B.2 energy questions are accounting problems. Track the power crossing the boundary of the Earth-atmosphere system: incoming solar radiation, reflected short-wave radiation, absorbed energy, and outgoing long-wave infrared radiation. The greenhouse effect changes the pathway and balance of infrared radiation; it does not create energy.

Conservation of energy means total energy is not created or destroyed, only transferred or transformed.

For a climate energy model, choose the system boundary before deciding which transfers cross it.

Incoming solar radiation may be reflected by albedo or absorbed by the surface and atmosphere.

A body at nonzero temperature emits thermal radiation, mainly infrared for Earth.

At radiative equilibrium, average absorbed power equals average emitted power; if not, temperature changes until a new balance is reached.

Choose the energy-balance statement that follows from conservation of energy.

Decision

Average temperature is steady over time.

Absorbed incoming power is greater than outgoing power.

Outgoing power is greater than absorbed incoming power.

State how conservation of energy is used in a simple Earth radiation-balance model.

Common mark losses are failing to define the system boundary, saying energy is created, or ignoring reflected radiation.

State how conservation of energy is used in a simple Earth radiation-balance model.

Choose

Emissivity modifies the Stefan-Boltzmann law for non-ideal surfaces. It is not a new kind of energy; it is a multiplier that compares a real emitter with an ideal black body at the same temperature and surface area. Because temperature is raised to the fourth power, T must be absolute temperature in kelvin.

Emissivity ε is a dimensionless measure of how efficiently a surface emits radiation compared with a perfect black body.

A perfect black body has ε = 1; real surfaces have 0 < ε < 1 in the ideal model.

The emitted power of a real body is P = εσAT^4.

For the same area and Kelvin temperature, lower emissivity means lower emitted power.

In climate models, effective emissivity helps describe how well Earth or the atmosphere emits infrared radiation to space.

Choose how changing emissivity changes thermal emission.

Decision

ε = 1 at a fixed A and T.

ε is reduced at fixed A and T.

T is given in degrees Celsius for P = εσAT^4.

Define emissivity and state how it modifies the Stefan-Boltzmann law for a real surface.

Common mark losses are omitting the comparison with a black body, giving ε a unit, or using Celsius temperature.

Define emissivity and state how it modifies the Stefan-Boltzmann law for a real surface.

Choose

Albedo controls the short-wave solar part of the energy balance. High-albedo surfaces such as ice and clouds reflect more incoming radiation, reducing absorbed power. Low-albedo surfaces such as dark ocean absorb more. In IB energy-balance equations, albedo usually appears through the factor 1 - α.

Albedo α is the fraction of incident radiation that is reflected or scattered back.

α = reflected power / incident power, so it is dimensionless.

A perfect reflector has α = 1 and absorbs none of the incident radiation in the simple model.

A perfect absorber has α = 0 and absorbs all incident radiation in the simple model.

The absorbed fraction used in energy-balance calculations is 1 - α.

Build the albedo relationship and the absorbed fraction.

Formula

Target formula α = P_reflected/P_incident; absorbed fraction = 1 - α

α

albedo, the fraction of incident radiation reflected

P_reflected

incoming power reflected or scattered back

W

P_incident

total incident incoming power

W

1 - α

fraction of incident power absorbed in the simple model

1Start with total incident incoming radiation.P_incident

2Identify the part reflected or scattered back.P_reflected

3Make albedo the reflected fraction of incident power.α = P_reflected/P_incident

4Find the fraction absorbed in a simple opaque model.1 - α

Build the formula

Define albedo and explain how albedo affects absorbed solar radiation in an Earth energy-balance model.

Common mark losses are describing albedo as absorption, giving it a unit, or forgetting the absorbed factor 1 - α.

Define albedo and explain how albedo affects absorbed solar radiation in an Earth energy-balance model.

Choose

The value 0.30 is a global average, not a fixed local constant. A cloudy polar region and a dark ocean region reflect very different fractions of incident sunlight. This variation matters in climate feedback: for example, melting ice can lower albedo, increasing absorbed solar radiation.

Earth’s average albedo is approximately 0.30, so about 30% of incoming solar radiation is reflected.

Clouds, snow, and ice usually increase albedo because they reflect more short-wave radiation.

Dark ocean, vegetation, and bare soil generally have lower albedo and absorb more radiation.

Albedo varies with cloud formation, surface cover, season, latitude, and solar angle.

Changing albedo changes absorbed solar power through the factor 1 - α.

Sort each Earth condition by its likely effect on albedo.

Sort

Unsorted

6

higher albedo

0

lower albedo

0

variation cue

0

State the approximate average albedo of Earth and explain two reasons why local albedo varies.

Common mark losses are quoting 0.30 without saying it is an average, or forgetting that higher albedo means more reflection and less absorption.

State the approximate average albedo of Earth and explain two reasons why local albedo varies.

Choose

S is an intensity at Earth’s orbit before averaging over the rotating spherical Earth. A flat detector facing the Sun receives S watts per square metre. In energy-balance models, Earth first intercepts SπR^2, then that power is distributed over the sphere for global average calculations.

The solar constant S is the solar power per unit area arriving at Earth’s orbital distance, measured outside the atmosphere.

It is defined for a surface perpendicular to the incoming solar radiation.

The unit of S is W m^-2; a commonly used value is about 1.36×10^3 W m^-2.

If the Sun has luminosity L_sun and Earth-Sun distance d, then S = L_sun/(4πd^2).

Earth intercepts solar power over its cross-sectional disk area πR^2, not over its full surface area.

Choose the correct interpretation of the solar constant.

Decision

A flat detector outside the atmosphere faces the Sun directly.

A model asks for incoming solar power averaged over Earth’s whole surface.

A problem asks for total power emitted by the Sun.

Define the solar constant S and explain why Earth intercepts power SπR^2 in a simple climate model.

Common mark losses are confusing S with luminosity, forgetting the perpendicular-area definition, or using 4πR^2 for the intercepted area.

Define the solar constant S and explain why Earth intercepts power SπR^2 in a simple climate model.

Choose

The Sun illuminates only Earth’s projected disk at any instant, but global climate models average over the entire spherical surface and over time. This gives the factor 1/4. After this geometric average, albedo is applied to find how much of the incoming radiation is absorbed.

Earth intercepts solar power SπR^2 because its shadow/cross-section is a disk of area πR^2.

For a whole-planet average, that power is spread over Earth’s spherical surface area 4πR^2.

The mean incoming solar intensity is therefore SπR^2/(4πR^2) = S/4.

Including albedo α, the mean absorbed solar intensity is (1 - α)S/4.

The S/4 factor is a geometry average, not a change in the solar constant itself.

Choose the correct geometry for mean solar intensity.

Decision

Find global mean incoming intensity before reflection.

Find global mean absorbed solar intensity with albedo α.

A flat panel outside the atmosphere faces the Sun directly.

Explain why the effective mean incident solar intensity on Earth is S/4, and state the mean absorbed intensity when albedo is α.

Common mark losses are using 4πR^2 as the intercepting area, treating 1/4 as albedo, or forgetting the absorbed factor 1 - α.

Explain why the effective mean incident solar intensity on Earth is S/4, and state the mean absorbed intensity when albedo is α.

Choose

A gas contributes to the greenhouse effect if its molecular energy levels allow it to absorb infrared radiation at wavelengths emitted by Earth. The key IB list is H2O, CO2, CH4, and N2O. Human activity can increase concentrations, but the gases themselves can also occur naturally.

The main greenhouse gases in the IB B.2 list are water vapour H2O, carbon dioxide CO2, methane CH4, and nitrous oxide N2O.

Greenhouse gases absorb some outgoing infrared radiation emitted by Earth’s surface and atmosphere.

After absorbing infrared radiation, greenhouse gas molecules re-emit radiation in many directions, including back toward the surface.

These gases have natural sources as well as human-related sources.

N2 and O2 make up most of the atmosphere but are not significant greenhouse gases in this model.

Choose the set and pathway that matches greenhouse gases.

Decision

CO2 concentration rises due to fossil fuel burning.

Water vapour is present naturally in the atmosphere.

A claim says N2 and O2 are the main greenhouse gases.

State the main greenhouse gases in the IB model and describe their role in the greenhouse effect.

Common mark losses are listing N2/O2 as greenhouse gases, omitting water vapour, or describing reflection of visible light instead of infrared absorption.

State the main greenhouse gases in the IB model and describe their role in the greenhouse effect.

Choose

The greenhouse effect is selective radiation absorption. Incoming solar radiation is mostly shorter wavelength, while Earth emits longer wavelength infrared. Greenhouse gases have molecular energy-level spacings that can absorb parts of this outgoing infrared spectrum, then redistribute and re-emit that energy in all directions.

Earth’s cooler surface emits mainly long-wave infrared radiation.

Greenhouse gas molecules can absorb infrared photons whose energies match changes in molecular rotational or vibrational energy levels.

The absorbed energy is later transferred by collisions or re-emitted as infrared radiation.

Re-emission occurs in many directions, including back toward Earth’s surface.

The atmosphere is relatively more transparent to incoming short-wave solar radiation than to some outgoing infrared wavelengths.

Choose the correct infrared absorption mechanism.

Decision

Incoming sunlight is mainly visible/short-wave radiation.

Earth emits long-wave infrared radiation upward.

An excited greenhouse gas molecule emits IR again.

Explain how greenhouse gases absorb outgoing infrared radiation and why this warms Earth’s surface.

Common mark losses are saying only “gases trap heat”, omitting molecular energy levels, or forgetting re-emission in all directions.

Explain how greenhouse gases absorb outgoing infrared radiation and why this warms Earth’s surface.

Choose

Greenhouse models are useful because they force an energy accounting structure. First choose the boundary and steady-state assumption, then decide how solar and infrared radiation interact with Earth and the atmosphere. More greenhouse absorption reduces net infrared loss to space for a given surface temperature, so the equilibrium temperature must change.

A simple greenhouse model treats Earth as an energy-balance system at steady average temperature.

Incoming solar radiation is modelled as short-wave radiation; Earth’s outgoing thermal radiation is long-wave infrared.

The atmosphere is often assumed to transmit most incoming short-wave radiation but absorb and re-emit some outgoing infrared radiation.

Albedo controls reflected incoming radiation, while emissivity or IR absorptivity controls outgoing thermal radiation.

Simple models use averages and idealisations, so they explain the mechanism but do not include all climate feedbacks or regional variation.

Choose the assumption that belongs in a simplified greenhouse model.

Decision

Incoming solar radiation in the model.

Thermal radiation emitted by Earth.

Average temperature is stable in the model.

Describe the assumptions in a simple greenhouse-effect model and explain one limitation of the model.

Common mark losses are using vague “trapped heat” language, failing to separate short-wave and long-wave radiation, or ignoring that the model is idealised.

Describe the assumptions in a simple greenhouse-effect model and explain one limitation of the model.

Choose

Enhanced does not mean the greenhouse effect is newly created; it means the existing infrared absorption pathway is strengthened. Human activities such as fossil fuel burning, agriculture, and land-use change can raise greenhouse gas concentrations. The physics answer should trace radiation and energy balance, not just state that “humans cause warming”.

The natural greenhouse effect is the warming caused by atmospheric absorption and re-emission of outgoing infrared radiation.

The enhanced greenhouse effect is an increase in this warming due to increased greenhouse gas concentrations, especially from human activity.

Higher concentrations of CO2, CH4, and N2O increase absorption of outgoing infrared radiation in relevant wavelength bands.

For the old surface temperature, less net infrared energy escapes to space, creating a radiative imbalance.

The surface-atmosphere system warms until outgoing power again balances absorbed incoming solar power at a higher equilibrium temperature.

Choose the correct enhanced-greenhouse pathway.

Decision

CO2 concentration increases due to fossil fuel combustion.

Water vapour and CO2 keep Earth warmer than a no-atmosphere model.

A response says the enhanced greenhouse effect is mainly ozone layer thinning.

Explain the enhanced greenhouse effect and distinguish it from the natural greenhouse effect.

Common mark losses are saying only “humans cause climate change”, confusing it with ozone depletion, or omitting the outgoing-infrared absorption step.

Explain the enhanced greenhouse effect and distinguish it from the natural greenhouse effect.

Choose

B.2 is a radiation-balance story. Start with incoming solar power, remove the reflected fraction using albedo, average over Earth’s sphere, then compare absorbed power with outgoing infrared emission. Greenhouse gases change the outgoing-infrared pathway, so enhanced concentrations shift the equilibrium temperature.

Use conservation of energy: at steady average temperature, absorbed incoming power equals outgoing infrared power.

Use emissivity ε in P = εσAT^4 and albedo α as reflected fraction, with absorbed fraction 1 - α.

Use the solar constant S as perpendicular solar flux at Earth orbit; global mean incoming intensity is S/4.

Use mean absorbed solar intensity (1 - α)S/4 in a simple Earth energy-balance model.

Identify H2O, CO2, CH4, and N2O as the main greenhouse gases in the IB model.

Explain greenhouse warming through molecular absorption and re-emission of outgoing infrared radiation.

State the assumptions and limits of simplified greenhouse models.

Distinguish the natural greenhouse effect from the enhanced greenhouse effect caused by increased greenhouse gas concentrations.

Match each B.2 retrieval cue to the physics move it should trigger.

Match

Reasons

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IB Physics SL B.2 Greenhouse effect Notes

Track Energy Through the System

Trace Emissivity

Trace Albedo

Trace Earth’s albedo variation

Trace Solar constant S

Trace Mean solar intensity

Trace Greenhouse gases

Trace Infrared absorption

Trace Greenhouse models

Trace Enhanced greenhouse effect

Retrieve the B.2 Greenhouse effect Model