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3. The natural greenhouse effect


This article is from the Climate Change FAQ, by Jan Schloerer jan.schloerer@medizin.uni-ulm.de with numerous contributions by others.

3. The natural greenhouse effect

The sun's radiation, much of it in the visible region of the spectrum,
warms our planet. On average, earth must radiate back to space the
same amount of energy which it gets from the sun. Being cooler than the
sun, earth radiates in the infrared. (An object, when getting warmer,
radiates more energy and at shorter wavelengths. On cooling, it emits
less and at longer wavelenghts. Lava or heated iron are examples.)
The wavelengths at which the sun and the earth emit are, for energetic
purposes, almost completely distinct. Often, solar radiation is called
shortwave, whereas terrestrial infrared is called longwave radiation.

Greenhouse gases in earth's atmosphere, while largely transparent to
incoming solar radiation, absorb most of the infrared emitted by earth's
surface. The air is cooler than the surface, emission declines with
temperature, so the air or, rather, its greenhouse gases emit less
infrared upwards than the surface. Moreover, while the surface emits
upwards only, the air's greenhouse gases radiate both up- and downwards,
so some infrared comes back down. Clouds also absorb infrared well.
Again, cloud tops are usually cooler and emit less infrared upwards
than the surface, while cloud bottoms radiate some infrared back down.
All in all, part of the infrared emitted by the surface gets trapped.

Satellites, viewing earth from space, tell us that the amount of
infrared going out to space corresponds to an `effective radiating
temperature' of about -18 o C. At -18 o C, about 240 watts per square
metre (W/m**2) of infrared are emitted. This is just enough to balance
the absorbed solar radiation. Yet earth's surface currently has a mean
temperature near 15 o C and sends an average of roughly 390 W/m**2 of
infrared upwards. After the absorption and emission processes just
outlined, 240 W/m**2 eventually escape to space; the rest is captured
by greenhouse gases and clouds. The `natural greenhouse effect' can
be defined as the 150 or so W/m**2 of outgoing terrestrial infrared
trapped by earth's preindustrial atmosphere. It warms earth's surface
by about 33 o C.

As an aside, note that garden glasshouses retain heat mainly by lack
of convection and advection [Jones]. The atmospheric `greenhouse'
effect, being caused by absorption and re-emission of infrared
radiation, is a misnomer. We won't get rid of it, though ;-)

Under clear sky, roughly 60-70 % of the natural greenhouse effect is
due to water vapor, which is the dominant greenhouse gas in earth's
atmosphere. Next important is carbon dioxide, followed by methane,
ozone, and nitrous oxide [IPCC 90, p 47-48].

Clouds are another big player in the game. Beginners please don't
confuse clouds with water vapor: clouds consist of water droplets or
ice particles or both. Under cloudy sky the greenhouse effect is
stronger than under clear sky. At the same time, cloud tops in the
sunshine look brilliantly white: they reflect sunlight. Globally and
seasonally averaged, clouds currently exert the following effects:

Outgoing terrestrial infrared trapped (warming) about 30 W/m**2
Solar radiation reflected back to space (cooling) nearly 50 W/m**2
Net cloud effect (cooling) roughly 20 W/m**2

Earth's present reflectivity or albedo (whiteness) is near 0.3. This
means that about 30 % or slightly over 100 W/m**2 of the sun's incoming
radiation is reflected back to space, while roughly 240 W/m**2 or about
70 % is absorbed. Almost half of earth's current albedo and perhaps
20 % of the natural greenhouse effect is caused by clouds. Quantities
involving clouds are hard to measure and may vary by a few W/m**2,
depending on whom you listen to.

Globally averaged, the surface constantly gains radiative energy,
whereas the atmosphere scores a loss. Sending up about 390 W/m**2,
the surface absorbs roughly 170 W/m**2 solar radiation and over 300
W/m**2 infrared back radiation from greenhouse gases and clouds.
The atmosphere, clouds included, radiates both up- and downwards,
altogether over 500 W/m**2. It absorbs roughly 70 W/m**2 solar
radiation and 350 W/m**2 terrestrial infrared.

The surface's radiative heating and the atmosphere's radiative
cooling are balanced by convection and by evaporation followed by
condensation. When evaporating, water takes up latent heat; when
water vapor condenses, as happens in cloud formation, latent heat is
released to the atmosphere. Information in this section comes from
[Berger] and [Hartmann, chapters 2-4], unless indicated otherwise.


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