Appearance
2.7 — Climate Zones and Biomes
A kangaroo, a bison, an antelope and a blackbuck have no close family relationship. But put photographs of their habitats side by side — the Australian interior, the American Great Plains, the African savanna, the Deccan grasslands — and the landscapes look like variations of one place: waist-high grass, scattered trees, a long dry season, large fast herbivores and something that hunts them.
Climate builds the same kind of ecosystem out of whatever species are available. That is why biomes repeat across continents that have been separated for a hundred million years, and it is the single most useful pattern in physical geography.
Weather, climate, and the two numbers that matter
Weather is what the atmosphere is doing now. Climate is the statistics of weather over decades — conventionally a thirty-year average, plus the variability around it. "It rained today" is weather. "This district averages 800 millimetres, almost all of it in four months, with a one-in-five chance of a year below 600" is climate, and it is the second statement that decides what can be farmed there.
Almost all of climate classification comes down to two things: how much heat, and how much water, and — crucially — when.
The when is not a refinement. Two places with 700 millimetres a year are completely different if one gets it spread evenly and the other gets all of it in ten weeks. Mediterranean climates get their rain in winter, when plants are least able to use it, which is why their vegetation is tough, small-leaved and drought-adapted despite reasonable annual totals.
The Köppen system
Wladimir Köppen, a Russian-German botanist and climatologist, published his classification in 1884, and it is still the standard. His insight was to classify climate by the vegetation it produces, since plants integrate temperature and rainfall over the whole year automatically — the plants are the instrument.
The system uses letters. The first is the main group.
A — Tropical. Every month averages above 18 °C. Subdivided by rainfall: Af rainforest, wet all year; Am monsoon, a short dry season but a very wet wet season; Aw savanna, a long dry season. Kerala's coast is Am. Most of central India is Aw.
B — Arid. Defined by evaporation exceeding precipitation, so the threshold depends on temperature — a cool place needs less rain to avoid being classed arid. BW is desert, BS is steppe or semi-arid. The Thar is BW; much of Rajasthan, Gujarat and interior Deccan is BS.
C — Temperate. Coldest month between −3 and 18 °C. Includes Cfa humid subtropical (much of eastern China, the American southeast, and north India's plains by some versions), Cfb oceanic (Britain, New Zealand), and Csa/Csb Mediterranean, with its dry summer.
D — Continental. Coldest month below −3 °C, warmest above 10 °C. Cold winters, warm summers, big annual range. Most of Russia, Canada, the northern United States, northeast China. There is essentially no D climate in the southern hemisphere, because there is no land at those latitudes — which is a nice demonstration that climate zones are about geography, not just latitude.
E — Polar. Warmest month below 10 °C, which is the treeline threshold. ET tundra, EF ice cap.
Highland climates are sometimes given their own group, because altitude compresses the entire latitudinal sequence into a few thousand metres. Going up a tropical mountain takes you through the equivalent of travelling to the pole, which is why the slopes of Kilimanjaro run from savanna through rainforest and moorland to permanent ice within about 5,000 metres of altitude, and why Ooty and Shimla are cool towns at latitudes that are otherwise hot.
The biomes, and why each is where it is
Tropical rainforest. Equatorial belt, where the tropical rain band sits most of the year (Chapter 2.2). Warm and wet all year, so growth never stops. The greatest biodiversity on Earth — a few hectares of Amazon forest can hold more tree species than all of Europe. And the soil is terrible, which surprises people. In continuous warmth and heavy rain, dead material is decomposed within weeks and the nutrients are taken straight back up by roots, while the rain leaches everything else away. The nutrients are in the living vegetation, not in the ground. That is why slash-and-burn farming there works for two or three years and then fails, and why clearing rainforest for agriculture yields poor land.
Tropical savanna. Just poleward of the rainforest, where the rain band visits for part of the year. Grass with scattered fire-tolerant trees. Fire is not a disturbance here, it is part of the system — regular burning kills tree seedlings and keeps the grassland open, and the grasses grow back from underground bases. This is where the great herds live, and — as Chapter 3.2 covers — it is where humans evolved.
Hot desert. The 30-degree belt, plus rain shadows and cold-current coasts. Plants solve the water problem in one of three ways: store it (cactus, and in the Old World the unrelated but similar-looking euphorbias), reach for it with very deep roots, or avoid the problem by living as seeds and completing a whole life cycle in the weeks after a rain.
Mediterranean scrub. West-facing coasts around 30–40 degrees, where the desert belt sits over them in summer and the westerlies reach them in winter. Hot dry summer, mild wet winter. Small tough evergreen leaves to survive the drought, and vegetation adapted to periodic fire. Found in only five places on Earth — the Mediterranean basin, California, central Chile, the South African Cape and southwestern Australia — and two of them, the Cape and southwestern Australia, are among the most species-rich plant regions in the world.
Temperate grassland. Continental interiors, too dry for forest and too wet for desert. Prairie, steppe, pampas, veld. These have the best soils on the planet, because grasses put most of their growth into roots that die and rot in place, building deep dark humus-rich earth over thousands of years. Every one of them has been ploughed up, and they are now the world's breadbaskets: American Midwest, Ukrainian and Russian black earth, Argentine pampas, northern China.
Temperate forest. Mid-latitudes with adequate rain year-round. Deciduous trees drop their leaves because in winter the roots cannot draw water from cold or frozen soil, so a broad leaf becomes a liability. Almost all of it in Europe, eastern China and eastern North America has been cleared at some point for farming, and much of what stands today is regrowth.
Boreal forest, or taiga. A belt across Canada, Scandinavia and Siberia — the largest land biome on Earth. Conifers, because a needle loses little water, sheds snow, and keeps its photosynthetic machinery ready for a very short growing season. Enormous carbon stores in the soil and in permafrost beneath, which is why this belt matters to Part 14 out of proportion to its productivity.
Tundra. Beyond the treeline. Permafrost — ground frozen for at least two consecutive years — prevents deep roots and blocks drainage, so the summer surface is waterlogged. Vegetation is mosses, lichens, sedges and dwarf shrubs. Decomposition is so slow that dead material accumulates, which is why tundra soils hold vast quantities of carbon.
India as a demonstration
India is unusually useful for this chapter because it contains almost every one of these zones within one country, and the reasons for each are visible.
- Tropical wet: the Western Ghats windward slopes and the northeast — orographic lifting of monsoon flow (Chapter 2.5).
- Tropical savanna: most of central and peninsular India — a strongly seasonal monsoon with a long dry season.
- Arid and semi-arid: the Thar and western Rajasthan — the Aravalli range runs parallel to the monsoon flow rather than across it, so it does not force the air up, and the region sits at the far dry end of the Arabian Sea branch.
- Humid subtropical: the Gangetic plain — monsoon rain plus cool winters with western disturbances.
- Highland and alpine: the Himalayan slopes, running through every zone with altitude.
- Cold desert: Ladakh and Spiti — in the Himalaya's rain shadow, high, and cut off from the monsoon entirely.
And the population map follows it. Bihar, Uttar Pradesh, West Bengal and Kerala are dense because they are wet and flat or wet and fertile. Rajasthan is sparse because it is dry. A map of Indian population density is close to a map of reliable water, and Chapter 6.1 builds Indian history on that observation.
Microclimate: the scale you actually live in
The global map is not what you experience. Local conditions vary enormously over metres.
Cities are hotter than their surroundings — the urban heat island effect — typically by 2 to 5 °C at night, sometimes more. The causes are all physical: dark asphalt and concrete absorb more sunlight than vegetation; those materials store heat during the day and release it at night; there is little evaporation from plants and soil to cool the air; tall buildings trap radiation in street canyons; and vehicles, air conditioners and industry add heat directly. Delhi's summer nights are measurably hotter than the surrounding countryside, and since heat deaths are strongly driven by night-time temperatures that fail to give the body a break, this is a public health matter, not a curiosity.
Slope aspect matters at village scale. In the northern hemisphere a south-facing slope receives far more sun than a north-facing one, so it is warmer, drier, and has different vegetation and different crops. In the Himalaya and the Alps, settlements and terraced fields sit on the sunny side and forests on the shaded side, and the pattern is visible from a plane.
Water moderates. Anywhere within a few kilometres of a large lake or the sea has smaller day-night and summer-winter swings, because of water's heat capacity (Chapter 1.11).
Climate zones are not fixed
The map above is a snapshot of 1980–2016 and it was different before and will be different later.
The Sahara was green. Between roughly 11,000 and 5,000 years ago, a shift in the Earth's orbit strengthened the North African monsoon and the Sahara held lakes, rivers, grassland, hippos and human populations. Rock art in the middle of the desert shows cattle herders and swimming figures. The drying was relatively rapid once it began, and it pushed populations toward the Nile — which is one of the reasons Egyptian civilisation concentrated where it did. Chapter 4.2 uses this.
The Thar has moved. Palaeochannel studies and satellite imagery show major river systems that once flowed through what is now desert Rajasthan and Cholistan. The relationship of these to the Indus civilisation's decline is covered honestly in Chapter 4.3.
And zones are shifting now. Observed poleward movement of climate belts, expansion of the subtropical dry zones, and upward movement of treelines and species ranges are documented across many regions. Part 14 handles the causes and the projections.
Where this shows up in your life
What grows where you live, and therefore what you eat. Rice needs standing water and warmth, so it belongs to the wet tropics and the monsoon lands. Wheat needs a cool growing season and a dry ripening period, so it belongs to the temperate zones and the north Indian rabi season. Millets tolerate drought and poor soil, which is why they were the traditional staple of the semi-arid Deccan — and why the recent push to bring them back is a climate adaptation argument as much as a nutrition one.
What your house is made of and shaped like. Steep roofs in Kerala and Meghalaya to shed heavy rain; flat roofs in Rajasthan where rain is rare and the roof is usable space; thick mud or stone walls in deserts for thermal mass against a huge day-night swing; courtyards and high ceilings in the hot plains for ventilation; small windows and heavy insulation in cold climates. Traditional architecture is climate physics worked out by trial and error over centuries, and much of it was abandoned in the twentieth century in favour of designs that only work with air conditioning.
Where diseases are. Malaria needs warmth for the parasite to develop inside the mosquito and standing water for larvae, which sets its climate boundaries. Dengue's mosquito is an urban container-breeder, which is why it tracks cities and water storage rather than forests. Volume V covers the biology; the geography of it is in this chapter.
What the next page covers
Everything so far has treated climate as a fixed background. It is not. For the last two and a half million years the Earth has swung repeatedly between ice ages and warm periods, and the last swing ended about 11,700 years ago — which is roughly when every human civilisation in this book begins. Chapter 2.8 covers the ice ages: what actually causes them, why the timing follows the shape of the Earth's orbit, how ice cores let us read the atmosphere of 800,000 years ago directly, why the last 11,700 years have been unusually stable, and what that stability had to do with the invention of farming.