B.2
Greenhouse Effect
The solar system, black body radiation, the greenhouse effect, the enhanced greenhouse effect and climate change.
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Key Concepts, Greenhouse Effect
The Sun as an Energy Source and Earth's Energy Balance
The Sun is the primary energy source for Earth. It emits energy predominantly as visible light and ultraviolet radiation. Of the solar radiation arriving at Earth, some is reflected back into space by clouds, ice, and other surfaces (a property quantified by albedo), and the rest is absorbed by the surface and atmosphere. Albedo is the fraction of incident radiation that is reflected: a perfectly white surface has an albedo of 1 and a perfectly black surface has an albedo of 0. Earth's average albedo is approximately 0.30, meaning around 30% of incoming solar radiation is reflected without being absorbed. The energy absorbed must eventually be re-radiated back to space as infrared radiation for Earth to maintain a stable temperature.
Black Body Radiation and the Stefan-Boltzmann Law
A black body is a theoretical object that absorbs all incident radiation and emits radiation across a continuous spectrum depending only on its temperature. Both the Sun and the Earth are modelled as approximate black bodies. The Stefan-Boltzmann law gives the total power radiated per unit surface area: P = σAT⁴, where σ = 5.67 × 10⁻⁸ W/m²/K⁴ is the Stefan-Boltzmann constant, A is the surface area, and T is the surface temperature in Kelvin. Power scales with the fourth power of temperature, so even a small increase in temperature produces a large increase in radiated power. For non-ideal bodies (emissivity ε < 1), the equation becomes P = εσAT⁴.
Wien's Displacement Law
Wien's displacement law states that the peak wavelength of radiation emitted by a black body is inversely proportional to its absolute temperature: λ_max × T = 2.90 × 10⁻³ m K. The Sun, at approximately 5,800 K, emits radiation that peaks in the visible region (around 500 nm). Earth, at approximately 255–288 K, is much cooler, so it emits radiation that peaks in the infrared region (around 10,000 nm). This difference in peak wavelength is crucial for understanding the greenhouse effect: the atmosphere is largely transparent to incoming visible radiation from the Sun but absorbs outgoing infrared radiation from Earth.
Earth's Equilibrium Temperature
Earth's equilibrium temperature can be derived by equating the power absorbed from the Sun with the power radiated by Earth back into space. The power absorbed is: P_absorbed = (1 - α) × (L_Sun / 4πd²) × πR², where α is Earth's albedo, d is the Earth-Sun distance, and R is Earth's radius. Setting this equal to the power radiated by a black body gives: (1 - α) × L_Sun / (4πd²) × πR² = σ × 4πR² × T⁴. Solving for T gives an equilibrium temperature of approximately 255 K (about -18°C). This is significantly lower than Earth's actual average surface temperature of about 288 K (15°C), and the difference is explained by the natural greenhouse effect.
The Natural Greenhouse Effect
The natural greenhouse effect is the process by which certain gases in Earth's atmosphere trap infrared radiation and warm the surface. When Earth re-radiates energy as infrared radiation, some of it is absorbed by greenhouse gases rather than escaping directly to space. These gases then re-emit the radiation in all directions, including back towards the surface, effectively providing an additional source of heating. Without this effect, Earth's average surface temperature would be approximately -18°C rather than the current +15°C, making much of the planet uninhabitable. The natural greenhouse effect is therefore essential for life as we know it. It should not be confused with the enhanced greenhouse effect.
Greenhouse Gases and How They Work
The main greenhouse gases are water vapour (H₂O), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃). These molecules absorb infrared radiation because their molecular bonds can vibrate at frequencies that correspond to infrared wavelengths. When a greenhouse gas molecule absorbs an infrared photon, it vibrates and then re-emits the radiation in a random direction, effectively scattering energy back towards Earth's surface. Nitrogen (N₂) and oxygen (O₂), which make up most of the atmosphere, do not absorb infrared radiation in this way and so are not greenhouse gases.
The Enhanced Greenhouse Effect and Climate Change
The enhanced greenhouse effect refers to the intensification of the natural greenhouse effect due to human activity, particularly the burning of fossil fuels, deforestation, and agriculture, which have significantly increased the concentrations of CO₂, CH₄, and N₂O in the atmosphere. Higher concentrations of greenhouse gases absorb more outgoing infrared radiation, reducing the rate at which Earth loses energy to space. This causes Earth's surface temperature to rise to a new, higher equilibrium. The resulting climate change includes rising global mean surface temperatures, melting ice caps and glaciers, rising sea levels, more frequent extreme weather events, and disruption to ecosystems. The scientific evidence, including ice core data, satellite measurements and global temperature records, strongly supports the link between rising greenhouse gas concentrations and rising temperatures.
Emissivity
Emissivity (ε) is a dimensionless quantity between 0 and 1 that describes how efficiently a real body emits radiation compared to a perfect black body at the same temperature. A perfect black body has ε = 1. Real surfaces have ε < 1. The modified Stefan-Boltzmann law for a real body is P = εσAT⁴. Emissivity also describes how efficiently a body absorbs radiation: a body with high emissivity absorbs and emits efficiently; a body with low emissivity (like a shiny metal) reflects most radiation and emits less. In climate science, Earth's effective emissivity is modified by the greenhouse effect: greenhouse gases reduce the effective emissivity of the atmosphere, causing the surface to reach a higher equilibrium temperature than the basic black body calculation predicts.
Responses to the Enhanced Greenhouse Effect
Methods to reduce the enhanced greenhouse effect fall into several categories. Switching to renewable energy sources (solar, wind, hydroelectric, geothermal) reduces CO₂ emissions from burning fossil fuels. Improving energy efficiency reduces total energy demand. Carbon capture and storage (CCS) technologies aim to remove CO₂ from power station emissions or directly from the atmosphere before it enters the atmosphere or after. Changes in land use, such as reforestation and reducing deforestation, increase the amount of CO₂ absorbed by vegetation. International policy frameworks, including emissions trading schemes and binding national targets, aim to coordinate global action. Each method involves trade-offs in cost, scalability, land use, and political feasibility.
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Frequently Asked Questions, IB Physics Greenhouse Effect
What is Greenhouse Effect in IB Physics? ↓
The solar system, black body radiation, the greenhouse effect, the enhanced greenhouse effect and climate change. This topic is part of Theme B (The Particulate Nature of Matter) in the current IB Physics syllabus.
Is Greenhouse Effect SL or HL in IB Physics? ↓
Greenhouse Effect is covered by both SL and HL students in the current IB Physics syllabus. HL students study additional depth and extension content beyond the SL core.
What equations do I need for IB Physics Greenhouse Effect? ↓
The key equations for Greenhouse Effect are covered in the concept tutorial above. For a structured set of notes with all equations, conditions and worked examples, the GradePod Exam Pack includes a revision note template for every topic.
What are common exam mistakes in IB Physics Greenhouse Effect? ↓
Common mistakes are covered in detail in the exam technique video above. The GradePod Exam Pack also includes exam-style questions with mark schemes so you can see exactly how marks are awarded and where students typically drop them.
How do I revise Greenhouse Effect for the IB Physics exam? ↓
Follow the GradePod three-step method. First, watch the concept tutorial and tick off each learning objective on the checklist above as you go. Second, watch the exam technique video to see how IB-style questions are answered under exam conditions. Third, use the Exam Pack to practise independently with knowledge questions, exam questions and mark schemes. That's it. It works. I promise.