Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

5.3.3Topic 5 · On the Wild Side

The Greenhouse Effect — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 23 of 34 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

5.3.3 The Greenhouse Effect — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe how greenhouse gases keep Earth warm, why the enhanced greenhouse effect changes climate, and compare the sources and potency of the main gases.Incoming solar radiation is mostly short-wavelength …

  • global warming potential
  • positive feedback
  • negative feedback
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3

Part 1 of 7The natural greenhouse effect

Incoming solar radiation is mostly short-wavelength (visible and ultraviolet).

About a third is reflected back to space by clouds, ice and the surface.
The rest passes through the atmosphere and is absorbed by the land and oceans, which warm up.
Infrared
The warm surface then emits energy as longer-wavelength infrared radiation.
Certain gases in the atmosphere absorb some of this infrared and re-emit it in all directions, including back toward the surface.
This keeps the lower atmosphere warmer than it would otherwise be: without it the average surface temperature would be about −18 °C rather than about +15 °C.
The natural greenhouse effect is therefore essential for life as we know it.

Part 2 of 7The enhanced greenhouse effect

Human activities have increased the concentrations of greenhouse gases. More infrared is absorbed and re-emitted toward the surface, less escapes to space, and the mean temperature rises. This is the enhanced greenhouse effect and it is the basis of current global warming.

Part 3 of 7The main greenhouse gases

GasMain human-linked sourcesNotes
Carbon dioxide, CO2Burning fossil fuels, deforestation, cement productionLargest contribution because of the amount released; stays in the atmosphere for a very long time
Methane, CH4Rice paddies, ruminant livestock (anaerobic digestion by gut microbes), landfill, fossil fuel extraction, thawing permafrostMuch more effective per molecule than CO2, but breaks down faster
Dinitrogen oxide (nitrous oxide), N2ONitrogen fertilisers, denitrification in soils, burning of biomassVery effective per molecule and long lived
Water vapourEvaporationAmount rises with temperature, which amplifies warming

The global warming potential of a gas compares its warming effect over a stated period with that of the same mass of CO2. Methane has a much higher potential over 100 years, and nitrous oxide is higher again. Even so, CO2 has caused the greatest total warming because the quantity emitted is vastly greater.

Part 4 of 7Feedback

A positive feedback amplifies the original change.

As the Arctic warms, ice melts and dark ocean or land is exposed,
  • which reflects less sunlight (lower albedo) and absorbs more; this warms the area further.
Warming can also thaw permafrost, releasing methane and CO2 from decaying organic matter, which causes further warming.
Negative feedback
A negative feedback reduces change: for instance, more atmospheric CO2 may stimulate plant growth and photosynthesis, drawing carbon from the air, though the benefit is limited by water, nutrients and temperature.
Models must include feedbacks, which is one source of uncertainty.

Part 5 of 7Links

The enhanced greenhouse effect is the proposed mechanism linking the rising CO2 concentration with temperature in the proxy and direct evidence. It underlies the models used to predict future climate and the policies aimed at reducing emissions, covered in later lessons.

Part 6 of 7Evidence that the effect is changing and other influences on temperature

The concentrations of greenhouse gases are measured directly at observatories such as Mauna Loa, where the CO2 record since 1958 shows a rising trend with a regular seasonal wiggle.

Evidence that the effect is changing and other influences on temperature
The wiggle arises because the northern hemisphere has more land vegetation: photosynthesis in spring and summer lowers the concentration,
  • and respiration and decay in autumn and winter raise it.
The steady upward trend is superimposed, and its size matches the amount of fossil fuel burned.
Isotope analysis shows that the additional carbon contains less carbon-13 and no carbon-14,
  • which is the signature of plant-derived fossil carbon, supporting a fossil fuel origin.

Greenhouse gases are not the only influence.

Evidence that the effect is changing and other influences on temperature
Aerosols from volcanoes and pollution reflect sunlight and cool the surface for a short time.
Variation in solar output has a small effect that does not explain the warming of recent decades.
Comparisons of models run with and without human emissions show that natural factors alone cannot reproduce the observed temperature rise,
  • which is an important part of the argument.

Exam tip on wordingsay that the surface emits infrared radiation, that greenhouse gases absorb it and re-emit it, and that some returns to the surface.

Evidence that the effect is changing and other influences on temperature
Avoid saying that heat is 'trapped' without naming the radiation.
Also note that the gases differ in how long they last: CO2 persists for centuries,
  • so reductions in emission take a long time to show up as reduced concentration.
Methane lasts about a decade, so cutting methane has a quicker effect on warming.

Part 7 of 7Exam-style questions

  1. Explain how the greenhouse effect keeps the Earth's surface warm. [4]4 marks

  2. Explain why melting Arctic ice is an example of positive feedback. [3]3 marks

  3. Suggest why methane from livestock is a concern even though its concentration in the atmosphere is much lower than that of CO2. [2]2 marks

Key terms

TermMeaning
Greenhouse gasA gas that absorbs and re-emits infrared radiation, warming the lower atmosphere.
Infrared radiationLong-wavelength radiation emitted by the warm surface.
Enhanced greenhouse effectAdditional warming caused by raised greenhouse gas concentrations from human activity.
Global warming potentialA measure of how much a gas warms the climate compared with the same mass of CO2.
AlbedoThe proportion of incoming sunlight reflected by a surface.
Positive feedbackA process that amplifies an initial change.
PermafrostGround that stays frozen all year and stores large amounts of organic carbon.

Avoid these mistakes

1
Confusing the greenhouse effect with ozone depletion. The ozone layer filters ultraviolet; greenhouse gases trap infrared.
2
Saying greenhouse gases reflect heat. They absorb and re-emit infrared radiation.
3
Saying the greenhouse effect is entirely bad. The natural effect makes Earth habitable; the enhanced effect is the problem.
4
Writing that CO2 is the most potent gas per molecule; it has the biggest total effect because of quantity, not potency.

Last updated: