Human Impacts on the Environment
Population growth causes; pollution table; greenhouse effect and global warming consequences; eutrophication six-step chain; acid rain causes and freshwater effects; deforestation impacts; biodiversity definition and importance; Extended conservation methods; and Extended sustainable development with fishing, forestry, and energy examples.
Human Impacts on the Environment
CORE EXTENDEDHuman Population Growth
The human population has grown exponentially, particularly since the Industrial Revolution (~1800). By 2025 the world population exceeds 8 billion.
Reduced death rate (not increased birth rate) is the primary driver:
- Improved medicine and healthcare (vaccines, antibiotics, surgery)
- Better nutrition and food production (agriculture, storage)
- Improved sanitation and clean water supply (reduces infectious disease)
- Improved living conditions reducing infant mortality
Birth rate has also remained high in many regions, widening the gap between births and deaths.
Increased demand for: land (deforestation, urbanisation); food (intensive agriculture); water; fossil fuels (energy); raw materials. These all increase pollution, habitat destruction, and biodiversity loss.
Pollution
| Pollutant | Source | Effect on environment |
|---|---|---|
| Carbon dioxide (CO₂) | Burning fossil fuels; deforestation; cement production | Enhanced greenhouse effect → global warming → climate change |
| Methane (CH₄) | Cattle farming; rice paddies; landfill; natural gas leaks | Potent greenhouse gas (~30× CO₂ over 100 years) → enhanced greenhouse effect |
| Sulfur dioxide (SO₂) + nitrogen oxides (NOₓ) | Burning coal, oil; vehicle exhausts | Dissolve in rainwater → acid rain → acidifies lakes and soils, damages forests, dissolves buildings |
| Nitrates and phosphates | Agricultural fertiliser runoff; untreated sewage | Eutrophication → algal bloom → O₂ depletion → fish kill |
| Pesticides | Agricultural use to control pests | Bioaccumulation and biomagnification up food chains; kills non-target species; reduces biodiversity |
| Plastic waste | Consumer packaging; fishing gear; industrial waste | Ingestion by marine animals; entanglement; microplastics enter food chains |
| Heavy metals (e.g. mercury, lead) | Industrial effluent; old pipework; mining | Toxic to aquatic organisms; accumulate in food chains; neurological damage |
Greenhouse Effect and Global Warming
Natural greenhouse effect: Greenhouse gases (CO₂, CH₄, H₂O vapour) in the atmosphere absorb infrared radiation re-emitted from Earth’s surface and radiate it back toward the Earth, keeping it ~33°C warmer than it would otherwise be. This is essential for life.
Enhanced greenhouse effect: Human activities (burning fossil fuels, deforestation, livestock farming) have increased concentrations of greenhouse gases → more infrared is trapped → average global temperatures rise → global warming.
| Consequence of global warming | Mechanism |
|---|---|
| Melting of polar ice caps and glaciers | Higher temperatures melt ice → raises sea levels → flooding of low-lying coastal regions and island nations |
| More extreme weather events | More energy in atmosphere → more intense storms, droughts, floods; disrupted rainfall patterns |
| Shifts in species distributions | Species move toward poles or higher altitude as temperatures increase; some species cannot adapt quickly enough → extinction |
| Coral bleaching | Warmer sea temperatures → coral expels symbiotic algae → bleaching and death |
| Disrupted agriculture | Changed growing seasons; droughts in breadbasket regions; flooding of farmland |
Eutrophication
1. Excess nitrates and phosphates from fertiliser runoff or sewage enter a water body.
2. Algae and cyanobacteria multiply rapidly → algal bloom covers the water surface.
3. Algae block sunlight to submerged aquatic plants → plants cannot photosynthesise → plants die.
4. Algae and dead plants are decomposed by aerobic bacteria.
5. Decomposers use up dissolved oxygen in aerobic respiration → hypoxia (O₂ depletion).
6. Fish and other aerobic organisms die (cannot survive low O₂ levels) → dead zone.
Acid Rain
Burning fossil fuels releases sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). These dissolve in atmospheric water to form sulfuric acid and nitric acid, which fall as acid rain (pH as low as 4).
| Effect | Mechanism |
|---|---|
| Lakes and rivers acidified | Acid rain lowers pH of freshwater → kills fish eggs and invertebrates → food chain collapses |
| Soils acidified | Leaches minerals (calcium, magnesium) from soil → nutrient deficiency in plants; releases toxic aluminium ions into soil water |
| Forest damage | Damages leaf cuticles and inhibits photosynthesis; weakens trees leaving them vulnerable to pests and disease |
| Buildings eroded | Acid dissolves limestone and marble in buildings and statues |
Deforestation
| Reason for deforestation | Consequence |
|---|---|
| Cleared for agriculture (cattle, soya) | Loss of habitat → biodiversity loss; species extinction |
| Timber for construction and paper | Increased CO₂ (burning/decomposition of felled trees) |
| Land for urban development | Reduced transpiration → less rainfall; increased surface runoff → flooding and soil erosion |
| Fuel wood (especially in developing nations) | Soil erosion as tree roots no longer anchor soil; nutrient loss from leaching |
Biodiversity and Conservation
Biodiversity is the variety of living organisms in an area — including the number of different species (species diversity), the genetic variation within species, and the variety of different ecosystems.
| Why biodiversity matters | Example |
|---|---|
| Ecosystem stability and resilience | More species = more redundant food web pathways; losing one species less damaging |
| Food and medicine | Many crop varieties, medicines derived from wild species |
| Ecological services | Pollination, water purification, carbon sequestration, soil formation |
| Intrinsic value | Species have value independent of human use |
Conservation Methods — Extended
| Method | How it helps |
|---|---|
| Protected areas / nature reserves | Safeguard habitats from development; provide undisturbed breeding grounds; maintain ecosystem processes |
| Captive breeding programmes | Breed endangered species in zoos/wildlife parks; maintain genetic diversity; reintroduce to wild when habitat is secure |
| Seed banks | Store seeds of wild and crop plants at low temperature and humidity; preserve genetic diversity; insurance against extinction |
| International agreements | CITES (Convention on International Trade in Endangered Species) — bans trade in endangered species; Paris Agreement — limits greenhouse gas emissions |
| Reforestation | Planting trees to restore habitats; sequesters carbon; restores water cycle; reduces soil erosion |
| Reducing pollution | Cleaner energy; catalytic converters; wastewater treatment; reduced fertiliser use |
| Education and awareness | Informs public and policymakers; changes consumer behaviour; supports conservation funding |
Sustainable Development — Extended
Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.
In practice, this means using natural resources at a rate no faster than they can be replenished, and managing waste so that it does not accumulate to harmful levels.
Set catch quotas below the maximum sustainable yield; enforce minimum net mesh size (allow juvenile fish to escape); establish marine protected areas; ban destructive fishing methods (bottom trawling in sensitive habitats).
Replant trees at the same rate they are harvested (managed rotation); certify sustainably sourced timber (FSC certification); protect old-growth forests; use selective logging rather than clear-cutting.
Replace fossil fuels with solar, wind, hydroelectric, and tidal energy. These do not deplete finite resources and produce little or no CO₂ during operation, reducing the enhanced greenhouse effect.
A farmer applies excess nitrogen fertiliser to fields near a lake. Which sequence correctly describes the subsequent changes in the lake?
- A. Nitrate runoff → fish kill → algal bloom → oxygen depletion
- B. Nitrate runoff → increased fish populations → algal bloom → oxygen increase
- C. Nitrate runoff → algal bloom → decomposer activity increases → oxygen depletion → fish kill
- D. Nitrate runoff → acid rain → pH decrease → fish kill
(a) Explain what is meant by ‘sustainable development’. [1 mark]
(b) Describe three methods used to conserve biodiversity and for each method explain how it helps. [6 marks]
- (a) Development that meets present needs without compromising the ability of future generations to meet their own needs [1 mark]
- (b) Any three methods from the conservation table, each with a valid explanation [2 marks each = 6 marks total] — e.g.:
Protected areas/reserves: prevent habitat destruction; undisturbed breeding; ecosystem processes maintained [2 marks]
Captive breeding: maintains population of endangered species; preserves genetic diversity; allows reintroduction to wild [2 marks]
Seed banks: preserve genetic diversity of plant species; insurance against extinction; can restore populations [2 marks]
International agreements (CITES): prevent exploitation through trade; legally binding commitment to protect species [2 marks]
Comprehensive Practice Questions
Mixed questions across Topic 20.
Which combination correctly identifies a greenhouse gas and one consequence of increasing its atmospheric concentration?
- A. Oxygen — acid rain
- B. Sulfur dioxide — global warming
- C. Methane — rising sea levels due to melting polar ice
- D. Nitrogen — increased crop yields
(a) State two reasons why tropical forests are being deforested. [2 marks]
(b) Describe three environmental consequences of large-scale deforestation. [3 marks]
(c) Define biodiversity and explain why high biodiversity is important for the stability of an ecosystem. [3 marks]
- (a) Any two of: cleared for cattle ranching/agriculture; timber extraction; urban development; fuel wood; mining [2 marks]
- (b) Any three of: loss of habitat → biodiversity loss/species extinction [1 mark]; increased atmospheric CO₂ from combustion/decomposition of felled trees → enhanced greenhouse effect [1 mark]; reduced transpiration → less local rainfall / disrupted water cycle [1 mark]; increased surface runoff → flooding and soil erosion / loss of topsoil nutrients [1 mark] — any 3 [3 marks]
- (c) Biodiversity = the variety of living organisms in an area / number of different species [1 mark]; high biodiversity means more species in the food web [1 mark]; if one species is lost, other species can fulfill its ecological role (redundancy) → ecosystem remains functional and stable [1 mark]
(a) Explain how burning coal in power stations contributes to acid rain and describe two effects of acid rain on freshwater ecosystems. [4 marks]
(b) A water sample from a polluted river contains very few dissolved oxygen molecules. Suggest a sequence of events that could explain this observation, starting with the addition of raw sewage to the river. [4 marks]
- Burning coal releases sulfur dioxide (and nitrogen oxides) into the atmosphere [1 mark]; these dissolve in rain water forming sulfuric acid (and nitric acid), lowering rain pH [1 mark]
- Effect 1: acid rain lowers the pH of lakes → fish eggs cannot hatch / kills invertebrates → food chains disrupted [1 mark]
- Effect 2: acid rain leaches minerals from soil / releases toxic aluminium ions → weakens aquatic and riparian plants [1 mark]
- Raw sewage contains nitrates and phosphates (nutrients) → stimulate rapid algal growth (algal bloom) [1 mark]
- Algae block light → submerged plants die; algae die when nutrients exhausted [1 mark]
- Decomposer bacteria break down the dead algae and plants by aerobic respiration [1 mark]
- Decomposers consume dissolved O₂ → hypoxia → very low dissolved oxygen concentration [1 mark]
High-Frequency Mistakes — Topic 20 Overall
- 🌞Confusing the greenhouse effect with ozone depletionThe greenhouse effect (global warming) is caused by CO₂, CH₄, and water vapour trapping infrared radiation. Ozone depletion is caused by CFCs destroying stratospheric ozone, increasing UV radiation reaching Earth’s surface. They are separate problems with different causes, different gases, and different effects. Do not mix them up.
- 🔴Stopping the eutrophication chain too earlyThe full chain is required: nutrients → algal bloom → algae die → decomposers multiply → decomposers consume O₂ in aerobic respiration → hypoxia → fish kill. Stopping at "algal bloom" or even "oxygen depletion" without explicitly stating decomposer respiration as the cause of O₂ depletion loses marks.
- 🌿Saying deforestation directly causes global warming (without explanation)Deforestation contributes to global warming through two mechanisms that must both be stated: (1) fewer trees = less CO₂ absorbed by photosynthesis; (2) burning or decomposing felled trees releases stored carbon as CO₂. Simply writing "deforestation increases CO₂" without explaining why scores only partial credit.
- ⛈Saying sulfur dioxide causes the greenhouse effectSO₂ causes acid rain — not the greenhouse effect. The greenhouse gases are CO₂, CH₄, and water vapour. SO₂ is a pollutant that forms sulfuric acid in rainwater. Mixing up acid rain and the greenhouse effect is one of the most common errors in this topic.
- 🌎Ext: Saying biodiversity only means “number of species”Biodiversity includes: (1) species diversity (number and relative abundance of different species); (2) genetic diversity (variation within a species); (3) ecosystem diversity (variety of different ecosystems). For full marks at Extended level, acknowledging that biodiversity is broader than just species count is important.
- 📈Ext: Confusing sustainable development with conservationConservation aims to protect existing species and habitats. Sustainable development allows economic development but manages resources so they are not permanently depleted for future generations. Sustainable development accepts some resource use; conservation may prioritise preservation over development. Both are needed but they are distinct concepts.
- 🔥Saying acid rain is caused by CO₂Acid rain is caused by sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) dissolving in rain to form acids. CO₂ also dissolves in water (forming carbonic acid, which slightly lowers pH) but this is a negligible contributor to acid rain compared to SO₂ and NOₓ. In exam answers, always name SO₂ and NOₓ as the acid rain precursors.
Highest-yield Core items: the complete eutrophication chain (six steps — every step must be present); greenhouse gases (CO₂ and CH₄) vs acid rain gases (SO₂ and NOₓ) — keep these two issues completely separate; consequences of global warming (at least three: sea level rise, extreme weather, species distribution shifts); deforestation effects (biodiversity loss, CO₂ increase, reduced rainfall, erosion); biodiversity definition. For Extended: conservation methods table (at least four methods with explanations); sustainable development definition; sustainable fishing and forestry specifics. Eutrophication and conservation methods are near-certain Paper 4 targets in most examination series.