Chapter 11 — World Climate and Climate Change

Koeppen's scheme is an empirical climate classification based on mean annual and mean monthly temperature and precipitation data. Koeppen identified a close relationship between vegetation distribution and climate, selected certain temperature and precipitation values, and introduced capital and small letters to designate climatic groups and types. Though developed in 1918 and modified over time, it is still widely used.

Koeppen recognised five major climatic groups: A (Tropical — coldest month average ≥ 18°C), B (Dry — potential evaporation exceeds precipitation), C (Warm Temperate/Mid-latitude — coldest month between −3°C and 18°C), D (Cold Snow Forest — coldest month ≤ −3°C), and E (Cold — all months below 10°C). Four groups are based on temperature and one (B) on precipitation.

The three types are: Af (Tropical Wet) — found near the equator, no dry season, temperature maximum around 30°C and minimum around 20°C, with tropical evergreen forests; Am (Tropical Monsoon) — found over the Indian sub-continent, North Eastern South America, and Northern Australia, with heavy summer rainfall and dry winter; and Aw (Tropical Wet and Dry) — found north and south of Af regions, shorter wet season and longer dry season, with deciduous forests and grasslands.

Dry climates are characterised by very low rainfall inadequate for plant growth, covering large areas from 15°–60° north and south of the equator. They are divided into steppe or semi-arid (BS) and desert (BW) climates, further subdivided as subtropical steppe (BSh) and subtropical desert (BWh) at 15°–35° latitudes, and mid-latitude steppe (BSk) and mid-latitude desert (BWk) at 35°–60° latitudes. The highest shade temperature of 58°C was recorded at Al Aziziyah, Libya.

Mediterranean climate (Cs) occurs around the Mediterranean Sea and along the west coast of continents in subtropical latitudes between 30°–40°, including Central California, Central Chile, and coastal southeastern and southwestern Australia. It is characterised by hot, dry summers (monthly average around 25°C) and mild, rainy winters (below 10°C), with annual precipitation ranging from 35–90 cm.

Cold Snow Forest climates occur in large continental areas of the northern hemisphere between 40°–70° north latitudes in Europe, Asia, and North America. They are divided into Df (cold climate with humid winter — poleward of marine west coast climate, with cold and snowy winters and a short frost-free season) and Dw (cold climate with dry winters — mainly over Northeastern Asia, with monsoon-like wind reversal, very low annual precipitation of 12–15 cm, and temperatures below freezing for up to seven months).

Polar climates exist poleward beyond 70° latitude and consist of two types: Tundra (ET) — named after low-growing mosses, lichens, and flowering plants; the subsoil is permanently frozen (permafrost); and Ice Cap (EF) — occurring over interior Greenland and Antarctica, where even summer temperatures remain below freezing, with very little precipitation causing accumulation of ice sheets that break off as icebergs.

Geological records show alternating glacial and inter-glacial periods. Geomorphological features in high altitudes show 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. In India, the Rajasthan desert had wet and cool climate around 8,000 B.C., and the period 2,000–1,700 B.C. was the centre of the Harappan civilisation. Historical records of crop failures, floods, and migration also reflect climate variability.

Astronomical causes include changes in solar output associated with sunspot activities (increased sunspots linked to cooler and wetter weather; decreased sunspots to warmer and drier conditions) and Millankovitch oscillations — cycles in Earth's orbital characteristics, wobbling, and axial tilt that alter insolation received. Terrestrial causes include volcanism (volcanic eruptions throw aerosols into the atmosphere that reduce solar radiation reaching Earth's surface) and anthropogenic effects, most importantly the increasing concentration of greenhouse gases.

The greenhouse effect refers to the warming of the atmosphere because greenhouse gases (GHGs) absorb outgoing long-wave radiation emitted by Earth's surface while allowing incoming short-wave solar radiation to pass through. The primary GHGs are carbon dioxide (CO₂), chlorofluorocarbons (CFCs), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃). Gases such as nitric oxide (NO) and carbon monoxide (CO) also react with GHGs and affect their concentration.

The Kyoto Protocol was proclaimed in 1997 and went into effect in 2005, ratified by 141 nations. It bound 35 industrialised countries to reduce their GHG emissions by the year 2012 to 5 per cent less than the levels prevalent in the year 1990. It represents one of the major international efforts to control GHG emissions and arrest the trend towards global warming.

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 annual average near-surface air temperature of the world is approximately 14°C. The greatest warming of the 20th century occurred during 1901–44 and 1977–99, with global temperatures rising about 0.4°C over each period. The year 1998 was the warmest year, not only for the 20th century but probably for the whole millennium.

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