World Climate, Biomes and Grasslands

Weather is what the sky is doing today; climate is the average of weather over a long stretch, usually about 30 years. A place may have a hot, dry day in a rainy climate, but its climate does not change because of one day. To understand why Cairo is dry, Singapore is wet and Oymyakon is freezing, we do not memorise lists. We ask which controls push temperature and rainfall up or down, and then we group places that share the same answers into climate types.

Maritime climate means near the sea: mild winters, cool summers, a small annual range and rain through the year. Continental climate means deep inside a large landmass: bitter winters, hot summers, a very large annual range and most rain in summer. The bigger the landmass, the stronger the continental effect, which is why the Northern Hemisphere has far more of it than the Southern.

To avoid describing every town separately, scientists classify climates. The most famous scheme is the one by Wladimir Köppen, a German climatologist born in Russia. He first published it in 1884 and kept refining it for decades; later it was revised with Rudolf Geiger, which is why you may also see the name Köppen–Geiger. His key idea was clever: natural vegetation is a mirror of climate, so he used monthly temperature and rainfall (and their seasons) as numbers that match the vegetation zones. Each climate gets a code of letters.

The tropical (A) group is the hot half of the world: even the coolest month is 18 °C or more, so there is no winter in the real sense. What separates the three tropical climates is not heat but rain: the equatorial climate has rain all year, the monsoon climate has one heavy wet season with a short dry spell, and the savanna has a wet summer and a long dry winter. Keep this one idea and the vegetation of each will follow by itself.

The equatorial rainforest. Because the sun is overhead and the air is moist, the ground heats up each morning, the air rises and by afternoon there is a thunderstorm. The mean temperature stays near 27 °C all year, so night is the winter here: the difference between day and night is larger than between the hottest and coolest month. The main regions are the Amazon basin (South America), the Congo basin (Africa), and the islands and peninsulas of South-East Asia such as Malaysia and Indonesia. The Amazon is the world's largest tropical rainforest and lies across nine countries and territories, with roughly three-fifths of it in Brazil.

A rainforest is rich in trees but poor in soil. Nutrients are not stored in the ground; they are locked inside the living plants and recycled very fast. Heavy rain washes the rest away (leaching), leaving red, iron-rich laterite soil. So when the forest is cleared, farming fails within a few years.

The monsoon forests. In the tropical monsoon climate the rain is concentrated in summer, and there is a clear dry season. Trees survive the dry months by shedding their leaves, so the forest is called deciduous. The best-known trees are teak and sal; the typical regions are India, Bangladesh, Myanmar, Thailand and the rest of mainland South-East Asia. In India, moist deciduous forests receive about 100–200 cm of rain and dry deciduous forests about 70–100 cm.

The savanna. Here the sun's overhead position and the rain belt move north and south with the seasons, so the savanna gets the rain belt in summer and the dry trade winds in winter. The vegetation is tall grass (elephant grass can reach 3–4 m) with scattered flat-topped trees, and it supports the world's richest herds of large wild animals. Because the dry season is long, animals migrate in search of water and grass, and fire is a regular part of the landscape.

A desert is not defined by sand or heat but by dryness: it is a region where yearly rainfall is very low, usually under about 25 cm, and evaporation is far greater than rain. Most deserts are not seas of sand at all; large parts are bare rock or gravel. Some deserts are scorching, like the Sahara, and some are bitterly cold, like the Gobi and Antarctica. In Köppen's scheme they are the BW climates, and the semi-arid fringe around them is BS (steppe).

Life and weather in a hot desert. With almost no cloud and no moisture to hold heat, the sand burns by day and loses heat quickly at night, so the daily range can exceed 30 °C. Rain is rare and falls in short violent showers that cause flash floods in dry valleys (called wadis in Arabia and Africa). Plants are xerophytes: cactus, date palm and thorny shrubs with deep roots, small leaves and waxy skins. Animals such as the camel store fat and survive without water for long periods. People gather at oases, where groundwater reaches the surface. The hottest air temperature reliably recorded on Earth is 56.7 °C at Death Valley (USA) in 1913; the 58 °C once claimed for Libya was rejected by the WMO in 2012.

Desertification is different from natural deserts. It means land turning degraded and desert-like in dry areas, mainly because of human pressure such as overgrazing, cutting of trees, bad irrigation and over-ploughing, worsened by drought and climate change. The Sahel, the belt south of the Sahara, is the most-watched case, and the African Union's Great Green Wall aims to restore land across it. The UN Convention to Combat Desertification (UNCCD) was adopted in 1994, and 17 June is observed as the World Day to Combat Desertification and Drought. India hosted UNCCD COP14 in September 2019 at Greater Noida.

The warm temperate (C) group has mild winters (the coolest month between 18 °C and −3 °C) and a warm to hot summer. It is the climate where most of the world's people live well, because it is neither too hot nor too cold. Three types matter for exams, and the key to separating them is where on the continent they sit: the Mediterranean type on the west side in the subtropics, the China type (humid subtropical) on the east side in the subtropics, and the British type (marine west coast) on the west side in the middle latitudes.

Why the Mediterranean has wet winters and dry summers. It is the only major climate with this pattern, and it comes from the shifting of the pressure and wind belts with the sun. In summer the belts move poleward, so the Mediterranean lands lie under the dry, sinking air of the subtropical high. In winter the belts shift equatorward, and the westerlies with their cyclones bring rain.

What grows in a Mediterranean climate. Plants must survive a hot, dry summer, so they are sclerophyllous: evergreen, with small, thick, leathery or waxy leaves and deep roots, such as olive, cork oak, cypress and pine, along with aromatic shrubs. Because summers are bright and sunny, the region is famous for citrus fruits (oranges, lemons), grapes (so wine), figs and olives, which earned it the name 'orchard of the world'. Wheat is grown as a winter crop, using the winter rain.

The China type. On the east side of continents, warm moist air flows in from the ocean in summer, so summers are hot and humid and rain is heavy. In Asia this rain is monsoonal, with a dry winter (Cwa) as in the Ganga plain and southern China; in the south-eastern USA, south Brazil and eastern Australia rain falls all year (Cfa). Crops include rice, tea, cotton, tobacco and sugarcane, and these regions support very dense populations. Tropical storms such as typhoons (China, Japan) and hurricanes (south-east USA) are a regular hazard.

The British type. Winds from the sea (westerlies) blow over warm water, so winters are mild and summers cool, the annual range is small, and the sky is often cloudy. Because of the North Atlantic Drift, ports of Norway such as Narvik and Hammerfest stay ice-free in winter even far inside the Arctic Circle. The natural cover is deciduous forest of oak, beech and elm, but much of it has been cleared for pasture, so dairy farming and mixed farming are typical. Regions: north-western Europe (British Isles, France, the Low Countries, Denmark, Norway's coast), British Columbia and the Pacific north-west of North America, southern Chile, Tasmania and New Zealand.

Move north across Canada or Russia and you can almost read the climate zones off the ground: the temperate forest thins out, then comes a wide belt of dark conifers, then a treeless plain of moss and lichen, and finally ice. These are the cold climates: D (cold snow-forest, the taiga), ET (tundra) and EF (ice cap). Winters are long and severe, summers are short, and the growing season is brief. The big reason this group is a Northern Hemisphere story is simple: at these latitudes the Northern Hemisphere has huge landmasses, while the Southern Hemisphere has mostly ocean.

The taiga (boreal forest). The taiga is a belt of coniferous forest across northern North America (Canada, Alaska), Scandinavia and Siberia, and it is the world's largest land biome. Winters are long, snowy and bitterly cold; summers are short and cool to mild. This gives the largest annual temperature range on Earth: Verkhoyansk in Siberia swings more than 60 °C between its coldest and warmest months, and Oymyakon is among the coldest permanently inhabited places. The trees are softwoods such as pine, spruce, fir and larch, with needle-like leaves and sloping branches that shed snow. The soil is acidic, thin and poor (podzol).

The tundra and the ice cap. The tundra lies mainly along the Arctic Ocean coasts of North America and Eurasia, and in parts of Greenland. There are no trees because the summer is too short and cool and the soil is frozen below the surface (permafrost). During the brief summer the top layer thaws, forming boggy ground, and plants flower in a hurry. Where even the warmest month stays below freezing, we get the ice cap climate, found in the interior of Greenland and over Antarctica. Antarctica is the coldest, windiest and driest continent and also the one with the highest average elevation; the lowest temperature ever recorded on Earth, −89.2 °C, was at Vostok Station there in 1983.

Why do the poles have six months of day and six months of night? The Earth's axis is tilted by 23.5° and always points the same way as the Earth goes round the Sun. So for half the year a pole leans towards the Sun and has the midnight sun, and for the other half it leans away and has polar night. The Arctic Circle and Antarctic Circle (66.5° N and S) mark where at least one such day occurs each year. Latitude and tilt, not distance from the Sun, decide this.

Highland climate (H). On a mountain, temperature falls about 6.5 °C for every 1,000 m, so climbing a few kilometres is like travelling thousands of kilometres towards the pole. Vegetation therefore arranges itself in belts, one above another. This is why the tropical Andes and the Himalaya both show everything from tropical forest at the base to ice at the top. In the Andes the zones are named by height: tierra caliente (hot), tierra templada (temperate), tierra fría (cold) and tierra helada (frozen).

A biome is a very large community of plants and animals that lives in one type of climate and looks alike wherever it occurs, whether in Africa or South America. If you know the climate, you can predict the biome. The big land biomes are forest (tropical rainforest, tropical deciduous, temperate and boreal), grassland, desert and tundra. The vegetation that grew by itself, without human help, is called natural (virgin) vegetation; crops and orchards are cultivated vegetation, and they are not part of it.

Why grass and not forest? Grasslands occupy the middle ground in rainfall: roughly 25–75 cm a year, too much for a desert but too little (or too unevenly spread) for a forest. Grass also recovers quickly from grazing and fire, since its growing point is close to the ground. The two big families are the tropical grasslands (the savanna: warm all year, rain in summer) and the temperate grasslands (hot summers, cold winters, found in the interiors of continents, rain mostly in summer). The soil under temperate grassland, rich in humus, is among the most fertile in the world; the black earth of the steppe is called chernozem.