Chapter 11 World Climate And Climate Change

Studying the climate of the entire world can be complex. To simplify this, scientists classify climates into smaller, manageable units based on shared characteristics. Various approaches exist for classifying climate:

• **Empirical Classification:** Based on observable climate data, primarily temperature and precipitation values. This focuses on the *effects* of climate.
• **Genetic Classification:** Attempts to classify climates based on the underlying *causes* of climate (e.g., atmospheric circulation patterns, latitude, proximity to oceans).
• **Applied Classification:** Designed for specific practical purposes (e.g., for agriculture, architecture, or engineering).

---

Koeppen’s Scheme Of Classification Of Climate

One of the most widely used climate classification systems is the **empirical scheme** developed by Russian-German climatologist **Wladimir Koeppen**. First introduced in 1918 and later refined, Koeppen's system is based on a perceived strong correlation between the distribution of vegetation types and climate conditions.

---

Koeppen used specific threshold values of average annual and average monthly **temperature** and **precipitation** to define climate zones, relating these numerical values to observed vegetation patterns globally. He used a system of capital and small letters to designate different climate groups and types.

---

Koeppen initially identified **five major climate groups**, based mostly on temperature characteristics, with one group defined by moisture deficiency (dryness). These major groups are designated by capital letters (Table 11.1):

Group	Characteristics
A - Tropical Humid Climates	Mean temperature of the coldest month is $18^\circ\text{C}$ or higher.
B - Dry Climates	Potential evaporation (amount of water that could evaporate) exceeds precipitation (water received). These are climates where dryness is the dominant feature.
C - Warm Temperate (Mid-latitude) climates	Average temperature of the coldest month is between $-3^\circ\text{C}$ and $18^\circ\text{C}$.
D - Cold Snow Forest Climates	Average temperature of the coldest month is $-3^\circ\text{C}$ or below.
E - Cold Climates	Average temperature for all months is below $10^\circ\text{C}$.
H - High Land Climates	Climates significantly influenced by elevation, typically resulting in lower temperatures regardless of latitude. (Often considered a separate category or a modifier).

---

These major groups (A, C, D, E, H - representing humid conditions; B - representing dry conditions) are further subdivided into specific climate types using small letters (Table 11.2).

---

Small letters often indicate precipitation patterns (seasonality of dryness):

• **f:** No dry season; sufficient precipitation throughout the year.
• **w:** Winter dry season.
• **s:** Summer dry season.
• **m:** Monsoon climate (short dry season, but heavy monsoon rain compensates).

---

Other small letters may indicate temperature severity, particularly in groups C and D:

• **a:** Hot summer (mean temperature of warmest month $\ge 22^\circ\text{C}$).
• **b:** Warm summer (mean temperature of warmest month $< 22^\circ\text{C}$, at least 4 months $\ge 10^\circ\text{C}$).
• **c:** Cool summer (less than 4 months $\ge 10^\circ\text{C}$, coldest month $\ge -38^\circ\text{C}$).
• **d:** Very cold winter (coldest month $< -38^\circ\text{C}$).

---

For Dry (B) climates, capital letters specify the degree of dryness:

• **S:** Steppe or semi-arid climate (grassland).
• **W:** Desert or arid climate (very dry).

---

These dry types (BS, BW) are then further subdivided using small letters (h, k) to indicate temperature:

• **h:** Hot dry (average annual temperature $\ge 18^\circ\text{C}$).
• **k:** Cold dry (average annual temperature $< 18^\circ\text{C}$).

---

Putting the letters together creates specific climate types (Table 11.2):

Group	Type Letter Code	Characteristics
A-Tropical Humid Climate	Af	Tropical wet, no dry season
	Am	Tropical monsoon, short dry season
	Aw	Tropical wet and dry, winter dry season
B-Dry Climate	BSh	Subtropical steppe (low-latitude semi-arid), hot dry
	BWh	Subtropical desert (low-latitude arid), hot dry
	BSk	Mid-latitude steppe, cold dry
	BWk	Mid-latitude desert, cold dry
C-Warm temperate (Mid-latitude) Climates	Cwa	Humid subtropical, winter dry, warm summer
	Cs	Mediterranean, dry hot summer, mild rainy winter
	Cfa	Humid subtropical, no dry season, mild winter, warm summer
	Cfb	Marine west coast, no dry season, warm and cool summer
D-Cold Snowforest Climates	Df	Cold climate with humid winter, severe winter
	Dw	Cold climate with dry winter, very severe winter
E-Cold Climates	ET	Tundra, no true summer (warmest month temp < 10°C)
	EF	Ice cap, perennial ice (all months temp < 0°C)
H-Highland	H	Highland climates influenced by elevation.

---

Let's examine some of these climate types in more detail.

---

Group A : Tropical Humid Climates

These climates are located in the tropics, generally between the Tropic of Cancer ($23.5^\circ$ N) and the Tropic of Capricorn ($23.5^\circ$ S). They are characterized by consistently **high temperatures** throughout the year (coldest month average $\ge 18^\circ\text{C}$) and **high humidity**. This is due to intense insolation (sun is high in the sky year-round) and the influence of the Inter Tropical Convergence Zone (ITCZ), a low-pressure zone with rising air and abundant rainfall. The annual range of temperature is very small in tropical climates, while annual rainfall is generally high. Group A is subdivided based on precipitation patterns:

• Af: Tropical Wet Climate
• Am: Tropical Monsoon Climate
• Aw: Tropical Wet and Dry Climate

---

Tropical Wet Climate (Af)

Found near the equator, this climate type is characterized by **high temperatures year-round** and **significant rainfall in every month**. Convectional thunderstorms are common in the afternoons, providing consistent moisture. Annual temperature range is minimal (e.g., daily maximum $\sim 30^\circ\text{C}$, daily minimum $\sim 20^\circ\text{C}$). Major regions include the Amazon Basin, equatorial Africa, and islands of Southeast Asia (East Indies). This climate supports dense tropical evergreen forests with high biodiversity.

---

Tropical Monsoon Climate (Am)

Distinguished by a pronounced **seasonal reversal of winds (monsoon)** and a distinct wet season (mostly summer) and a dry season (winter). Despite a dry season, total annual rainfall is usually high, supporting vegetation similar to tropical wet climates. Key regions include the Indian subcontinent, parts of Northeast South America, and Northern Australia.

---

Tropical Wet And Dry Climate (Aw)

Located north and south of the equatorial Af regions. Also known as the **savanna climate**. Characterized by high temperatures throughout the year, but with a marked **winter dry season** and a wet summer season. Total annual rainfall is lower and more variable than Af or Am climates, and droughts can be severe in the dry season. Diurnal (daily) temperature ranges are highest during the dry season. Found in extensive areas of Brazil (north and south of the Amazon), parts of Central and Southern Africa (Sudan, south of Central Africa), and borders dry climates to the west and temperate climates to the east. Vegetation typically consists of deciduous forests and grasslands with scattered trees.

---

Dry Climates : B

These climates are defined by a deficit of moisture, where **potential evaporation is greater than precipitation**. This means that even if some rain falls, the rate at which water evaporates and transpires from plants is higher, leading to dry conditions that limit plant growth. Dry climates cover vast areas, found in both low latitudes ($15^\circ - 30^\circ$) and mid-latitudes ($35^\circ - 60^\circ$).

---

In low latitudes, they occur in the belts of **subtropical high pressure**, where sinking air suppresses rainfall. On the western margins of continents in these latitudes, cold ocean currents contribute to dryness by stabilizing the air. In mid-latitudes, dry climates are often located deep in the interior of continents, far from oceanic moisture sources, or are surrounded by mountains that create rain shadows. Dry climates are subdivided into:

• BS: Steppe or semi-arid (receives some rain, supporting grasslands).
• BW: Desert or arid (very little rain, sparse vegetation).

---

Further subdivision uses temperature (h for hot, k for cold):

• BSh: Subtropical Steppe (hot semi-arid).
• BWh: Subtropical Desert (hot arid).
• BSk: Mid-latitude Steppe (cold semi-arid).
• BWk: Mid-latitude Desert (cold arid).

---

Subtropical Steppe (BSh) And Subtropical Desert (BWh) Climates

These hot dry climates are typically located between $15^\circ$ and $35^\circ$ latitude. BSh regions receive slightly more rainfall than BWh, supporting sparse grasslands (steppe) compared to the very scarce vegetation of deserts. Rainfall in both is highly variable and often occurs as intense, short-lived thunderstorms. In coastal deserts adjacent to cold ocean currents, fog can be common. Summer temperatures are extremely high; world record high temperatures are often found in BWh regions. Both climates experience high annual and diurnal (daily) temperature ranges.

---

Warm Temperate (Mid-Latitude) Climates-C

Found mainly on the eastern and western sides of continents between approximately $30^\circ$ and $50^\circ$ latitude. These climates are characterized by a distinct seasonality, with warm or hot summers and mild winters (coldest month average between $-3^\circ\text{C}$ and $18^\circ\text{C}$). They are influenced by both tropical air masses in summer and polar air masses/westerlies in winter. Four main types exist:

• Cwa: Humid Subtropical with dry winter.
• Cs: Mediterranean.
• Cfa: Humid Subtropical with no dry season.
• Cfb: Marine West Coast.

---

Humid Subtropical Climate (Cwa)

Located poleward of the tropics, primarily in inland parts of large continents in subtropical latitudes, especially in Asia (e.g., North Indian plains, South China interior). Similar to the Aw climate, it has a wet summer and a dry winter, but the key difference is that winters are warm (coldest month average $> -3^\circ\text{C}$). It is influenced by monsoon circulation in many areas.

---

Mediterranean Climate (Cs)

Found on the **west coasts** of continents in subtropical latitudes ($30^\circ - 40^\circ$), famously around the Mediterranean Sea. Other examples include Central California, Central Chile, and parts of southwestern Australia. This climate is controlled by subtropical highs during summer (causing hot, dry conditions) and influenced by the westerlies in winter (bringing mild, rainy conditions). Summers are hot and dry (average $\sim 25^\circ\text{C}$), and winters are mild and wet (average $< 10^\circ\text{C}$). Annual precipitation ranges from 35-90 cm.

---

Humid Subtropical (Cfa) Climate

Located on the **eastern parts** of continents in subtropical latitudes ($30^\circ - 40^\circ$). Examples include the southeastern USA, southeastern China, southern Japan, northeastern Argentina, and eastern Australia. These regions are influenced by moist, unstable air masses from the western side of subtropical high-pressure systems, resulting in rainfall distributed throughout the year, without a distinct dry season. Summers are warm to hot (mean monthly $\sim 27^\circ\text{C}$), and winters are mild (mean monthly $5^\circ - 12^\circ\text{C}$). Thunderstorms are common in summer, and frontal rain occurs in winter. Daily temperature range is small.

---

Marine West Coast Climate (Cfb)

Situated on the **west coasts** of continents poleward of the Mediterranean climate ($40^\circ - 50^\circ$, extending higher in Europe due to ocean influence). Major areas include northwestern Europe, the west coast of North America (north of California), southern Chile, southeastern Australia, and New Zealand. Characterized by the strong moderating influence of the ocean and prevailing westerlies, resulting in moderate temperatures year-round. Winters are mild for their latitude ($4^\circ - 10^\circ\text{C}$), and summers are warm to cool ($15^\circ - 20^\circ\text{C}$). Precipitation is distributed throughout the year, and annual/daily temperature ranges are small.

---

Cold Snow Forest Climates (D)

These climates are found in large continental areas of the Northern Hemisphere between $40^\circ$ and $70^\circ$ latitude. They are characterized by cold, snowy winters. The severity of winter is more pronounced at higher latitudes and further inland. Cold snow forest climates are divided into two main types based on winter precipitation:

• Df: Cold climate with humid winter (no dry season).
• Dw: Cold climate with dry winter.

---

Cold Climate With Humid Winters (Df)

Found poleward of marine west coast climates and mid-latitude steppes. Winters are very cold with significant snowfall and frost. There is no distinct dry season (precipitation throughout the year). Summers are short and cool. Annual temperature ranges are large due to continentality. Weather changes can be abrupt.

---

Cold Climate With Dry Winters (Dw)

Occurs primarily over northeastern Asia. Characterized by a pronounced winter anticyclone (high pressure) that brings very cold, dry conditions. Summers are relatively warm and receive most of the precipitation. Winters are extremely cold, with temperatures remaining below freezing for many months. Annual precipitation is low (12-15 cm), concentrated in summer.

---

Polar Climates (E)

These extremely cold climates are found poleward of $70^\circ$ latitude. They are defined by average temperatures for all months being below $10^\circ\text{C}$. Polar climates are divided into two types:

• ET: Tundra Climate.
• EF: Ice Cap Climate.

---

Tundra Climate (Et)

Located in high latitudes, characterized by very cold temperatures where the warmest month has an average temperature between $0^\circ\text{C}$ and $10^\circ\text{C}$. Precipitation is low, often falling as snow. A key feature is **permafrost**, where the ground remains permanently frozen beneath a thin surface layer that thaws in summer. The short, cool summers and waterlogging (due to permafrost preventing drainage) only support low-growing vegetation like mosses, lichens, and some flowering plants. Tundra regions experience very long daylight hours in summer.

---

Ice Cap Climate (Ef)

The most severe climate, found over the interior ice sheets of **Greenland and Antarctica**. All months have average temperatures **below $0^\circ\text{C}$**. Precipitation is very low (often falling as snow), but snow and ice accumulate over time due to the lack of melting. The immense weight of the ice causes it to flow outwards, forming glaciers and eventually breaking off into icebergs that float in polar waters.

---

Highland Climates (H)

These climates are not primarily determined by latitude but by **high elevation (altitude)** and mountainous topography. Temperature decreases significantly with height (following the lapse rate). Precipitation patterns and intensity also vary greatly due to orography (mountains forcing air to rise). Highland regions often exhibit a **vertical zonation** of climate and vegetation, where different climate types (similar to those found at increasing latitudes) occur in distinct bands as elevation increases on a mountainside.

---

Climate Change

While climate classification describes current or recent average climate conditions, it's crucial to understand that Earth's climate is not static but has undergone significant **changes** throughout its history. The climate experienced today, roughly stable for the last 10,000 years (the Holocene epoch), is just one phase in a constantly changing system.

---

Evidence for past climate change comes from various sources:

• **Geological Records:** Show alternating periods of extensive glaciation (glacial periods or ice ages) and warmer periods (inter-glacial periods).
• **Geomorphological Features:** Landforms in high mountains and high latitudes (like glacial valleys, moraines) show evidence of past glacier advances and retreats.
• **Sediment Deposits:** Layers of sediment in lakes and oceans contain clues about past environmental and climatic conditions (e.g., fossil pollen, types of micro-organisms).
• **Tree Rings:** Annual growth rings in trees provide proxy data about past wet and dry periods, as growth rates are influenced by moisture availability and temperature.
• **Historical Records:** Written accounts, such as descriptions of crop failures, floods, or migrations, provide qualitative information about climate variations in the recent past.

---

These lines of evidence demonstrate that climate change is a **natural and continuous process** on Earth. For example, archaeological findings in India suggest that the Rajasthan region, now a desert, had a wetter and cooler climate around 8,000 BC and experienced higher rainfall around 3,000-1,700 BC (the period of the Harappan civilization), with conditions becoming drier thereafter. Globally, the Pleistocene epoch (2.6 million to 11,700 years ago) was marked by repeated glacial-interglacial cycles. The last major glacial peak was around 18,000 years ago, and the current inter-glacial period began approximately 10,000 years ago.

---

Climate In The Recent Past

Even within the current inter-glacial period, climate has shown variability over shorter timescales. The late 20th century, particularly the 1990s, saw an increase in extreme weather events and recorded the warmest temperatures of the century. Examples of recent climate variability include severe droughts (like the Sahel region in the 1960s-70s or the Dust Bowl in the US in the 1930s) and periods of significant warming or cooling recorded in historical documents (e.g., the Medieval Warm Period, the Little Ice Age in Europe from 1550-1850). Global temperature records show a warming trend from the late 19th century, though with fluctuations (e.g., a slight cooling period after 1940).

---

Causes Of Climate Change

Climate change can be attributed to various factors, broadly categorized as **astronomical** and **terrestrial** causes.

• **Astronomical Causes:**
  • **Changes in Solar Output (Sunspot Activity):** Sunspots are darker, cooler areas on the sun's surface that occur in cycles (roughly 11 years). Variations in the number of sunspots are associated with slight changes in the amount of energy emitted by the sun. Some studies suggest links between sunspot cycles and variations in Earth's temperature and storminess, though these links are not always statistically significant for long-term climate change.
  • **Milankovitch Oscillations:** These are long-term, cyclical changes in the Earth's orbital parameters: (i) the eccentricity of Earth's orbit (how elliptical vs. circular it is, with cycles of $\sim 100,000$ years); (ii) the obliquity or tilt of Earth's axis (which varies between $22.1^\circ$ and $24.5^\circ$ over $\sim 41,000$ years); and (iii) the precession of the equinoxes (a wobble in the Earth's axis direction, with cycles of $\sim 26,000$ years). These orbital variations alter the amount and distribution of solar radiation received at different latitudes and seasons, influencing the timing and severity of glacial and inter-glacial periods.
• **Terrestrial Causes:**
  • **Volcanism:** Large volcanic eruptions inject significant amounts of dust and sulfate aerosols (tiny particles) into the stratosphere. These aerosols can remain in the upper atmosphere for several years, reflecting incoming solar radiation back to space. This reduces the amount of sunlight reaching the Earth's surface, leading to a temporary cooling effect (e.g., after the Pinatubo eruption in 1991, global temperatures cooled slightly for a few years).
  • **Changes in Atmospheric Composition (Greenhouse Gases):** This is considered the most important cause of recent climate change, largely driven by **anthropogenic** (human) activities. The increasing concentration of greenhouse gases (GHGs) in the atmosphere enhances the greenhouse effect, trapping more heat and leading to global warming.
  • **Changes in Land Use:** Deforestation, urbanization, and changes in land cover alter the Earth's surface properties (albedo, roughness, evaporation), influencing local and regional climate. Deforestation also reduces the removal of $\text{CO}_2$ from the atmosphere.

---

Global Warming

The Earth's atmosphere naturally contains gases that absorb and re-emit longwave radiation, keeping the planet warmer than it would be otherwise. This is the **natural greenhouse effect**, essential for life. However, increasing the concentration of these **greenhouse gases (GHGs)** in the atmosphere enhances this effect, leading to the trapping of more heat and a rise in the Earth's average temperature, a phenomenon known as **global warming**.

---

The analogy used is a **greenhouse** made of glass. Glass is transparent to incoming shortwave sunlight, which warms the interior. However, the glass is opaque to the outgoing longwave heat radiated by the plants and surfaces inside. This heat is trapped, warming the greenhouse interior. Similarly, certain atmospheric gases are transparent to incoming solar radiation but absorb outgoing terrestrial radiation, trapping heat in the atmosphere.

---

Examples of the greenhouse effect in everyday life include how the interior of a car parked in the sun becomes much hotter than the outside air, even with windows closed, because glass traps the outgoing heat.

---

Greenhouse Gases(Ghgs)

The main greenhouse gases of concern in the context of recent climate change due to human activities are:

• **Carbon Dioxide ($\text{CO}_2$):** The most abundant anthropogenic GHG. Primarily released by the burning of fossil fuels (coal, oil, natural gas) for energy, transportation, and industry, as well as by deforestation and land-use change. Forests and oceans act as major natural 'sinks' for $\text{CO}_2$, absorbing it from the atmosphere. However, emissions now exceed the capacity of these sinks. Atmospheric $\text{CO}_2$ concentration has been rising steadily, currently increasing by about 0.5% annually. Its atmospheric lifetime is long (20-50 years for a pulse, longer for full removal).
• **Methane ($\text{CH}_4$):** Produced by sources like wetlands, livestock digestion, rice cultivation, landfill decomposition, and natural gas leaks. It is a more potent GHG per molecule than $\text{CO}_2$, though its atmospheric concentration is much lower and its lifetime shorter (around 12 years).
• **Nitrous Oxide ($\text{N}_2\text{O}$):** Released from sources including agricultural soils (fertilizer use), livestock manure, fossil fuel combustion, and industrial processes. It is also a potent GHG with a long atmospheric lifetime (over 100 years).
• **Chlorofluorocarbons (CFCs):** Synthetic chemicals previously used in refrigerants, aerosols, and foam blowing. They are very potent GHGs with very long lifetimes (decades to centuries). CFCs also deplete the stratospheric ozone layer.
• **Ozone ($\text{O}_3$):** While stratospheric ozone is beneficial for absorbing UV radiation, **tropospheric ozone** (in the lower atmosphere) is a pollutant formed by reactions involving other pollutants. It is a GHG and harmful to respiratory health and plants. Its lifetime is relatively short (days to weeks).

---

Other gases like carbon monoxide ($\text{CO}$) and nitric oxide ($\text{NO}$) are not direct GHGs but influence the concentration of GHGs through chemical reactions in the atmosphere.

---

The overall impact of a GHG on climate depends on its concentration increase, its ability to absorb radiation at certain wavelengths, and its lifespan in the atmosphere. CFCs, despite lower concentrations than $\text{CO}_2$, are very effective heat absorbers. The longer a GHG molecule remains in the atmosphere, the longer its warming effect persists and the harder it is for the climate system to recover.

---

A major concern related to CFCs is the **depletion of the stratospheric ozone layer**, particularly over Antarctica, creating an **ozone hole**. While distinct from global warming (GHGs cause warming, CFCs cause ozone depletion), both are serious atmospheric issues. The ozone layer absorbs UV rays; its depletion allows more UV rays to reach the surface. International efforts, like the **Montreal Protocol** (not mentioned in text, but key alongside Kyoto for atmospheric protection), successfully phased out most CFCs.

---

International agreements aim to address GHG emissions. The **Kyoto Protocol**, adopted in 1997 and effective from 2005, committed industrialized countries (listed in Annex I) to reduce their collective GHG emissions by 5% below 1990 levels by the period 2008-2012. While its effectiveness and future remain debated, it was a landmark step in international climate policy.

---

Observed warming trends support concerns about global warming. The Earth's average near-surface air temperature is currently around $14^\circ\text{C}$. The 20th century saw a clear warming trend, particularly during 1901-1944 and 1977-1999, with temperatures rising about $0.4^\circ\text{C}$ in each period. The globally averaged annual mean temperature at the end of the 20th century was approximately $0.6^\circ\text{C}$ higher than at the end of the 19th century. The 1990s were notably warm, with 1998 being the warmest year in recent records at the time of the text's writing. Continued increases in GHG concentrations are projected to cause further warming in the future.

---

Global warming is predicted to have widespread and potentially severe adverse effects on life support systems and human societies. Consequences include rising sea levels (due to thermal expansion of warming water and melting glaciers/ice sheets), which threaten coastal areas and islands with inundation, leading to social and economic disruption. Changes in precipitation patterns, increased frequency of extreme weather events, impacts on ecosystems and agriculture are also major concerns. Addressing global warming requires significant international cooperation to reduce GHG emissions and transition towards sustainable practices to ensure a livable planet for future generations.

---

Answer:

---

Exercises

Multiple Choice Questions

(Exercise questions are not included as per instructions.)

---

Answer The Following Questions In About 30 Words

(Exercise questions are not included as per instructions.)

---

Answer The Following Questions In About 150 Words

(Exercise questions are not included as per instructions.)

---

Project Work

(Project work details are not included as per instructions.)