Chapter 11 – World Climate and Climate Change | CBSE Notes

GEOGRAPHY | CLASS XI | NCERT

Book: Fundamentals of Physical Geography | Chapter 11

World Climate and Climate Change

⭐ Topper Level 💬 Easy Language 📌 Point-Wise 🌍 Koeppen’s Classification

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1. Learning Objectives

After reading these notes, you will be able to:

Understand three approaches of climate classification — Empirical, Genetic and Applied.

Explain Koeppen’s scheme of climate classification — 5 major groups (A, B, C, D, E) and their sub-types.

Describe characteristics of each climate type — Tropical, Dry, Warm Temperate, Cold Snow Forest, Polar.

Understand causes of climate change — astronomical and terrestrial, including sunspots and Millankovitch oscillations.

Explain greenhouse effect, greenhouse gases and global warming — causes and consequences.

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2. Approaches to Climate Classification

The world climate can be studied by organising information on climate and synthesising them in smaller units for easy understanding, description and analysis. Three broad approaches have been adopted for classifying climate.

📊 1. Empirical Classification

Based on observed data, particularly on temperature and precipitation. Koeppen’s scheme is the most famous empirical classification. It is data-driven — uses actual measured values.

🔬 2. Genetic Classification

Attempts to organise climates according to their causes. Focuses on the origin and reasons behind the climate type rather than just observed data.

🎯 3. Applied Classification

Classification done for a specific purpose — for example, for agriculture, aviation, or military use. It is customised to suit a particular practical application.

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3. Koeppen’s Scheme of Climate Classification

The most widely used classification of climate is the empirical climate classification scheme developed by V. Koeppen. Koeppen identified a close relationship between the distribution of vegetation and climate. He selected certain values of temperature and precipitation related to vegetation distribution. It is based on mean annual and mean monthly temperature and precipitation data. Developed in 1918, modified over time — still popular and in use.

📌 Key Features of Koeppen’s Scheme

Koeppen used capital letters (A, B, C, D, E) to designate climatic groups and small letters to designate sub-types.
A, C, D, E = Humid climates  |  B = Dry climates.

Small letters for dryness: f = no dry season, m = monsoon, w = winter dry, s = summer dry.

Small letters for temperature severity: a, b, c, d.

For B (Dry): S = steppe/semi-arid, W = desert.

📋 Five Major Climatic Groups — Koeppen

Group	Name	Key Characteristic
A	Tropical Humid	Average temperature of the coldest month ≥ 18°C
B	Dry Climates	Potential evaporation exceeds precipitation
C	Warm Temperate (Mid-latitude)	Coldest month avg temp: above –3°C but below 18°C
D	Cold Snow Forest	Average temperature of coldest month is –3°C or below
E	Cold Climates	Average temperature for all months is below 10°C

📋 All Climate Types — Quick Reference Table

Group	Type	Code	Key Feature
A	Tropical wet	Af	No dry season
	Tropical monsoon	Am	Monsoonal, short dry season
	Tropical wet and dry	Aw	Winter dry season
B	Subtropical steppe	BSh	Low-latitude semi-arid
	Subtropical desert	BWh	Low-latitude arid/dry
	Mid-latitude steppe	BSk	Mid-latitude semi-arid
	Mid-latitude desert	BWk	Mid-latitude arid/dry
C	Humid subtropical	Cfa	No dry season, warm summer
	Mediterranean	Cs	Dry hot summer
	Marine west coast	Cfb	No dry season, cool summer
D	Humid continental	Df	No dry season, severe winter
D	Subarctic	Dw	Winter dry, very severe
E	Tundra	ET	No true summer
E	Polar ice cap	EF	Perennial ice

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4. Group A — Tropical Humid Climates

Tropical humid climates exist between Tropic of Cancer and Tropic of Capricorn. The Sun being overhead throughout the year and the presence of ITCZ make the climate hot and humid. Annual range of temperature is very low; annual rainfall is high.

🌧️ Af — Tropical Wet Climate

Found near the equator. Major areas: Amazon Basin (South America), western equatorial Africa, islands of East Indies. Significant rainfall every month — thunder showers in the afternoon. Temperature uniformly high; annual range negligible. Max temp ~30°C; Min ~20°C. Tropical evergreen forests with dense canopy and large biodiversity.

🌦️ Am — Tropical Monsoon Climate

Found over Indian sub-continent, North-Eastern South America, Northern Australia. Heavy rainfall in summer; winter is dry. Short dry season. Characteristic of monsoon regions.

🌿 Aw — Tropical Wet and Dry Climate

Occurs north and south of Af type. Found in Brazil (north and south of Amazon), Sudan, south of Central Africa. Annual rainfall less and variable compared to Af/Am. Wet season shorter; dry season longer; drought more severe. Temperature high throughout year; diurnal range greatest in dry season. Deciduous forest and tree-shredded grasslands.

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5. Group B — Dry Climates

Dry climates are characterised by 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 in the area of subtropical high where subsidence and temperature inversion prevent rainfall. In middle latitudes (35°–60°), confined to interior of continents where maritime humid winds don’t reach, often surrounded by mountains.

Steppe / Semi-arid — slightly more rainfall, sparse grassland possible

Desert — extremely low rainfall, arid conditions

BSh / BWh

Subtropical — latitudes 15°–35°, h = hot

BSk / BWk

Mid-latitude — latitudes 35°–60°, k = kalt (cold)

📌 Key Features of B Climates

Rainfall is highly variable. Variability affects steppe life more than desert — often causes famine. Rain in deserts: short intense thundershowers — ineffective in building soil moisture. Fog common in coastal deserts bordering cold currents. Maximum temperature in summer very high. Highest shade temperature: 58°C at Al Aziziyah, Libya — 13 September 1922. Annual and diurnal temperature ranges are also high.

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6. Group C — Warm Temperate (Mid-Latitude) Climates

Warm temperate climates extend from 30°–50° latitude mainly on the eastern and western margins of continents. Generally have warm summers with mild winters.

🌾 Cwa — Humid Subtropical (dry winter)

Occurs poleward of Tropic of Cancer and Capricorn. Found mainly in North Indian plains and South China interior plains. Similar to Aw climate except temperature in winter is warm. Dry winter, hot summer.

🫒 Cs — Mediterranean Climate

Occurs around Mediterranean Sea, west coast of continents in subtropical latitudes (30°–40°). Examples: Central California, Central Chile, SW and SE Australia. Under subtropical high in summer → westerly wind in winter. Characterised by hot dry summer and mild rainy winter. Summer avg ~25°C; winter below 10°C. Annual precipitation: 35–90 cm.

🌱 Cfa — Humid Subtropical (no dry season)

On eastern parts of continents in subtropical latitudes. Air masses generally unstable → rainfall throughout the year. Areas: Eastern USA, southern/eastern China, southern Japan, NE Argentina, eastern Australia. Annual precipitation: 75–150 cm. Thunderstorms in summer, frontal precipitation in winter. Summer mean ~27°C; winter 5°–12°C.

🌊 Cfb — Marine West Coast Climate

Located poleward from Mediterranean climate on west coast. Areas: NW Europe, west coast N. America (north of California), southern Chile, SE Australia, New Zealand. Due to marine influence, temperature is moderate. Summer: 15°–20°C; winter: 4°–10°C. Annual and daily temperature ranges are small. Precipitation throughout the year — 50–250 cm.

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7. Group D — Cold Snow Forest Climates

Cold snow forest climates occur in large continental areas in the Northern Hemisphere between 40°–70° N latitudes in Europe, Asia and North America. Divided into two types: Df (humid winter) and Dw (dry winter).

❄️ Df — Cold Climate with Humid Winters

Occurs poleward of marine west coast climate and mid-latitude steppe. Winters are cold and snowy. Frost-free season is short. Annual temperature ranges are large. Weather changes are abrupt and short. Poleward, winters are more severe.

🌨️ Dw — Cold Climate with Dry Winters

Occurs mainly over Northeastern Asia. Development of pronounced winter anticyclone and its weakening in summer → monsoon-like reversal of wind. Winter temperatures extremely low — many locations below freezing for up to 7 months. Precipitation in summer. Annual precipitation: 12–15 cm (very low).

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8. Group E — Polar Climates

Polar climates exist poleward beyond 70° latitude. Consist of two types: ET (Tundra) and EF (Ice Cap).

🌿 ET — Tundra Climate

Named after the types of vegetation — low growing mosses, lichens and flowering plants. Region of permafrost — subsoil is permanently frozen. Short growing season and water logging support only low growing plants. During summer, tundra regions have very long duration of daylight.

🧊 EF — Ice Cap Climate

Occurs over interior Greenland and Antarctica. Even in summer, temperature is below freezing point. Receives very little precipitation. Snow and ice accumulate → mounting pressure → deformation of ice sheets → they break → move as icebergs floating in Arctic and Antarctic waters. Example: Plateau Station, Antarctica (79°S).

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9. Climate Change

The type of climate we experience now might be prevailing over the last 10,000 years with minor fluctuations. The planet has witnessed many climate variations since the beginning. Change in climate is a natural and continuous process.

🕰️ Evidence of Climate Change (Past Records)

Geological records: Show alternation of glacial and inter-glacial periods. Geomorphological features in high altitudes/latitudes show traces of advances and retreats of glaciers.
Sediment deposits in glacial lakes: Reveal occurrence of warm and cold periods.
Tree rings: Provide clues about wet and dry periods.
Historical records: Describe vagaries in climate.
India: Rajasthan desert experienced wet and cool climate around 8,000 B.C.; period 3,000–1,700 B.C. had higher rainfall; Harappan civilisation (2,000–1,700 B.C.) centred in this region.
Geological past: Earth was warm 500–300 million years ago (Cambrian, Ordovician, Silurian periods). Last major peak glacial period was ~18,000 years ago. Present inter-glacial period started 10,000 years ago.

📰 Climate in the Recent Past

The 1990s recorded the warmest temperature of the century and some of the worst floods around the world.
1967–1977: Worst devastating drought in the Sahel region, south of the Sahara desert.
1930s: Severe drought in southwestern Great Plains of USA — called the dust bowl.
Europe witnessed warm and dry conditions in 10th–11th centuries when Vikings settled in Greenland.
Europe witnessed “Little Ice Age” from 1550 to ~1850.
1885–1940: World temperature showed an upward trend. After 1940, rate of increase slowed down.

🔭 Causes of Climate Change

🌞 Sunspot Activities

Sunspots = dark and cooler patches on the Sun that increase and decrease cyclically. When sunspots increase → cooler, wetter weather, greater storminess. When sunspots decrease → warm and drier conditions. (These findings are not statistically significant.)

🌍 Millankovitch Oscillations

Astronomical theory — infers cycles in variations in Earth’s orbital characteristics around the Sun, wobbling of the Earth, and changes in Earth’s axial tilt. All these alter the amount of insolation received from the Sun → bearing on climate.

🌋 Volcanism

Volcanic eruptions throw up lots of aerosols into the atmosphere. Aerosols remain in atmosphere for a considerable period → reduce Sun’s radiation reaching Earth’s surface. After Pinatoba and El Cion eruptions, average temperature of Earth fell to some extent for some years.

🏭 Anthropogenic Effect

Most important anthropogenic effect — increasing concentration of greenhouse gases (GHGs) in the atmosphere, which is likely to cause global warming. Human activity = main driver of recent rapid climate change.

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10. Greenhouse Effect and Global Warming

Due to the presence of greenhouse gases, the atmosphere behaves like a greenhouse. The atmosphere transmits incoming solar radiation but absorbs the vast majority of long-wave radiation emitted upwards by Earth’s surface. The gases that absorb long-wave radiation are called greenhouse gases (GHGs). The processes that warm the atmosphere are collectively called the greenhouse effect.

📌 Greenhouse Analogy

A greenhouse is made of glass — transparent to incoming short-wave solar radiation but opaque to outgoing long-wave radiation. Glass allows more radiation in but prevents long-wave radiation from going out → temperature inside warmer than outside. Similarly, Earth’s atmosphere traps outgoing radiation → warming of Earth.

☁️ Primary Greenhouse Gases (GHGs)

CO₂

Carbon Dioxide — largest concentration. From fossil fuel combustion. Rising ~0.5% annually

CFCs

Chlorofluorocarbons — highly effective. Products of human activity. Destroys ozone in stratosphere

CH₄

Methane — from agriculture, livestock, landfills, natural gas

N₂O

Nitrous Oxide — from agriculture and industrial processes

O₃

Ozone — absorbs UV in stratosphere; absorbs terrestrial radiation in lower troposphere

NO & CO

Nitric oxide & Carbon monoxide — react with GHGs and affect their concentration

🌲 CO₂ — Important Points

Largest concentration of GHGs in atmosphere is carbon dioxide.
Emission mainly from fossil fuel combustion (oil, gas, coal).
Forests and oceans are the sinks for CO₂ — forests use CO₂ in growth.
Deforestation due to changes in land use also increases CO₂ concentration.
Time taken for atmospheric CO₂ to adjust = 20–50 years. Rising at about 0.5% annually.
Doubling of CO₂ over pre-industrial level is used as an index for estimating climate change in climatic models.

🕳️ Ozone Hole

📌 CFCs and Ozone Depletion

Ozone occurs in the stratosphere where ultra-violet rays convert oxygen into ozone — thus UV rays do NOT reach Earth’s surface. The CFCs (products of human activity) drift into the stratosphere and destroy the ozone. Large depletion of ozone occurs over Antarctica. The depletion of ozone concentration in the stratosphere is called the ozone hole. This allows UV rays to pass through the troposphere — harmful to life.

🌐 Kyoto Protocol

📌 International Effort
Kyoto Protocol — proclaimed in 1997. Went into effect in 2005. Ratified by 141 nations. Bound 35 industrialised countries to reduce their emissions by the year 2012 to 5% less than the levels of 1990.

🌡️ Effects of Global Warming

The annual average near-surface air temperature of the world is approximately 14°C.
Greatest warming in the 20th century during 1901–44 and 1977–99 — each period rose by about 0.4°C.
In between, there was a slight cooling — more marked in the Northern Hemisphere.
Globally averaged annual mean temperature at end of 20th century was about 0.6°C above that at end of 19th century.
Seven warmest years in 1856–2000 were all recorded in the last decade. 1998 was the warmest year — probably for the whole millennium.
Rise in sea level due to melting of glaciers and ice-caps and thermal expansion of the sea → may inundate large parts of coastal areas and islands.
Effect of global warming may not be uniform everywhere. Once set in, it will be difficult to reverse.

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Summary — Quick Revision

3 Approaches: Empirical (observed data — Koeppen), Genetic (causes), Applied (specific purpose). Koeppen’s is the most widely used — developed 1918, based on temperature and precipitation.

5 Koeppen Groups: A (Tropical, coldest month ≥18°C), B (Dry, evaporation > precipitation), C (Warm Temperate, coldest month –3° to 18°C), D (Cold Snow Forest, coldest month ≤ –3°C), E (Polar, all months <10°C).

Group A types: Af (tropical wet, near equator, no dry season, evergreen forests), Am (monsoon, Indian subcontinent, heavy summer rain), Aw (wet and dry, deciduous forest and grasslands).

Group B types: BS (steppe/semi-arid), BW (desert). Subtropical (h) = 15°–35°; Mid-latitude (k) = 35°–60°. Highest temperature: 58°C at Al Aziziyah, Libya (1922).

Group C types: Cwa (humid subtropical, N. Indian plains), Cs (Mediterranean — hot dry summer, mild rainy winter), Cfa (humid subtropical, eastern continents), Cfb (marine west coast, moderate temperature).

Group D: Df (humid winter, NW Europe/N. America), Dw (dry winter, NE Asia — monsoon reversal, annual precip only 12–15 cm). Group E: ET (tundra, permafrost, low plants), EF (ice cap, interior Greenland and Antarctica, always below freezing).

Causes of Climate Change: Sunspot activities (cyclical), Millankovitch oscillations (orbital changes), Volcanism (aerosols reduce insolation), Anthropogenic (GHGs — most important today).

Primary GHGs: CO₂ (largest, from fossil fuels, rising 0.5%/yr), CFCs (destroys ozone), CH₄, N₂O, O₃. Forests and oceans = CO₂ sinks. CO₂ adjustment time = 20–50 years.

Ozone Hole: CFCs destroy ozone in stratosphere over Antarctica → UV rays pass through troposphere. Kyoto Protocol (1997, effective 2005) — 141 nations, 35 industrialised countries to cut emissions 5% below 1990 levels by 2012.

Global Warming facts: World avg temperature ~14°C. Greatest warming: 1901–44 and 1977–99 (+0.4°C each). By end of 20th century: +0.6°C above 19th century. 1998 = warmest year ever. Sea level rise due to glacier melt and thermal expansion of sea = major concern.

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Important Terms to Remember

Empirical Classification: Climate classification based on observed data — especially temperature and precipitation. Koeppen’s scheme is the most famous example.
Genetic Classification: Classification based on the causes of climate rather than observed data.
Applied Classification: Classification done for a specific practical purpose such as agriculture, aviation or military use.
Koeppen’s Scheme: Most widely used climate classification — developed by V. Koeppen in 1918. Uses capital letters (A, B, C, D, E) for groups and small letters for sub-types. Based on mean annual and mean monthly temperature and precipitation.
Tropical Wet Climate (Af): Found near equator. Rainfall every month. Uniformly high temperature. Annual range negligible. Tropical evergreen forests. Amazon Basin, W. equatorial Africa, East Indies.
Tropical Monsoon Climate (Am): Heavy rainfall in summer, dry winter. Found over Indian sub-continent, NE South America, N. Australia.
Tropical Wet and Dry Climate (Aw): Wet season shorter, dry season longer. Deciduous forests and grasslands. North and south of Amazon in Brazil, Sudan, Central Africa.
Dry Climates (B): Potential evaporation exceeds precipitation. BS = steppe/semi-arid. BW = desert. h = subtropical (hot), k = mid-latitude (cold).
Mediterranean Climate (Cs): Hot dry summer, mild rainy winter. Along west coasts of continents at 30°–40° latitude. Central California, Central Chile, around Mediterranean Sea.
Marine West Coast Climate (Cfb): Poleward of Mediterranean climate, west coast of continents. Moderate temperature due to marine influence. Precipitation throughout the year.
Cold Snow Forest Climate (D): Northern Hemisphere, 40°–70° N. Df = humid winter, Dw = dry winter (NE Asia, monsoon reversal, extremely cold winters).
Tundra Climate (ET): Poleward beyond 70°. Permafrost — subsoil permanently frozen. Low growing mosses, lichens, flowering plants. Very long summer daylight.
Ice Cap Climate (EF): Interior Greenland and Antarctica. Temperature below freezing even in summer. Icebergs form when ice sheets break. Very little precipitation.
Permafrost: Subsoil that is permanently frozen — found in tundra regions. Limits plant growth to low growing vegetation only.
Climate Change: The variation in Earth’s global climate over time. Natural and continuous process. Evidence from geological records, tree rings, sediment deposits, historical records.
Sunspots: Dark and cooler patches on the Sun that increase and decrease cyclically. Linked to variations in solar output and associated weather changes.
Millankovitch Oscillations: Astronomical theory — cycles in variations in Earth’s orbital characteristics, wobbling of Earth, and changes in Earth’s axial tilt — alter insolation received.
Aerosols: Tiny particles thrown into the atmosphere by volcanic eruptions. Remain in atmosphere for a long time, reducing solar radiation reaching Earth’s surface — causing temporary cooling.
Greenhouse Effect: Process by which greenhouse gases absorb and re-emit long-wave radiation from Earth’s surface, warming the lower atmosphere. Named after analogy to a glass greenhouse.
Greenhouse Gases (GHGs): Gases that absorb long-wave radiation — CO₂, CFCs, CH₄, N₂O, O₃. CO₂ has the largest concentration. CFCs are the most effective per molecule.
Carbon Dioxide (CO₂): Most abundant GHG. From fossil fuel combustion. Forests and oceans are sinks. Rising at 0.5% annually. Adjustment time in atmosphere = 20–50 years.
Chlorofluorocarbons (CFCs): Products of human activity. Drift into stratosphere and destroy ozone. Large-scale ozone depletion over Antarctica = ozone hole.
Ozone Hole: Depletion of ozone concentration in the stratosphere — mainly over Antarctica — caused by CFCs. Allows harmful UV radiation to reach Earth’s surface.
Global Warming: Increasing trend in Earth’s average temperature due to rising GHG concentrations. World avg ~14°C. By end of 20th century: 0.6°C above 19th century levels. 1998 = warmest year on record.
Kyoto Protocol: International agreement proclaimed in 1997, effective 2005. Ratified by 141 nations. Bound 35 industrialised countries to reduce GHG emissions by 5% below 1990 levels by 2012.
Dust Bowl: Severe drought in the southwestern Great Plains of the USA in the 1930s — a historical example of climate variability affecting human life.
Little Ice Age: A period of significant cooling experienced in Europe from approximately 1550 to 1850 — a historical example of natural climate change.