Astrophysics (Index)About

greenhouse effect

(mechanism that boosts temperature through selective blocking of EMR)

The term greenhouse effect is used for atmospheres, referring to the same mechanism by which greenhouses can remain warmer than surrounding air, i.e., that the energy carried in by incoming electromagnetic radiation (EMR) is not efficiently carried out by outgoing EMR. The imbalance leads to energy gathering in the form of heat, the temperature rising to the point that it supports sufficient EMR emission to reach a balance. In the case of a greenhouse, the effect is because of the glass's differing transparency at different wavelengths, whereas for a planet, the effect is due to differing transparency of the atmosphere. In both cases, this transparency is easily measured and is necessarily taken into account for much technology (atmospheric windows) and the effect is easily produced in lab experiments.

The Earth's temperature is in part maintained by a greenhouse effect, due to the atmosphere's efficient transmission of much of the Sun's visible light down to the surface, and its opacity in some infrared wavelengths, which is what would carry away the bulk of the energy given the Earth's temperature. The components of the atmosphere that block EMR are known as greenhouse gases. Changes in the atmosphere's makeup would affect the greenhouse effect, such as the addition of greenhouse gases causing the Earth's surface temperature to rise. In fact, the ever-increasing use of technology that burns fossil fuels is adding a greenhouse gas (carbon dioxide). Whether a rise in the global temperature that can be ascribed to this effect has been detected and what effect such a discovery should have on policy and practice is of much current interest in politics.

Runaway greenhouse effect occurs in cases where a rising temperature increases the presence of greenhouse gases, which in turn raise the temperature, i.e., positive feedback, resulting in a large rise in temperature. An example of such a runaway greenhouse effect might be a rise in temperature causing an increase in water vapor (a greenhouse gas) in the atmosphere, i.e., if the warming of surface-water (oceans) caused sufficient additional water to evaporate into the atmosphere to increase the temperature, further warming the surface-water, causing more such evaporation, etc.

The greenhouse effect affects other worlds to varying degrees: Venus has a temperature of 730 K (about 850°F) whereas without the greenhouse effect, its temperature would be roughly the equilibrium temperature, 328 K (about 160°F). Earth's, in contrast, raises the temperature on the order of 10°F, and the difference is apparently that in Venus's history, a runaway greenhouse effect occurred.

The analysis of extra-solar planet and solar system planet and moon atmospheres and temperature must take the effect into account.

Further reading:

Referenced by pages:
equilibrium temperature (Teq)
radiative forcing (RF)
runaway process
silicate weathering feedback
thermal runaway
atmospheric window