World Climate and Climate Change

2.1 Core concepts

• Three approaches to climate classification: Empirical (based on observed data, particularly temperature and precipitation), Genetic (organises climates according to their causes), and Applied (for a specific purpose). Koeppen's scheme is empirical (NCERT §Koeppen's Scheme, p. 91).
• Koeppen's scheme is "the most widely used classification of climate" — V. Koeppen identified a close relationship between vegetation distribution and climate, selected certain values of temperature and precipitation, and used these to delineate groups; he introduced the use of capital letters (for groups/major sub-divisions) and small letters (for type-level seasonality and temperature severity). Developed in 1918 and modified since (NCERT p. 91).
• Koeppen's five climatic groups (Table 11.1, p. 91):
• A — Tropical: Average temperature of the coldest month is 18°C or higher.
• B — Dry Climates: Potential evaporation exceeds precipitation.
• C — Warm Temperate (Mid-latitude): Average temperature of the coldest month is higher than −3°C but below 18°C.
• D — Cold Snow Forest Climates: Average temperature of the coldest month is −3°C or below.
• E — Cold (Polar) Climates: Average temperature for all months is below 10°C.
• Capital letters A, C, D and E delineate humid climates; B is dry.
• Small-letter subdivisions (for seasonality of precipitation): f = no dry season; m = monsoon (short dry season); w = winter dry season; s = summer dry season. The small letters a, b, c, d refer to the degree of severity of temperature. For B climates, capital S = steppe/semi-arid; W = desert/arid (NCERT p. 92).
• Climatic types (Table 11.2, p. 92): Af (Tropical wet — no dry season), Am (Tropical monsoon — short dry season), Aw (Tropical wet and dry — winter dry), BSh (Subtropical steppe — low-latitude semi-arid), BWh (Subtropical desert — low-latitude arid), BSk (Mid-latitude steppe — semi-arid), BWk (Mid-latitude desert — arid), Cfa (Humid subtropical — no dry season, warm summer), Cs (Mediterranean — dry hot summer), Cfb (Marine west coast — no dry season, warm and cool summer), Df (Humid continental — no dry season, severe winter), Dw (Subarctic — winter dry and very severe), ET (Tundra — no true summer), EF (Polar ice cap — perennial ice).
• Group A — Tropical Humid Climates: Exist between the Tropic of Cancer and the Tropic of Capricorn; sun overhead throughout the year; presence of the ITCZ keeps them hot and humid; annual range of temperature is very low and annual rainfall high. Three types:
• Af — Tropical Wet: Near the equator — Amazon Basin, western equatorial Africa, East Indies islands; significant rainfall every month as thunder showers in the afternoon; uniformly high temperature; daily max ~30°C, min ~20°C; tropical evergreen forests with dense canopy and large biodiversity (NCERT p. 92).
• Am — Tropical Monsoon: Indian subcontinent, NE South America, Northern Australia; heavy rainfall in summer; dry winter.
• Aw — Tropical Wet and Dry: North and south of the Amazon forest in Brazil and parts of Bolivia/Paraguay, Sudan and central Africa; annual rainfall less than Af/Am and variable; wet season shorter, dry season longer; deciduous forest and tree-shredded grasslands (NCERT p. 93).
• Group B — Dry Climates: Very low rainfall not adequate for plant growth; cover a very large area extending from 15°–60° N and S of the equator. At low latitudes (15°–30°) they occur under subtropical highs where subsidence and inversion prevent rainfall; on the western margins of continents (adjoining cold currents) they extend equatorwards (e.g., west coast of South America); in mid-latitudes (35°–60°) they sit in continental interiors beyond the reach of maritime-humid winds, often ringed by mountains. Divided into steppe (BS) and desert (BW) types and further into subtropical (BSh, BWh) 15°–35° and mid-latitude (BSk, BWk) 35°–60° (NCERT p. 93).
• Subtropical Steppe (BSh) and Subtropical Desert (BWh): Located in the transition zone between humid and dry climates; steppe receives slightly more rainfall (enough for sparse grassland); both have highly variable rainfall — variability hits the steppe harder and causes famine; rain in deserts falls as short intense thundershowers and is ineffective for soil moisture; fog is common in coastal deserts bordering cold currents; the highest shade temperature ever recorded — 58°C at Al Aziziyah, Libya on 13 September 1922 — comes from this group.
• Group C — Warm Temperate (Mid-Latitude) Climates: Extend from 30°–50° of latitude mainly on the eastern and western margins of continents; warm summers with mild winters; four types:
• Cwa — Humid Subtropical (winter dry, hot summer): poleward of Tropics of Cancer/Capricorn, mainly in N Indian plains and S China interior plains; similar to Aw but warm winter.
• Cs — Mediterranean: west coast in subtropical latitudes 30°–40° — Central California, Central Chile, SE and SW Australia; under subtropical high in summer and westerlies in winter; hot dry summer and mild rainy winter; summer monthly average ~25°C, winter <10°C; annual precipitation 35–90 cm.
• Cfa — Humid Subtropical (no dry season): eastern parts of continents in subtropical latitudes — eastern USA, S and E China, southern Japan, NE Argentina, coastal S Africa, eastern Australia; thunderstorms in summer and frontal precipitation in winter; summer mean ~27°C, winter 5°–12°C; annual precipitation 75–150 cm.
• Cfb — Marine West Coast: poleward of the Mediterranean climate — NW Europe, west coast N America (north of California), southern Chile, SE Australia, NZ; marine influence makes temperatures moderate; summer 15°–20°C, winter 4°–10°C; precipitation throughout the year, varying greatly 50–250 cm (NCERT pp. 93–94).
• Group D — Cold Snow Forest Climates: Large continental areas of the northern hemisphere between 40°–70° N latitude in Europe, Asia and North America; severity of winter rises with latitude.
• Df — Cold humid winter: poleward of marine west coast and mid-latitude steppe; winters cold and snowy; frost-free season short; large annual range; abrupt weather changes.
• Dw — Cold dry winter (Subarctic): mainly NE Asia; pronounced winter anticyclone whose summer weakening produces a monsoon-like reversal of wind; winter temperatures extremely low — many locations below freezing for up to seven months; precipitation in summer; annual precipitation 12–15 cm (NCERT p. 94).
• Group E — Polar Climates: Exist poleward beyond 70° latitude; two types:
• ET — Tundra: named after the low-growing mosses, lichens and flowering plants; region of permafrost (permanently frozen sub-soil); short growing season and water-logging support only low-growing plants; very long summer daylight.
• EF — Ice Cap: interior Greenland and Antarctica; temperature below freezing even in summer; very little precipitation; accumulated ice deforms and breaks under mounting pressure, forming icebergs that float in Arctic/Antarctic waters; Plateau Station, Antarctica, 79°S portrays this climate.
• Climate Change — long-run perspective: The current climate has prevailed for the last ~10,000 years (the present inter-glacial); geological records show alternation of glacial and inter-glacial periods; geomorphological features at high altitudes/latitudes show glacier advances and retreats; sediment deposits in glacial lakes reveal warm/cold periods; tree rings provide clues about wet/dry periods; historical records describe vagaries in climate; change in climate is a "natural and continuous process" (NCERT §Climate Change, p. 95).
• India in the geological past: Rajasthan desert experienced wet and cool climate around 8,000 BC; the period 3,000–1,700 BC had higher rainfall and 2,000–1,700 BC was the centre of the Harappan civilisation; dry conditions accentuated since (NCERT p. 95).
• Geological deep time: the earth was warm 500–300 million years ago through the Cambrian, Ordovician and Silurian periods; during the Pleistocene epoch glacial and inter-glacial periods occurred; the last major peak glacial period 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; the 1967–77 Sahel drought south of the Sahara; the 1930s "dust bowl" in the southwestern Great Plains of the USA. Europe witnessed warm/wet/cold/dry episodes — warm-dry conditions in the 10th–11th centuries when Vikings settled in Greenland; "Little Ice Age" in Europe from 1550 to about 1850. From 1885–1940 world temperature showed an upward trend; after 1940 the rate slowed (NCERT pp. 95, 97).
• Causes of Climate Change — Astronomical: (i) Sunspots — dark, cooler patches on the sun that increase/decrease in a cyclical manner; some meteorologists hold that more sunspots produce cooler and wetter weather with greater storminess, while fewer sunspots produce warm and drier conditions — though these findings are "not statistically significant". (ii) Millankovitch oscillations — cycles in variations in Earth's orbital characteristics around the sun, the wobbling of the earth, and changes in Earth's axial tilt; all alter the amount of insolation received (NCERT pp. 95–96).
• Causes of Climate Change — Terrestrial: (i) Volcanism — eruptions throw aerosols into the atmosphere; aerosols remain for a long period and reduce solar radiation reaching the surface; Pinatoba and El Cion eruptions caused average temperature to fall for some years. (ii) Anthropogenic — the most important effect is the increasing concentration of greenhouse gases (NCERT p. 96).
• Greenhouse Effect: The atmosphere transmits incoming short-wave solar radiation but absorbs most of the long-wave radiation emitted upwards by the Earth's surface; gases that absorb long-wave radiation are called GHGs; the warming processes are collectively the greenhouse effect. Analogy: a greenhouse made of glass is transparent to incoming short-wave but opaque to outgoing long-wave radiation; everyday examples are a closed car in summer or a closed vehicle in winter (NCERT box, p. 96).
• Greenhouse Gases (GHGs): The primary GHGs of concern today are CO₂, CFCs, CH₄, N₂O and O₃; other gases — NO and CO — react with GHGs and affect their concentration. Effectiveness depends on concentration, atmospheric life-time and the wavelength absorbed; CFCs are highly effective; ozone absorbs UV in the stratosphere but absorbs terrestrial radiation in the lower troposphere. CO₂ has the largest concentration; from fossil fuel combustion (oil, gas, coal); forests and oceans are CO₂ sinks; deforestation also raises concentration; CO₂ adjusts to source-sink changes over 20–50 years and is rising at ~0.5% annually; doubling of CO₂ over pre-industrial level is used as an index for estimating climate change in climatic models (NCERT pp. 96).
• Ozone Hole: Ozone occurs in the stratosphere where UV rays convert oxygen into ozone; CFCs (products of human activity) drift into the stratosphere and destroy ozone; large depletion occurs over Antarctica — the ozone hole — allowing UV rays to pass through the troposphere (NCERT p. 96).
• Kyoto Protocol: Proclaimed in 1997; went into effect in 2005; ratified by 141 nations; binds 35 industrialised countries to reduce GHG emissions by 2012 to 5% less than 1990 levels (NCERT p. 96).
• Global Warming consequences: Rise in sea level due to melting glaciers, ice caps and thermal expansion of the sea may inundate large parts of coastal areas and islands; annual average near-surface air temperature of the world is ~14°C; the greatest warming of the 20th century was during 1901–44 and 1977–99, with global temperatures rising by ~0.4°C in each period and a slight cooling in between (more marked in the Northern Hemisphere); globally averaged annual mean temperature at the end of the 20th century was ~0.6°C above that recorded at the end of the 19th century; the seven warmest years during 1856–2000 were recorded in the last decade of that period; 1998 was the warmest year — probably not only for the 20th century but for the whole millennium; the Gangotri glacier is receding at ~23 m per year on average (NCERT p. 97 and news clipping).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Table 11.1 — Climatic Groups According to Koeppen (p. 91): Five rows A-B-C-D-E with their temperature/precipitation thresholds — the single most testable summary.
• Table 11.2 — Climatic Types According to Koeppen (p. 92): Full code list Af / Am / Aw / BSh / BWh / BSk / BWk / Cfa / Cs / Cfb / Df / Dw / ET / EF with one-line characteristic per code.
• World distribution map (implicit in Table 11.1): A in equatorial belt; B in subtropical highs and continental interiors 15–60°; C in warm-temperate margins 30–50°; D in northern continental interiors 40–70°N; E poleward of 70°.
• Greenhouse Effect process (p. 96): Short-wave solar radiation passes through atmosphere → absorbed by Earth's surface → Earth emits long-wave radiation upward → GHGs absorb this long-wave radiation → atmosphere warms. Analogy: glass of a greenhouse / closed car or bus.
• Millankovitch cycle concept (pp. 95–96): Three orbital variables — eccentricity of orbit around sun, axial tilt (obliquity), and wobble (precession) — each operating on different time scales, collectively altering insolation.
• News clippings (p. 97): "Greenhouse gases rising alarmingly" (Antarctic ice cores), "Gangotri is shrinking 23 m every year", "Ice Age cometh" and "Warming Arctic could affect global weather" — used as visual evidence of recent climate change.

2.5 Key data table (chapter facts at a glance)

2.4 Common confusions / NTA trap points

• A vs. E threshold trap: Group A needs the coldest month ≥18°C; Group E has ALL months below 10°C. NTA often swaps 18°C with 10°C or inserts "hottest month" as a distractor.
• B-climate sub-types confusion: S = steppe (semi-arid), W = desert (arid); lowercase h = subtropical (hot), k = mid-latitude (cool). Students mix up capital S/W with lowercase s/w (which indicate dry season in other groups).
• Sunspot relationship reversal: More sunspots → cooler and wetter (not warmer). Counter-intuitive and a favourite trap.
• Kyoto Protocol dates: Proclaimed 1997, came into effect 2005, ratified by 141 nations, but binds only 35 industrialised countries to emission cuts. NTA frequently conflates these numbers.
• Ozone vs. Greenhouse distinction: Ozone layer depletion (UV problem, caused by CFCs in the stratosphere) is separate from the greenhouse effect (long-wave radiation trapping, caused by CO₂, CH₄ etc.).
• Indian Peninsula is "Am" (Tropical Monsoon) — Exercise 1(iii) answer; not Af, BSh or Cfb.
• Highest temperature on Earth — Al Aziziyah, Libya, 58°C, 13 Sept 1922; located in a B-climate (subtropical desert), not A.
• The C and A type vegetations are commonly mixed — A has tropical evergreen/deciduous; Cs has Mediterranean evergreen scrub; Cfa has mixed broadleaf.
• Plateau Station (Antarctica, 79°S) is the EF (Ice Cap) example — not ET.
• The list of "humid" climate groups is A-C-D-E (Exercise 1(v)); B is dry — so any "humid" foursome that includes B is wrong.
• 1998 = warmest year of the 20th century AND the millennium (Exercise 1(iv)), not 1990, 1885 or 1950.