EVSC10001 · The Global Environment
The Greenhouse Effect, Recent & Future Climate
This chapter ties together the physics that sets Earth's temperature: the planetary energy balance (absorbed sunlight = outgoing infrared), how greenhouse gases raise the surface temperature through back-radiation, the carbon cycle reservoirs and fluxes, and how recent and future change is attributed and projected. It is the climax of the David Noone block and a Part-A favourite — almost every short-answer here is built on a single labelled diagram (the energy-balance cartoon, a feedback loop, the carbon cycle, or a shifting distribution), so being able to draw and annotate these from memory is the difference between a pass and a top mark.
What this chapter covers
- 01Planetary energy balance: (S/4)(1-albedo) = sigma*T^4
- 02The greenhouse effect: selective IR absorption & back-radiation
- 03Which gases are greenhouse gases (and why N2/O2 are not)
- 04Climate feedbacks: positive amplify, negative damp
- 05The carbon cycle: reservoirs vs fluxes & the human perturbation
- 06Attribution: indicators, radiative forcing & models
- 07Future change: warming proportional to cumulative CO2
- 08Extremes: percentile shift & event attribution
Draw and explain a positive climate feedback (ice-albedo)
- +1Define the term: a feedback is a process triggered by an initial change that loops back to affect that same change. A positive feedback amplifies the initial change; a negative feedback damps it back toward equilibrium.
- +1Draw the diagram: a closed loop of boxes joined by arrows, each box one step in the chain, with a '+' sign on the returning arrow to show the loop reinforces itself. Title the loop 'Ice-albedo feedback (positive)'.
- +1Label the chain in order around the loop: (1) initial warming -> (2) bright ice/snow melts -> (3) darker land/ocean is exposed -> (4) surface albedo falls -> (5) more incoming solar is absorbed (less reflected) -> back to (1) more warming.
- +1State explicitly why the sign is positive: the final step (more absorbed sunlight) pushes in the SAME direction as the initial warming, so each pass round the loop adds to the warming rather than cancelling it.
- +1Add the contrast for credit: this is unlike the planetary energy-balance negative feedback, where a warmer surface emits more outgoing IR (sigma*T^4) and so cools back toward balance.
Key terms
- Planetary energy balance
- The equilibrium condition where absorbed solar radiation equals outgoing infrared: (S/4)(1-albedo) = sigma*T^4. The factor 1/4 arises because Earth intercepts sunlight over its disc (pi*R^2) but emits over its whole sphere (4*pi*R^2).
- Albedo
- The fraction of incoming radiation reflected by a surface. Ice, fresh snow, cloud and bright desert are high-albedo (reflect strongly); ocean and forest are low-albedo (absorb strongly). Earth's planetary albedo is about 0.3.
- Greenhouse effect
- The warming caused when greenhouse gases absorb outgoing longwave infrared and re-emit it in all directions, including back down to the surface (back-radiation). This raises the surface temperature needed to balance incoming sunlight; it is radiative, not the convection-trapping of a real glasshouse.
- Greenhouse gas
- A gas whose molecules can absorb and emit infrared because they have vibrational modes that change the molecular dipole, e.g. H2O, CO2 (strong 15-micrometre band), CH4, N2O, O3. The symmetric diatomics N2 and O2 lack such modes and are not greenhouse gases despite dominating the air.
- Carbon cycle
- The movement of carbon between reservoirs (atmosphere, ocean, biosphere/soils, rocks) along fluxes such as photosynthesis, respiration, air-sea exchange, volcanism and burial. The rock reservoir is by far the largest; burning fossil carbon adds a new fast one-way flux the slow sinks cannot offset.
- Radiative forcing
- A measure (in watts per square metre) of how strongly a driver pushes the energy balance away from equilibrium. CO2 is the largest positive anthropogenic forcing; aerosols exert a partly offsetting negative forcing.
The Greenhouse Effect, Recent & Future Climate FAQ
Why do we divide the solar constant by four in the energy balance?
Earth intercepts sunlight only across its shadow disc (area pi*R^2) but radiates infrared from its entire surface (area 4*pi*R^2). The ratio pi*R^2 / 4*pi*R^2 = 1/4, so the globally and time-averaged incoming flux is S/4. Forgetting this factor of 4 is the classic energy-balance slip.
Why aren't nitrogen and oxygen greenhouse gases when they make up most of the air?
A molecule can only absorb infrared if it has a vibrational mode that changes its dipole moment. N2 and O2 are symmetric diatomics with no such mode, so they are infrared-inactive. The greenhouse work is done by trace gases like CO2 and H2O whose bending and stretching modes do change the dipole.
What is the difference between a forcing and a feedback?
A forcing is an external push on the energy balance, such as rising CO2 from burning fossil fuels, volcanic aerosols, or a change in solar output. A feedback is the system's own response that then amplifies (positive) or damps (negative) that push, such as water vapour, ice-albedo or cloud. CO2 is the human forcing; water vapour is the feedback it triggers.
How do scientists attribute warming to human activity if there is only one Earth?
Because we cannot run a no-emissions control experiment on the real planet, climate models supply the missing control: the same model is run with and without human greenhouse-gas forcing. Observed warming since the mid-20th century is reproduced only when human forcing is included, which is the basis of attribution. Many independent indicators (air and ocean temperature, sea level, shrinking ice) point the same way.
Why does future warming depend on cumulative CO2 rather than the rate of emission?
CO2 persists in the atmosphere for a very long time, so warming tracks the total amount emitted, not the rate in any single year. Across emission scenarios this gives a near-linear relationship: warming is approximately proportional to cumulative CO2, meaning every additional tonne adds to the eventual peak warming.
Exam move
Treat this chapter as a set of four diagrams you can reproduce from memory, because in Part A the labelled sketch carries most of the marks. Master the energy-balance cartoon first (incoming shortwave, reflected albedo, outgoing IR, the greenhouse-gas layer re-emitting up and down, back-radiation, and the 8-12 micrometre window), then the feedback loop (with a + or - sign and one line justifying it), the carbon cycle (boxes are reservoirs, arrows are fluxes, the bold combustion arrow is the new human flux driving CO2 from ~280 to over 420 ppm), and the two overlapping distributions for extremes. For each, drill the discipline-correct vocabulary: say 'radiative' not 'traps heat', 'effective temperature ~255 K = -18 C' for the bare rock, and frame attribution as a change in likelihood rather than a single cause. Practise drawing fast, then writing your sentences to your own labels.
