Introduction
Climate is the average pattern of weather (temperature, precipitation, winds, humidity) over a long period (at least 30 years) at a location. Weather is the short-term state of the atmosphere. This chapter explores how climates are classified, what controls global climate patterns, and how human activities are now altering the climate system.
Koppen's Climate Classification
The most widely used classification is Koppen's system (modified by Trewartha), which groups climates primarily by temperature and precipitation patterns, using letter codes.
- Major groups:
- A (Tropical): All months average above 18 degrees C; high rainfall. Subdivisions: Af (rainforest, rain every month), Am (monsoon), Aw (savanna, dry winter).
- B (Dry): Evaporation exceeds precipitation. BS (steppe) and BW (desert, W = waste). Temperature prefix: h (hot), k (cold).
- C (Temperate/Mesothermal): Coldest month between -3 and 18 degrees C. Cf (humid, no dry season), Cs (Mediterranean, dry summer), Cw (dry winter).
- D (Cold/Microthermal): Coldest month below -3 degrees C, warmest above 10 degrees C. Df (humid continental), Dw (dry winter — Siberian).
- E (Polar): No month averages above 10 degrees C. ET (tundra, warmest month 0–10 degrees C), EF (ice cap, always below 0 degrees C).
- H (Highland): Complex climate variations due to altitude.
World Climate Types — Key Features
Tropical Rainforest (Af): Near equator (0–10 degrees). High temperature (~27 degrees C all year), very high rainfall (>2000 mm), no distinct seasons. Dense evergreen forest (Amazon, Congo, SE Asia).
Tropical Monsoon (Am): Strong seasonal rainfall; short dry season compensated by heavy monsoon. South Asia, West Africa.
Tropical Savanna (Aw): Distinct wet and dry seasons; grasslands with scattered trees. Interior of Africa, Brazil.
Hot Desert (BWh): Sub-tropical high pressure; very low rainfall (<250 mm); extreme temperature range. Sahara, Arabian, Thar deserts.
Mediterranean (Cs): West coasts 30–45 degrees; hot dry summers, mild wet winters. Due to seasonal shift of pressure belts. Mediterranean basin, California, SW Australia.
Temperate Oceanic (Cfb): West coasts 45–60 degrees; mild, wet throughout year; no extreme temperatures. Western Europe.
Humid Continental (Df): Continental interiors, 40–60 degrees N; cold winters, warm summers, year-round precipitation. Great Plains (USA), Eastern Europe.
Tundra (ET): 60–75 degrees; very cold winters, brief cool summers; permafrost; mosses, lichens. Northern Canada, Siberia.
Factors Controlling World Climate
- Latitude: Controls temperature and insolation (equatorial hot, polar cold).
- Altitude: Temperature decreases with height; highlands cooler.
- Distance from sea: Continental climates have extreme ranges; maritime climates are moderate.
- Ocean currents: Warm currents bring rain; cold currents create coastal deserts and fog.
- Prevailing winds: Onshore winds bring moisture; offshore winds bring dry conditions.
- Relief (mountains): Create rain shadows and temperature gradients.
Climate Change
Climate change refers to long-term shifts in global or regional temperature, precipitation, and other climatic variables. While natural climate change has occurred throughout Earth's history (ice ages, warm periods), the current change is primarily driven by human activities since the Industrial Revolution (~1750).
- 1.Causes of current climate change:
- 2.Enhanced greenhouse effect: Burning of fossil fuels releases CO2; deforestation reduces CO2 absorption; agriculture releases CH4 and N2O. These gases trap more outgoing radiation, warming the atmosphere.
- 3.Urbanisation: Urban heat islands replace vegetation with concrete, raising local temperatures.
- 4.Land use change: Deforestation alters albedo and evapotranspiration.
- 5.Industrial processes: Release aerosols and pollutants.
- Evidence of climate change:
- Rising global mean temperatures (approximately +1.1–1.2 degrees C above pre-industrial levels as of recent decades).
- Melting of glaciers and ice sheets (Greenland, Antarctica, Himalayan glaciers).
- Rising sea levels (thermal expansion + melt water; ~3.3 mm/year average).
- Increasing frequency of extreme weather events (floods, droughts, heatwaves, intense cyclones).
- Shifts in plant and animal ranges (species moving poleward or to higher altitudes).
- Ocean acidification (CO2 dissolves in seawater, forming carbonic acid).
- Consequences of climate change:
- Submergence of low-lying coastal areas and island nations.
- Disruption of agriculture and water supplies.
- Increased vector-borne diseases (malaria, dengue moving to new areas).
- Biodiversity loss and ecosystem collapse.
- More frequent and intense natural disasters.
- International responses:
- UNFCCC (1992): Framework for international climate action.
- Kyoto Protocol (1997): Binding emission targets for developed countries.
- Paris Agreement (2015): Target to limit warming to well below 2 degrees C above pre-industrial levels; countries set Nationally Determined Contributions (NDCs).
- IPCC: Intergovernmental Panel on Climate Change — scientific body assessing climate research.
Common mistakes
- Climate and weather are NOT the same — climate is a long-term average (30+ years), weather is day-to-day.
- The greenhouse effect itself is natural and essential; the problem is the enhanced greenhouse effect from human emissions.
- Mediterranean climates have dry summers and wet winters — the opposite of what many students expect.
- The Koppen B (dry) group is defined by evaporation exceeding precipitation, NOT by temperature alone — cold deserts exist.
Summary
World climates are classified using Koppen's system based on temperature and precipitation. Each climate type results from a combination of controls — latitude, altitude, ocean currents, winds, and distance from sea. Human-induced enhancement of the greenhouse effect is causing measurable warming, sea-level rise, and ecosystem disruption, prompting international climate agreements.