The importance of infra-red to the greenhouse effect can be seen in
the following table.
Percent solar constant at aircraft altitude:
Lambda (nm) Cum % % Range
0 - 10 *** less than 0.00044 percent***
10 - 400 8.725 8.725 UV
400 - 700 46.879 38.154 Visible
700 - 100000 99.999 53.120 IR
100000 - 1000000 *** less than .000998 percent ***
Derived from page 14-10 of the 74th Edition of The CRC Handbook.
Most of the incoming power is in already the form of infra-red. Most
of the remaining incoming power is in the visible range. The
downshifting of the visible range to infra-red is key to the
greenhouse effect.
When molecules or atoms absorb radiation, they sometimes simply re-
emit the radiation at the same frequency, though not necessarily in
the same direction. However, excited atoms and molecules can also re-
emit the radiation in steps. This results in a down-shifting of the
absorbed and then re-emitted energy. In many such cases net momentum
is absorbed by the molecule in the interaction, and the light re-
emitted is further down-shifted. The resulting molecular momentum
change means a velocity change which on balance results in a
temperature increment.
Hot material emits radiation in proportion to the 4th power of its
temperature. As the Earth heats up a highly non-linear relationship
between infra-red radiation comes into play. As ice and snow is
reduced, more of the Earth's cooling is dependent upon infra-red
making it back to space. Less visible light is reflected to space as
ice disappears. H2O and CO2 thus play a highly non-linear role in
retarding the process of compensating for the loss of albedo due to
polar melting. As polar ice disappears, albedo is lost, more water
is available to the polar atmosphere, as is more methane. This
results in a feedback effect which is even more powerful than a 4th
power law. To some degree, compensation occurs due to cloud
formation, which reflects visible light. However, high altitude
water vapor concectration, that water vapor which is above any clouds
that form, has a highly non-linear relationship with temperature. As
temperature above the clouds increases, the ability of water vapor to
exist there, without forming ice, increases dramatically.
All this non-linearity ultimately means many people are in for some
severe surprises. Some parts of the atmosphere are already in a
runaway regime. The rapidity with which the other parts can enter
that regime will doubtless be a surprise even to many scientists.
Horace Heffner
