Geoscience Reference
In-Depth Information
Table 8.1. Heating and cooling effects of clouds.
Parameter
High clouds
Middle clouds
All low
Total
clouds
Thin
Thick
Thin
Thick
Global fraction (%)
10.1
8.6
10.7
7.3
26.6
63
Forcing relative to clear sky (W/m 2 )
Albedo (visible)
4.1
15.6
3.7
9.9
20.2
53.5
Outgoing IR
þ 6.5
þ 8.6
þ 4.8
þ 2.4
þ 3.5
þ 25.8
Net forcing
þ 2.4
7.0
þ 1.1
7.5
16.7
27.7
The overall effect of cloudiness is production of a net cooling effect. As M&M
pointed out, even if these values are only approximate, they show that changes in
cloud cover can affect climate significantly.
If there are forces acting on the Earth that produce quasi-periodic changes in
cloud cover, these forces could produce periods of glaciation interspersed by
deglaciation periods. Three such models are discussed briefly in the following
sections. One model has to do with interplanetary dust affecting stratospheric
conductivity and, thereby, affecting cloud formation via a complex process.
Another is based on quasi-periodic changes in cosmic ray penetration of the
Earth's atmosphere affecting cloud formation. A third model is based on time lags
in the interaction between the ocean and the atmosphere, leading to quasi-periodic
changes in cloud cover. None of these models are very convincing to this writer,
but as M&M said: ''The atmosphere, especially the upper atmosphere, is not a
well understood system, so even the craziest ideas may prove to be right—and
maybe even important.''
8.7.1 Extraterrestrial dust accretion
M&M provided an extensive discussion of interplanetary dust centered on the
ecliptic plane of planets' orbits. This dust is probably the result of collisions
between asteroids. The main bulk of the dust lies within 5 of the ecliptic but
can reach 15 from the ecliptic plane. The distribution of dust around the Earth
is highly non-uniform. As the Earth traverses through space, its orbital parameters
vary, the tilt of the Earth's orbital plane wobbles relative to the ecliptic, and the
Earth intercepts varying amounts of this interplanetary dust. Calculation of this
variability is complex. M&M claim that it follows a cycle with a period of 100,000
years. Dust particles that impinge on the Earth's atmosphere tend to vaporize and
recondense as very tiny particles called ''smoke.''
M&M estimated the reduction in solar intensity reaching the Earth due to
reflection from high-altitude dust. They found that the density levels were too low
 
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