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World Climate and Climate Change

NCERT Class 11 · Geography Based on NCERT Class 11 Geography textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains how climate is classified globally, focusing on Koeppen's empirical classification scheme based on temperature and precipitation. It also covers the causes and evidence of climate change, including natural factors like sunspots and volcanism, and anthropogenic factors like greenhouse gas emissions. The one big idea a student must walk away with is that climate is not static; it varies naturally and is now being significantly altered by human activities, particularly through global warming.
Key Concepts (12)
Empirical classification
A classification of climate based on observed data, particularly on temperature and precipitation.
e.g. Koeppen's scheme uses mean annual and mean monthly temperature and precipitation data to classify climates.
Genetic classification
A classification that attempts to organise climates according to their causes.
e.g. Classifying climates based on whether they are caused by tropical or polar air masses.
Applied classification
A classification of climate for a specific purpose.
e.g. Classifying climate for agricultural planning or building design.
Koeppen's scheme
An empirical climate classification scheme developed by V. Koeppen, based on mean annual and mean monthly temperature and precipitation data, and the relationship between vegetation and climate.
e.g. Using capital letters A, C, D, E for humid climates and B for dry climates.
Inter Tropical Convergence Zone (ITCZ)
A zone near the equator where the trade winds converge, causing hot and humid conditions with high rainfall.
e.g. The ITCZ is responsible for the heavy rainfall in the Amazon Basin and equatorial Africa.
Greenhouse effect
The process by which the atmosphere transmits incoming solar radiation but absorbs the vast majority of long wave radiation emitted upwards by the earth's surface, warming the atmosphere.
e.g. A car with closed windows on a sunny day becomes hotter inside than outside due to the greenhouse effect.
Greenhouse gases (GHGs)
Gases that absorb long wave radiation emitted by the earth's surface, causing the greenhouse effect.
e.g. Carbon dioxide (CO2), methane (CH4), and chlorofluorocarbons (CFCs) are greenhouse gases.
Ozone hole
The depletion of ozone concentration in the stratosphere, allowing ultraviolet rays to pass through the troposphere.
e.g. Large depletion of ozone occurs over Antarctica, creating the ozone hole.
Kyoto protocol
An international treaty proclaimed in 1997 that bounds 35 industrialised countries to reduce their greenhouse gas emissions by 5% less than 1990 levels by 2012.
e.g. The Kyoto protocol went into effect in 2005 and was ratified by 141 nations.
Millankovitch oscillations
Cycles in the variations in the earth's orbital characteristics around the sun, the wobbling of the earth, and changes in the earth's axial tilt that alter the amount of insolation received from the sun.
e.g. These oscillations are an astronomical cause of climate change.
Sunspots
Dark and cooler patches on the sun which increase and decrease in a cyclical manner, associated with changes in solar output.
e.g. An increase in sunspots is associated with cooler and wetter weather and greater storminess.
Permafrost
Permanently frozen subsoil found in tundra regions.
e.g. In the Tundra climate (ET), the subsoil is permanently frozen, supporting only low growing plants like mosses and lichens.
How It Works
Koeppen's Climate Classification Process
  1. Step 1: Identify the five major climatic groups (A, B, C, D, E) based on temperature and precipitation criteria.
  2. Step 2: For group A, C, D, and E (humid climates), use capital letters; for B (dry climates), use capital letters S (steppe) and W (desert).
  3. Step 3: Subdivide groups into types using small letters: f (no dry season), m (monsoon), w (winter dry), s (summer dry) for precipitation seasonality.
  4. Step 4: Further subdivide using small letters a, b, c, d for the degree of severity of temperature.
  5. Step 5: Assign the final code (e.g., Af, BSh, Cfa) based on the combination of group and type characteristics.
💡 Students often forget that 'f' means no dry season, not 'frequent rain'. Also, 'w' and 's' refer to dry seasons, not wet seasons.
Greenhouse Effect Mechanism
  1. Step 1: Incoming short-wave solar radiation passes through the atmosphere and reaches the earth's surface.
  2. Step 2: The earth's surface absorbs this radiation and emits long-wave (infrared) radiation upward.
  3. Step 3: Greenhouse gases in the atmosphere (e.g., CO2, CH4, CFCs) absorb this long-wave radiation.
  4. Step 4: The absorbed energy warms the atmosphere, which then re-emits radiation in all directions, including back toward the earth's surface.
  5. Step 5: This trapping of heat keeps the earth's surface warmer than it would be without the greenhouse gases.
💡 Students often confuse the greenhouse effect with global warming. The greenhouse effect is a natural process; global warming is the enhanced effect due to increased GHGs from human activities.
Ozone Depletion by CFCs
  1. Step 1: CFCs (chlorofluorocarbons) are released into the atmosphere from human activities.
  2. Step 2: CFCs drift into the stratosphere.
  3. Step 3: In the stratosphere, CFCs destroy ozone molecules.
  4. Step 4: This depletion of ozone concentration is called the ozone hole.
  5. Step 5: The ozone hole allows ultraviolet rays to pass through the troposphere, reaching the earth's surface.
💡 Students often think the ozone hole is a literal hole. It is actually a thinning or depletion of ozone concentration, not a complete absence.
Volcanism as a Cause of Climate Change
  1. Step 1: A volcanic eruption throws up lots of aerosols into the atmosphere.
  2. Step 2: These aerosols remain in the atmosphere for a considerable period.
  3. Step 3: The aerosols reduce the sun's radiation reaching the Earth's surface.
  4. Step 4: This leads to a temporary cooling of the earth's average temperature.
💡 Students often think volcanism only warms the climate. In fact, volcanic aerosols can cause short-term cooling by blocking sunlight.
Exam Traps — Don't Make These Mistakes
⚠ Confusing 'f' with 'frequent rain'
✗ Af means frequent rain all year.
→ In Koeppen's scheme, 'f' stands for 'no dry season', not 'frequent'. It means precipitation is distributed throughout the year without a distinct dry period.
✓ Af indicates a tropical wet climate with no dry season.
⚠ Mixing up 'w' and 's' for dry seasons
✗ Aw means summer dry season.
→ In Koeppen's scheme, 'w' stands for winter dry season, and 's' stands for summer dry season. Aw has a dry winter, not a dry summer.
✓ Aw indicates a tropical wet and dry climate with a winter dry season.
⚠ Confusing greenhouse effect with global warming
✗ The greenhouse effect is bad and causes global warming.
→ The greenhouse effect is a natural and essential process that keeps the earth warm enough for life. Global warming refers to the enhanced greenhouse effect due to increased concentrations of greenhouse gases from human activities.
✓ The greenhouse effect is a natural process that warms the earth; global warming is the accelerated warming due to anthropogenic emissions of GHGs.
⚠ Thinking the ozone hole is a literal hole
✗ The ozone hole is a hole in the ozone layer through which UV rays enter.
→ The ozone hole is not a literal hole but a region of significantly depleted ozone concentration, particularly over Antarctica.
✓ The ozone hole refers to the depletion of ozone concentration in the stratosphere, allowing more UV radiation to reach the earth.
⚠ Misidentifying the warmest year
✗ The warmest year of the 20th century was 1990.
→ According to the chapter, the year 1998 was the warmest year, not only for the 20th century but possibly for the whole millennium.
✓ The year 1998 recorded the warmest temperature, probably for the entire millennium.
Compare & Contrast
Basis of classification
Empirical classification
Genetic classification
Empirical classification is based on observed data (e.g., temperature and precipitation), while genetic classification is based on the causes of climate (e.g., air masses).
Seasonality of rainfall
Tropical Wet Climate (Af)
Tropical Monsoon Climate (Am)
Af has no dry season with rainfall occurring every month, while Am has a short dry season and heavy rainfall mostly in summer.
Amount of rainfall
Steppe (BS) climate
Desert (BW) climate
Steppe (semi-arid) receives slightly more rainfall than desert (arid), adequate for sparse grasslands, while desert receives very low rainfall inadequate for plant growth.
Summer temperature
Tundra (ET) climate
Ice Cap (EF) climate
In Tundra climate, summer temperatures are above freezing allowing low-growing vegetation, while in Ice Cap climate, even summer temperatures are below freezing with no vegetation.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 SA (3 marks), 1 LA (5 marks)
Most asked: Koeppen's classification scheme and its groups (A, B, C, D, E) · Greenhouse gases and the greenhouse effect · Causes of climate change (astronomical and terrestrial) · Global warming and its effects
Don't skip: Explain Koeppen's classification of climate with examples of each group.; What are greenhouse gases? Explain the greenhouse effect and its role in global warming.; Differentiate between empirical and genetic classification of climate.
One-Line Revision (read aloud before the exam)
  1. Koeppen's empirical classification uses mean annual and mean monthly temperature and precipitation data.
  2. The five major climatic groups in Koeppen's scheme are A (Tropical), B (Dry), C (Warm Temperate), D (Cold Snow Forest), and E (Cold).
  3. Group A climates have an average temperature of the coldest month of 18°C or higher.
  4. Group B climates are dry, where potential evaporation exceeds precipitation.
  5. Group C climates have the coldest month temperature between -3°C and 18°C.
  6. Group D climates have the coldest month temperature at -3°C or below.
  7. Group E climates have average temperature for all months below 10°C.
  8. The greenhouse effect is the natural process where greenhouse gases absorb long-wave radiation and warm the atmosphere.
  9. Carbon dioxide (CO2) is the largest concentration of greenhouse gases in the atmosphere.
  10. The Kyoto protocol (1997) bounds industrialised countries to reduce emissions by 5% below 1990 levels by 2012.
  11. The year 1998 was the warmest year recorded in the 20th century and possibly the whole millennium.
  12. Volcanic eruptions can cause short-term cooling by releasing aerosols that block sunlight.
  13. Millankovitch oscillations refer to cycles in earth's orbital characteristics that affect insolation.
  14. The ozone hole is the depletion of ozone in the stratosphere, allowing UV rays to reach the earth.

MCQs — 10 Questions with Answers

Q1. According to Koeppen's classification, what is the defining characteristic of Group A (Tropical) climates?

  • A. The average temperature of the coldest month is 18°C or higher. ✓
  • B. Potential evaporation exceeds precipitation throughout the year.
  • C. The average temperature of all months is below 10°C.
  • D. The coldest month has an average temperature between -3°C and 18°C.

Answer: A — Group A tropical climates are defined by an average coldest month temperature of 18°C or above, indicating year-round warm conditions.

Q2. Which of the following statements about Tropical Wet Climate (Af) is correct?

  • A. It occurs between 35°–60° latitude and has seasonal dry periods.
  • B. It is found near the equator with rainfall in every month from afternoon thundershowers and uniform high temperatures. ✓
  • C. It has a pronounced winter dry season and supports deciduous forests.
  • D. It occurs in subtropical high-pressure zones with very limited annual rainfall.

Answer: B — Af climate is equatorial with year-round afternoon thundershowers, uniformly high temperatures (20–30°C), and dense evergreen forest vegetation.

Q3. Subtropical steppe (BSh) and subtropical desert (BWh) climates are located at the same latitude range. How do they differ?

  • A. BSh has more precipitation and represents a transition zone between humid and dry climates. ✓
  • B. BWh receives more annual rainfall due to monsoon influence.
  • C. BSh occurs only on coastal regions while BWh occurs in continental interiors.
  • D. BWh has lower average temperatures due to increased cloud cover and moisture.

Answer: A — Subtropical steppe (BSh) receives slightly more precipitation than subtropical desert (BWh) and forms the transition zone between humid and dry climates.

Q4. At 15°–30° latitude, dry climates (B) are primarily caused by which atmospheric phenomenon?

  • A. Strong monsoon winds that reverse seasonally and block precipitation.
  • B. Subtropical high-pressure zones with subsidence and temperature inversion that prevent rainfall. ✓
  • C. Cold ocean currents that increase evaporation rates dramatically.
  • D. Permanent ITCZ (Inter Tropical Convergence Zone) that diverts moisture poleward.

Answer: B — Subtropical high-pressure systems at 15–30° latitude create subsidence and temperature inversion, which suppress cloud formation and prevent rainfall.

Q5. In Koeppen's classification system, what do the small letters 'f', 'w', 's', and 'm' represent?

  • A. The latitude bands where each climate type is found.
  • B. The degree of temperature severity within each climatic group.
  • C. The seasonality and type of dry season (or lack thereof) in the climate. ✓
  • D. The percentage of annual precipitation that falls in each season.

Answer: C — Small letters f (no dry season), m (monsoon), w (winter dry), and s (summer dry) indicate the precipitation seasonality pattern in Koeppen's system.

Q6. Which climate type would you expect to find in the Amazon Basin and parts of equatorial Africa, and what vegetation would dominate?

  • A. Aw climate with deciduous forests and grasslands due to pronounced seasonality.
  • B. Af climate with dense evergreen forests due to year-round rainfall and high temperatures. ✓
  • C. Am climate with monsoon-driven rainfall and coniferous forests.
  • D. BSh climate with savanna grasslands adapted to semi-arid conditions.

Answer: B — The Amazon Basin and equatorial Africa have Af (tropical wet) climate with continuous rainfall, high temperatures, and dense evergreen forest vegetation.

Q7. Tropical Monsoon Climate (Am) differs from Tropical Wet and Dry (Aw) in several ways. Which statement about this difference is NOT correct?

  • A. Am receives heavy rainfall concentrated in summer with a defined dry winter, whereas Aw has a shorter wet season and longer dry season.
  • B. Am supports dense evergreen forests in all regions, while Aw supports deciduous forests and grasslands. ✓
  • C. Am is found over the Indian subcontinent and northeastern South America, while Aw occurs north and south of equatorial regions.
  • D. Both Am and Aw have high temperatures throughout the year with low annual temperature range.

Answer: B — Am does not support dense evergreen forests everywhere; vegetation depends on the length and intensity of the dry season, and both climates have high year-round temperatures.

Q8. Koeppen's classification system is described as 'empirical.' What does this mean about his approach? (Assertion-style question)

  • A. He classified climates based on the underlying causes of atmospheric circulation patterns.
  • B. He organized climates according to the genetic origin of air masses.
  • C. He based his classification on observed data of temperature and precipitation, then linked them to vegetation distribution. ✓
  • D. He developed theoretical models of climate without reference to real-world observational data.

Answer: C — Empirical classification relies on observed temperature and precipitation data rather than theoretical causes; Koeppen selected specific thresholds and related them to vegetation.

Q9. Study the climate characteristics: Annual rainfall = 450 mm, coldest month = 22°C, wettest season = summer, driest season = winter. Which Koeppen climate type best matches this profile? (Numerical/analytical MCQ)

  • A. Af (Tropical Wet) because temperature is above 18°C and rainfall occurs.
  • B. Aw (Tropical Wet-Dry) because the coldest month is above 18°C with winter dry season and relatively low annual rainfall of 450 mm. ✓
  • C. Am (Tropical Monsoon) because heavy summer rainfall dominates and temperature remains high.
  • D. Cw (Warm Temperate) because there is a dry winter and moderate rainfall.

Answer: B — A coldest month of 22°C (≥18°C) places it in Group A; 450 mm with winter dry season and shorter wet season fits Aw rather than Af or Am.

Q10. Why do dry climates (B) occur not only at 15°–30° latitude but also at 35°–60° latitude in continental interiors? (HOTS: multi-step reasoning)

  • A. Because both latitude bands receive direct overhead sun at some point during the year.
  • B. At 15–30°, subtropical highs cause subsidence; at 35–60°, continents are geographically isolated from maritime-humid winds and surrounded by mountains that create rain shadow effects. ✓
  • C. Cold ocean currents dominate both latitude ranges, causing extreme evaporation.
  • D. The ITCZ shifts to both latitude bands seasonally, preventing moisture transport.

Answer: B — Low-latitude dryness results from atmospheric subsidence at subtropical highs; mid-latitude dryness results from continental isolation and orographic barriers, showing how different mechanisms produce similar dry climates at different latitudes.

Flashcards

What are the three main approaches to climate classification?

Empirical (observed temperature and precipitation data), genetic (causes of climate), and applied (for specific practical purposes).

What relationship did Koeppen identify between climate and geography?

Koeppen found a close relationship between climate distribution and vegetation distribution, using temperature and precipitation values to classify climates.

What do capital letters A, B, C, D, E represent in Koeppen's scheme?

A, C, D, E represent the five major climatic groups based on temperature and precipitation characteristics, with B specifically denoting dry climates.

What is the characteristic feature of Tropical Wet Climate (Af)?

Found near the equator with rainfall every month from afternoon thunder showers, uniform high temperatures (20–30°C range), and supporting dense evergreen forests.

How does Tropical Monsoon Climate (Am) differ from Tropical Wet (Af) in precipitation?

Am has heavy rainfall concentrated in summer with a dry winter, whereas Af receives significant rainfall throughout all months of the year.

Where is Tropical Wet and Dry Climate (Aw) located geographically?

Aw occurs north and south of Af climate regions, found extensively around the Amazon Basin, Sudan, and Central Africa with a shorter wet and longer dry season.

Why do dry climates (B) occur at 15°–30° latitude?

Subtropical high-pressure zones at 15°–30° latitude cause subsidence and temperature inversion, which prevent rainfall formation.

What is the difference between subtropical steppe (BSh) and subtropical desert (BWh)?

Subtropical steppe (BSh) receives slightly more precipitation than subtropical desert (BWh), occupying the transition zone between humid and dry climates.

What does the small letter 'w' signify in Koeppen's climate designation system?

The small letter 'w' indicates a winter dry season in the climate classification.

What vegetation is characteristic of Tropical Wet and Dry Climate (Aw)?

Deciduous forests and tree-shredded grasslands are found in Aw climates due to the distinct wet and dry seasons.

Important Board Questions

Define Tropical Wet Climate (Af) and state two key characteristics that distinguish it from Tropical Wet-Dry Climate (Aw). [2 marks]

Af: equatorial location with year-round rainfall from thundershowers; Aw: north/south of equator with winter dry season and shorter wet period. Compare annually vs seasonally.

Explain why dry climates (B) occur at both low latitudes (15°–30°) and mid-latitudes (35°–60°), describing the atmospheric mechanisms responsible for each. Provide one geographic example for each latitude band. [5 marks]

Low-latitude dryness: subtropical high-pressure zones, subsidence, temperature inversion (e.g., Sahara at 15–30° N). Mid-latitude dryness: continental interior isolation, orographic barriers blocking maritime moisture (e.g., Central Asia at 35–60° N). Show how different causes produce same result.

Koeppen's climate classification system uses both capital and small letters to designate climate groups and types. Explain the meaning of capital letters, and then discuss how the small letters 'f', 'w', 's', and 'm' refine the classification. Using Tropical climates (Af, Am, Aw) as examples, demonstrate how small letters distinguish between different seasonal precipitation patterns and resulting vegetation types. [6 marks]

Capital letters (A, B, C, D, E) = primary climate groups based on temperature/precipitation thresholds. Small letters = seasonality: f (no dry), m (monsoon), w (winter dry), s (summer dry). Af = year-round rain → evergreen forest; Am = summer heavy, winter dry → monsoon forest; Aw = winter dry, shorter wet → deciduous forest + grassland. Link each to vegetation outcome.

Assertion-Reason Questions for World Climate and Climate Change

Q1. Assertion (A):

Tropical wet climate (Af) has a negligible annual temperature range with maximum temperatures around 30°C and minimum temperatures around 20°C.

Reason (R):

The overhead sun position and Inter Tropical Convergence Zone (ITCZ) throughout the year maintain consistently high temperatures with minimal seasonal variation in equatorial regions.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why Af climate experiences negligible temperature range. The consistent sun angle and ITCZ presence directly cause the uniform high temperatures characteristic of Af climates.

Q2. Assertion (A):

Dry climates at middle latitudes (35° to 60°) occur primarily in continental interiors surrounded by mountains, while low-latitude dry climates occur in subtropical high-pressure zones.

Reason (R):

Dry climates at all latitudes form due to cold ocean currents that reduce evaporation and prevent moisture transport to land.

Answer: C — A is true but R is false — The assertion is true and correctly identifies different mechanisms for dry climates at different latitudes. However, the reason is false—cold currents contribute to some coastal aridity but are not the primary cause of continental interior deserts. The reason misrepresents the text's explanation of continentality and orographic effects.

Q3. Assertion (A):

Köppen's climate classification scheme uses both capital letters and small letters in a hierarchical system to organize climate information from broad categories to specific types.

Reason (R):

Capital letters designate the five major climatic groups (A, B, C, D, E) based on temperature and precipitation characteristics, while small letters indicate subdivisions based on seasonality of precipitation and temperature severity.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason provides the correct explanation for why Köppen used this hierarchical lettering system. The capital-to-lowercase progression from broad climate groups to specific types is the fundamental organizational principle of his scheme.

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Case-Based Questions (CBQ) for World Climate and Climate Change

Case Study:

A meteorologist studying climate patterns across South America collected data from three locations at different latitudes. Location 1 (near Manaus, Brazil, 3°S) showed consistent rainfall throughout the year averaging 240mm monthly with temperatures consistently 25-30°C. Location 2 (12°S of the Amazon, in Brazil) recorded highest rainfall from November to March (400mm+) and very low rainfall June-August (20mm), with annual rainfall around 1600mm and temperatures of 20-32°C. Location 3 (25°S, in Paraguay) showed annual rainfall of 1200mm concentrated in summer months, with winter temperatures dropping to 5°C and summer temperatures reaching 28°C. Using Köppen's classification scheme, these three locations represent different climate responses to latitudinal position and seasonal atmospheric circulation patterns.

Q1. Based on the rainfall and temperature data, Location 1 would be classified as which Köppen climate type?

  • A. Af (Tropical Wet) ✓
  • B. Am (Tropical Monsoon)
  • C. Aw (Tropical Wet and Dry)
  • D. BSh (Subtropical Steppe)

Answer: A

Q2. Why does Location 2 experience a pronounced dry season (June-August) while Location 1, at a similar longitude but slightly higher latitude, receives consistent rainfall?

  • A. Location 2 is outside the Amazon Basin while Location 1 is within it, affecting moisture availability
  • B. The ITCZ remains over Location 1 year-round but shifts seasonally away from Location 2, causing the dry season ✓
  • C. Location 2 has higher elevation which creates orographic drying effects
  • D. Temperature differences of more than 10°C between the locations determine rainfall distribution

Answer: B

Q3. How would Location 3 be classified, and what atmospheric feature primarily influences its climate pattern?

  • A. Cfa climate influenced by year-round maritime tropical air masses
  • B. Aw climate influenced by the seasonal shift of the ITCZ and subtropical high-pressure systems ✓
  • C. BWh climate influenced by the South Atlantic subtropical high-pressure zone
  • D. Cfb climate influenced by mid-latitude westerly winds

Answer: B

Q4. Which Köppen classification principle best explains the progression from Location 1 (Af) through Location 2 (Aw) to Location 3 (Cfa or Cfb)?

  • A. Increasing elevation creates cooler temperatures and reduced rainfall as one moves south
  • B. The ITCZ intensity weakens systematically with each degree of latitude change
  • C. Ocean currents become colder toward higher latitudes, reducing evaporation and changing climate types
  • D. Decreasing tropical influence and increasing influence of subtropical and mid-latitude systems as latitude increases, combined with seasonal changes in wind patterns ✓

Answer: D

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