World Climate and Climate Change, Class 11 Geography

What World Climate and Climate Change is about

Key concepts

Approaches and Koeppen's scheme

NCERT section Koeppen's Scheme of Classification of Climate

Three approaches to classifying climate:

Approach	Basis
Empirical	Observed data, especially temperature and precipitation
Genetic	The causes of climates
Applied	A specific purpose

Koeppen's scheme:

• The most widely used, empirical, developed by V. Koeppen in 1918 and modified over time.
• Found a close relationship between vegetation and climate; chose threshold values of temperature and precipitation that match vegetation boundaries.
• Uses mean annual and mean monthly temperature and precipitation.
• Capital letters = groups; small letters = types.

Five groups (Table 11.1):

Group	Defining condition
A Tropical	Coldest month's average 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 of all months below 10°C

Four groups are based on temperature, one (B) on precipitation. A, C, D, E are humid; B is dry.

Letter codes: f = no dry season; m = monsoon; w = winter dry season; s = summer dry season; a, b, c, d = severity of temperature; for B: S = steppe (semi-arid), W = desert; h = low-latitude (subtropical), k = mid-latitude.

Worked example

A station's coldest month averages 2°C and its warmest 22°C, with rain in all seasons. To which Koeppen group does it belong? (1 mark)

Try it first, then show the solutionSolution

1. Coldest month 2°C is above -3°C and below 18°C.
2. That defines group C, warm temperate; rain in all seasons adds f (no dry season).

Answer: Group C (a Cf type).

Tropical (A) and dry (B) climates

NCERT section Koeppen's Scheme of Classification of Climate

Group A: tropical humid (between the tropics; overhead sun and the ITCZ make it hot and humid; very low annual range; high rainfall)

Type	Where	Features
Af tropical wet	Near the equator: Amazon Basin, western equatorial Africa, East Indies	Rain every month as afternoon thunder showers; max about 30°C, min about 20°C; negligible annual range; tropical evergreen forests, dense canopy, high biodiversity
Am tropical monsoon	Indian subcontinent, north-eastern South America, northern Australia	Heavy summer rain, dry winter
Aw tropical wet and dry	North and south of Af; north and south of the Amazon forest (Brazil, Bolivia, Paraguay), Sudan, south of Central Africa	Less, variable rain; shorter wet and longer dry season; greatest diurnal range in the dry season; deciduous forests and tree-shredded grasslands

Group B: dry (rainfall too low for plant growth; 15°-60° N and S)

• 15°-30°: in the subtropical high, where subsidence and inversion prevent rain; on western margins next to cold currents (e.g. west coast of South America) they reach the coast and extend equatorward.
• 35°-60°: in continental interiors beyond maritime winds, often ringed by mountains.
• Types: BSh subtropical steppe and BWh subtropical desert (15°-35°); BSk mid-latitude steppe and BWk mid-latitude desert (35°-60°).
• Steppe gets slightly more rain (sparse grassland); rainfall is highly variable, hitting the steppe harder (famine). Desert rain comes in short intense showers, ineffective for soil moisture; fog in coastal deserts by cold currents; very high summer maxima (the chapter cites 58°C at Al Aziziyah, Libya); high annual and diurnal ranges.

Worked example

Why do dry climates reach the coast on the west coast of South America in low latitudes? (2 marks)

Try it first, then show the solutionSolution

1. At 15°-30° dry climates lie under the subtropical high, where subsidence and inversion prevent rain.
2. On western margins next to cold currents the air is further stabilised, so the dry belt extends equatorward right onto the coast.

Answer: Subtropical high pressure plus the adjoining cold current keep the coast dry.

Warm temperate (C), cold snow forest (D) and polar (E) climates

NCERT section Koeppen's Scheme of Classification of Climate

Group C: warm temperate (30°-50°, mainly eastern and western margins; warm summers, mild winters)

Type	Where	Features
Cwa humid subtropical, dry winter	Poleward of the tropics: north Indian plains, south China interior plains	Like Aw but warm winter
Cs Mediterranean	Around the Mediterranean Sea; west coasts at 30°-40°: central California, central Chile, SE and SW Australia	Subtropical high in summer, westerlies in winter: hot dry summer, mild rainy winter; summer about 25°C, winter below 10°C; 35-90 cm
Cfa humid subtropical, no dry season	Eastern parts of continents: eastern USA, southern and eastern China, southern Japan, NE Argentina, coastal South Africa, east coast of Australia	Unstable air, rain all year (75-150 cm); summer thunderstorms, winter frontal rain; summer about 27°C, winter 5°-12°C
Cfb marine west coast	Poleward of Mediterranean on west coasts: NW Europe, west coast of North America north of California, southern Chile, SE Australia, New Zealand	Moderate; winters warmer than latitude suggests; summer 15°-20°C, winter 4°-10°C; rain all year, 50-250 cm

Group D: cold snow forest (northern hemisphere, 40°-70°N in Europe, Asia, North America)

• Df (humid winter): poleward of marine west coast and mid-latitude steppe; cold snowy winters, short frost-free season, large annual range.
• Dw (dry winter): mainly north-eastern Asia; strong winter anticyclone gives a monsoon-like reversal of wind; up to seven months below freezing; summer rain, only 12-15 cm a year.

Group E: polar (beyond 70°)

• ET tundra: named after low mosses, lichens and flowering plants; permafrost; short growing season, waterlogging; very long summer daylight.
• EF ice cap: interior Greenland and Antarctica; below freezing even in summer; little precipitation; accumulating ice deforms and breaks into icebergs (e.g. Plateau Station, Antarctica, 79°S).

Worked example

Why does the Mediterranean (Cs) climate have dry summers and rainy winters? (2 marks)

Try it first, then show the solutionSolution

1. In summer these west-coast areas at 30°-40° come under the subtropical high, which brings dry, sinking air.
2. In winter the belts shift and the westerlies bring rain.

Answer: Subtropical high in summer, westerlies in winter.

Climate change: evidence and causes

NCERT section Climate Change

Change is natural and continuous. Today's type of climate has prevailed for about 10,000 years with minor, occasionally wide, fluctuations.

Evidence:

• Geological records of alternating glacial and inter-glacial periods.
• Landforms in high altitudes and latitudes showing glacier advances and retreats.
• Sediments in glacial lakes revealing warm and cold periods.
• Tree rings showing wet and dry periods.
• Historical records of climate's vagaries, crop yields and failures, floods and migrations.

Examples from the chapter:

• India: Rajasthan desert wet and cool around 8,000 BC; higher rainfall 3,000-1,700 BC; centre of the Harappan civilisation 2,000-1,700 BC; dry conditions since.
• Geological past: warm 500-300 million years ago (Cambrian, Ordovician, Silurian); Pleistocene glacials and inter-glacials; last major glacial peak about 18,000 years ago; present inter-glacial began 10,000 years ago.
• Recent past: extreme events in the 1990s (warmest temperatures of the century, severe floods); Sahel drought 1967-1977; dust bowl in the south-western Great Plains of the USA in the 1930s; Vikings settled in Greenland in the warm, dry 10th-11th centuries; Little Ice Age in Europe 1550-1850; world temperatures rose 1885-1940, more slowly after 1940.

Causes:

• Astronomical: (i) sunspots: dark, cooler patches that rise and fall cyclically; some meteorologists link more sunspots with cooler, wetter, stormier weather and fewer with warmer, drier weather, but the findings are not statistically significant; (ii) Milankovitch oscillations: cycles in the earth's orbit, its wobble and its axial tilt, which alter insolation.
• Terrestrial: volcanism: eruptions throw up aerosols that stay for long and reduce sunlight reaching the surface (after the Pinatubo and El Chichon eruptions, the earth's average temperature fell for some years).
• Anthropogenic: the most important is the rising concentration of greenhouse gases, likely to cause global warming.

Worked example

What climatic conditions might follow an increase in sunspots, and how reliable is this link? (2 marks)

Try it first, then show the solutionSolution

1. Some meteorologists associate more sunspots with cooler, wetter weather and greater storminess.
2. But NCERT notes the findings are not statistically significant, so the link is uncertain.

Answer: Cooler, wetter, stormier weather, but the evidence is not statistically significant.

Greenhouse effect, greenhouse gases and global warming

NCERT section Global Warming

Greenhouse effect: 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); their warming effect is the greenhouse effect. The name comes from a glass greenhouse: glass is transparent to short waves and opaque to long waves, so it stays warmer inside (as does a closed car in summer or winter).

Main GHGs: carbon dioxide (CO2), chlorofluorocarbons (CFCs), methane (CH4), nitrous oxide (N2O), ozone (O3). Nitric oxide (NO) and carbon monoxide (CO) react with GHGs and affect their concentration.

Effectiveness of a GHG depends on the rise in its concentration, its lifetime in the atmosphere and the wavelength it absorbs. CFCs are highly effective; ozone is very effective at absorbing terrestrial radiation in the lower troposphere. The longer a GHG stays, the longer the system takes to recover.

Carbon dioxide: the largest concentration among GHGs; from fossil fuel combustion (oil, gas, coal); forests and oceans are sinks; deforestation raises CO2. It takes 20-50 years to adjust to changes in sources and sinks; it is rising about 0.5 per cent a year. Doubling of CO2 over pre-industrial levels is used as an index in climate models.

CFCs and the ozone hole: CFCs are man-made; in the stratosphere ultraviolet rays convert oxygen into ozone, which stops UV reaching the surface. CFCs that drift up destroy ozone; large depletion over Antarctica is the ozone hole, letting UV pass into the troposphere.

International effort: the Kyoto Protocol (proclaimed 1997, in force 2005, ratified by 141 nations) bound 35 industrialised countries to cut emissions by 2012 to 5 per cent below 1990 levels.

Effects: once warming sets in it is hard to reverse; effects will not be uniform but will harm life-support systems; sea-level rise from melting glaciers and ice caps and thermal expansion may flood coasts and islands, causing social problems.

Temperature record: world average near-surface air temperature about 14°C; greatest warming in 1901-44 and 1977-99 (about 0.4°C each); end of the 20th century about 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.

Worked example

Why does cutting down forests increase the concentration of carbon dioxide? (2 marks)

Try it first, then show the solutionSolution

1. Forests are sinks for CO2: trees use it in their growth.
2. Removing forests (changes in land use) removes this sink, so more CO2 stays in the atmosphere.

Answer: Forests absorb CO2; deforestation removes a sink.

World Climate and Climate Change in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Climate Change: Evidence, Natural and Human Causes, the Greenhouse Effect and Global Warming
• Koeppen's Scheme: Classifying the Climates of the World into A, B, C, D and E Groups

World Climate and Climate Change: key facts

Koeppen groups

• •A coldest month ≥ 18°C
• •B evaporation > precipitation
• •C -3°C < coldest month < 18°C
• •D coldest month ≤ -3°C
• •E all months < 10°C

A, C, D, E humid; B dry.

Small letters

f no dry season; m monsoon; w winter dry; s summer dry; a-d temperature severity

B uses S (steppe) and W (desert); h low latitude, k mid-latitude.

Latitude bands

A between the tropics; B 15°-60°; C 30°-50°; D 40°-70°N; E beyond 70°

Approximate, from the chapter.

Climate history

Warm 500-300 Ma; last glacial peak 18,000 years ago; inter-glacial from 10,000 years ago; Little Ice Age 1550-1850

Ma = million years ago.

Causes of change

$Astronomical(sunspots,Milankovitch)+volcanism(aerosols)+anthropogenic(GHGs)$

Sunspot findings not statistically significant.

GHGs

CO2, CFCs, CH4, N2O, O3 (NO and CO affect them)

CO2 largest concentration; rising about 0.5% a year.

Warming record

Mean about 14°C; +0.4°C in 1901-44 and in 1977-99; about +0.6°C over the 20th century

1998 warmest year.

Kyoto Protocol

1. 1.Proclaimed 1997
2. 2.in force 2005
3. 3.141 nations
4. 4.35 industrialised countries cut to 5% below 1990 by 2012

International effort on GHGs.

World Climate and Climate Change: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Tricks to remember World Climate and Climate Change

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Koeppen groups

  'All Bears Can Dance Elegantly': A tropical, B dry, C warm temperate, D cold snow forest, E cold.

  Use it when: Order of groups.

• Season letters

  'f = full year wet, m = monsoon, w = winter dry, s = summer dry'.

  Use it when: Small letter codes.

• GHGs

  'Carbon Can Make Nasty Ozone': CO2, CFCs, Methane, Nitrous oxide, Ozone.

  Use it when: Listing GHGs.

• Causes of change

  'Spots, Swings, Smoke, Smog': sunspots, Milankovitch swings, volcanic aerosols, human GHGs.

  Use it when: Causes of climate change.

Where students lose marks in World Climate and Climate Change

The slips examiners see most often, and what to write instead.

• ✗Mistake: Calling Koeppen's scheme genetic

  It explains climates, so students think it is cause-based.

  Wrong working

  Koeppen's classification is genetic.

  Right working

  It is empirical: based on observed temperature and precipitation linked to vegetation.

  ✓Do this: Koeppen counted, not explained.

  Usually costs: Accuracy in the project report and viva.

• ✗Mistake: Listing B among humid groups

  Five letters, one odd one out.

  Wrong working

  Humid groups: A, B, C, D.

  Right working

  Humid: A, C, D, E. Dry: B.

  ✓Do this: B for Barren (dry).

  Usually costs: Accuracy in the viva.

• ✗Mistake: Mixing up the ozone hole and global warming

  Both are atmospheric problems involving gases.

  Wrong working

  The ozone hole is caused by carbon dioxide and causes global warming.

  Right working

  The ozone hole is caused by CFCs destroying stratospheric ozone (letting UV through); global warming is caused by rising GHGs trapping long-wave radiation.

  ✓Do this: CFCs eat ozone; GHGs trap heat.

  Usually costs: Accuracy in the project report.

• ✗Mistake: Overstating the sunspot link

  It sounds like a firm law.

  Wrong working

  More sunspots always cause cool, wet weather.

  Right working

  Some meteorologists associate them, but the findings are not statistically significant.

  ✓Do this: Add 'not statistically significant'.

  Usually costs: Accuracy in the presentation.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What are the three approaches to classifying climate?Show answer

   Empirical, genetic and applied.

2. 2.When did Koeppen develop his scheme?Show answer

   1918 (modified later).

3. 3.What defines Koeppen's group B?Show answer

   Potential evaporation exceeds precipitation.

4. 4.Where is Dw climate found?Show answer

   Mainly north-eastern Asia.

5. 5.When was the Little Ice Age in Europe?Show answer

   From 1550 to about 1850.

6. 6.What are Milankovitch oscillations?Show answer

   Cycles in the earth's orbit, its wobble and axial tilt that alter insolation.

7. 7.What creates the ozone hole?Show answer

   CFCs that drift into the stratosphere and destroy ozone, especially over Antarctica.

8. 8.What is the world's average near-surface air temperature?Show answer

   About 14°C.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Assessed only through a project and presentation in 2026-27.
• ☐Approaches: empirical, genetic, applied; Koeppen (1918) is empirical, linked to vegetation.
• ☐A ≥ 18°C coldest month; B dry; C -3°C to 18°C; D ≤ -3°C; E all months < 10°C.
• ☐f, m, w, s for seasonality; S and W for steppe and desert; h and k for latitude.
• ☐Af Amazon; Am India; Aw savanna; Cs Mediterranean; Cfb NW Europe; Dw NE Asia; ET tundra; EF ice caps.
• ☐Present climate about 10,000 years old; Little Ice Age 1550-1850; Rajasthan wet around 8,000 BC.
• ☐Causes: sunspots, Milankovitch oscillations, volcanic aerosols, GHGs.
• ☐GHGs: CO2, CFCs, CH4, N2O, O3; CO2 from fossil fuels, rising 0.5%/yr; forests and oceans are sinks.
• ☐CFCs make the ozone hole over Antarctica; Kyoto Protocol 1997 (in force 2005).
• ☐Mean temperature about 14°C; about 0.6°C warmer over the 20th century; 1998 warmest.

World Climate and Climate Change: frequently asked questions