tropical Rainforests: Hot, Wet and Bursting With Life

Near the Equator, where it is warm and rainy all year, grow the tropical rainforests. These are the richest biomes on the planet, home to more kinds of plants and animals than anywhere else on Earth.

Rain falls almost daily, and the warmth never lets up, so plants grow with incredible energy. The tallest trees spread their leaves into a green roof called the canopy, and beneath it a tangle of vines, ferns and smaller trees fights for the scraps of light that filter through. Jaguars, monkeys, sloths, brilliant birds and countless insects make their homes at every level.

The greatest rainforest of all is the Amazon in South America. Rainforests are sometimes called the "lungs of the Earth" because their billions of trees help clean our air. To explore this biome up close, read the companion book Life in the Rainforest.

Deserts: Lands of Little Rain

A desert is not defined by heat but by dryness. A desert is simply a place that gets very little rain — less than a small amount each year. Many deserts, like the Sahara, are scorching by day; but some, like the cold deserts of Central Asia or even icy Antarctica, are freezing.

Deserts often form where dry, sinking air keeps the skies clear, or in the rain shadow behind mountains. Life here must master the art of saving water. Cacti store water in fat, waterproof stems; many desert animals hide from the heat by day and come out only in the cool of night; some can go for astonishing lengths of time without a drink.

Far from being empty, healthy deserts are full of cleverly adapted life. To discover how plants and animals survive these harsh lands, see The World's Deserts.

Grasslands: Seas of Waving Grass

Between the wet rainforests and the dry deserts lie the grasslands — wide open regions where there is enough rain for grass to thrive but too little, or too many fires, for thick forest to take over. Grasses roll on for as far as the eye can see, like a green or golden sea.

Tropical grasslands near the Equator are called savannas. The great African savanna is one of the most famous places on Earth for wildlife, home to grazing herds of zebras, wildebeest and antelope, hunted by lions, cheetahs and wild dogs. In cooler parts of the world, temperate grasslands like the North American prairies and the Asian steppes once thundered with bison and wild horses.

Grasslands are also the world's great farmlands. The deep, rich soils that grow wild grass also grow our wheat, maize and grazing cattle, so this biome helps feed much of humanity.

Forests of the Temperate and Cold Lands

Move away from the Equator into the temperate zones and you reach great forests that change with the seasons.

In temperate deciduous forests, found in places like Europe, eastern North America and East Asia, broad-leaved trees such as oak, beech and maple burst into leaf in spring, give shade in summer, blaze orange and gold in autumn, and stand bare in winter. Deer, foxes, owls and bears live among them.

Further north stretches a colder, darker forest of conifers — pines, spruces and firs with needle-like leaves — known as the taiga or boreal forest. It forms a vast green belt across the top of the world and is home to wolves, moose, lynx and bears. Conifers keep their needles all year and are shaped like cones so snow slides off their branches. To learn how forests work as living communities, dip into The Secret Life of Trees.

Tundra and the Frozen Edge

At the very edges of the land — in the far north and high on tall mountains — lies the tundra, the coldest biome where plants can still grow. Here it is too cold for trees, so the tundra is open and treeless, carpeted with tough mosses, lichens, grasses and tiny flowers.

Beneath the surface, the ground stays frozen solid for much or all of the year. This permanently frozen soil is called permafrost. For a few short weeks of summer, the top thaws and the tundra bursts briefly into life, with flowers blooming and clouds of insects feeding migrating birds.

Animals here are built for cold: reindeer, Arctic foxes, hares and musk oxen, all wrapped in thick fur. Beyond the tundra, where even hardy plants cannot survive, lie the permanent ice of the polar regions. You can explore that frozen world in The Arctic and Antarctica.

What We Have Learned

We have circled the planet through its great patterns of life. We learned the difference between weather and climate, and that the angle of the Sun — set by your latitude — is the master switch that makes the tropics hot and the poles cold. We saw how oceans, mountains and winds fine-tune the climate everywhere.

Then we journeyed through the great biomes: the steaming rainforests, the dry deserts, the rolling grasslands, the seasonal temperate forests and cold taiga, and finally the frozen tundra. In each one, the plants and animals are perfectly adapted to their home.

Now, whenever you look at a world map, you can read it like a story — and explain why each region looks, feels and lives the way it does.

Keep exploring the living world: walk into the green in Life in the Rainforest, or cross the dry lands in The World's Deserts.