This chapter deals with
The world climate can be studied by organizing information and data on
climate and synthesizing them in smaller units for easy understanding,
description and analysis. Three broad approaches have been adopted for classifying
climate. They are empirical, genetic and applied. Empirical classification is based on observed data,
particularly on temperature and precipitation. Genetic classification attempts to organize climates
according to their causes. Applied classification is for specific purpose.
KOEPPEN‟ S SCHEME OF CLASSIFICATION OF CLIMATE
Koeppen
The most widely
used classification
of climate is the
empirical climate
classification
scheme developed
by V. Koeppen.
Koeppen identified a
close relationship between the distribution of vegetation and climate. He selected certain values of
temperature and precipitation and related them to the distribution of vegetation
and used these values for classifying the climates.
It is an empirical classification based on mean annual and mean monthly temperature and
precipitation data. He introduced the use of capital and small letters to designate climatic groups
and types. Although developed in 1918 and modified over a period of time, Koeppen‘s scheme is still
popular and in use.
Koeppen recognized five major climatic groups, four of them are based on temperature and one on
precipitation. Table 12.1 lists the
climatic groups and their characteristics according to Koeppen.
The capital letters : A,C, D and E delineate humid climates and B dry climates.
The climatic groups are subdivided into types, designated by small letters, based on
seasonality of precipitation and temperature characteristics.
CHAPTER-12 WORLD CLIMATE AND CLIMATE CHANGE
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The seasons of dryness are indicated by the small letters : f, m, w and s, where f corresponds to no
dry season,
m - monsoon climate, w- winter dry season and s - summer dry season.
The small letters a, b,c and d refer to the degree of severity of temperature.
The B- Dry Climates are subdivided using the capital letters S for steppe or semi-arid and W for
deserts.
Group A : Tropical Humid Climates
1. Tropical humid climates exist between Tropic of Cancer and Tropic of Capricorn. 2. The sunbeing
overhead throughout the year and the presence of Inter Tropical Convergence Zone(ITCZ) make the
climate hot and humid.
3. Annual range of temperature is very low and annual rainfall is high.
4. The tropical group is divided into three types, namely
(i) Af- Tropical wet climate;
(ii) Am - Tropical monsoon climate;
(iii) Aw- Tropical wet and dry climate.
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Tropical Wet Climate (Af)
1. Tropical wet climate is found near the equator.
2. The major areas are the Amazon Basin in South
America, western equatorial Africa and the islands of
East Indies.
3. Significant amount of rainfall occurs in every month
of the year as thunder showers in the afternoon.
4. The temperature is uniformly high and the annual
range of temperature is negligible.
5. The maximum temperature on any day is around
30°C while the minimum temperature is around 20°C.
6. Tropical evergreen forests with dense canopy cover
and large biodiversity are found in this climate
Tropical Monsoon Climate (Am)
Tropical monsoon climate (Am) is found over the Indian
sub-continent, North Eastern part of South America
and Northern Australia. Heavy rainfall occurs mostly
in summer. Winter is dry.
Tropical Wet and Dry Climate (Aw)
Tropical wet and dry climate occurs north and south of Af type climate regions. It borders with dry
climate on the western part of the continent and Cf or Cw on the eastern part.
Extensive Aw climate is found to the north and south of
the Amazon forest in Brazil and adjoining parts of Bolivia
and Paraguay in South America, Sudan and south of
Central Africa. The annual rainfall in this climate is
considerably less than that in Af and Am climate types
and is variable also.
The wet season is shorter and the dry season is longer
with the drought being more severe.
Temperature is high throughout the year and diurnal
ranges of temperature are the
greatest in the dry season. Deciduous forest and treeshredded
grasslands occur in this climate.
Dry Climates : B
Dry climates are characterized by very low rainfall that is not adequate for the growth of plants.
These climates cover a very large areaof the planet
extending over large latitudes from 15° - 60° north and south
of the equator.
At low latitudes, from 15° - 30°, they occur in the area of
subtropical high where subsidence and inversion of
temperature do not produce rainfall.
On the western margin of the continents, adjoining the cold
current, particularly over the west coast of South\ America,
they extend more equator wards and occur on the coast land.
In middle latitudes, from 35° - 60° north and south of
equator, they are confined to the interior of continents where maritime-humid winds do not reach
and to areas often surrounded by mountains.
Dry climates are divided into steppe or semi-arid climate (BS) and desert climate (BW).
They are further subdivided as subtropical steppe (BSh) and subtropical desert (BWh) at latitudes
from 15° - 35° and mid-latitude steppe (BSk) and mid-latitude desert (BWk) at latitudes between
35° - 60°.
Subtropical Steppe (BSh) and Subtropical
Desert (BWh) ClimatesSubtropical steppe (BSh) and
subtropical desert (BWh) have common precipitation and
temperature characteristics.
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Located in the transition zone between humid and dry climates, subtropical steppe receives slightly
more rainfall than the desert, adequate enough for the growth of sparse grasslands.
The rainfall in both the climates is highly variable.
The variability in the rainfall affects the life in the steppe much more than in the desert, more
often causing famine.
Rain occurs in short intense thundershowers in deserts and is ineffective in building soil moisture.
Fog is common in coastal deserts bordering cold currents.
Maximum temperature in the summer is very high.
The highest shade temperature of 58° C was recorded at Al Aziziyah, Libya on
13 September 1922. The annual and diurnal ranges of temperature are also high.
Warm Temperate (Mid-Latitude)
Climates-C
Warm temperate (mid-latitude) climates
extend from 30° - 50° of latitude mainly on
the eastern and western margins of
continents.
These climates generally have warm
summers with mild winters.
They are grouped into four types:
(i) Humid subtropical, i.e. dry in winter
and hot in summer (Cwa);
(ii) Mediterranean (Cs);
(iii) Humid subtropical, i.e. no dry season and mild winter (Cfa);
(iv) Marine west coast climate(Cfb).
Humid Subtropical Climate (Cwa)
Humid subtropical climate occurs pole ward
of Tropic of Cancer and Capricorn, mainly in
North Indian plains and South China interior
plains.
The climate is similar to Aw climate except that
the temperature in winter is warm.
Mediterranean Climate (Cs)
As the name suggests, Mediterranean climate
occurs around Mediterranean sea, along the west
coast of continents in subtropical latitudes
between 30° - 40° latitudes e.g. — Central
California, Central Chile, along the coast in south
eastern and south western Australia.
These areas come under the influence of sub
tropical high in summer and westerly wind in
winter. Hence, the climate is characterised by hot,
dry summer and mild, rainy winter.
Monthly average temperature in summer is around
25° C and in winter below 10°C. The annual precipitation ranges between 35 - 90 cm.
Humid Subtropical (Cfa) Climate
Humid subtropical climate lies on the eastern
parts of the continent in subtropical latitudes.
In this region the air masses are generally
unstable and cause rainfall throughout the
year.
They occur in eastern United States of
America, southern and eastern China, southern
Japan, northeastern Argentina, coastal south
Africa and eastern coast of Australia.
The annual averages of precipitation vary from 75-150 cm. Thunderstorms in
summer and frontal precipitation in winter are common.
Mean monthly temperature in summer is around 27°C, and in winter it varies
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from 5°-12° C. The daily range of temperature is small.
Marine West Coast Climate (Cfb)
Marine west coast climate is located poleward
from the Mediterranean climate on the west coast of
the continents.
The main areas are:
Northwestern Europe, west coast of North
America, north of California, southern Chile,
southeastern Australia and New Zealand.
Due to marine influence, the temperature is moderate and in winter, it is warmer its latitude.
The mean temperature in summer months ranges from 15°-20°C and in winter
4°-10°C. The annual and daily ranges of temperature are small. Precipitation occurs throughout the
year.
Precipitation varies greatly from 50-250cm.
Cold Snow Forest
Climates (D)
Cold snow forest climates
occur in the large
continental area in the
northern hemisphere
between 40°-70° north latitudes in Europe,
Asia and North America. Cold snow forest climates are
divided into two types:
(i) Df- cold climate with humid
winter;
(ii) (ii) Dw- cold climate with
dry winter.
(iii) The severity of winter is
more pronounced in higher
latitudes.
Cold Climate with Humid Winters (Df)
Cold climate with humid winter occurs pole ward of marine west coast climate and mid latitude
steppe.
The winters are cold and snowy.
The frost free season is short.
The annual ranges of temperature are large.
The weather changes are abrupt and short.Poleward, the winters are more severe.
Cold Climate with Dry Winters (Dew)
Cold climate with dry winter occurs mainlyover North eastern Asia. The development of pronounced
winter anticyclone and its weakening in summer sets in monsoon like reversal of wind in this region.
Pole ward summer temperatures are lower and
winter temperatures are extremely low with many
locations experiencing below freezing point
temperatures for up to seven months in a year.
Precipitation occurs in summer. The annual
precipitation is low from 12-15 cm.
polar Climates (E)
Polar climates exist poleward beyond 70°latitude.
Polar climates consist of two types:
(i) Tundra (ET); (ii) Ice Cap (EF).
Tundra Climate (ET)
The tundra climate (ET) is so called after the types of vegetation, like
low growing mosses, lichens and flowering plants.
This is the region of permafrost where the sub soil is permanently
frozen. The short growing season and waterlogging support only low growing plants.
During summer, the tundra regions have very long duration of day light.
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Ice Cap Climate (EF)
The ice cap climate (EF) occurs over interior Greenland and Antarctica. Even in summer, the
temperature is below freezing point.
This area receives very little precipitation.
The snow and ice get accumulated and the mounting pressure causes the deformation of the ice
sheets and they break.
They move as icebergs that float in the Arctic and Antarctic waters.
Plateau Station Antarctica ,79°S, portray this climate. HIGH LATITUDE CLIMATE
ICE CAP CLIMATE
Highland Climates (H)
Highland climates are governed by topography. In high mountains, large changes in mean
temperature occur over short distances.
Precipitation types and intensity also vary spatially across high lands. There is vertical zonation of
layering of climatic types with elevation in the mountain environment.
CLIMATE CHANGE
The type of climate we experience now might be prevailing over the last 10,000 years with minor
and occasionally wide fluctuations.
India also witnessed alternate wet and dry periods.
Archaeological findings show that the Rajasthan desert experienced wet and cool climate around
8,000 B.C.
The period 3,0001,700 B.C. had higher rainfall. From about 2,000-1,700 B.C.,
this region was the centre of the Harappan civilization.
Dry conditions since then.
In the geological past, the earth was warm some 500-300 million years ago, through the Cambrian,
Ordovician and Silurian periods.
During the Pleistocene epoch, glacial and inter-glacial periods occurred, the last major peak glacial
period ago.
The present inter-glacial period started 10,000 years ago.
Climate in the recent past
Variability in climate occurs all the time. The 1990s recordedthe warmest temperature of the
century andsome of the worst floods around the world.
The worst devastating drought in the Sahel region, south of the Sahara desert, from 1967-1977is one
such variability.
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During the 1930s, severe drought occurred in southwestern Great Plains of the United States,
described as the dust bowl.
Historical records of crop yield or crop failures, of floods and migration of people tell about the
effects of changing climate.
A number of times Europe witnessed warm, wet, cold and dry periods, the significant episodes were
the warm and dry conditions in the tenth and eleventh centuries,
when the Vikings settled in Greenland. Europe witnessed ―Little Ice Age‖ from 1550 to about 1850.
From about 1885-1940 world temperature showed an upward trend. After 1940, the rate of increase
in temperature slowed down.
Causes of Climate Change
The causes for climate change are many.
They can be grouped into Astronomical 2.Terrestrial causes.
The astronomical causes area.
A. the changes in solar output associated with sunspot activities. Sunspots are dark and cooler
patches on the sun which increase and decrease in a cyclical manner. B. According to some
meteorologists, when the number of sunspots increase, cooler and wetter weather and greater
storminess occur.
C. decrease in sunspot numbers is associated with warm and drier conditions. Yet, these findings are
not statistically significant.
D. An another astronomical theory is Millankovitch oscillations, which infer cycles
in the variations in the earth‘s orbital characteristics around the sun, the wobbling
of the earth and the changes in the earth‘s axial tilt.
E. All these alter the amount of insolation received from the sun, which in turn, might have a
bearing on the climate.
F. Volcanism is considered as another cause for climate change. Volcanic eruption throws up lots of
aerosols into the atmosphere. These aerosols remain in the atmosphere for a considerable period of
time reducing the sun‘s radiation reaching the Earth‘s surface.
After the recent Pinatoba and El Cion volcanic eruptions, the average temperature of the earth fell
to some extent for some years.
The most important anthropogenic effect on the climate is the increasing trend in concentration of
greenhouse gases in the atmosphere which is likely to cause global
warming.
Global Warming
Due to the presence of greenhouse gases, the atmosphere is behaving like a greenhouse. The
atmosphere also transmits the incoming solar radiation but absorbs the vast majority of long wave
radiation emitted upwards by the earth‘s surface. The gases that absorb long wave radiation are
called greenhouse gases. The processes that warm the atmosphere are often collectively referred to
as the greenhouse effect.
The term greenhouse is derived from the analogy to a greenhouse used in cold areas for preserving
heat. A greenhouse is made up of glass. The glass which is transparent to incoming short wave solar
radiation is opaque to outgoing long wave radiation. The glass, therefore, allows in more radiation
and prevents the long wave radiation going outside the glass house, causing the temperature inside
the glasshouse structure warmer than outside.
Greenhouse Gases (GHGs)
The primary GHGs of
concern today are
carbondioxide (CO2),
Chlorofluorocarbons
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NITROUS OXIDE METHANE
(NO2) CH4
and ozone(O3). Some other gases such as nitric oxide (NO)and carbon monoxide (CO) easily react
with GHGs and affect their concentration in the atmosphere.
The effectiveness of any given GHG will depend on the magnitude of the Increase in its
concentration, its life time in the atmosphere and the wavelength of radiation that it absorbs. The
chlorofluorocarbons(CFCs) are highly effective. Ozone which absorbs ultra violet radiation in the
stratosphere is very effective in absorbing terrestrial radiation when it is present in the lower
troposphere. Another important point to be noted is that the more time the GHG molecule remains
in the atmosphere, the longer it will take for earth‘s atmospheric system to recover from any
change brought about by the latter.
The largest concentration of GHGs in the atmosphere is carbon dioxide.
1. The emission of CO2 comes mainly from fossil fuel combustion (oil, gas and coal). 2. Forests and
oceans are the sinks for the carbon dioxide.
3. Forests use CO 2 in their growth.
4. So, deforestation
5. due to changes in land use, also increases the concentration of Co 2
The time taken for atmospheric CO2 to adjust to changes in sources to sinks is 20-50 years. It is
rising at about 0.5 per cent annually.
Doubling of concentration of over pre-industrial levelis used as an index for estimating the
changes in climate in climatic models.
Chlorofluorocarbons (CFCs) are products of human activity. Ozone occurs in the stratosphere where
ultra-violet rays convert oxygen into ozone. Thus, ultra violet rays do not reach the earth‘s surface.
The CFCs which drift into the stratosphere destroy the ozone .Large depletion of ozone occurs over
Antarctica. The depletion of ozone
concentration in the stratosphere is called the ozone hole.
This allows the ultra violet rays to pass through the troposphere.
International efforts have been initiated for reducing the
emission of GHGs into the atmosphere. The most important one is the
Kyoto protocol proclaimed in 1997.
This protocol went into effect in 2005, ratified by 141 nations.
Kyoto protocol bounds the 35 industrialized countries to reduce their emissions by the year 2012 to 5
per cent less than the levels prevalent in the year 1990.
The increasing trend in the concentration of GHGs in the atmosphere may, in the long run, warm up
the earth.
Once the global warming sets in, it will be difficult to reverse it. The effect of global warming may
not be uniform everywhere.
Nevertheless, the adverse effect due to global warming will adversely affect the life supporting
system.
Rise in the sea level due to melting of glaciers and ice-caps and thermal expansion of the sea may
inundate large parts of the coastal area and islands, leading to social problems.
This is another cause for serious concern for the world community. Efforts have already been
initiated to control the emission of GHGs and to arrest the trend towards global warming.
One of the major concerns of the world today is global warming.
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.The annual average near-surface air temperature of the world is approximately 14°C. The greatest
warming of the 20th century was during the two periods, 1901-44 and 1977-99.
Over each of these two periods, global temperatures rose by about 0.4°C. In between, there was a
slight cooling, which was more marked in the Northern Hemisphere.
The globally averaged annual mean temperature at the end of the 20th century was about 0.6°C
above that recorded at the end of the 19th century.
The seven warmest years during the 1856-2000 were recorded in the last decade.
The year 1998 was the warmest year, probably not only for the 20th century but also for the whole
millennium.