Climate Change

Climate Classification: The Big Picture

Climate is the long-term average of weather conditions, but the world's climates are so varied that studying them requires organisation. Geographers classify climates by grouping information on temperature and precipitation into smaller, understandable units. Three broad approaches exist. Empirical classification is based on observed data, especially temperature and precipitation. Genetic classification organises climates according to their causes. Applied classification serves a specific purpose, such as agriculture or building design.

Koeppen's Scheme of Classification

The most widely used climate classification is the empirical scheme developed by V. Koeppen. He noticed a close relationship between the distribution of vegetation and climate. He selected particular values of temperature and precipitation, related them to where vegetation grows, and used these values to classify climates. The scheme is based on mean annual and mean monthly temperature and precipitation data. Koeppen introduced capital and small letters to designate climatic groups and types. Though developed in 1918 and modified since, it remains popular.

Koeppen recognised five major climatic groups, four based on temperature and one on precipitation. The capital letters A, C, D and E delineate humid climates, while B designates dry climates. Groups are subdivided into types using small letters, based on the seasonality of precipitation and temperature.

The small letters f, m, w and s indicate seasons of dryness: f means no dry season, m means monsoon climate, w means winter dry season, and s means summer dry season. The letters a, b, c and d refer to the degree of severity of temperature. The B dry climates are subdivided using capital letters: S for steppe or semi-arid, and W for deserts.

Group A: Tropical Humid Climates

Tropical humid climates exist between the Tropic of Cancer and the Tropic of Capricorn. The sun being overhead throughout the year and the presence of the Inter Tropical Convergence Zone (ITCZ) make the climate hot and humid. The annual range of temperature is very low and annual rainfall is high. The group divides into three types: Af (tropical wet), Am (tropical monsoon), and Aw (tropical wet and dry).

Tropical Wet Climate (Af) is found near the equator. Major areas include the Amazon Basin in South America, western equatorial Africa, and the islands of the East Indies. Significant rainfall occurs every month as afternoon thunder showers. Temperature is uniformly high and the annual range is negligible. Maximum temperature on any day is around 30°C, minimum around 20°C. Tropical evergreen forests with dense canopy cover and large biodiversity grow here.

Tropical Monsoon Climate (Am) is found over the Indian sub-continent, northeastern South America, and northern Australia. Heavy rainfall occurs mostly in summer; winter is dry.

Tropical Wet and Dry Climate (Aw) occurs north and south of the Af regions. It borders dry climate on the western part of the continent and Cf or Cw on the eastern part. Extensive Aw climate is found north and south of the Amazon forest in Brazil and adjoining parts of Bolivia and Paraguay, in Sudan, and south of Central Africa. Annual rainfall is considerably less than in Af and Am and is variable. The wet season is shorter, the dry season longer, and drought more severe. Temperature is high throughout the year, and diurnal ranges are greatest in the dry season. Deciduous forest and tree-shredded grasslands occur here.

Dry Climates: B

Dry climates have very low rainfall, inadequate for plant growth. They cover a vast area extending over latitudes from 15° to 60° north and south of the equator. At low latitudes (15°–30°), they occur in the area of subtropical high pressure, where subsidence and temperature inversion prevent rainfall. On the western margins of continents adjoining cold currents, especially the west coast of South America, they extend more equatorwards and occur on the coast. In middle latitudes (35°–60°), they are confined to the interiors of continents where maritime humid winds do not reach, and to areas often surrounded by mountains.

Dry climates divide into steppe or semi-arid climate (BS) and desert climate (BW). These further subdivide into subtropical steppe (BSh) and subtropical desert (BWh) at latitudes 15°–35°, and mid-latitude steppe (BSk) and mid-latitude desert (BWk) at latitudes 35°–60°.

Subtropical Steppe (BSh) and Subtropical Desert (BWh) share common precipitation and temperature characteristics. Located in the transition zone between humid and dry climates, the steppe receives slightly more rainfall than the desert, enough for sparse grasslands. Rainfall in both is highly variable. This variability affects life in the steppe more than in the desert, often causing famine. Rain in deserts occurs as short, intense thundershowers, ineffective in building soil moisture. Fog is common in coastal deserts bordering cold currents. Summer maximum temperatures are very high — the highest shade temperature of 58°C was recorded at Al Aziziyah, Libya, on 13 September 1922. Annual and diurnal ranges of temperature are also high.

Warm Temperate (Mid-Latitude) Climates: C

Warm temperate climates extend from 30° to 50° latitude, mainly on the eastern and western margins of continents. They generally have warm summers with mild winters. Four types exist: Cwa (humid subtropical, dry in winter, hot in summer), Cs (Mediterranean), Cfa (humid subtropical, no dry season, mild winter), and Cfb (marine west coast).

Humid Subtropical Climate (Cwa) occurs poleward of the Tropic of Cancer and Capricorn, mainly in the North Indian plains and South China interior plains. It resembles the Aw climate except that winter temperatures are warm.

Mediterranean Climate (Cs) occurs around the Mediterranean Sea and along the west coasts of continents in subtropical latitudes between 30° and 40°, for example central California, central Chile, and the coasts of southeastern and southwestern Australia. These areas come under the influence of the subtropical high in summer and westerly winds in winter. The climate is characterised by hot, dry summers and mild, rainy winters. Monthly average summer temperature is around 25°C, winter below 10°C. Annual precipitation ranges between 35 and 90 cm.

Humid Subtropical Climate (Cfa) lies on the eastern parts of continents in subtropical latitudes. Air masses here are generally unstable and cause rainfall throughout the year. It occurs in the eastern United States, southern and eastern China, southern Japan, northeastern Argentina, coastal South Africa, and the eastern coast of Australia. Annual precipitation averages 75–150 cm. Thunderstorms in summer and frontal precipitation in winter are common. Mean monthly summer temperature is around 27°C; winter varies from 5° to 12°C. The daily range of temperature is small.

Marine West Coast Climate (Cfb) is located poleward from the Mediterranean climate on the west coasts of continents. Main areas include northwestern Europe, the west coast of North America north of California, southern Chile, southeastern Australia, and New Zealand. Due to marine influence, temperature is moderate, and winters are warmer than expected for the latitude. Mean summer temperatures range from 15° to 20°C, winter from 4° to 10°C. Annual and daily ranges are small. Precipitation occurs throughout the year and varies greatly, from 50 to 250 cm.

Cold Snow Forest Climates: D

Cold snow forest climates occur in the large continental areas of the northern hemisphere between 40° and 70° north latitudes in Europe, Asia, and North America. They divide into two types: Df (cold climate with humid winter) and Dw (cold climate with dry winter). The severity of winter increases with latitude.

Cold Climate with Humid Winters (Df) occurs poleward of the marine west coast climate and mid-latitude steppe. Winters are cold and snowy, the frost-free season is short, and annual ranges of temperature are large. Weather changes are abrupt and short. Poleward, winters become more severe.

Cold Climate with Dry Winters (Dw) occurs mainly over northeastern Asia. The development of a pronounced winter anticyclone and its weakening in summer sets up a monsoon-like reversal of wind. Poleward summer temperatures are lower, and winter temperatures are extremely low, with many locations experiencing below-freezing temperatures for up to seven months a year. Precipitation occurs in summer. Annual precipitation is low, from 12 to 15 cm.

Polar Climates: E

Polar climates exist poleward beyond 70° latitude and consist of two types: Tundra (ET) and Ice Cap (EF).

Tundra Climate (ET) is named after its vegetation — low-growing mosses, lichens, and flowering plants. This is the region of permafrost, where the subsoil is permanently frozen. The short growing season and waterlogging support only low-growing plants. During summer, the tundra regions have very long durations of daylight.

Ice Cap Climate (EF) occurs over interior Greenland and Antarctica. Even in summer, temperature remains below freezing. This area receives very little precipitation. Snow and ice accumulate, and the mounting pressure deforms the ice sheets, causing them to break. They move as icebergs floating in Arctic and Antarctic waters. Plateau Station, Antarctica, at 79°S, portrays this climate.

Climate Change

The climate we experience now may have prevailed over the last 10,000 years, with minor and occasionally wide fluctuations. The planet has witnessed many variations since its beginning. Geological records show alternation of glacial and inter-glacial periods. Geomorphological features, especially in high altitudes and latitudes, exhibit traces of glacier advances and retreats. Sediment deposits in glacial lakes reveal warm and cold periods. Tree rings provide clues about wet and dry periods. Historical records describe vagaries in climate. All these evidences indicate that climate change is a natural and continuous process.

India also witnessed alternate wet and dry periods. Archaeological findings show the Rajasthan desert experienced a wet and cool climate around 8,000 B.C. The period 3,000–1,700 B.C. had higher rainfall. From about 2,000–1,700 B.C., this region was the centre of the Harappan civilisation. Dry conditions accentuated 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 was about 18,000 years ago. The present inter-glacial period started 10,000 years ago.

Climate in the Recent Past

Climate variability occurs all the time. The 1990s recorded the warmest temperature of the century and some of the worst floods around the world. The worst devastating drought in the Sahel region, south of the Sahara desert, from 1967–1977, is one such variability. During the 1930s, a severe drought occurred in the southwestern Great Plains of the United States, described as the dust bowl. Historical records of crop yields, crop failures, floods, and migration of people tell of the effects of changing climate.

Europe witnessed warm, wet, cold, and dry periods. Significant episodes included warm and dry conditions in the tenth and eleventh centuries, when the Vikings settled in Greenland. Europe witnessed the "Little Ice Age" from 1550 to about 1850. From about 1885–1940, world temperature showed an upward trend. After 1940, the rate of increase slowed down.

Causes of Climate Change

The causes of climate change are many and can be grouped into astronomical and terrestrial causes.

Astronomical causes include changes in solar output associated with sunspot activities. Sunspots are dark, cooler patches on the sun that increase and decrease cyclically. According to some meteorologists, when sunspot numbers increase, cooler and wetter weather and greater storminess occur; a decrease is associated with warm and drier conditions. Yet these findings are not statistically significant. Another astronomical theory is the Milankovitch oscillations, which infer cycles in variations in the earth's orbital characteristics around the sun, the wobbling of the earth, and changes in the earth's axial tilt. All these alter the amount of insolation received from the sun, which in turn may bear on climate.

Terrestrial causes include volcanism. Volcanic eruptions throw up lots of aerosols into the atmosphere. These aerosols remain for a considerable period, reducing the sun's radiation reaching the earth's surface. After the recent Pinatubo and El Chichon volcanic eruptions, the average temperature of the earth fell to some extent for some years.

The most important anthropogenic effect on climate is the increasing trend in the 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 behaves like a greenhouse. The atmosphere transmits 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 collectively referred to as the greenhouse effect. …