Greenhouse Effect and Atmospheric Absorption

How water vapor, carbon dioxide, and other gases absorb and re-emit thermal radiation, warming the surface above its radiative equilibrium temperature.

Definition

The greenhouse effect is the warming of a planet's surface caused by atmospheric gases that are transparent to incoming sunlight but absorb and re-emit outgoing thermal radiation, raising the surface temperature above the value set by solar heating alone.

Scope

This topic covers the physical basis of the greenhouse effect: the absorption and emission of longwave radiation by molecular gases, the spectral bands in which they act, and the way the effective radiating level of the atmosphere controls surface temperature. It treats the roles of water vapor, carbon dioxide, methane, nitrous oxide, and ozone, the concept of radiative forcing from changes in their concentrations, and the saturation and band-overlap effects that govern how forcing scales with concentration.

Core questions

• Why are some gases effective absorbers of thermal radiation while others are not?
• How does the greenhouse effect raise the surface temperature above radiative equilibrium?
• How does radiative forcing change as a greenhouse gas concentration increases?
• What are the relative contributions of water vapor and carbon dioxide?

Key theories

Effective emission level

Greenhouse gases raise the altitude from which the planet effectively radiates to space, and because that level is colder than the surface, the surface must warm to emit enough energy to balance incoming sunlight.

Logarithmic forcing of carbon dioxide

Because the central absorption bands of carbon dioxide are nearly saturated, its radiative forcing grows approximately with the logarithm of concentration, so each doubling adds a similar amount of forcing.

Mechanisms

Molecules with the right vibrational and rotational transitions absorb outgoing infrared radiation and re-emit it in all directions, including back toward the surface; this raises the altitude at which radiation finally escapes to space, and since temperature falls with height in the troposphere, the surface warms until the colder emission level radiates enough to balance the absorbed sunlight. Adding more gas thickens the absorbing layer and shifts the emission level higher and colder, increasing the forcing.

Clinical relevance

The radiative forcing from rising carbon dioxide, methane, and other greenhouse gases is the direct physical driver of human-caused climate change, making this mechanism central to understanding and projecting global warming.

Evidence & guidelines

The IPCC Sixth Assessment Report quantifies the effective radiative forcing of each major greenhouse gas and attributes the dominant share of observed warming since the pre-industrial era to these gases.

History

Tyndall demonstrated in the nineteenth century that water vapor and carbon dioxide absorb thermal radiation, and Arrhenius made the first quantitative estimate of the warming from doubling carbon dioxide; twentieth-century spectroscopy and radiative-convective modeling refined these ideas into the modern quantitative theory of radiative forcing.

Debates

Historical claims of band saturation

Early objections that carbon dioxide absorption was already saturated were resolved by recognizing that adding gas raises the cold emission level, so forcing continues to increase rather than plateau.

Key figures

• Svante Arrhenius
• John Tyndall
• Raymond Pierrehumbert
• Syukuro Manabe

Seminal works

• arrhenius1896
• pierrehumbert2010

Frequently asked questions

Which gas contributes most to the greenhouse effect?

Water vapor is the largest contributor overall, but it acts as a feedback that responds to temperature, while carbon dioxide is the main long-lived gas that humans are increasing.

Why does carbon dioxide still matter if its bands are nearly saturated?

Adding carbon dioxide raises the altitude from which the planet radiates to space to a colder level, so the warming effect keeps growing with each doubling of concentration.