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5. The enhanced greenhouse effect. Radiative forcing


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

5. The enhanced greenhouse effect. Radiative forcing

Since around 1800 and especially during the past few decades, human
activities have increased the atmospheric levels of several greenhouse
gases. To name a few: Carbon dioxide (CO2) went up from about 280 ppmv
(parts per million by volume) in the year 1800 via 315 ppmv in 1958
to about 358 ppmv in 1994 [IPCC 95, p 16, 78] [Keeling]. Methane (CH4)
increased from roughly 0.8 ppmv in 1800 to more than 1.7 ppmv in 1992.
Nitrous oxide (N2O) rose from a preindustrial level of about 0.275 ppmv
to 0.310 or so ppmv in 1992 [IPCC 94, p 87-8, 91-2].

The resulting enhanced greenhouse effect is often expressed in terms of
`radiative forcing'. To get a feeling for this notion, suppose that
greenhouse gas levels go up, while anything else, including temperature,
is kept fixed. Adding greenhouse gases renders the atmosphere more
opaque to outgoing infrared radiation. Thus the mean altitude from
which infrared emitted upwards makes it to space (5 or so km) rises.
As mentioned, the troposphere gets cooler with height. With rising
emission altitude, both earth's effective radiating temperature and,
consequently, the amount of infrared emitted to space decline. The
influx of solar radiation, to which greenhouse gases are almost trans-
parent, changes little. So the net influx (the difference between
what goes in and out) is now positive instead of being zero.

Radiative forcing means a _change_ in the net downward flux of radia-
tion, in W/m**2, at the tropopause, the borderline between troposphere
and stratosphere. Eventually the climate system must respond and re-
adjust the net flux to zero, but temporarily this flux may get positive
or negative. Given some perturbation like a change in greenhouse gas
or aerosol levels, radiative forcing is estimated with tropospheric and
surface temperatures (the response of which takes decades) _kept fixed_
at their unperturbed values [IPCC 94, p 169-71]. Rising greenhouse gas
levels cause positive radiative forcing. Aerosols, to be described
later, can cause negative radiative forcing.

Radiative forcing due to human-made greenhouse gases is currently
estimated at about 2.5 W/m**2. CO2 causes roughly 1.6 W/m**2 of this,
while methane contributes about 0.5 W/m**2. Doubling the CO2 level
from its preindustrial 280 to 560 ppmv amounts to a radiative forcing
of a bit over 4 W/m**2. If business goes on as usual, the combined
effect of the rising greenhouse gas levels is likely to reach the
equivalent of a CO2 doubling around the year 2050 and will hardly
stop there [IPCC 90, p 52] [IPCC 95, p 108-18, 321].

An enhanced greenhouse effect disturbs earth's radiation balance:
less infrared gets out, while the sun keeps shining. This cannot last,
the balance must be restored. At least one of the following things
must happen: earth's surface and troposphere may warm (lapse rate
remaining unchanged), earth's albedo may go up, the amount of the mean
tropospheric lapse rate may drop (the latter, though, might also rise
and thus enhance surface warming), or other changes in earth's climate
system may curb the enhanced greenhouse effect.

In short, something has to give. Monkeying with earth's radiation
balance will change the climate in some way. Earth's surface will most
probably warm, although it is uncertain by how much and how swiftly.
In addition, there will probably be a gamut of other changes, some
of which, like changes in the water cycle, are even harder to predict
and may become more troublesome than warming [IPCC 95] [Morgan].


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