Climate vs weather

· Climate = average atmospheric conditions over about 30 years.
· Weather = short-term atmospheric conditions at a particular time, including temperature, humidity, air pressure and wind speed.
· In exams, link climate to long-term biome distribution and weather to short-term variation.

Biomes: definition and controls

· A biome = a group of comparable ecosystems that develop under similar climatic conditions.
· Main controls of terrestrial biome distribution are temperature, precipitation and insolation.
· Abiotic factors determine which terrestrial biome is likely to develop in a location.
· For any given pattern of rainfall and temperature, one natural ecosystem type is generally favoured.
· Be able to interpret a temperature–precipitation biome graph and link climate to productivity and biodiversity.

This diagram shows how temperature and rainfall determine the likely terrestrial biome. It is useful for explaining why deserts, tundra and tropical rainforests occur under very different climatic conditions. In exams, use it to justify biome distribution from climate data. Source

This map shows the global distribution of major world biomes. It helps link latitude, climate and continental position to where different biomes are found. Use it to support explanations of why biomes form broad global belts. Source

Major biome groups

· Broad biome groups include freshwater, marine, forest, grassland, desert and tundra.
· Each biome has characteristic limiting factors, productivity and biodiversity.
· Tropical rainforest: high temperature + high rainfall → very high productivity and very high biodiversity; limiting factors are often linked more to soil nutrients than climate.
· Hot desert: very low rainfall → low productivity and low biodiversity; main limiting factor is water.
· Tundra: low temperatures, short growing season, often permafrost → low productivity and low biodiversity; limiting factors are temperature and water availability in frozen soils.
· Temperate forest: moderate temperature + moderate/high rainfall → moderate to high productivity and moderate biodiversity.
· Grassland: rainfall often seasonal or insufficient for continuous forest growth; fire, grazing and water availability help maintain the biome.
· In essays, compare biomes using limiting factor → productivity → biodiversity.

Tricellular model and global biome pattern

· The tricellular model of atmospheric circulation explains global patterns of heat and precipitation and therefore biome distribution.
· There are three cells in each hemisphere: Hadley cell, Ferrel cell and Polar cell.
· At the equator, intense heating causes air to rise, cool and condense → high rainfall → favours tropical rainforest.
· Around 30° N/S, air descends, warms and becomes drier → low rainfall → favours hot deserts.
· Around 60° N/S, air rises again where air masses meet → more precipitation, supporting temperate and boreal biomes.
· Near the poles, cold descending air contributes to very low temperatures and low precipitation → polar/tundra conditions.
· Key exam link: global circulation controls rainfall belts, and rainfall belts strongly influence biomes.

This diagram shows the tricellular model and the major global belts of rising and sinking air. It is ideal for explaining why rainforests occur near the equator and deserts around 30° latitude. Use it to connect latitude, pressure belts and biome distribution. Source

Oceans and heat redistribution

· The oceans absorb solar radiation and help redistribute heat around the world.
· Ocean currents move warm and cool water, influencing regional climates.
· Because oceans redistribute heat, areas at the same latitude may still have different climates.
· Ocean heat transfer contributes to the global pattern of temperature, which helps shape biomes.
· At SL, know that oceans redistribute heat; detailed thermohaline circulation is HL only elsewhere, but the climate link is important here.

This map shows large-scale ocean circulation moving warm surface water and cold deep water around the planet. It helps explain how oceans redistribute heat and influence regional climate patterns. Use it to support the idea that ocean circulation affects the distribution of biomes. Source

Climate change and shifting biomes

· Global warming is causing climates to change and biomes to shift.
· The general trend is for biomes to move poleward and to higher altitudes.
· As temperatures rise, species adapted to cooler conditions may be forced into smaller ranges or displaced.
· Biome shifts can alter productivity, species composition and biodiversity.
· In data questions, look for evidence of changing temperature and rainfall leading to changes in biome boundaries.

Exam shortcuts

· Equator = rising air, high rainfall, rainforest.
· 30° latitude = sinking air, dry conditions, desert.
· Higher latitude / altitude = colder conditions, shorter growing season, tundra/polar biomes.
· More rain + more warmth generally = higher productivity and often higher biodiversity.
· Climate change shifts biome boundaries; human activity can override the biome expected from climate alone.