WORLD CLIMATE AND CLIMATE CHANGE

Introduction

Climate refers to the average weather conditions of a place over a long period of time. To understand the world climate properly, information related to temperature, rainfall, humidity, pressure, winds, and vegetation is organised and classified into different climatic regions.There are three major approaches used for climate classification:

1. Empirical Classification – Based on observed climatic data such as temperature and rainfall.
2. Genetic Classification – Based on the causes responsible for climate.
3. Applied Classification – Developed for specific practical purposes.

KOEPPEN’S SCHEME OF CLIMATE CLASSIFICATION

The most widely accepted climatic classification was developed by Vladimir Koeppen.Koeppen established a close relationship between:

• Climate
• Temperature
• Rainfall
• Natural Vegetation

It is an empirical classification based on:

• Mean annual temperature
• Mean monthly temperature
• Mean annual precipitation
• Mean monthly precipitation

Koeppen used:

• Capital letters for major climatic groups
• Small letters for sub-types and seasonal characteristics

Although introduced in 1918, the scheme still remains widely used.

MAJOR CLIMATIC GROUPS ACCORDING TO KOEPPEN

LETTER SYMBOLS USED IN KOEPPEN CLASSIFICATION

Small Letters for Rainfall Seasonality

Small Letters for Temperature Severity

Symbols Used in Dry Climate

GROUP A : TROPICAL HUMID CLIMATES

General Characteristics

Tropical humid climates occur between:

• Tropic of Cancer
• Tropic of Capricorn

Major Features

• Sun remains overhead throughout the year.
• Influence of Inter Tropical Convergence Zone (ITCZ).
• Climate remains hot and humid.
• Annual temperature range is very low.
• Rainfall is very high.

Types of Tropical Humid Climate

1. Af – Tropical Wet Climate
2. Am – Tropical Monsoon Climate
3. Aw – Tropical Wet and Dry Climate

TROPICAL WET CLIMATE (Af)

Distribution

Found near the equator in:

• Amazon Basin
• Western Equatorial Africa
• East Indies

Characteristics

• Rainfall occurs throughout the year.
• Afternoon convectional thunderstorms are common.
• Temperature remains uniformly high.
• Annual temperature range is negligible.
• Maximum temperature around 30°C
• Minimum temperature around 20°C

Natural Vegetation

• Tropical evergreen forests
• Dense canopy
• High biodiversity

TROPICAL MONSOON CLIMATE (Am)

Distribution

Found in:

• Indian subcontinent
• Northeastern South America
• Northern Australia

Characteristics

• Heavy rainfall during summer
• Dry winter season

TROPICAL WET AND DRY CLIMATE (Aw)

Distribution

Occurs north and south of Af climate regions.Major regions include:

• Bolivia
• Paraguay
• Sudan
• Southern Central Africa

Characteristics

• Rainfall less than Af and Am climates
• Rainfall highly variable
• Short wet season
• Long dry season
• Severe drought conditions
• High temperature throughout the year
• Large diurnal temperature range during dry season

Natural Vegetation

• Deciduous forests
• Tree-shredded grasslands

GROUP B : DRY CLIMATES

General Characteristics

Dry climates receive very low rainfall which is insufficient for plant growth.

Latitudinal Distribution

• Between 15°–60° North and South

Causes

In Low Latitudes (15°–30°)

• Subtropical high pressure
• Descending air
• Temperature inversion

On Western Continental Margins

• Influence of cold ocean currents

In Mid-Latitudes (35°–60°)

• Interior continental location
• Absence of maritime winds
• Mountain barriers

TYPES OF DRY CLIMATES

Further Subdivisions

SUBTROPICAL STEPPE (BSh) AND DESERT (BWh)

Characteristics

• Located in transition zone between humid and dry climates.
• Steppe receives slightly more rainfall than desert.
• Rainfall highly variable.
• Frequent drought and famine.
• Rainfall occurs through intense thunderstorms.
• Rain ineffective in retaining soil moisture.
• Fog common in coastal deserts adjoining cold currents.

Temperature

• Very high summer temperature
• High annual temperature range
• High diurnal temperature range

Important Fact

• Highest shade temperature:
  58°C at Al Aziziyah, Libya
  on 13 September 1922

GROUP C : WARM TEMPERATE (MID-LATITUDE) CLIMATES

Distribution

Located between:

• 30°–50° latitude

Mainly on eastern and western continental margins.

General Characteristics

• Warm summers
• Mild winters

TYPES OF WARM TEMPERATE CLIMATES

HUMID SUBTROPICAL CLIMATE (Cwa)

Distribution

• North Indian plains
• South China interior plains

Characteristics

• Similar to Aw climate
• Winters are warmer

MEDITERRANEAN CLIMATE (Cs)

Distribution

Found around:

• Mediterranean Sea
• Central California
• Central Chile
• Southwestern Australia
• Southeastern Australia

Climatic Control

• Subtropical high pressure in summer
• Westerlies in winter

Characteristics

• Hot and dry summer
• Mild and rainy winter
• Summer temperature around 25°C
• Winter temperature below 10°C
• Annual rainfall: 35–90 cm

HUMID SUBTROPICAL CLIMATE (Cfa)

Distribution

• Eastern USA
• Southern and Eastern China
• Southern Japan
• Northeastern Argentina
• Coastal South Africa
• Eastern Australia

Characteristics

• Rainfall throughout the year
• Unstable air masses
• Summer thunderstorms
• Winter frontal rainfall
• Annual rainfall: 75–150 cm
• Summer temperature around 27°C
• Winter temperature between 5°–12°C
• Small daily temperature range

MARINE WEST COAST CLIMATE (Cfb)

Distribution

• Northwestern Europe
• West coast of North America
• Southern Chile
• Southeastern Australia
• New Zealand

Characteristics

• Strong marine influence
• Moderate temperature
• Mild winters
• Small annual temperature range
• Rainfall throughout the year

Temperature

• Summer: 15°–20°C
• Winter: 4°–10°C

Rainfall

• 50–250 cm

GROUP D : COLD SNOW FOREST CLIMATES

Distribution

Found between:

• 40°–70° North Latitude

Mainly in:

• Europe
• Asia
• North America

Types

COLD CLIMATE WITH HUMID WINTER (Df)

Characteristics

• Cold and snowy winters
• Short frost-free season
• Large annual temperature range
• Abrupt weather changes
• Severe winters in higher latitudes

COLD CLIMATE WITH DRY WINTER (Dw)

Distribution

Mainly in Northeastern Asia.

Characteristics

• Monsoon-like wind reversal
• Strong winter anticyclone
• Extremely cold winters
• Temperatures below freezing for up to seven months
• Summer rainfall
• Annual precipitation: 12–15 cm

GROUP E : POLAR CLIMATES

Distribution

Poleward of:

• 70° latitude

Types

TUNDRA CLIMATE (ET)

Characteristics

• Presence of permafrost
• Permanently frozen subsoil
• Very short growing season
• Waterlogged soil
• Long summer daylight duration

Vegetation

• Mosses
• Lichens
• Flowering plants

ICE CAP CLIMATE (EF)

Distribution

• Interior Greenland
• Antarctica

Characteristics

• Temperature below freezing even in summer
• Very low precipitation
• Snow and ice accumulation
• Formation of icebergs

Important Example

• Plateau Station, Antarctica (79°S)

GROUP H : HIGHLAND CLIMATES

Characteristics

• Controlled by topography and altitude
• Rapid temperature variation with elevation
• Spatial variation in rainfall
• Vertical zonation of climate and vegetation

CLIMATE CHANGE

Climate change refers to long-term variation in climatic conditions over time.The Earth has experienced:

• Glacial periods
• Inter-glacial periods
• Wet and dry climatic phases

Climate change is a natural and continuous process.

EVIDENCES OF CLIMATE CHANGE

Geological Evidence

• Advance and retreat of glaciers
• Glacial lake sediments

Biological Evidence

• Tree rings indicating wet and dry periods

Historical Evidence

• Historical records of climatic irregularities

CLIMATE CHANGE IN INDIA

Rajasthan Desert

Around 8000 B.C.

• Wet and cool climate existed.

3000–1700 B.C.

• Higher rainfall occurred.

2000–1700 B.C.

• Region became centre of Harappan civilisation.

Later Period

• Increasing dryness developed.

GEOLOGICAL HISTORY OF CLIMATE

500–300 Million Years Ago

Earth remained warm during:

• Cambrian Period
• Ordovician Period
• Silurian Period

Pleistocene Epoch

• Alternating glacial and inter-glacial periods

Important Fact

• Last major glacial peak: 18,000 years ago
• Present inter-glacial period began: 10,000 years ago

CLIMATE VARIABILITY IN RECENT PAST

1990s

• Warmest decade of the century
• Severe floods worldwide

Sahel Drought (1967–1977)

• Severe drought south of Sahara Desert

Dust Bowl (1930s)

• Severe drought in Southwestern USA

Europe

10th–11th Century

• Warm and dry climate
• Viking settlement in Greenland

1550–1850

• Little Ice Age

1885–1940

• Rising global temperature trend

After 1940

• Temperature rise slowed

CAUSES OF CLIMATE CHANGE

1. Astronomical Causes

Sunspot Theory

• Increase in sunspots linked with cooler and wetter climate
• Decrease linked with warmer and drier climate

Milankovitch Oscillations

Changes in:

• Earth’s orbit
• Axial tilt
• Earth’s wobbling

These affect incoming solar radiation.

2. Volcanism

Volcanic eruptions release aerosols into atmosphere.

Effects

• Reduction in solar radiation reaching Earth
• Temporary cooling of Earth

Important Examples

• Pinatubo eruption
• El Cion eruption

3. Anthropogenic Causes

Most Important Cause

Increase in greenhouse gases (GHGs) leading to global warming.

GLOBAL WARMING

The atmosphere behaves like a greenhouse.

Greenhouse Effect

• Atmosphere allows incoming shortwave solar radiation.
• Atmosphere absorbs outgoing longwave terrestrial radiation.
• This traps heat and warms the atmosphere.

EXAMPLES OF GREENHOUSE EFFECT

Greenhouse Structure

• Glass allows solar radiation inside.
• Prevents escape of terrestrial radiation.
• Temperature inside remains higher.

Closed Vehicles in Summer

• Cars and buses become hotter when windows remain closed.

GREENHOUSE GASES (GHGs)

Major Greenhouse Gases

Other gases like:

• Nitric oxide (NO)
• Carbon monoxide (CO)

also influence greenhouse gas concentration.

CHARACTERISTICS OF GHGs

The effectiveness of GHGs depends on:

• Increase in concentration
• Atmospheric life span
• Radiation absorption wavelength

CARBON DIOXIDE (CO₂)

Major Sources

• Fossil fuel combustion
• Coal
• Oil
• Gas

Sinks

• Forests
• Oceans

Effects of Deforestation

• Increase in atmospheric CO₂ concentration

Important Facts

• Atmospheric adjustment time: 20–50 years
• Annual increase: 0.5%

CHLOROFLUOROCARBONS (CFCs) AND OZONE

Ozone Layer

• Formed in stratosphere
• Absorbs ultraviolet radiation

Impact of CFCs

• Destroy ozone layer
• Cause ozone depletion

Ozone Hole

Large ozone depletion observed over:

• Antarctica

INTERNATIONAL EFFORTS TO CONTROL GLOBAL WARMING

Kyoto Protocol (1997)

Implemented

• Came into force in 2005

Ratification

• Ratified by 141 nations

Objective

• 35 industrialised countries to reduce emissions by 5% below 1990 levels by 2012

IMPACTS OF GLOBAL WARMING

Major Impacts

• Melting glaciers and ice caps
• Sea level rise
• Thermal expansion of oceans
• Coastal inundation
• Threat to islands
• Social and environmental problems

GLOBAL TEMPERATURE TREND

Average Global Temperature

• Approximately 14°C

Major Warming Phases

Overall Rise

• End of 20th century temperature was 0.6°C higher than late 19th century.

Important Facts

• Seven warmest years recorded in the last decade of 1856–2000.
• 1998 was the warmest year of the millennium according to records.