The Biosphere and Global Biomes

The biosphere as a life-support system

The biosphere encompasses all living organisms on Earth and the ecosystems in which they exist. It regulates:

• Atmospheric composition: photosynthesis converts CO₂ → O₂; respiration and decomposition cycle carbon.
• Water cycle: evapotranspiration from vegetation returns water to the atmosphere; roots hold soil and slow runoff.
• Nutrient cycles: decomposers break down dead matter → nutrients returned to soil → taken up by plants.
• Climate regulation: forests store carbon; wetlands regulate local temperature; ocean phytoplankton absorb CO₂.
• Food supply: all human food (directly or indirectly) comes from photosynthetic organisms in the biosphere.

Disruption of the biosphere (deforestation, ocean acidification, biodiversity loss) threatens these services.

Major biomes: distribution and characteristics

A biome is a large-scale ecosystem defined by climate (temperature + precipitation) and its characteristic vegetation and wildlife. The distribution of biomes closely mirrors climate zones, which in turn reflect the global atmospheric circulation system (Hadley, Ferrel and Polar cells — T1.1 link).

1. Tropical Rainforest

• Location: 5°N–5°S; Amazon basin (Brazil), Congo basin (DRC), SE Asia (Borneo, Sumatra, Papua New Guinea), Central America.
• Climate: Hot (~27 °C all year), very wet (>2,000 mm/year), no dry season; near the equator within the ITCZ.
• Structure: layered — emergents (>40 m), canopy (~30 m, continuous), understorey (~15 m), shrub layer, forest floor.
• Characteristics: very high biodiversity (~50% of world's species); rapid nutrient cycling (hot + wet = fast decomposition); thin, infertile soils (nutrients locked in biomass, not soil).
• Resources: timber (mahogany, teak), rubber, food (cacao, bananas, Brazil nuts), medicines (25% of modern drugs derived from rainforest plants), tourism.

2. Tropical Savanna (tropical grassland with trees)

• Location: 5°–15° N/S; sub-Saharan Africa (Serengeti, Sahel), northern Australia, Brazil (Cerrado), India.
• Climate: Hot all year; distinct wet season (ITCZ overhead) and dry season (>6 months); 500–1,500 mm/year.
• Characteristics: grassland with scattered trees (acacia, baobab); animals adapted to seasonal drought; high density of large mammals (Africa's Serengeti: wildebeest, lions, elephants).
• Resources: grazing for cattle; subsistence farming; trophy hunting; tourism (safari).

3. Hot Desert

• Location: ~20°–30° N/S (beneath subtropical high-pressure belts formed by descending air in Hadley cells); Sahara, Arabian Desert, Atacama, Australian Outback, Sonoran.
• Climate: Very hot days (>40 °C), cold nights (< 0 °C); extremely arid (<250 mm/year); clear skies, intense insolation.
• Characteristics: sparse xerophyte vegetation (cacti, succulents, thorn scrub); extreme diurnal temperature range; thin, nutrient-poor soils; high wind erosion.
• Resources: minerals (phosphate, oil, gold), solar energy potential, tourism (Atacama, Sahara), date palm agriculture in oases.

4. Mediterranean (shrubland / chaparral)

• Location: 30°–40° N/S on western coasts of continents; Mediterranean basin, California, Chile, SW Australia, South Africa.
• Climate: Hot dry summers; mild wet winters; 300–900 mm/year.
• Characteristics: fire-adapted shrubs (maquis/garrigue, chaparral); drought-resistant leaves (waxy, hard, small).
• Resources: wine grapes, olives, citrus fruits; high tourism value; wildfire risk is a hazard.

5. Temperate Forest (deciduous/mixed)

• Location: 40°–60° N; Western/Central Europe, Eastern USA, Eastern China, Japan.
• Climate: Warm summers, cool winters; 600–1,200 mm/year, no prolonged dry season.
• Characteristics: broad-leaved deciduous trees (oak, beech, ash, maple) shed leaves in winter; rich soils (brown earths — moderate decomposition, nutrient accumulation); moderate biodiversity.
• Resources: high-quality timber; agriculture (the UK's farmland was largely temperate forest); hunting; recreation.

6. Taiga (Boreal Forest)

• Location: 50°–70° N; Russia (Siberia), Canada, Scandinavia — the world's largest biome by area (~17% of Earth's land surface).
• Climate: Very cold winters (-30 °C), short warm summers; 300–900 mm/year (mostly snow).
• Characteristics: coniferous trees (pine, spruce, fir, larch) — evergreen, needle leaves reduce water loss and snow loading; low biodiversity; slow decomposition (cold) → deep peat/organic layer; acidic podsol soils.
• Resources: softwood timber (paper, construction), fur trapping, minerals (Russian gas, nickel, Canadian oil sands), hydroelectric power.

7. Tundra

• Location: > 60°–70° N; Arctic coasts of Canada, Russia, Alaska, Greenland; and Antarctic fringes.
• Climate: Extremely cold (-30 to -40 °C winters), cool summers (0–10 °C); very low precipitation (<250 mm/year); permafrost (permanently frozen ground layer).
• Characteristics: no trees; low-growing plants (sedges, mosses, lichens, dwarf shrubs); very low biodiversity; extremely slow decomposition; permafrost stores vast carbon reserves.
• Resources: oil and gas (Alaska/Russia, Arctic drilling), minerals, reindeer herding (Sámi people), wildlife tourism.

8. Polar Ice

• Location: Antarctica, Arctic Ocean, Greenland.
• Characteristics: no vegetation; supported by algae under sea ice (base of marine food chain); polar bears, penguins, seals.
• Resources: fishing (Southern Ocean krill, cod), potential minerals; critical for global climate regulation.

Biosphere resources under pressure

Human demand for biosphere resources has accelerated since industrialisation:

• Food: global population 8 billion and growing; calorie demand rising; land conversion for agriculture.
• Fuel: wood fuel (3 billion people rely on solid biomass for cooking/heating); biofuels replacing rainforest.
• Medicine: ~25% of pharmaceuticals derive from wild plant species; biopiracy and habitat loss threaten undiscovered cures.
• Building materials: tropical hardwood logging (mahogany, teak, merbau) drives deforestation.
• Rising demand: middle-class growth in China, India, Brazil → meat consumption up → 7 kg of grain to produce 1 kg of beef → 80% of deforestation is for agriculture.

Edexcel B exam tip

Biome questions often include a map or climate graph. Describe the climate graph (temperature range, precipitation pattern, seasonality) → identify the biome → explain vegetation adaptations → link to resource use pressures. Use the six characteristics to differentiate biomes: climate, location, soils, vegetation structure, biodiversity, and human resource use.