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Chapter 12 World Climate And Climate Change
Understanding the world's diverse climates involves organizing vast amounts of meteorological information and data into simpler, manageable units. Climate classifications provide a framework for describing, analyzing, and comparing different climatic regions across the globe. Broadly, there are three main approaches to classifying climate:
- Empirical Classifications: Based on observed data, primarily focusing on measurable elements like temperature and precipitation.
- Genetic Classifications: Attempt to classify climates based on the underlying causes or factors that create them, such as air mass types or atmospheric circulation patterns.
- Applied Classifications: Designed for specific practical purposes, such as agriculture, building design, or tourism.
Koeppen’s Scheme Of Classification Of Climate
One of the most widely used empirical climate classification systems was developed by Russian-German climatologist Vladimir Koeppen. First published in 1918 and subsequently modified, Koeppen's scheme remains popular today.
Koeppen recognized a strong correlation between the distribution of natural vegetation types and climatic characteristics. He selected specific temperature and precipitation values that seemed to correspond well with the boundaries of different vegetation zones and used these values as criteria for defining his climate types. The classification is based on quantitative data, specifically mean annual and mean monthly temperature and precipitation figures.
Koeppen used a system of capital and small letters to designate major climatic groups and more specific climatic types. He defined five main climatic groups, four based primarily on temperature thresholds and one based on precipitation (dryness).
The primary climatic groups are designated by capital letters:
| Group Code | Group Name | Defining Characteristic |
|---|---|---|
| A | Tropical Humid Climates | Average temperature of the coldest month is $18^\circ C$ or higher. |
| B | Dry Climates | Potential evaporation (amount of water that could evaporate) exceeds actual precipitation (indicating a moisture deficit). |
| C | Warm Temperate (Mid-latitude) Climates | Average temperature of the coldest month is higher than $-3^\circ C$ but below $18^\circ C$. |
| D | Cold Snow Forest Climates | Average temperature of the coldest month is $-3^\circ C$ or below. |
| E | Cold Climates | Average temperature for all months is below $10^\circ C$. |
| H | Highland | Dominated by the effect of altitude (cold temperatures due to elevation). |
Groups A, C, D, and E represent humid climates (where precipitation generally exceeds potential evaporation), while Group B represents dry climates.
These main groups are further divided into specific climatic types using additional small letters, which indicate seasonality of precipitation and temperature characteristics. The small letters for precipitation seasonality are:
- f: No dry season (adequate precipitation throughout the year).
- w: Winter dry season (most precipitation occurs in summer).
- s: Summer dry season (most precipitation occurs in winter).
- m: Monsoon climate (a specific type of tropical climate with a short dry season and very heavy summer rainfall).
For temperature severity (especially in C and D climates), small letters include:
- a: Hot summer (mean temperature of warmest month $> 22^\circ C$).
- b: Warm summer (mean temperature of warmest month $< 22^\circ C$, at least 4 months $> 10^\circ C$).
- c: Cool summer (less than 4 months $> 10^\circ C$, coldest month $> -38^\circ C$).
- d: Very cold winter (coldest month $< -38^\circ C$).
Dry (B) climates are subdivided using capital letters: S for Steppe (semi-arid) and W for Desert (arid), combined with small letters (h or k) to indicate temperature conditions (h for hot dry, k for cold dry).
The main climatic types resulting from these combinations are listed below:
| Group | Type Code | Characteristics |
|---|---|---|
| A - Tropical Humid | Af | Tropical Wet: No dry season, high rainfall year-round. |
| Am | Tropical Monsoon: Monsoonal rainfall pattern, short dry season. | |
| Aw | Tropical Wet and Dry (Savanna): Distinct winter dry season, wet summer. | |
| B - Dry Climate | BSh | Subtropical Steppe: Low-latitude semi-arid, sparse grassland. |
| BWh | Subtropical Desert: Low-latitude arid, very dry. | |
| BSk | Mid-latitude Steppe: Mid-latitude semi-arid, sparse grassland, cold winters. | |
| BWk | Mid-latitude Desert: Mid-latitude arid, very dry, cold winters. | |
| C - Warm Temperate | Cfa | Humid Subtropical: No dry season, hot summer. |
| Cs | Mediterranean: Dry hot summer, mild rainy winter. | |
| Cfb | Marine West Coast: No dry season, mild to warm summers, cool but not freezing winters. | |
| D - Cold Snow Forest | Df | Humid Continental (No Dry Season): Severe, snowy winters, precipitation year-round. |
| Dw | Humid Continental (Winter Dry): Severe, snowy winters with minimal precipitation, wet summers. | |
| E - Cold Climates | ET | Tundra: No true summer (warmest month $< 10^\circ C$), permafrost. |
| EF | Ice Cap: Perennially covered by ice and snow, all months below $0^\circ C$. | |
| H - Highland | H | Highland: Altitude-governed, varying greatly over short distances. |
Generalized map illustrating the global distribution patterns of the major climate groups and types according to Koeppen's classification scheme.
Group A : Tropical Humid Climates
These climates are found in the tropical belt, primarily between the Tropic of Cancer and the Tropic of Capricorn ($23.5^\circ$ N and $23.5^\circ$ S latitude). They are characterized by uniformly high temperatures throughout the year and high humidity due to the overhead sun and the presence of the Inter-Tropical Convergence Zone (ITCZ). The annual temperature range is very small, and annual rainfall is high. Group A is divided into three types based on precipitation seasonality:
Tropical Wet Climate (Af)
This climate type is found near the equator, in major regions like the Amazon Basin (South America), the Congo Basin and western equatorial Africa, and the islands of the East Indies. A key characteristic is significant rainfall throughout the year, often occurring as intense convective thunderstorms in the afternoon. Temperatures are consistently high, with a very small annual range. Daily temperature ranges (difference between maximum and minimum in a day) are also small, typically from around $20^\circ C$ at night to $30^\circ C$ during the day. This climate supports dense tropical evergreen forests with high biodiversity and a thick canopy.
Tropical Monsoon Climate (Am)
Tropical monsoon climate is found in regions influenced by monsoon wind systems, such as the Indian subcontinent, parts of Southeast Asia, northeastern South America, and northern Australia. It is characterized by heavy rainfall concentrated mostly in the summer months during the wet monsoon season, followed by a distinct dry winter season. Although there is a dry period, the total annual rainfall is typically high enough to support tropical forests. This type is similar to Af but with a pronounced seasonal rainfall pattern.
Tropical Wet And Dry Climate (Aw)
Also known as Savanna climate, this type is found poleward of the Af climate regions, bordering dry climates (Group B) in the west and some humid mid-latitude climates (C types) in the east. Major areas include parts of Brazil bordering the Amazon basin, Sudan, and southern Central Africa. This climate has a distinct wet season and a longer, more severe winter dry season compared to Am. Annual rainfall is considerably lower and more variable than in Af or Am climates. Temperatures remain high throughout the year, but the diurnal temperature range is greatest during the dry season. The characteristic vegetation is deciduous forest and tree-studded grasslands (savanna), adapted to the seasonal dryness.
Dry Climates : B
Dry climates are defined by a condition where the potential amount of water that could evaporate from surfaces and be transpired by plants (potential evapotranspiration) is greater than the actual amount of precipitation received. This results in a significant moisture deficit, limiting plant growth. Dry climates cover extensive areas globally, from subtropical latitudes (15-30°) to mid-latitudes (35-60°) in both hemispheres.
- In low latitudes, they often occur in the areas dominated by subtropical high-pressure systems, where sinking air inhibits rainfall. Coastal deserts in these latitudes can be intensified by the presence of cold ocean currents offshore (e.g., Atacama Desert on the west coast of South America).
- In mid-latitudes, dry climates are typically found in the interior of continents, far from oceanic moisture sources, or in rain-shadow areas on the leeward side of mountain ranges.
Dry climates are subdivided into two major types based on the degree of aridity: Steppe or semi-arid (BS) and Desert (BW). These are further divided into hot (h) and cold (k) subtypes based on temperature.
Subtropical Steppe (BSh) And Subtropical Desert (BWh) Climates
These dry climates are found in the subtropical belt (15-35° latitude). The key difference is the amount of rainfall:
- Subtropical Steppe (BSh): Semi-arid climate. Receives slightly more rainfall than the desert, enough to support sparse grasslands (steppe vegetation). Rainfall is highly variable, and periodic droughts are common, significantly impacting life.
- Subtropical Desert (BWh): Arid climate (true desert). Receives very low and highly unpredictable rainfall, insufficient for significant plant growth. Rainfall often occurs as intense, short-duration thunderstorms that cause flash floods but do not significantly replenish soil moisture. Coastal deserts bordering cold currents may experience frequent fog. Temperatures are extremely high in summer (records above $50^\circ C$ exist), and both annual and diurnal temperature ranges are very large (hot days, cold nights).
Warm Temperate (Mid-Latitude) Climates-C
Warm temperate climates, also known as mid-latitude climates, are located between approximately 30° and 50° latitude, typically on the eastern and western margins of continents. They are characterized by a significant seasonal variation in temperature, with generally warm to hot summers and mild (not extremely cold) winters. The average temperature of the coldest month is above $-3^\circ C$ but below $18^\circ C$. They are grouped into four types:
Humid Subtropical Climate (Cwa)
This subtype is found poleward of the tropics, mainly in regions like the plains of northern India and interior southern China. The "w" indicates a winter dry season, similar to the Aw climate, but the winter temperatures are warm rather than hot. Summers are typically hot and wet due to monsoon influence (in Asia) or convection. The mean temperature of the coldest month is typically above $0^\circ C$.
Mediterranean Climate (Cs)
Located on the west coasts of continents in subtropical latitudes (30-40°), including areas around the Mediterranean Sea, parts of California, central Chile, and coastal areas of southwestern and southeastern Australia. This climate is unique because it is influenced by subtropical highs (dry conditions) in summer and prevailing westerlies (bringing moisture) in winter. The defining characteristics are hot, dry summers and mild, rainy winters (the "s" denotes a summer dry season). Average summer temperatures are around $25^\circ C$, while winter temperatures are below $10^\circ C$. Annual precipitation varies from 35 to 90 cm. Vegetation is adapted to dry summers (e.g., evergreen shrubs, scattered trees).
Humid Subtropical (Cfa) Climate
This climate is found on the eastern parts of continents in subtropical latitudes. Areas include the southeastern United States, southern and eastern China, southern Japan, northeastern Argentina, coastal South Africa, and the east coast of Australia. It is a humid climate with no dry season (indicated by "f"), receiving rainfall throughout the year. Summer months are hot (average around $27^\circ C$, "a" indicates hot summer), and winters are mild (average 5-12$^\circ C$). The daily temperature range is small. Precipitation is often high (75-150 cm annually), with summer thunderstorms and winter frontal precipitation.
Marine West Coast Climate (Cfb)
Situated poleward of the Mediterranean climate on the west coasts of continents, typically between 40° and 60° latitude. Examples include northwestern Europe, the west coast of North America north of California, southern Chile, southeastern Australia, and New Zealand. This climate is strongly influenced by the ocean ("Marine"), resulting in moderate temperatures year-round. Winters are mild (warmer than typical for their latitude, 4-10$^\circ C$), and summers are mild to warm (15-20$^\circ C$, "b" indicates warm summer, not hot). Both annual and daily temperature ranges are small. Precipitation is distributed throughout the year ("f"), often brought by frontal systems associated with the westerlies, varying significantly depending on orography (50-250 cm annually).
Cold Snow Forest Climates (D)
These climates are characteristic of large continental interiors in the high middle and subpolar latitudes (typically 40-70° N), found across Eurasia and North America. They are defined by having an average temperature of the coldest month below $-3^\circ C$, indicating severe winters. They are humid climates with sufficient precipitation for forest growth. They are divided based on winter precipitation:
Cold Climate With Humid Winters (Df)
This type is found poleward of the Marine West Coast and some mid-latitude steppe areas. It has cold, snowy winters and precipitation occurring throughout the year ("f"). The frost-free season is short. Annual temperature ranges are large, with significant differences between cold winters and warm summers. Weather changes can be abrupt.
Cold Climate With Dry Winters (Dw)
This climate occurs mainly in northeastern Asia (like much of Siberia). It is characterized by extremely cold, dry winters ("w" indicates winter dry season) due to the dominance of strong winter high-pressure systems (anticylones). Summer temperatures are lower in more northerly areas, but winters are exceptionally severe, with prolonged periods below freezing (up to 7 months). Precipitation is low annually (12-15 cm) and concentrated in the warmer summer months.
Polar Climates (E)
Polar climates are found in the highest latitudes, beyond 70°. They are characterized by extremely low temperatures, with the average temperature of all months below $10^\circ C$. These are the coldest climates on Earth and support minimal or no vegetation. They are divided into two types:
Tundra Climate (ET)
This climate occurs in coastal areas of polar regions, such as the northern fringes of North America, Europe, and Asia, and some polar islands. It is characterized by having at least one month with an average temperature between $0^\circ C$ and $10^\circ C$ (no true summer defined by temperatures above $10^\circ C$). The ground beneath a shallow surface layer experiences permafrost (permanently frozen ground). Vegetation is limited to low-growing forms like mosses, lichens, grasses, and dwarf shrubs that can survive the short, cool growing season and waterlogged conditions (as surface layer melts but water cannot drain through permafrost). These regions experience very long daylight hours during the short summer.
Ice Cap Climate (EF)
This is the most severe climate, found over the vast ice sheets of interior Greenland and Antarctica. It is defined by having the average temperature of all months below freezing ($0^\circ C$). These regions receive very little precipitation, but snow and ice accumulate over vast periods due to the continuous below-freezing temperatures. The immense weight of the ice causes it to flow slowly, forming ice sheets that can deform and break off as icebergs at the margins. Plateau Station in Antarctica is a representative location for this climate.
Highland Climates (H)
Highland climates are strongly influenced by the altitude and topography of mountainous regions. Temperature decreases significantly with increasing elevation (environmental lapse rate), and precipitation patterns are highly variable, often increasing with altitude up to a certain point and varying greatly depending on wind direction relative to mountain ranges (orographic effect). As a result, mountainous areas often exhibit a vertical zonation of climate types, with different climatic conditions and vegetation zones found at different elevations over short horizontal distances, mimicking the climate changes seen across different latitudes. Due to the complex local factors, highlands are grouped as a separate category (H) in the Koeppen system rather than fitting neatly into the other broad latitudinal zones.
Climate Change
Climate is not static; it has varied significantly throughout Earth's long history and continues to change. The current climate we experience, roughly stable over the last 10,000 years (the Holocene epoch), has still undergone minor and occasional large fluctuations.
Evidence for past climate changes comes from various sources:
- Geological Records: Show alternations between cold glacial periods (ice ages) and warmer inter-glacial periods. Landforms in high mountains and high latitudes show marks of past glacier advances and retreats.
- Paleoclimatic Archives: Sediment cores from lakes and oceans, ice cores from polar ice sheets, tree rings (dendroclimatology), and fossil records provide detailed information about past temperatures, precipitation, atmospheric composition, and vegetation patterns over thousands to millions of years.
- Historical Records: Non-instrumental records like accounts of crop yields, famines, floods, droughts, and human migrations provide qualitative evidence of climate variability over recent centuries and millennia.
These diverse records confirm that climate change is a natural, ongoing process of the Earth system.
Even within India, evidence points to past climate shifts. Archaeological studies suggest that the Rajasthan desert region experienced a wetter and cooler climate around 8,000 B.C. Rainfall was higher during the period of the Harappan civilization (3,000-1,700 B.C.), after which drier conditions became more prevalent.
Looking further back, the Earth was significantly warmer during periods like the Cambrian to Silurian (500-300 million years ago). More recently, the Pleistocene epoch (last 2.6 million years) was characterized by multiple cycles of glaciation and deglaciation. The last major glacial maximum occurred about 18,000 years ago, and the current inter-glacial period, the Holocene, began approximately 10,000 years ago.
Climate In The Recent Past
While long-term trends are evident, climate also exhibits considerable variability over shorter periods (decades or centuries). The late 20th century saw an increase in extreme weather events. The 1990s were recorded as the warmest decade of the century, alongside severe floods and droughts in various parts of the world. The devastating drought in the Sahel region (southern edge of the Sahara) from 1967-1977 is a notable example of significant decadal variability. The severe drought and dust storms of the 1930s in the Great Plains of the USA ("Dust Bowl") is another historical example.
Historical accounts from Europe mention periods of notable climate shifts, such as a warm and dry period in the 10th-11th centuries (allowing Viking settlement in Greenland) and a subsequent "Little Ice Age" from roughly 1550 to 1850, characterized by cooler temperatures and expanded glaciers. Global temperature records from the late 19th century show a warming trend from 1885 to 1940, a slight cooling until the 1970s, followed by renewed and significant warming towards the end of the century.
Causes Of Climate Change
Climate change is driven by a combination of natural and human-induced factors. The causes can be broadly categorized into:
- Astronomical Causes: Related to changes in the Earth's position and orientation relative to the Sun.
- Solar Output Variations: Changes in the amount of energy emitted by the Sun. Sunspot activity (darker, cooler areas on the Sun's surface) follows cycles (approximately 11 years). Some theories suggest that variations in sunspot numbers can correlate with minor fluctuations in solar output, potentially influencing Earth's climate, although the impact on major climate shifts is debated and statistically not highly significant over long terms.
- Milankovitch Oscillations: Long-term cyclical changes in the Earth's orbital characteristics, including: (1) the eccentricity of Earth's orbit (shape of the orbit around the Sun, changing over cycles of about 100,000 and 400,000 years); (2) the obliquity or tilt of Earth's axis (angle of tilt relative to its orbit plane, changing over about 41,000 years); and (3) the precession or wobble of Earth's axis (change in the direction of the axis's tilt, affecting when Earth is closest/furthest from the Sun during specific seasons, cycling over about 26,000 years). These orbital changes alter the distribution and total amount of insolation received at different latitudes and seasons over long timescales, considered a primary driver of glacial-interglacial cycles.
- Terrestrial Causes: Related to processes occurring on Earth or within its atmosphere.
- Volcanism: Major volcanic eruptions can inject large quantities of aerosols (tiny particles and gas droplets) into the stratosphere. These aerosols can reflect incoming solar radiation back to space, reducing the amount reaching the surface and causing a temporary global cooling effect (e.g., after the Pinatubo (1991) and El Chichón (1982) eruptions, global temperatures dropped slightly for a few years).
- Anthropogenic Causes: Human activities that alter the composition of the atmosphere or the Earth's surface. The most significant anthropogenic influence today is the increasing concentration of greenhouse gases in the atmosphere.
Global Warming
Global warming refers to the observed increase in the Earth's average surface temperature over recent decades. This warming is strongly linked to the enhanced greenhouse effect caused by rising concentrations of greenhouse gases (GHGs) in the atmosphere.
The atmosphere naturally acts like a greenhouse. It is largely transparent to incoming shortwave solar radiation, allowing most of it to reach the Earth's surface. The Earth's surface absorbs this energy and re-radiates it as longwave infrared radiation (heat). Certain gases in the atmosphere, known as greenhouse gases, absorb this outgoing longwave radiation and re-emit some of it back towards the Earth's surface. This process traps heat in the lower atmosphere, warming the planet. This natural greenhouse effect is essential for life, keeping the Earth's average temperature much warmer than it would be otherwise.
The analogy comes from actual greenhouses used in cold climates to grow plants. Glass panels are transparent to sunlight (shortwave radiation) but largely opaque to the outgoing infrared radiation (longwave radiation) from inside. Sunlight enters and heats the interior, but the glass traps the heat inside, making the greenhouse warmer than the outside environment. Similarly, air inside a closed car heats up significantly in sunlight because the windows allow solar radiation in but trap the outgoing heat.
Enhanced Greenhouse Effect: The current concern about global warming arises because human activities, primarily the burning of fossil fuels and deforestation, are significantly increasing the concentration of GHGs in the atmosphere, intensifying the natural greenhouse effect and causing additional warming.
Greenhouse Gases(GHGs)
Greenhouse gases (GHGs) are atmospheric gases that absorb and emit radiant energy within the thermal infrared range, causing the greenhouse effect. The most significant GHGs influenced by human activity are:
- Carbon Dioxide ($CO_2$): The most abundant anthropogenic GHG. Primarily emitted from the combustion of fossil fuels (coal, oil, natural gas) for energy, industry, and transport, as well as from deforestation and land-use change. CO2 is removed from the atmosphere by natural "sinks" such as forests (through photosynthesis) and oceans (by dissolving in seawater). However, emissions currently exceed the capacity of these sinks to absorb CO2, leading to its accumulation in the atmosphere. Atmospheric CO2 concentration has been rising steadily, increasing by about 0.5% annually. The time required for atmospheric CO2 levels to adjust to changes in sources and sinks is estimated to be 20-50 years, but some effects last much longer. Doubling of pre-industrial CO2 levels is often used as a benchmark in climate models to estimate potential future warming.
- Methane ($CH_4$): Produced from natural sources (wetlands, termites) and anthropogenic activities (livestock farming, rice cultivation, natural gas leaks, coal mining, waste decomposition). Methane is a more potent GHG per molecule than CO2 but has a shorter atmospheric lifetime.
- Nitrous Oxide ($N_2O$): Emitted from natural processes in soils and oceans and anthropogenic sources like agriculture (fertilizers, livestock manure), fossil fuel combustion, and industrial processes. It is a potent GHG with a long atmospheric lifetime.
- Chlorofluorocarbons (CFCs): Synthetic chemicals previously used widely in refrigerants, aerosols, and foam blowing. While their production has been largely phased out due to their role in stratospheric ozone depletion, they are also very powerful GHGs with long atmospheric lifetimes.
- Ozone ($O_3$): Exists in two layers: Stratospheric ozone is beneficial, absorbing UV radiation and naturally formed. However, Tropospheric ozone (in the lower atmosphere) is a pollutant formed by reactions involving emissions from vehicles and industry. Tropospheric ozone is a potent GHG and harmful to human health and ecosystems.
Other gases like Nitric Oxide ($NO$) and Carbon Monoxide ($CO$) are not direct GHGs but can influence the concentration of other GHGs through chemical reactions.
The impact of a specific GHG molecule on warming depends on its concentration increase, its effectiveness at absorbing infrared radiation (radiative efficiency), and its atmospheric lifetime. CFCs are particularly effective absorbers despite their low concentrations. Tropospheric ozone, although the same molecule as beneficial stratospheric ozone, acts as a GHG in the lower atmosphere by absorbing terrestrial radiation.
The longer a GHG molecule remains in the atmosphere, the more lasting its warming influence, and the longer it would take for the climate system to recover if emissions were reduced.
Concerns about global warming have led to international efforts to reduce GHG emissions. The Kyoto Protocol (adopted in 1997, entered force in 2005) was an international treaty that committed industrialized countries to reduce their collective GHG emissions by 5% below 1990 levels by 2012.
The increasing concentration of GHGs poses a significant risk of warming the planet, potentially leading to irreversible changes. Although the effects of global warming may vary regionally, the overall adverse impacts on Earth's life support systems are a major concern. A key consequence is the rise in global sea level due to the melting of glaciers and ice sheets and the thermal expansion of warming seawater. Rising sea levels threaten to inundate low-lying coastal areas and islands, potentially displacing large populations and causing significant social and economic problems. This prompts urgent global cooperation to control emissions and transition towards sustainable practices to ensure a livable planet for future generations.
Analysis of temperature records confirms that the Earth's average near-surface air temperature has increased. Data available from the mid-19th century show a clear warming trend in the 20th century. Relative to the 1961-1990 average, global temperatures rose by about $0.4^\circ C$ during the periods 1901-1944 and 1977-1999. Despite a slight cooling between these periods (more noticeable in the Northern Hemisphere), the overall warming trend resulted in the average global temperature at the end of the 20th century being approximately $0.6^\circ C$ higher than at the end of the 19th century. Notably, seven of the warmest years between 1856 and 2000 occurred in the last decade of the 20th century, with 1998 potentially being the warmest year of the millennium.
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