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4. Tropospheric lapse rate




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

4. Tropospheric lapse rate

At any given location, the temperature profile of the air column varies
between day and night, from winter to summer. At times and places the
air may get warmer higher up (an inversion). Globally averaged, the
troposphere, the lower about 10 to 15 km of our atmosphere, gets cooler
with height. A typical value cited is 6.5 o C cooling / km of altitude.
This is the so-called global mean tropospheric lapse rate. Some people
attach a plus, others attach a minus sign to this rate [Hartmann, p 3,
69] [Sinha]. In any case, it indicates the average rate of cooling
with height. For illustration, if the amount of the mean tropospheric
lapse rate should increase by 1 o C / km, then the mean air temperature
at 5 km altitude would drop by 5 o C.

Basically, earth's surface temperature and the greenhouse effect tend
to go up and down with the amount of the tropospheric lapse rate. To
see why, recall that infrared emitted from the surface rarely reaches
space directly: greenhouse gases and clouds absorb most of it. Earth's
effective radiating temperature of -18 o C corresponds to an apparent
radiating altitude of 5 or so km. The bulk of the infrared escaping
to space comes from the middle and upper troposphere. On its way up,
little of this radiation gets caught: still higher up the air is thin,
there are few greenhouse gases and clouds [Hartmann, p 28, 59-60].

Now imagine that the amount of the global mean tropospheric lapse rate
goes up, while anything else remains equal (a wild simplification, but
never mind). Then the middle and upper troposphere get cooler and emit
less infrared to space. The sun keeps shining, so earth's radiation
budget gets out of balance. The surface (and troposphere) must warm
until they emit enough infrared to restore the balance under the enhan-
ced lapse rate. The difference between surface emission and emission
to space, that is: the greenhouse effect, increases. Vice versa, if
the magnitude of the global mean tropospheric lapse rate drops, then
the middle and upper troposphere warm and emit more infrared to space.
To regain the balance, the greenhouse effect must decline.

Once again, this is simplified in order to convey the basic idea.
The mean tropospheric lapse rate is a balance between many processes
of energy transfer, like radiation, convection, evaporation, cloud
formation, and large scale air motions. Data from the midlatitudes
and tropics suggest that local lapse rate changes currently tend to
amplify local variations of surface temperature and of the greenhouse
effect. It is unclear whether and how the global mean tropospheric
lapse may change with a changing global climate [Sinha] [Soden].

Finally, note that if the surface warms, while the lapse rate remains
unchanged, then the troposphere will warm by the same amount as the
surface. Infrared emission to space will rise accordingly.

 

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