Previous Chapter: 1 - Introduction and Summary
The greenhouse effect is the process by which heat escaping from the Earth's surface is partially absorbed in the atmosphere before being re-emitted or transferred to surrounding molecules. To understand climate models and their limitations, it is essential to have a clear and intuitive grasp of how this mechanism operates. Although it involves quantum mechanics and detailed spectroscopy, the basic principle is straightforward: heat must move upward from the surface into the atmosphere, and greenhouse gases influence how much of that heat escapes to space.
The surface emits heat in the form of infrared photons. As these photons travel upward, many encounter greenhouse-effect molecules (GEMs) such as water vapour, carbon dioxide, methane, nitrous oxide, and ozone. When a photon matches the vibrational frequency of one of these molecules, it may be absorbed, raising the molecule to an excited state. This is the fundamental process behind the greenhouse effect.
However, the atmosphere cools rapidly with altitude. At the surface the average temperature is about +15°C, but by 10 to 12 km altitude temperatures fall to around -60°C. At the same time, the abundance of greenhouse gases decreases sharply with height. This means that most photon absorption occurs in the lower atmosphere, where both temperature and greenhouse-gas density are highest.
The probability that an upward-travelling photon will be absorbed depends on three main factors:
GEMs absorb photons only when the wavelength closely matches their vibrational frequency. If the match is not close enough, no absorption occurs. When a photon is absorbed near the surface, the excited molecule typically collides with another air molecule within a nanosecond, transferring the energy and raising the air temperature. Near the top of the atmosphere, where collisions are less frequent and temperatures are lower, absorbed photons are more likely to be re-emitted rather than converted to heat.
Because the absorption behaviour is wavelength-specific, large portions of the infrared spectrum pass straight through the atmosphere. Around 30 percent of surface-emitted photons escape directly to space without interacting with any greenhouse gas. The remainder form complex absorption patterns that depend on the detailed spectra of water vapour, carbon dioxide, and other gases.
Rather than dealing with thousands of individual spectral lines, it is helpful to imagine a simplified absorption band. In such an idealised view:
Although simplistic, this picture captures the essential behaviour: some wavelengths are strongly absorbed, others are partially absorbed, and some are barely affected at all. Real spectra consist of many overlapping peaks from different gases, which the more detailed models in later chapters attempt to describe or approximate.
Understanding the vertical structure of the atmosphere is crucial for understanding the greenhouse effect. The troposphere, where nearly all weather occurs, extends from the surface to roughly 10 or 12 km altitude. Temperatures decline steeply through this region. Most photon absorption and heat transfer takes place here because the density of the atmosphere is high and molecular collisions are frequent.
Above the tropopause, the stratosphere exhibits different temperature behaviour. Here, certain wavelengths are absorbed primarily by ozone, and temperatures increase with height. However, the stratosphere contains far fewer greenhouse-effect molecules, so its contribution to the total greenhouse effect is much smaller.
Between January 1985 and March 2022, satellites measured the total infrared radiation escaping to space each month. Combined with estimates of surface emission, these observations give us the total greenhouse effect for 459 consecutive months. These measurements form the backbone of model testing in later chapters. They provide reliable, consistent data against which any model must be judged.
Chapter 2 therefore lays out the physical mechanism underlying the greenhouse effect, providing the foundation for evaluating the three models introduced earlier. With this understanding, the reader is prepared to examine how models attempt to represent the mechanism and how well they match observed behaviour.
Next Chapter: 3 - The IPCC Model - Summarised: Construction and Outcomes