The Greenhouse Effect and Radiative Forcing
How atmospheric trapping of longwave radiation warms the surface, and how changes in composition perturb the planetary energy balance.
Definition
The greenhouse effect is the warming of a planetary surface caused by atmospheric absorption and re-emission of longwave radiation; radiative forcing is the change in net irradiance at the tropopause caused by a perturbation such as a change in greenhouse-gas concentration.
Scope
Covers the physical basis of the greenhouse effect, the role of water vapour, carbon dioxide, methane and other greenhouse gases, the distinction between the natural and enhanced greenhouse effect, and the concept of radiative forcing as a standardized measure of an agent's perturbation to the top-of-atmosphere energy balance.
Core questions
- Which atmospheric constituents are responsible for the greenhouse effect?
- How is radiative forcing defined and quantified?
- Why does adding carbon dioxide warm the surface even when its absorption bands are partly saturated?
Key theories
- Radiative forcing concept
- A framework that quantifies the perturbation to Earth's energy balance from an agent as a flux change in watts per square metre, enabling comparison of different climate drivers.
- Arrhenius's carbon-dioxide warming
- The first quantitative estimate that changes in atmospheric carbon dioxide alter surface temperature through longwave absorption, establishing the link between composition and climate.
Mechanisms
Greenhouse gases are largely transparent to incoming solar radiation but absorb outgoing thermal infrared, re-emitting it in all directions including downward. This raises the effective emission level to higher, colder altitudes, so the surface must warm to restore top-of-atmosphere balance. Increasing a gas's concentration deepens absorption in the wings of its bands, producing additional forcing that for carbon dioxide grows approximately logarithmically with concentration.
Clinical relevance
Radiative forcing is the central metric linking emissions to climate change in scientific assessments and underlies emission-equivalence metrics and climate-policy targets.
History
Fourier suggested the atmosphere retains heat in the 1820s, Tyndall measured the infrared absorption of gases in the 1860s, and Arrhenius produced the first quantitative carbon-dioxide sensitivity estimate in 1896. The radiative-forcing concept was formalized in the late twentieth century and codified in successive IPCC assessments.
Key figures
- Joseph Fourier
- John Tyndall
- Svante Arrhenius
Related topics
Seminal works
- arrhenius1896
- myhre2013
Frequently asked questions
- Is the natural greenhouse effect harmful?
- No. The natural greenhouse effect keeps Earth's surface roughly 33 degrees Celsius warmer than it would otherwise be and makes the planet habitable; concern centres on the additional, human-enhanced forcing.