Introduction
World climate is studied by organising climate data into smaller units. Three broad approaches are used to classify climate:
Empirical — based on observed data, especially temperature and precipitation.Genetic — organises climates according to their causes.Applied — for a specific purpose.The chapter describes Koeppen's scheme and then discusses climate change and global warming.
Key Concepts
1. Koeppen's Scheme of Classification
Developed by V. Koeppen, the most widely used empirical scheme. He found a close relationship between vegetation and climate, and used selected values of temperature and precipitation to classify climates.Based on mean annual and mean monthly temperature and precipitation.Developed in 1918, modified over time and still popular.Uses capital letters for climatic groups and small letters for types.Five major groups (four based on temperature, one on precipitation):
A – Tropical: average temperature of the coldest month is 18°C or higher.B – Dry: potential evaporation exceeds precipitation.C – Warm temperate (mid-latitude): coldest month above –3°C but below 18°C.D – Cold snow forest: coldest month –3°C or below.E – Cold: average temperature of all months below 10°C.The book groups A, C, D and E as humid climates and B as dry.
Small letters:
Seasons of dryness: f (no dry season), m (monsoon), w (winter dry season), s (summer dry season).a, b, c, d show the severity of temperature.B climates use S (steppe or semi-arid) and W (desert).Climatic types: Af (tropical wet), Am (tropical monsoon), Aw (tropical wet and dry), BSh (subtropical steppe), BWh (subtropical desert), BSk (mid-latitude steppe), BWk (mid-latitude desert), Cfa (humid subtropical), Cs (Mediterranean), Cfb (marine west coast), Df (humid continental), Dw (subarctic), ET (tundra), EF (polar ice cap).
2. Group A – Tropical Humid Climates
Between the Tropic of Cancer and Tropic of Capricorn; the overhead sun and the ITCZ make them hot and humid, with a very low annual range and high rainfall.
Af – Tropical wet: near the equator — Amazon Basin, western equatorial Africa, East Indies islands. Rain every month as afternoon thunder showers. Max ~30°C, min ~20°C; negligible annual range. Tropical evergreen forests with dense canopy and huge biodiversity.Am – Tropical monsoon: Indian sub-continent, north-eastern South America, northern Australia. Heavy rain mostly in summer; dry winter.Aw – Tropical wet and dry: north and south of Af regions — around the Amazon forest in Brazil, Bolivia, Paraguay, Sudan and south of Central Africa. Less and more variable rain than Af/Am; shorter wet season, longer, more severe dry season; greatest diurnal range in the dry season. Deciduous forest and tree-shredded grasslands.3. Group B – Dry Climates
Very low rainfall, not enough for plant growth; they cover large areas from 15°–60° N and S.At 15°–30° they lie in the subtropical high, where subsidence and inversion prevent rain. On western margins next to cold currents (e.g., the west coast of South America) they reach the coast.At 35°–60° they lie in continental interiors away from moist winds, often surrounded by mountains.BSh and BWh (15°–35°): steppe gets slightly more rain than desert, enough for sparse grasslands. Rain is highly variable, which affects the steppe more, often causing famine. Desert rain comes in short, intense thundershowers that don't build soil moisture. Fog is common in coastal deserts bordering cold currents. The book records the highest shade temperature as 58°C at Al Aziziyah, Libya, on 13 September 1922. Annual and diurnal ranges are high.BSk and BWk: mid-latitude steppe and desert at 35°–60°.4. Group C – Warm Temperate (Mid-Latitude) Climates
From 30°–50°, mainly on eastern and western margins of continents; warm summers, mild winters.
Cwa – Humid subtropical (dry winter, hot summer): poleward of the tropics, mainly the North Indian plains and South China interior plains. Like Aw but with a warm winter.Cs – Mediterranean: around the Mediterranean Sea and west coasts at 30°–40° — Central California, Central Chile, south-eastern and south-western Australia. Subtropical high in summer and westerlies in winter give hot, dry summers and mild, rainy winters. Summer ~25°C, winter below 10°C; rain 35–90 cm.Cfa – Humid subtropical (no dry season): eastern parts of continents — eastern USA, southern and eastern China, southern Japan, north-eastern Argentina, coastal South Africa, eastern Australia. Rain 75–150 cm; summer thunderstorms and winter frontal rain. Summer ~27°C, winter 5°–12°C; small daily range.Cfb – Marine west coast: poleward of the Mediterranean on west coasts — north-western Europe, west coast of North America north of California, southern Chile, south-eastern Australia, New Zealand. Moderate temperatures (summer 15°–20°C, winter 4°–10°C); small ranges; rain all year, 50–250 cm.5. Group D – Cold Snow Forest Climates
In large continental areas of the northern hemisphere, 40°–70° N (Europe, Asia, North America).
Df – Cold, humid winter: poleward of marine west coast and mid-latitude steppe; cold snowy winters, short frost-free season, large annual range, abrupt weather changes.Dw – Cold, dry winter: mainly north-eastern Asia. A strong winter anticyclone that weakens in summer gives a monsoon-like reversal of winds. Very low winter temperatures, below freezing for up to seven months in places; summer precipitation; annual precipitation only 12–15 cm.6. Group E – Polar Climates
Poleward of 70°.
ET – Tundra: named after its vegetation — mosses, lichens, flowering plants. Permafrost (permanently frozen subsoil); a short growing season and waterlogging allow only low plants; very long daylight in summer.EF – Ice cap: interior Greenland and Antarctica; below freezing even in summer; very little precipitation. Accumulated ice deforms, breaks and moves as icebergs. Plateau Station, Antarctica (79° S) portrays this climate.7. Climate Change
Present climate may have prevailed for about the last 10,000 years, with fluctuations. Climate change is a natural and continuous process.Evidence: glacial and inter-glacial periods in geological records; glacier advances and retreats; sediments in glacial lakes; tree rings (wet and dry periods); historical records.India: the Rajasthan desert was wet and cool around 8,000 B.C.; the period 3,000–1,700 B.C. had higher rainfall; around 2,000–1,700 B.C. the region was the centre of the Harappan civilisation; dry conditions have increased since.Geological past: the earth was warm 500–300 million years ago (Cambrian, Ordovician, Silurian). In the Pleistocene, glacial and inter-glacial periods occurred; the last major peak glacial was about 18,000 years ago; the present inter-glacial began 10,000 years ago.Climate in the recent past
The 1990s recorded the warmest temperatures of the century and some of the worst floods.Sahel drought south of the Sahara, 1967–1977.Dust bowl — severe drought in the south-western Great Plains of the USA in the 1930s.Europe: warm, dry conditions in the 10th–11th centuries when the Vikings settled in Greenland; the Little Ice Age from 1550 to about 1850.World temperature rose from about 1885–1940; after 1940 the rate slowed.8. Causes of Climate Change
Astronomical causes
Changes in solar output associated with sunspots (dark, cooler patches that rise and fall in cycles). Some meteorologists link more sunspots with cooler, wetter weather and more storms, and fewer sunspots with warm, drier conditions — but these findings are not statistically significant.Milankovitch oscillations — cycles in the earth's orbit, its wobble and changes in axial tilt that alter insolation.Terrestrial causes
Volcanism — eruptions throw aerosols into the atmosphere that reduce sunlight reaching the surface. After the Pinatubo and El Chichón eruptions, average temperature fell for some years.The most important human effect: rising greenhouse gas concentration, likely to cause global warming.9. Global Warming and Greenhouse Gases
The atmosphere transmits incoming solar radiation but absorbs most long-wave radiation from the earth. Gases that absorb long-wave radiation are greenhouse gases (GHGs); this warming is the greenhouse effect — like a glasshouse, or a closed car in summer.Primary GHGs: carbon dioxide (CO₂), chlorofluorocarbons (CFCs), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃). Nitric oxide (NO) and carbon monoxide (CO) react with GHGs and affect their concentration.A GHG's effectiveness depends on its increase in concentration, lifetime in the atmosphere, and the wavelength it absorbs. CFCs are highly effective. Ozone absorbs UV in the stratosphere but absorbs terrestrial radiation in the lower troposphere.CO₂ has the largest concentration; it comes mainly from fossil fuel combustion. Forests and oceans are its sinks, so deforestation raises it. CO₂ takes 20–50 years to adjust to changes in sources and sinks, and is rising about 0.5% annually. Doubling of CO₂ over the pre-industrial level is used as an index in climate models.Ozone hole: CFCs drifting into the stratosphere destroy ozone, especially over Antarctica, letting UV rays through.Kyoto Protocol: proclaimed in 1997, in effect from 2005, ratified by 141 nations. It bound 35 industrialised countries to cut emissions by 2012 to 5% below 1990 levels.Effects: rising sea levels from melting glaciers and ice caps and thermal expansion may flood coasts and islands.Temperature records
Average near-surface air temperature of the world ≈ 14°C.Greatest 20th-century warming: 1901–44 and 1977–99, each about 0.4°C, with slight cooling in between.End of 20th century ≈ 0.6°C warmer than the end of the 19th.The seven warmest years of 1856–2000 were in the last decade; 1998 was the warmest year, probably of the millennium.Diagrams to Know
A world map of Koeppen climate regions (Af along the equator, B in subtropical west coasts and interiors, Cs around the Mediterranean, D across northern Eurasia and North America, E near the poles).The greenhouse effect — short-wave radiation in, long-wave radiation trapped by GHGs.Key-Facts Sheet
Koeppen: A ≥ 18°C coldest month; C –3°C to 18°C; D ≤ –3°C; E all months < 10°C; B evaporation > precipitation.Little Ice Age: 1550–1850; Sahel drought 1967–77; Dust bowl 1930s.Kyoto: 1997 proclaimed, 2005 in force, 141 nations, 35 countries, 5% below 1990 by 2012.20th-century warming ≈ 0.6°C; 1998 the warmest year.Exam Focus
MCQ from the exercise: Koeppen's scheme is empirical; most of peninsular India is Am; the warmest year was 1998; humid groups are A–C–D–E. For the "A" climate question, the book's definition is coldest month 18°C or higher (so every month is above 18°C).30-word: Genetic vs empirical. Outline: empirical uses observed temperature and rainfall data; genetic groups climates by their causes.What happens if sunspots increase? Outline: some meteorologists link it to cooler, wetter, stormier weather, though not statistically significant.150-word: List greenhouse gases. Outline: define GHGs; CO₂, CFCs, CH₄, N₂O, O₃; sources and sinks; effects; Kyoto Protocol.Key Terms
Empirical classification (आनुभविक वर्गीकरण)Genetic classification (जननिक वर्गीकरण)Steppe (स्टेपी)Mediterranean climate (भूमध्यसागरीय जलवायु)Tundra (टुंड्रा)Permafrost (स्थायी तुषार भूमि)Ice cap (हिमटोपी)Climate change (जलवायु परिवर्तन)Sunspots (सौर कलंक)Greenhouse gases (हरित गृह गैसें)Global warming (भूमंडलीय तापन)Ozone hole (ओज़ोन छिद्र)Kyoto Protocol (क्योटो प्रोटोकॉल)Common Mistakes
Forgetting that B is the only group based on precipitation; the others are based on temperature.Confusing Cs (dry summer, rainy winter) with Cfa (rain all year).Writing that Kyoto came into force in 1997 — it was proclaimed in 1997 and came into effect in 2005.Saying greenhouse gases block incoming sunlight — they trap outgoing long-wave radiation.Treating the sunspot–weather link as proven — the book says it is not statistically significant.