WORLD CLIMATE AND CLIMATE CHANGE 

Three broad approaches have been adopted for classifying climate. They are empirical, genetic and applied.

1. Empirical classification is based on observed data on temperature and precipitation.
2. Genetic classification attempts to organize climates based on their causes.
3. Applied classification is for a specific purpose.

1. Koeppen’s scheme of classification of climate

Koeppen developed a scheme for empirical classification of climate in 1918. It is the most popular classification of climate even today. Koeppen identified a close relationship between the distribution of vegetation and climate. He introduced the use of capital and small letters to designate climatic groups and types. Koeppen recognized five major climatic groups four of them based on temperature and one on precipitation. A, C, D and E delineate humid climates and B dry climates.

Climate groups according to Koeppen

The climate groups are further divided into types, designated by small letters, based on the seasonality of precipitation and temperature. The seasons of dryness are indicated by the small letters; f, m, w and s.

f- No dry season,

m- Monsoon climate,

w- Winter dry season,

s- Summer dry season.

The small letters a, b, c and d are 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.

Climate types according to the Koeppen

Group A: Tropical humid climates

It exists between the Tropic of Cancer and the Tropic of Capricorn. The sun is overhead throughout the year and the presence of the Inter-Tropical Convergence Zone [ITCZ] makes the climate hot and humid. The annual temperature is very low and the annual rainfall is high. The tropical groups are divided into three;

1. Tropical wet climate- Af: It is found near the equator, the major areas are the Amazon basin in South America, western equatorial Africa and the islands of the East Indies. Rainfall occurs every month of the year as thundershowers in the afternoon. The maximum day temperature is 30°C and the minimum is 20°C. Tropical evergreen forests are rich in density and biodiversity.
2. Tropical monsoon climate- Am: It is found in the Indian sub-continent, the North Eastern part of South America and Northern Australia. Heavy rainfall occurs mostly in summer, winter is dry.
3. Tropical wet and dry climate- Aw: It occurs north and south of Af-type climate regions. Borders include a 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-central Africa. Annual rainfall is less than the Af and Am climates. The wet season is shorter and the dry season is longer. Delicious forest and tree-shredded grasslands occur in this climate.

Dry climates: B

Dry climates are very low rainfall that is not adequate for the growth of plants. This climate covers 15°-60° North and south of the equator at the large latitudes and 15°-30°C low latitudes. Dry climates are divided into steppe or semi-arid climates [BS] and desert climates [BW]. These are further sub subdivided into subtropical steppe[BSh] and subtropical desert [BWh] at latitudes steppe [BSk] and mid-latitude desert [BWk] at a latitude between 35°-60°.

Subtropical steppe [BSh] and subtropical desert [BWh] climates

It has common precipitation and temperature characteristics. Located in the transition zone between humid and dry climates, the subtropical steppe receives slightly more rainfall than the desert, sufficient for the growth of sparse grasslands. The rainfall is high in both climates. The variability in the rainfall affects life on the steppe much more than in the desert causing famine. Rain occurs in the short intense thundershowers in deserts that are insufficient for building soil moisture. Fog is common in coastal deserts bordering cold currents. The temperature in the summer is very high. The highest shade temperature of 58°C was recorded at AI Aziziyah in Libya on 13^th September 1922. The annual and diurnal ranges of temperature are also high.

Warm temperate [mid-latitude] climates-c:

It extends from 30°-50°C latitude mainly on the eastern and western margins of continents. These climates generally have warm summers and mild winters. There are four kinds

1. Humid subtropical I.e. dry in winter and hot in summer [CWa]: It occurs in the poleward tropic of Cancer and Capricorn, mainly in north Indian plains and south China interior plains. It is similar to the Aw climate except that the temperature in winter is warm.
2. Mediterranean [Cs]: The Mediterranean climate occurs around the Meditation Sea- central California, central Chile, along the coast in southeastern and southwestern Australia. It is a hot, dry summer and mild rainy winter. The monthly average temperature in summer is around 25°C and in winter below 10°C. The annual precipitation ranges from 35 to 90 cm.
3. Humid sub-tropical [i.e. no dry season and mild winter] [Cfa]: It occurs in the eastern United States of America, southern and eastern China, southern Japan, northeastern Argentina, coastal South Africa and the eastern coast of Australia. Rainfall is throughout the year varying from 75-150 cm. The monthly average temperature in summer is 27°C, and in winter varies from 5°-12°C.
4. Marine west coast climate [Cfb]: It is located in northwestern Europe, the west coast of North America, north of California, southern Chile, southeastern Australia and New Zealand. The mean temperature in summer months ranges from 15°-20°C and in winter is 4°-10°C. Precipitation has a great variation from 50-250 cm.

Cold Snow Forest Climates [D]

Cold snow forest climate occurs 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: Df- cold climate with a humid winter; and Dw- cold climate with a dry winter. The severity of winter is more pronounced in higher latitudes.

Cold climates with humid winters [Df]

Cold climate with a humid winter occurs poleward of marine west coast climate and mid-latitude steppe.

1. The winters are cold and snowy.
2. The frost-free season is short.
3. The annual ranges of temperature are large.
4. The weather changes are abrupt and short, poleward the winters are more severe.

Cold climate with dry winters [Dw]

A cold climate with dry winter occurs mainly over northeastern Asia. The development of a pronounced winter anticyclone and its weakening in summer sets in a monsoon-like reversal of wind in this region. 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;

1. Tundra [ET]: This is the region of permafrost where the subsoil is permanently frozen. Tundra has low-growing mosses, lichens and flowering plants are different vegetation. During summer the tundra regions have a very long duration of daylight.
2. Icecap [EF]: It occurs over interior Greenland and Antarctica. Even in summer, the temperature is below freezing point. This area receives very little precipitation. The snow and the ice get collected and mounting pressure causes the deformation of ice sheets and they break. Icebergs float toward the Arctic and Antarctic waters.  Plateau station Antarctica 79°S portrays this climate.

High land climate [H]

High land climates are governed by topography. In high mountains, temperature occurs over short distances. Precipitation types and intensity also vary spatially across highlands. There is vertical zonation of layering of climatic types with elevation in the mountain environment.

2. Climatic change

• The type of climate we experience now might be prevailing over the last 10,000 years with minor and occasionally wide fluctuations.
• The Earth has witnessed many variations in climate since the beginning.
• The High altitudes and high latitudes exhibit traces of advances and retreats of glaciers.
•  The sediment deposits in glacial lakes reveal the occurrence of warm and cold periods.
• Changes in the climate are a natural and continuous process.
• India also witnessed alternate wet and dry periods.
• Rajasthan desert experienced wet a and cool climate around 8,000 B.C.
• The period 3,000-1700 B.C., Had higher rainfall. From about 2,000-1,700 B.Cat, in this centre of The Harappa civilization, dry conditions highlighted since then.
• The earth was warm some 500-300 million years ago, through the Cambrian, Ordovician and Silurian periods.
• During the Pleistocene epoch, glacial and interglacial periods occurred.
• The last major peak glacial period was about 18,000 years ago.
• The present interglacial period started 10,000 years ago.

The climate in the recent past

• Changes in the climate occur all the time.
• The nineties decade of the last century witnessed extreme weather events.
• 1990 recorded the warmest temperature of the century and some of the worst floods around the world.
• The worst devastating drought occurred in the Sahel region, south of the Sahara desert.
• During the 1930s severe drought [dust bowl] occurred in the southwestern Great Plains of the United States.
• Climate change affects crop yield or crop failures, floods and migration of people.
• Europe witnessed warm, wet, cold and dry periods in the 10^th and 11^th centuries.
• The Little Ice Age of Europe is from 1550 to 1850.
• From 1885 to 1940 world temperature showed an upward trend.

Causes of climate change

• The causes of climate change are many. They are grouped into Astronomical causes and terrestrial causes.
• Astronomical causes are the changes in solar output associated with sunspot activities.
• Sunspots are dark and cooler patches on the sun which increase and decrease cyclically.
• When the number of sunspots increases, cooler and wetter weather and greater storminess occur, according to some meteorologists.
• Milankovitch oscillations astronomical theory describes variations in climate that are due to variations in the receipt of solar radiation associated with

1. The precession of the equinoxes and solstices;
2. The varying tilt of the earth’s rotational axis;
3. The varying eccentricity of the earth’s orbit.

Volcanism is considered another cause of Simatic change. Volcanic eruption throws up lots of aerosols into the atmosphere to reduce the sun’s radiation reaching the earth’s surface. The anthropogenic effect on the climate is increasing greenhouse gases in the atmosphere causing global warming.

3. Global warming

The atmosphere transmits the incoming solar radiation but absorbs the vast majority of longwave radiation emitted upward the earth's surface. The term greenhouse is derived from the analogy of a greenhouse used in cold areas for preserving heat. The gases that absorb long-wave radiation are called greenhouse gases. The processes that warm the atmosphere are collectively referred to as the greenhouse effect.

Greenhouse gases [GHGs]

• The primary GHGs of concern today are carbon dioxide [co₂], chlorofluorocarbons [CFCs], methane [CH₄], Nitrous oxide [N₂O] and ozone [O₃]. Some other gases are nitric oxide [NO] and carbon monoxide [CO].
• The effectiveness of any given GHG molecule will depend on the magnitude of the increase in its concentration. The chlorofluorocarbons are highly effective.
• Ozone observes that ultra-violate radiation in the stratosphere is very effective in absorbing terrestrial radiation when it is present in the lower troposphere.
• The largest concentration of GHGs is carbon dioxide. The emissions come from fossil fuel combustion [oil, gas and coal].
• Forests and oceans are the sinks of carbon dioxide. The time taken for atmospheric CO₂ to adjust to changes in sources to sinks is 20-50 years. It is rising 0.5 per cent annually. Chlorofluorocarbons [CFCs] are products of human activity.
• Ozone occurs in the stratosphere where ultraviolet rays convert oxygen into ozone. 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 said to be an ozone hole. This allows the ultraviolet rays to pass through the troposphere.
• International efforts have been initiated to reduce the emission of GHGs. The Kyoto Protocol was signed in 1997 and went into effect in 2005, ratified by 141 nations. 35 industrialised countries reduce their emissions to 5 per cent by the year 2012. Global warming will change the rise in sea level, melting of glaciers and ice-caps and thermal expansion of the sea. The greatest warming of the 20^th century increased by0.4°C, during two periods. 1901-44 and 1977-99. The year 1988 was the warmest year of the whole millennium.