Preview
IB DP Physics — Unit B: The Particulate Nature of Matter | SL & HL | B.2 Greenhouse Effect
📋 What You Will Learn
- Observe and describe how solar radiation interacts with Earth's atmosphere and surface to maintain a habitable temperature
- Define greenhouse gases and explain the molecular mechanism by which they absorb and re-emit infrared radiation
- Distinguish between the natural greenhouse effect and the enhanced greenhouse effect caused by human activity
- Apply the Stefan–Boltzmann law and the concept of emissivity to model Earth's surface temperature
- Analyse graphs of atmospheric CO₂ concentration and global mean temperature to identify trends and correlations
- Evaluate the evidence for anthropogenic climate change and connect this topic to energy, waves, and thermal physics across the IB course
🌍 Real-World Context On a clear summer night, deserts cool dramatically — sometimes by 30 °C or more — while a humid tropical night stays warm and oppressive. The difference is not the Sun; it has set for both. The difference is the atmosphere above. Water vapour, carbon dioxide, and methane in the tropical air act like an invisible thermal blanket, trapping outgoing heat and radiating it back to the surface. This same mechanism, operating on a planetary scale, is what keeps Earth's average surface temperature at a life-sustaining +15 °C rather than a frozen −18 °C. Understanding exactly how this blanket works — and what happens when we thicken it — is one of the most consequential questions in modern physics.
🔍 Guiding Question How do the physical properties of greenhouse gas molecules allow them to absorb infrared radiation, and what are the consequences of increasing their atmospheric concentration for Earth's energy balance?
Physical Setup: Earth's Energy Balance
To understand the greenhouse effect, we must first picture the flow of energy through Earth's atmosphere. The Sun, with a surface temperature of approximately 5 800 K, radiates energy predominantly as visible light and near-infrared radiation. This radiation travels through space, passes through the atmosphere largely unimpeded, and is absorbed by Earth's surface. The warmed surface then re-radiates energy — but because the surface is far cooler than the Sun (around 288 K), it radiates at much longer wavelengths in the far-infrared region of the spectrum.
This difference in wavelength is the key to everything that follows. The atmosphere is largely transparent to incoming short-wave solar radiation, but certain gases in the atmosphere are highly effective at absorbing outgoing long-wave infrared radiation. These gases then re-emit the absorbed energy in all directions — including back toward Earth's surface — effectively slowing the escape of thermal energy to space.
Notice in the diagram that the atmosphere acts as a selective filter: it is nearly transparent to the short-wave incoming radiation but partially opaque to the long-wave outgoing radiation. The net effect is that the surface must warm to a higher temperature than it would in the absence of an atmosphere, in order to radiate enough energy to maintain equilibrium. This is the natural greenhouse effect — and without it, liquid water and life as we know it could not exist on Earth.
Core Concepts and Definitions
Before we can quantify the greenhouse effect, we need to build a precise vocabulary. The concepts below are introduced in the order they are needed — from the most fundamental (blackbody radiation) to the more specific (greenhouse gas, emissivity, albedo). Each definition emerges from the physical reasoning above rather than being stated in isolation.
📖 Key Definition — Blackbody A blackbody is an idealised object that absorbs all electromagnetic radiation incident upon it and emits the maximum possible radiation at every wavelength for a given temperature. A blackbody emits a continuous spectrum whose peak wavelength and total power depend only on its temperature.
Real objects are not perfect blackbodies — they absorb and emit less than the theoretical maximum. The ratio of actual emission to blackbody emission is called emissivity. For Earth's surface, emissivity is close to 1 (approximately 0.95–0.99), making the