• Soil erosion is topsoil being worn away by wind or water — made worse by deforestation, overgrazing, over-cultivation, population growth and over-abstraction of groundwater.
• Desertification is when semi-arid land degrades to become desert-like, caused by a vicious cycle of natural drought/rainfall extremes plus human overuse of the land.
• Global warming happens naturally (Milankovitch cycles, volcanoes, sunspots) but has been massively sped up by humans adding extra greenhouse gases — this is the "enhanced greenhouse effect."
• The enhanced greenhouse effect causes rising sea levels, more extreme weather, ecosystem shifts, and knock-on effects on health, farming, homes, and jobs.
• International agreements (Kyoto, Paris, COP) try to coordinate a global response through mitigation (reducing the cause) and adaptation (coping with the effects).
• Pollution comes in five flavours: air, water, land, visual, and noise — each with distinct sources, impacts, and management solutions.
• Resource conservation means using resources wisely now so they're still available later — through recycling, protected areas, quotas, laws, and individual behaviour change.
• Case study: the Pearl River Delta, China — a megacity region suffering acid rain, heavy metal contamination, and eutrophication from rapid industrial growth.

The core idea

Think of topsoil like the icing on a cake — it's thin, it's on top, and it's the part that actually has all the good stuff (nutrients) that plants need. Soil erosion is the wearing away of that topsoil by wind or water. It's a completely natural process — wind and rain have always moved soil around — but human activity strips away the soil's natural protection (tree roots, vegetation cover) and leaves it exposed, so erosion happens much faster than it would naturally.

The key thing to understand is that erosion isn't really about the wind or the rain themselves — it's about what's missing when they arrive. Roots normally hold soil particles together like a net. Leaves and canopy normally break the force of rainfall before it hits the ground (this is called interception). Remove the vegetation, and suddenly bare soil is sitting there completely exposed to whatever the weather throws at it.

Causes of soil erosion — and WHY each one works

Salinisation — a special case

This is a sneaky one that trips students up. In semi-arid areas, high evaporation rates pull groundwater upward, like a wick pulling water up through a candle. As that water evaporates at the surface, it leaves the salts it was carrying behind — either in or on top of the soil. Over time the land becomes so salty that crops simply can't grow on it anymore. It's a totally different mechanism from wind/water erosion, but it has the same end result: land that's lost its usefulness.

When erosion spirals out of control

Desertification is the process by which semi-arid land turns desert-like — and this is the single most important thing to remember: it is NOT the expansion of existing deserts. It's degraded land in a totally different location becoming desert-like because of human activity and unsustainable land use, sometimes hundreds of kilometres from any actual desert.

It's caused by a feedback loop of natural and human factors reinforcing each other:

• Natural: erratic rainfall → drought → vegetation dies → less rainfall interception → more intense runoff during rare heavy rain → even less vegetation.
• Human: overgrazing, over-cultivation, deforestation and population growth all strip vegetation and nutrients faster than the land can recover.

Fixing what's broken

Every management strategy here works by tackling one of the root causes above. Notice the pattern — each solution either protects existing vegetation, replaces lost vegetation, or reduces the pressure that's causing the damage in the first place.

Agroforestry

Combining agriculture with forestry — some trees are kept standing among the crops. This decreases deforestation, provides shade, increases infiltration and interception (reducing erosion), and drops organic matter (leaves) that adds nutrients back to the soil.

Afforestation

Deliberately planting new trees — the flagship example is the Great Green Wall across the Sahel region of Africa. Roots bind soil together, canopy prevents drying and reduces rainfall impact, falling leaves replace nutrients, and increased animal/insect activity improves soil quality overall.

Contour stones and terraces

Large stones laid in lines along the contour (following the slope) of the land. They physically block soil and organic matter from being washed or blown away, and they slow down overland flow, giving water more time to soak (infiltrate) into the ground instead of rushing off and eroding the surface.

The climate has always changed — but slowly

Here's the thing students often miss: the Earth's climate has always swung between cold and warm periods, long before humans existed. We're currently in the Quaternary period (the last 2.6 million years), which has seen roughly 60 cold periods and warmer interglacial periods in between. The last ice age ended about 25,000 years ago. We know this from evidence like ice cores (which trap ash, air bubbles and microbes), preserved pollen, tree rings, and historical sources like diaries and art.

Three natural drivers

• Milankovitch cycles — long-term wobbles in Earth's orbit and tilt:
  • Orbit shape changes every 100,000 years (circular = cooler, elliptical = warmer)
  • Axial tilt varies every 40,000 years (greater tilt = hotter summers, colder winters)
  • Earth "wobbles" on its axis every 24,000 years, affecting seasonal temperatures
• Volcanic eruptions — large eruptions blast ash into the atmosphere, which blocks solar radiation and cools the planet.
• Sunspot activity — increased sunspot activity is linked to higher average temperatures.

Why we NEED the greenhouse effect (and why we're breaking it)

This is the concept the whole chapter hinges on, so let's slow right down. Imagine the atmosphere as a glass greenhouse roof. Sunlight arrives as short-wave radiation — it passes straight through the glass (atmosphere) and warms the ground inside. The ground then re-radiates that energy back out as long-wave radiation (heat). But long-wave radiation doesn't pass back through glass as easily — greenhouse gases absorb some of it and trap it, keeping the greenhouse warm.

Without this natural effect, Earth's average temperature would be a bone-chilling -18°C — nothing could live here. So the greenhouse effect itself is a good thing, essential to life. The problem is the enhanced greenhouse effect: human activity has pumped so many extra greenhouse gases into the atmosphere that too much heat is now being trapped, and the planet is warming beyond its natural balance.

CO₂ levels have risen by more than 100 parts per million (ppm) to 420 ppm in 2020, and average global temperatures have increased by over 1°C since pre-industrial times.

Who and what gets hit — and how

This is a huge, sprawling topic — the trick to remembering it is to sort impacts into categories rather than memorising a random list.

Health

• Diseases spread to new areas as temperatures rise — e.g. Italy has had malaria cases since 2017, an area previously malaria-free. Up to 280 million more people could be at risk of malaria.
• Waterborne diseases spread more easily; heat waves bring heat stroke, dehydration, sunburn.
• Stagnant air during heat waves increases air pollution → worsens respiratory and cardiopulmonary illness.
• Food shortages → malnutrition and famine.

Homes & settlements

• Sea level rise + more severe storms = increased flooding = displacement of people.
• Low-lying settlements may need to be abandoned, or need costly flood defences.

Agriculture

• Farmers may be forced to switch crops as conditions change.
• Coastal flooding causes salt intrusion into farmland.
• Reduced water availability limits irrigation → food shortages, malnutrition, famine.

Employment

• Tourism decline (e.g. ski resorts closing from lack of snow; coastal resorts at flood risk).
• Farmers changing or abandoning livelihoods.

Sea level rise

Warmer water expands (thermal expansion) and melting ice adds extra volume. Average sea levels have risen 23 cm since 1880, forecast to rise a further 30 cm by 2050. Low-lying areas like the Maldives could be uninhabitable by 2050. This also increases beach erosion and threatens coastal ecosystems like coral reefs and mangroves, plus contaminates freshwater with saltwater.

Ecosystem change

• Biomes shift north (Northern Hemisphere) or south (Southern Hemisphere) chasing suitable climate — polar and tundra biomes are most at risk since they have nowhere further to shift to.
• Ocean acidification + rising sea temperatures cause coral bleaching.
• Animal migration/behaviour patterns shift — e.g. fish moving to cooler waters, reduced hibernation.
• Food webs break down as species numbers fall.

Natural hazards

• More frequent and severe storms (2020's Atlantic hurricane season was the most active on record).
• Longer, more frequent droughts → increased wildfire risk (Australia, Greece, South Africa, Brazil, USA all hit hard in 2021).
• Rising seas + more storms = compounded flooding risk.

Mitigation vs. Adaptation

Because climate change affects the whole planet, it needs an international response. Responses fall into two categories: mitigation (tackling the cause — reducing emissions) and adaptation (coping with the effects that are already happening, e.g. building flood defences).

Reducing emissions — practical strategies

• Reducing deforestation + increasing afforestation
• Improving energy efficiency in industry and appliances
• Improving public transport and switching to electric vehicles
• Greater use of renewable energy and nuclear power

One word, five very different problems

"Pollution" is the introduction of harmful contamination into the environment. Rather than treating it as one giant topic, break it into its five types — each has its own sources and impacts, and exam questions will often ask you to focus on just one.

Air pollution

Sources: petrol/diesel vehicles, factories, fossil-fuel power stations, forest fires, open fires and wood-burning stoves.

Impacts: contributes to 12% of annual deaths worldwide (highest in middle/low-income countries); indoor pollution from fuelwood use in LEDCs worsens health; adds to the enhanced greenhouse effect.

Water pollution

Sources: fuel spillages, agricultural waste (pesticides/fertilisers), leaching from waste, sewage disposal, boat oil/diesel, soil erosion runoff, plastic waste (microplastics), radioactive waste.

Impacts: less clean drinking water, reduced crop yields, damaged ecosystems, wildlife death and abnormalities, toxins building up the food chain, spread of diseases like cholera and typhoid.

Land pollution

Sources: solid waste (domestic/industrial), agricultural waste, mining waste (toxic heavy metals like mercury), demolition/construction waste.

Impacts: reduced crop yields, toxins leaching into water systems, pollutants entering the food chain via plants, ecosystem/habitat destruction, economic losses.

Visual pollution

Sources: buildings, power stations/wind turbines, billboards, pylons/phone masts, waste.

Impacts: lower quality of life, stress/anxiety, negative economic impact, distraction that can cause accidents.

Noise pollution

Sources: roads/airports, construction, industry, nightlife (clubs/bars), wind turbines/power stations.

Impacts: hearing loss, wildlife disturbance (affects feeding/breeding), sleep disturbance, stress/anxiety, high blood pressure/heart disease, harm to child development.

Matching the fix to the type

Making resources last

Conservation is the protection and rational use of the environment and natural resources so they're still around for future generations. The underlying logic is simple: many resources are finite (non-renewable) — once they're gone, they're gone. Conservation is about stretching out how long we can keep using what we have.

Key conservation measures

• Recycling and reuse of materials like clothing, glass, plastics
• Energy-efficient appliances — less energy and water needed to run them
• Protected areas — National Parks, Areas of Outstanding Natural Beauty (AONB)
• Renewable energy investment
• Quotas on resource use — e.g. fishing quotas to prevent overfishing
• Protected ecosystems — Sites of Special Scientific Interest (SSSI), the Antarctica Treaty
• Laws, fines, and rationing to discourage overuse and damage

NGOs playing a role here include the World Wildlife Fund, Greenpeace, Friends of the Earth, and the Woodland Trust.

Who does what

Pearl River Delta, China

The Pearl River is China's third-longest river, and where it meets the South China Sea (southeast of Guangzhou) lies the Pearl River Delta — the largest urban area (megacity) in the world, home to over 60 million people, and China's biggest economic hub, producing over 9% of China's total GDP.

Pollution sources

• Gas- and coal-fired power plants emitting sulphur dioxide, carbon dioxide and nitrogen oxide
• Toxic waste discharged from factories
• Sewage from an enormous population released into rivers

Impacts

• The highest level of acid rain in China — this acidifies groundwater and increases heavy metal uptake in crops and drinking water, linked to higher stomach cancer rates
• Elevated heavy metals (lead, zinc, chromium, copper, arsenic, mercury) in river water and sediment — these pass through the food chain; rice grown in the region has been found contaminated with arsenic
• Pesticides accumulate in the food chain, impacting human health
• Fertilisers cause eutrophication (excess nutrients causing algal blooms that starve water of oxygen)