(d) Human influences on the environment
- Syllabus
- 2024
- Topic
- —
- Level
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Learning objectives
4.12Air pollution consequencesUnderstand the biological consequences of pollution of air by sulfur dioxide and carbon monoxide.4.13Greenhouse gasesUnderstand that water vapour, carbon dioxide, nitrous oxide, methane and CFCs are greenhouse gases.4.14Human sources of greenhouse gasesUnderstand how human activities contribute to greenhouse gases.4.15Enhanced greenhouse effectUnderstand how an increase in greenhouse gases results in an enhanced greenhouse effect and that this may lead to global warming and its consequences.4.16Sewage pollution consequencesUnderstand the biological consequences of pollution of water by sewage.4.17Eutrophication consequencesUnderstand the biological consequences of eutrophication caused by leached minerals from fertiliser.418B Deforestation effectsUnderstand the effects of deforestation, including leaching, soil erosion, disturbance of evapotranspiration and the carbon cycle, and the balance of atmospheric gases.
Explain harm from sulfur dioxide and carbon monoxide
Sulfur dioxide and carbon monoxide harm organisms by different mechanisms: sulfur dioxide forms acidic pollution, while carbon monoxide reduces oxygen transport in blood.
| Pollutant | Route and biological consequences |
|---|---|
| sulfur dioxide, SO₂ | dissolves in atmospheric water and contributes to acid rain; acidic water and soil damage leaves and roots, kill aquatic organisms, disrupt food webs and reduce biodiversity; the gas also irritates airways |
| carbon monoxide, CO | binds strongly to haemoglobin to form carboxyhaemoglobin, leaving less haemoglobin able to carry oxygen; cells receive less oxygen for aerobic respiration, causing tiredness, unconsciousness or death at high exposure |
Pollution-sensitive lichens may have lower coverage near a sulfur-dioxide source, but distance data show correlation; other conditions must be controlled before attributing the pattern solely to pollution.
Carbon monoxide is toxic but is not one of the greenhouse gases named in objective 4.13. Sulfur dioxide's acid-rain mechanism is distinct from carbon dioxide's greenhouse effect.
Identify the five greenhouse gases in scope
Greenhouse gases absorb outgoing infrared radiation and re-emit it, reducing the rate at which energy escapes from Earth to space.
| Greenhouse gas required in this syllabus | Formula or name cue |
|---|---|
| water vapour | H₂O in the atmosphere |
| carbon dioxide | CO₂ |
| nitrous oxide | N₂O; not nitrogen gas, N₂ |
| methane | CH₄ |
| chlorofluorocarbons | CFCs |
Oxygen, nitrogen and carbon monoxide are not members of this required list. A gas can be an air pollutant without being a greenhouse gas, and vice versa.
Link human activities to greenhouse-gas emissions
Human activities raise greenhouse-gas concentrations by releasing stored carbon and nitrogen compounds, creating methane, removing carbon sinks and emitting manufactured gases.
| Human activity | Main greenhouse-gas contribution |
|---|---|
| burning coal, oil and natural gas for electricity, heating, industry and transport | carbon dioxide from combustion |
| deforestation and burning cleared vegetation | carbon dioxide released, plus less CO₂ removed by photosynthesis |
| cattle and other ruminants, rice fields and anaerobic decay in landfill | methane |
| nitrogen fertilisers and disturbed agricultural soils | nitrous oxide |
| leakage from older refrigeration, air-conditioning and aerosol systems | CFCs |
A complete explanation names both the activity and the process. For example, more petrol cars means more fuel combustion, so more carbon dioxide enters the atmosphere.
Electric vehicles have no tailpipe carbon dioxide, but their total contribution depends on how electricity and materials are produced. Do not claim an activity has zero emissions without defining the system boundary.
Trace the enhanced greenhouse effect to global warming
Increasing greenhouse-gas concentrations enhances the natural greenhouse effect, raising Earth's average temperature and potentially causing global warming and wider climate change.
| Stage | Energy or biological consequence |
|---|---|
| 1 | incoming short-wave solar radiation reaches Earth's surface and warms it |
| 2 | the warm surface emits long-wave infrared radiation |
| 3 | greenhouse gases absorb and re-emit some infrared radiation |
| 4 | higher greenhouse-gas concentrations slow net energy loss to space |
| 5 | the climate system warms until incoming and outgoing energy balance again |
| 6 | warming can melt land ice, raise sea level, alter rainfall, increase drought or flooding, shift habitats, disrupt food webs, bleach coral and change species distributions |
A biological explanation links the physical change to organisms: melting sea ice can reduce the surface available for microscopic producers, reducing food for krill and then whales.
The greenhouse effect is a natural warming mechanism; the enhanced greenhouse effect is its strengthening by increased gas concentrations. Greenhouse gases absorb infrared radiation rather than simply trapping incoming sunlight.
Explain the biological consequences of sewage pollution
Untreated sewage adds pathogens, organic matter and mineral nutrients to water, so it can cause disease and deplete dissolved oxygen.
| Sewage component | Biological consequence |
|---|---|
| pathogenic microorganisms | water-borne disease can spread among humans and other animals |
| organic matter and faeces | decomposer bacteria multiply and respire while breaking it down; biochemical oxygen demand rises and dissolved oxygen falls |
| nitrates and phosphates | stimulate excessive algal and plant growth, adding a eutrophication route |
| low dissolved oxygen | fish and many aquatic invertebrates cannot respire aerobically enough; populations fall or die |
| community change | tolerant decomposers may increase while sensitive species disappear, lowering biodiversity |
Bacteria do not remove oxygen merely by being present: their increased aerobic respiration consumes it while organic sewage is decomposed. Sewage pollution also has a direct pathogen hazard beyond eutrophication.
Trace fertiliser leaching through eutrophication
Eutrophication begins when soluble mineral ions from excess fertiliser leach or run off into water and ends with oxygen depletion and loss of aquatic life.
| Stage | Causal change |
|---|---|
| 1 | rain washes nitrate and phosphate ions from soil into a river or lake |
| 2 | extra mineral nutrients cause rapid algal growth—an algal bloom |
| 3 | the bloom shades submerged plants, reducing photosynthesis; plants and algae die |
| 4 | decomposer bacteria break down the dead organic matter and their populations increase |
| 5 | bacterial aerobic respiration consumes dissolved oxygen |
| 6 | oxygen-sensitive fish and invertebrates die or leave; food webs simplify and biodiversity falls |
Downstream of a fertilised field, repeated samples may show lower mean dissolved oxygen than upstream. The position and flow direction support the mechanism, while repeats strengthen reliability.
Fertiliser does not poison fish directly in the standard eutrophication chain. The critical link is extra nutrients → bloom and death → decomposer respiration → oxygen depletion.
Connect deforestation to soil, water and atmospheric change
Removing a forest removes canopy, roots and photosynthetic biomass, so deforestation changes soil retention, water transfer, the carbon cycle and atmospheric gases together.
| Lost forest function | Consequence of deforestation |
|---|---|
| canopy intercepts rain; roots bind soil | more surface runoff and soil erosion; sediment can enter and block waterways |
| roots take up mineral ions; litter returns nutrients | soluble ions leach from exposed soil, reducing fertility |
| leaves transpire water | less evapotranspiration can reduce atmospheric water vapour and local rainfall, disturbing the water cycle |
| photosynthesis removes CO₂ and releases O₂ | less uptake of carbon dioxide and less oxygen production |
| biomass stores carbon | decay or combustion releases carbon dioxide, strengthening the enhanced greenhouse effect |
| trees provide habitat and food | populations and food-web links are lost, reducing biodiversity |
Replanting can reduce flooding because roots increase soil stability and water uptake, foliage intercepts rain, transpiration returns water to the atmosphere, and slower runoff reduces rapid river rise.
Deforestation changes stocks and flows: carbon already stored may be released, while future CO₂ removal falls. Soil erosion is physical loss of soil; leaching is dissolved mineral ions being washed through or out of it.