What you'll learn
This topic covers the differences between weather and climate, global atmospheric circulation patterns, and the distribution and characteristics of major world ecosystems. You'll explore the climate graphs, nutrient cycles and adaptations of organisms within tropical rainforests and hot deserts, along with the causes and impacts of deforestation and desertification.
Key terms and definitions
Weather — the day-to-day atmospheric conditions in a specific place, including temperature, precipitation, wind speed and cloud cover.
Climate — the average weather conditions recorded over a period of at least 30 years for a particular location.
Ecosystem — a community of living organisms (plants and animals) interacting with each other and their non-living environment (soil, water, air).
Biome — a large-scale ecosystem covering a significant area of the Earth's surface, characterised by distinctive climate, vegetation and wildlife.
Biomass — the total mass of living organisms (plants and animals) in a given area, usually measured per unit area.
Nutrient cycling — the continuous transfer of nutrients between living organisms, soil, water and atmosphere within an ecosystem.
Desertification — the degradation of land in arid and semi-arid areas, resulting in desert-like conditions, primarily caused by human activities and climate change.
Deforestation — the permanent removal of forest cover, usually for agriculture, logging or urban development.
Core concepts
Weather vs climate and measuring atmospheric conditions
Weather describes short-term atmospheric conditions, while climate represents long-term patterns. The UK experiences variable weather due to its position between warm tropical and cold polar air masses.
Key measurements include:
- Temperature (thermometer, measured in °C)
- Precipitation (rain gauge, measured in mm)
- Wind speed (anemometer, measured in km/h or knots)
- Wind direction (wind vane, recorded as compass bearing)
- Air pressure (barometer, measured in millibars/mb)
- Cloud cover (visual observation, measured in oktas)
Climate graphs display temperature (line graph) and precipitation (bar chart) data for a location across 12 months, enabling identification of climate type and seasonal patterns.
Global atmospheric circulation
The Earth's surface is heated unevenly by the sun, creating three circulation cells in each hemisphere that redistribute heat and moisture:
Hadley Cell (0°–30° latitude):
- Hot air rises at the equator, creating low pressure and heavy rainfall
- Air moves poleward at high altitude, cooling and sinking at 30°N/S
- Creates high pressure zones and arid conditions (major deserts located here)
- Surface winds return towards equator as trade winds
Ferrel Cell (30°–60° latitude):
- Driven by the Hadley and Polar cells
- Air rises at 60°N/S, creating low pressure and rainfall
- Air sinks at 30°N/S
- Surface winds move poleward as westerlies (the UK is located in this zone)
Polar Cell (60°–90° latitude):
- Cold dense air sinks at the poles, creating high pressure
- Air moves towards 60°N/S at surface, where it rises
- Creates cold, dry conditions at poles
This circulation creates distinct climate zones: tropical (equator), arid (30°N/S), temperate (40°–60°N/S), and polar (poles).
Tropical rainforest ecosystems
Tropical rainforests are located between the Tropics of Cancer and Capricorn, including the Amazon Basin, Congo Basin and South-East Asia.
Climate characteristics:
- High temperatures (26–28°C) year-round
- High annual rainfall (over 2000mm)
- No distinct dry season
- High humidity (80%+)
Vegetation structure:
- Emergent layer (45m+) — tallest trees receiving most sunlight
- Canopy layer (30–45m) — continuous cover forming a 'roof'
- Under-canopy (10–30m) — smaller trees in shade
- Shrub layer (0–10m) — sparse vegetation due to limited light
- Forest floor — decomposing leaf litter, limited plant growth
Plant adaptations:
- Drip-tip leaves channel water off quickly, preventing algae growth
- Buttress roots provide stability in shallow soils
- Smooth bark allows water to run off
- Lianas (climbing plants) reach sunlight without building thick trunks
- Epiphytes grow on tree branches to access light
Animal adaptations:
- Camouflage for predator/prey relationships
- Loud calls to communicate through dense vegetation
- Prehensile tails for climbing (e.g. spider monkeys)
- Bright colours for mating displays and warnings
Nutrient cycle: The rainforest has rapid nutrient cycling. Heavy rainfall quickly washes nutrients from soil (leaching), but the warm, humid conditions mean decomposition is extremely fast. Most nutrients are stored in the biomass rather than soil. When trees are removed, the soil quickly becomes infertile.
Hot desert ecosystems
Hot deserts are located around 30°N and 30°S latitude (Tropic zones), including the Sahara, Arabian, Kalahari and Australian deserts.
Climate characteristics:
- Very hot days (over 40°C) due to clear skies
- Cold nights (below 0°C) — no cloud blanket to retain heat
- Very low annual rainfall (under 250mm)
- High evaporation rates
- Large diurnal (daily) temperature range
Plant adaptations:
- Long tap roots to reach deep water sources
- Extensive shallow root systems to absorb surface water quickly
- Small leaves or spines reduce water loss through transpiration
- Waxy leaf coatings reduce evaporation
- CAM photosynthesis (opening stomata at night) minimises water loss
- Water storage in stems (cacti) or leaves (succulents)
Animal adaptations:
- Nocturnal behaviour avoids hottest temperatures
- Large ears (fennec fox) for heat loss
- Light-coloured fur reflects sunlight
- Concentrated urine and dry faeces conserve water
- Ability to obtain water from food (kangaroo rat)
- Burrowing underground during day for cooler temperatures
Nutrient cycle: Desert nutrient cycling is very slow due to limited decomposition (lack of moisture and decomposers). Most nutrients are stored in soil rather than biomass, as vegetation cover is sparse.
Deforestation: causes and impacts
Causes of tropical rainforest deforestation:
- Commercial agriculture (cattle ranching, soya, palm oil)
- Subsistence farming (slash-and-burn agriculture)
- Logging (hardwoods for furniture and construction)
- Mineral extraction (gold, iron ore, bauxite)
- Road building and infrastructure development
- Population growth increasing demand for land
Impacts of deforestation:
Environmental impacts:
- Reduced biodiversity and species extinction
- Soil erosion without tree roots to bind soil
- Increased greenhouse gas emissions (CO₂ released when trees burned)
- Disrupted water cycle and reduced rainfall
- Loss of medicine sources (many drugs derived from rainforest plants)
Economic impacts:
- Short-term: jobs in logging, mining, agriculture
- Long-term: loss of sustainable resources (rubber, nuts, medicines)
- Loss of tourism revenue
- Reduced income for indigenous communities
Social impacts:
- Displacement of indigenous tribes
- Loss of traditional cultures and knowledge
- Potential conflicts over land rights
- Improved infrastructure may benefit some communities
Desertification: causes and impacts
Desertification occurs primarily along desert margins in regions like the Sahel (southern Sahara edge).
Causes:
- Overgrazing — livestock remove vegetation faster than it regenerates
- Overcultivation — intensive farming exhausts soil nutrients
- Deforestation — trees removed for fuelwood, exposing soil
- Climate change — reduced rainfall and higher temperatures
- Population pressure — more people demanding resources from fragile land
Impacts:
- Reduced agricultural productivity and crop failure
- Food shortages and malnutrition
- Migration away from affected areas (environmental refugees)
- Soil erosion by wind when vegetation cover lost
- Loss of biodiversity as habitats destroyed
- Increased poverty in rural communities
- Conflicts over remaining resources (water, grazing land)
Worked examples
Example 1: Climate graph interpretation
Question: Study the climate graph for Manaus, Brazil. Describe the climate shown. (4 marks)
Mark scheme answer:
- Temperatures are high throughout the year (1), averaging around 27°C (1)
- Temperature range is small/only 2°C variation (1)
- Rainfall is high throughout the year/over 2000mm annually (1)
- There is no distinct dry season/rain falls every month (1)
- Wettest period is March–April/driest is August (1)
Examiner note: Any 4 relevant points score marks. Include data from the graph to support statements.
Example 2: Ecosystem adaptation
Question: Explain how plants in hot deserts are adapted to their environment. (6 marks)
Mark scheme answer:
- Plants have long tap roots (1) to reach water stored deep underground (1)
- Small leaves or spines (1) reduce surface area for water loss through transpiration (1)
- Waxy coatings on leaves/stems (1) reduce evaporation rates (1)
- Some plants store water in their stems like cacti (1) enabling survival during long dry periods (1)
- Widespread shallow roots (1) quickly absorb water when rare rainfall occurs (1)
- Seeds remain dormant until rain falls (1) then rapidly germinate and flower (1)
Examiner note: Level marked. For top marks, give detailed explanations linking features to function. Aim for 3 developed points.
Example 3: Deforestation impacts
Question: Assess the impacts of deforestation in tropical rainforests. (9 marks + 3 SPaG)
Level 3 answer (7–9 marks):
Deforestation has significant negative environmental impacts. When trees are removed and often burned, large amounts of CO₂ are released into the atmosphere, contributing to climate change and global warming. Additionally, the rainforest acts as a carbon sink, so removing trees reduces the amount of CO₂ absorbed from the atmosphere, making the problem worse. Biodiversity is severely affected as habitats are destroyed — the Amazon contains around 10% of Earth's species, many found nowhere else, so their extinction would be permanent.
However, there are some economic benefits to deforestation. Logging creates jobs in the forestry industry and provides valuable hardwood timber for export, generating foreign currency for developing countries like Brazil. Mining for minerals such as gold and iron ore also provides employment and economic development. Large-scale agriculture like cattle ranching and soya farming produces food and income for the country.
Despite these economic advantages, the long-term impacts are predominantly negative. Soil erosion occurs rapidly without tree roots to bind the soil, and heavy tropical rainfall washes away the thin topsoil. This makes the land infertile within a few years, so cleared areas cannot sustain agriculture long-term. Indigenous communities lose their homes and traditional ways of life, creating social problems. Overall, whilst deforestation brings short-term economic gains, the environmental and long-term economic costs are far greater.
Examiner note: This answer shows balanced assessment, uses specific examples, explains consequences, and reaches a judgement.
Common mistakes and how to avoid them
- Confusing weather and climate — remember weather is short-term (today is sunny) whilst climate is long-term average (the UK has mild, wet winters)
- Describing climate graphs without using data — always quote specific temperatures (in °C) and rainfall figures (in mm) from the graph
- Listing adaptations without explaining them — don't just state "cacti have thick stems"; explain "cacti have thick stems which store water for use during long dry periods"
- Forgetting to give located examples — name specific rainforests (Amazon, Congo) or deserts (Sahara, Kalahari) to demonstrate knowledge
- Only discussing negative impacts — most impact questions expect balance; consider economic benefits of deforestation alongside environmental costs
- Mixing up nutrient storage — in rainforests most nutrients are in the biomass; in deserts most are in the soil
Exam technique for "Weather, Climate and Ecosystems"
- Command words matter: "Describe" requires stating what you see; "Explain" requires giving reasons why; "Assess" or "Evaluate" requires weighing up different viewpoints and reaching a judgement
- Climate graph questions: systematically work through temperature (high/low, annual range), then precipitation (total, seasonal pattern, wettest/driest months), using specific figures from axes
- Extended writing (6–9 marks): plan before writing; include located examples; explain consequences fully; for "assess" questions, write balanced arguments with a conclusion
- SPaG marks available: on extended answers (usually 3 marks), so write in paragraphs, use specialist vocabulary correctly, and check spelling of key terms like desertification and deforestation
Quick revision summary
Weather describes daily atmospheric conditions; climate shows long-term averages over 30+ years. Global atmospheric circulation creates three cells per hemisphere, producing distinct climate zones. Tropical rainforests occur near the equator with high rainfall and temperatures; organisms show adaptations like drip-tip leaves and buttress roots. Hot deserts at 30°N/S have extreme temperature ranges and low rainfall; adaptations include water storage and nocturnal behaviour. Deforestation causes biodiversity loss and climate change but provides economic benefits. Desertification results from overgrazing, overcultivation and climate change, leading to food insecurity and migration.