Geography (0460) | Climate and natural vegetation

Welcome to Climate and Natural Vegetation!

Hello IGCSE Geographers! This chapter is all about understanding how the weather over a long time (climate) controls where different types of plants and animals (natural vegetation and ecosystems) can thrive.

Understanding this link is crucial because it explains why certain parts of the world look the way they do—from the dense, steamy jungle to the bone-dry desert. Ready to dive in? Let's break down these amazing environments!

Section 1: What Shapes Our Climate?

Climate is the average weather measured over many years (usually 30 years). It dictates life on Earth. But what causes such different climates around the globe?

The syllabus requires you to know the factors influencing climate. Think of them as the 'CLIMATE CONTROLS':

Factors Influencing Climate Characteristics

We can remember the major controls using a simple acronym: L-A-D-O-P-W (Latitude, Altitude, Distance from Sea, Ocean Currents, Pressure systems, Winds).

• Latitude:

The most important factor. Areas near the Equator (0° latitude) receive more concentrated solar radiation (sunlight) and are hotter than areas near the Poles (90° latitude).
Analogy: If you shine a flashlight directly (like at the equator), the light is intense. If you tilt it (like at the poles), the light is spread out and weak.

• Altitude (Height):

As you go higher up a mountain, the temperature drops. The atmosphere is thinner and less able to hold heat.
Rule of thumb: Temperature decreases by about 6.5°C for every 1000m rise in height (known as the Lapse Rate).

• Distance from the Sea (Continentality):

Areas far inland (in the middle of a continent) have a high annual temperature range. They heat up quickly in summer and cool down quickly in winter. Coastal areas have a small annual range because the sea takes longer to heat up and cool down, acting like a temperature regulator.

• Ocean Currents:

Currents can be warm or cold. Warm currents (like the North Atlantic Drift) raise the temperature and increase rainfall in coastal areas they pass. Cold currents decrease air temperature and prevent rainfall (often linked to hot deserts).

• Pressure Systems and Winds:

Low Pressure: Air rises, cools, condenses, and forms rain (wet climate). (e.g., Equatorial regions).
High Pressure: Air sinks, warms up, preventing cloud formation (dry climate). (e.g., Hot Desert regions).
Winds also carry moisture (or lack thereof) from the areas they travel over.

Section 2: The Hot and Wet World – Equatorial Climate

2.1 Characteristics of the Equatorial Climate

This climate is found very close to the Equator (roughly 0° to 10° North and South).

• Temperature: High and constant (around 26°C to 28°C) all year.
• Annual Temperature Range: Very small (often less than 3°C). The temperature barely changes throughout the year.
• Precipitation (Rainfall): Very high (over 2000 mm per year). Rain falls almost every afternoon.

Did you know? In equatorial regions, people often talk about the 'Diurnal Range' (daytime vs. nighttime temperature difference) instead of the annual range, because the daily change is much greater than the seasonal change.

2.2 Why is the Equatorial Climate like this?

1. High Latitude (0°): Intense heating from the sun all year round, leading to consistently high temperatures.

2. Low Pressure System (ITCZ): The heat causes the air to rise rapidly in a process called convection. As the warm, moist air rises, it cools, condenses, and forms massive rain clouds. This is called convectional rainfall, which happens almost every day in the late afternoon.

3. Distance from Sea: Not a major factor, as the heat and moisture generation are mostly controlled by the intense solar energy and the resulting atmospheric processes.

2.3 The Tropical Rainforest Ecosystem

The consistent heat and high rainfall create the perfect conditions for the Tropical Rainforest, an environment characterized by extreme biodiversity.

A. Natural Vegetation: Structure and Adaptations

The forest has a distinct vertical structure, often described as having four layers:

1. Emergent Layer: Talls trees (up to 50m) that break through the canopy. (e.g., Kapok)
2. Canopy Layer: Dense, continuous layer (20-30m high). This layer blocks about 90% of the sunlight, creating dark conditions below.
3. Under-Canopy Layer: Shorter trees and young saplings waiting for a gap in the canopy.
4. Shrub/Forest Floor Layer: Very dark and sparse vegetation, except along river banks or clearings.

Plant Adaptations:

• Drip Tips: Pointed leaves allow heavy rain to run off quickly, preventing fungi and moss growth.
• Buttress Roots: Huge, triangular roots at the base of trees, providing stability in the shallow, often unstable rainforest soil.
• Lianas/Epiphytes: Plants that climb or grow on other trees to reach sunlight.

B. Soil Characteristics (Latossols / Ferralsols)

Don't be fooled by the lush forest! Rainforest soils are surprisingly infertile.

• Leaching: Heavy rainfall washes away soluble nutrients (leaching), leaving behind mainly insoluble minerals like iron and aluminium (giving the soil a reddish color).
• Biomass and Nutrient Cycling: Nearly all the nutrients are stored in the plants themselves (the biomass). When a leaf or branch falls, it decomposes incredibly quickly (due to heat and moisture) and the nutrients are immediately taken up by the shallow tree roots. This is known as rapid nutrient cycling. If the trees are removed, the nutrient source is gone, and the soil quickly becomes useless for long-term agriculture.

• Wildlife: The complex vertical structure provides millions of habitats, leading to the world's highest level of biodiversity. Most animals live high in the canopy layer.

Section 3: The Hot and Dry World – Hot Desert Climate

3.1 Characteristics of the Hot Desert Climate

Hot deserts are typically found between 15° and 30° North and South of the Equator. (e.g., Sahara, Arabian Desert).

• Temperature: Very high daytime temperatures (can exceed 40°C).
• Diurnal Temperature Range: Very high. Nights are cold (can drop below freezing) because there are no clouds to trap the heat.
• Precipitation (Rainfall): Extremely low (often less than 250 mm annually). Rainfall is rare, unpredictable, and sometimes non-existent for years.

3.2 Why is the Hot Desert Climate like this?

1. High Pressure Systems: Deserts sit under permanent bands of high atmospheric pressure. Air is sinking here, warming up as it descends. Sinking air means no clouds, no condensation, and therefore, no rain.

2. Distance from the Sea (Continentality): Many major deserts (like the Gobi) are deep inland, far from the moisture sources, resulting in extremely dry air.

3. Cold Ocean Currents: Deserts on the western coasts of continents (like the Atacama or Namib) are often bordered by cold ocean currents. These currents cool the air above the ocean, stabilizing it and preventing it from rising to form rain-bearing clouds over the land.

3.3 The Hot Desert Ecosystem

Desert environments demand extreme adaptations to cope with aridity (dryness) and huge temperature swings.

A. Natural Vegetation: Adaptations (Xerophytes)

Desert plants (xerophytes) are typically sparse and widely spaced. They must minimize water loss (transpiration) and maximize water uptake.

• Waxy Cuticles: Thick, waxy skin (e.g., on cacti) reduces evaporation.
• Spines/Thorns: These reduce surface area and deter animals from eating the plant for its stored water.
• Succulence: Ability to store vast quantities of water in stems or roots (e.g., Cacti).
• Deep or Wide Root Systems: Deep roots access groundwater; wide roots quickly absorb moisture from short, infrequent rainfall events.

B. Soil Characteristics (Aridisols)

Desert soils are generally infertile for crops but rich in minerals because there is no water to leach them away.

• Rocky and Sandy: Often coarse texture.
• Lack of Organic Matter: Very little plant life means very little humus (decomposed organic matter).
• High Mineral Content: Lack of leaching means salts and minerals are near the surface.

• Wildlife: Many desert animals (e.g., jerboas, snakes, scorpions) are nocturnal—they stay hidden during the extreme daytime heat and emerge only at night. Animals like the camel have special adaptations (fat storage, ability to endure severe dehydration) to survive the harsh conditions.

Section 4: Threats to the Tropical Rainforest (Deforestation)

The Tropical Rainforest ecosystem is fragile and is rapidly shrinking due to human activity, mainly deforestation (the removal of forests).

4.1 Causes of Deforestation

1. Cattle Ranching: Often the largest cause. Land is cleared (burned) to create grazing pasture for beef cattle. This is driven by global demand for cheap meat.

2. Subsistence Farming (Slash and Burn): Local people clear small patches to grow crops. The nutrient-poor soil is quickly exhausted (after 2-3 years), forcing the farmers to move and clear new land.

3. Logging: Trees are cut down for timber (expensive hardwoods) and pulp (paper). This often leads to further destruction as logging tracks open up the forest to farmers and miners.

4. Mining: Large areas are cleared for mining iron ore, gold, or bauxite. This pollutes rivers and destroys habitat.

5. Hydroelectric Power (HEP): Large rivers are dammed to produce electricity, flooding vast areas of forest behind the dam wall.

4.2 Effects of Tropical Rainforest Deforestation

The impacts of deforestation are felt everywhere—locally, globally, and by people.

A. Effects on the Natural Environment (Local)

• Soil Erosion: Without the protective canopy and root systems, the heavy convectional rainfall hits the ground directly. The exposed, thin soil is easily washed away (eroded), especially on slopes.

• Loss of Biodiversity: Destroying the forest eliminates the habitats for millions of plant and animal species, many of which are unique to that area (endemic). Species extinction rates increase dramatically.

• River Silting: Eroded soil is washed into rivers, making them shallower and muddier (silting). This affects water quality and can increase the risk of local flooding.

B. Effects on the Natural Environment (Globally)

• Contribution to Climate Change: Rainforests are called the "lungs of the Earth" because they absorb massive amounts of carbon dioxide (\(CO_2\)) for photosynthesis. They are huge carbon sinks. When they are cut down and burned, the stored carbon is released into the atmosphere as \(CO_2\), contributing significantly to enhanced global warming (the greenhouse effect).

• Disruption of Global Water Cycle: The forest releases huge amounts of moisture through evapotranspiration. Removing the trees can lead to regional drying and changes in global weather patterns.

C. Effects on People

• Loss of Indigenous Cultures: Forest tribes lose their homes, food sources, and traditional way of life. They are often forced to move or face conflict with outside settlers.

• Economic Loss: Although logging makes money in the short term, the long-term potential for sustainable products (like medicines and natural materials) is lost forever when the forest is destroyed.

• Hazard Increase: Deforestation increases the risk of natural disasters like landslides and local droughts.

Section 5: Understanding Climatic Graphs

You must be able to describe and interpret graphs showing temperature and rainfall for both Equatorial and Hot Desert climates.

Key Interpretation Skills

1. Temperature (Line Graph): Look at the shape of the line.

• Equatorial: The line will be nearly flat and high (indicating consistently high temperatures and a very low annual range).
• Hot Desert: The line will be high but show a greater dip in the winter months (higher annual range than the equator).

2. Rainfall (Bar Graph): Look at the height and distribution of the bars.

• Equatorial: Bars will be tall, showing high rainfall every month, often with two peak seasons (due to the ITCZ moving north and south).
• Hot Desert: Bars will be extremely short, sometimes zero, showing very low overall rainfall.

Tip for Struggling Students: When describing a climate from a graph, always state the exact values for the highest/lowest temperature and the annual total rainfall!