What is greenhouse effect

The greenhouse effect, discovered by Josephbalance not only the absorbed solar flux but also
Fourier in 1824 and first investigated quantitativelythis downward infrared flux emitted by the
by Svante Arrhenius in 1896, is the process inatmosphere. The surface temperature will rise
which the emission of infrared radiation by anuntil it generates thermal radiation equivalent to
atmosphere warms a planet's surface. The namethe sum of these two incident radiation streams.
comes from an incorrect analogy with theA more realistic picture taking into account the
warming of air inside a greenhouse compared toconvective and latent heat fluxes is somewhat
the air outside the greenhouse. The Earth'smore complex. But the following simple model
average surface temperature is about 25°Ccaptures the essence. The starting point is to
warmer than it would be without the greenhousenote that the opacity of the atmosphere to
effect [1]. In addition to the Earth, Mars andinfrared radiation determines the height in the
especially Venus have greenhouse effects.atmosphere from which most of the photons
In common usage, "greenhouse effect" may referemitted to space are emitted. If the atmosphere
either to the natural greenhouse effect due tois more opaque, the typical photon escaping to
naturally occurring greenhouse gases, or to thespace will be emitted from higher in the
enhanced (anthropogenic) greenhouse effectatmosphere, because one then has to go to
which results from gases emitted as a result ofhigher altitudes to see out to space in the infrared.
human activities (see also global warming, scientificSince the emission of infrared radiation is a
opinion on climate change and attribution of recentfunction of temperature, it is the temperature of
climate change).the atmosphere at this emission level that is
The basic mechanismeffectively determined by the requirement that
The Earth receives energy from the Sun in thethe emitted flux balance the absorbed solar flux.
form of radiation. The Earth reflects about 30%But the temperature of the atmosphere generally
of the incident solar flux; the remaining 70% isdecreases with height above the surface, at a
absorbed, warming the land, atmosphere andrate of roughly 6.5 °C per kilometer on
oceans.average, until one reaches the stratosphere 10-15
To the extent that the Earth is in a steady state,km above the surface. (Most infrared photons
the energy stored in the atmosphere and oceanescaping to space are emitted by the
does not change in time, so energy equal to thetroposphere, the region bounded by the surface
absorbed solar radiation must be radiated back toand the stratosphere, so we can ignore the
space. Earth radiates energy into space asstratosphere in this simple picture.) A very simple
black-body radiation, which maintains a thermalmodel, but one that proves to be remarkably
equilibrium. This thermal, infrared radiationuseful, involves the assumption that this
increases with increasing temperature. One cantemperature profile is simply fixed, by the
think of the Earth's temperature as beingnon-radiative energy fluxes. Given the
determined by the infrared flux needed to balancetemperature at the emission level of the infrared
the absorbed solar flux.flux escaping to space, one then computes the
The visible solar radiation heats the surface, notsurface temperature by increasing temperature
the atmosphere, whereas most of the infraredat the rate of 6.5 °C per kilometer, the
radiation escaping to space is emitted from theenvironmental lapse rate, until one reaches the
upper atmosphere, not the surface. The infraredsurface. The more opaque the atmosphere, and
photons emitted by the surface are mostlythe higher the emission level of the escaping
absorbed by the atmosphere and do not escapeinfrared radiation, the warmer the surface, since
directly to space.one then needs to follow this lapse rate over a
reason this warms the surface is most easilylarger distance in the vertical. While less intuitive
understood by starting with a simplified model ofthan the purely radiative greenhouse effect, this
a purely radiative greenhouse effect that ignoresless familiar radiative-convective picture is the
energy transfer in the atmosphere by convectionstarting point for most discussions of the
(sensible heat transport) and by the evaporationgreenhouse effect in the climate modeling
and condensation of water vapor (latent heatliterature.
transport). In this purely radiative case, one canThe term "greenhouse effect" is a source of
think of the atmosphere as emitting infraredconfusion in that actual greenhouses do not warm
radiation both upwards and downwards. Theby this same mechanism
upward infrared flux emitted by the surface must