Clouds, Cloudiness, Surface Temperature and Global Climate Change
http://www.applet-magic.com/cloudinesspod.htm
applet-magic.com
Thayer Watkins
Silicon Valley
& Tornado Alley
USA
Clouds, Cloudiness, Surface Temperature
and Global Climate Change
The Net Effect of Cloudiness on Surface Temperatures
The greenhouse effect is not only produced by the greenhouse gases, clouds absorb long
wavelength radiation from the surface of the Earth and radiate some of it back down. In
addition to this absorption and re-radiation of long wavelength radiation from the Earth's
surface they may simply reflect it back to the surface.
Clouds also have a major role in reflecting some of the Sun's short wavelength radiation back
into space. Thus clouds share a role with the greenhouse gases and also share a role with the
ice and snow fields of the high latitudes. Water in its three forms as vapor, droplets of liquid,
and particles of ice is the overwhelmingly dominant substance in Earth's climate.
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The effects of cloud cover on temperature is a familiar experience. Without a cloud cover in
an area the temperature drops sharply at night whereas with clouds the temperature drop is
noticably more moderate. On the other hand in the daytime in the summer with no clouds the
temperature goes much higher than it does when there is a cloud cover.
The effect of clouds on surface temperature is the net effect of their reflecting sunlight and
their greenhouse effect of absorbing and reradiating downward the thermal radiation of the
Earth's surface. At night the reflection effect is zero so the greenhouse effect dominates and
thus clouds have a warming effect. In the daytime the reflection effect can dominate the
greenhouse effect and thus clouds could have a net cooling effect. The cooling effect of a cloud
shadow is familar to everyone.
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An even more homey illustration is the effect of a hat or cap on head temperature. A head
covering keeps in the body heat. It has essentially a greenhouse effect. But despite that
greenhouse effect, in the bright sunlight one is cooler with a hat or cap than without one, as
one can see by the amount of sweat produced. Of course, in the shade the reverse is true.
The effect of clouds in the daytime depends upon cloud type and their height. Thin clouds
reflect less sunlight so their net effect may be a slight net warming. The thick, puffy,
beautifully white cumulus are highly reflective and so they have a net cooling effect in the
daytime but a net warming at night. The dark rain-laden clouds are not reflective but they
nevertheless intercept nearly all of the Sun's radiation and prevent it from reaching the
surface. The dark clouds themselves would be warmed by the absorbed radiation and some
of absorbed energy would be re-radiated toward the surface. The dark clouds could be net
warmers or net coolers depending upon conditions but the general perception is that dark
clouds are net coolers in the daytime.
The effects of cloudiness on surface temperature as a function of cloud type are summarized
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Clouds, Cloudiness, Surface Temperature and Global Climate Change
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below:
Net Effect on Surface Temperature
of Various Cloud Types by Time of Day
Thick white Thick dark Thin
Daytime
cooling
cooling
warming
warming
warming
warming
Night-time
The effect of the thick clouds would shift from net cooling to net warming as the Sun's angle
changes during the day.
Cloudiness and the Climate Models
The climate models used by the Intergovernmental Panel on Climate Change (IPCC) are not
very good at replicating the current cloudiness as seen from the following diagram. The black
line is the observed values and the colored lines are for the various climate models used by
IPCC.
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Source: IPCC, Third Asssessment Report: Climate Change, 2001
This is the observed latitudinal profile of the proportion of cloudiness during the Northern
Hemispheric winter (DJF).
J.R. Houghton expressed the situation as follows:
Clouds are, in fact, probably the dominant influence in the
radiative budget of the lower atmosphere but adequately
taking them into account raises many problems […]
The Physics of Atmospheres, p. 41.
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It is perplexing that the models do so poorly at replicating the current cloudiness
characteristics yet they are supposed to be more accurate at replicating the latitude profile of
temperature as shown in the following graph.
Source: IPCC, Third Asssessment Report: Climate Change, 2001
The answer to how the 15 IPCC climate models can appear to do so well in reproducing the
current latitudinal profile on temperature and do so poorly for the latitudinal profile of cloud
cover involves two parts. One is that many of the IPCC climate models have what is called a
flux adjustment, which cynics call a fudge factor. If a climate model get the right general shape
then the flux adjustment can be adjusted to move the graph profile up or down to make it fit
the actual profile more closely. That is the case with the temperature results. For the cloud
cover results the models do not get the right shape so moving the graph of the results up or
down down does not improve the fit significantly. The second part of the explanation is that
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the climate models deal in terms of energy flows. The role of temperature is to determine the
thermal radiation from the Earth. This radiation is proportional to the fourth power of the
absolute temperature. This fourth power relationship means that temperature is the fourth
root of the thermal radiation flow and thus if the radiation flow is in error by 20 percent the
temperature is in error by only one fourth this much or five percent. Thus a model could be
in error by a whopping 40 percent and yet the temperature seems to have a modest error of
only ten percent.
The role of cloud cover in the determining albedo is illustrated as follows. Albedo is the
proportion of incoming radiation that is reflected from surface back to where it came from.
The average albedo for the Earth is about 37 percent. The albedo of an area depends upon
the nature of its surface and the angle of inclination of the Sun. Snow has a high albedo and
black earth has a low albedo. White clouds have an albedo comparable to a snow field as
airplane passengers often observe.
If a surface area has an albedo of 10 percent and clouds an albedo of 90 percent then with 70
percent cloudiness the albedo for the area is 0.7(0.9)+0.3(0.1)=0.66=66 percent. This means
that 34 percent of the Sun's radiation for the area is not reflected back into space. If a climate
model computes the cloudiness to be 40 percent then the albedo is 0.4(0.9)+(0.6)(0.1)=0.42=42
percent. This means the model would be using a solar input of energy to the area that was
42/66=0.636 of the actual, or roughly 64 percent. It is at first hard to see how the model could
have an accurate computation of the temperature if the energy input was so far off. The
answer as previously noted is that the temperature depends upon the fourth root of the
energy input. The fourth root of 0.636 is about 0.893 or roughly 0.9, so the error in
temperature would be about 10 percent. Thus an error of only about 10 percent in
temperature corresponds to an error of 34.4 percent in energy flow.
There is vastly more of the Sun's radiation reflected from clouds than from the polar ice caps.
This is because there is vastly more area covered by clouds outside of the polar regions than
the ice caps. It is also because angle of incidence in the polar regions is so low compared with
the other regions of the world.
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The situation is shown below for the Northern Hemispheric winter (DJF).
The area of the Earth's total surface is about 718 million square kilometers. The area covered
by clouds during December, January and February is 453 million square kilometers. The
total amount of sea ice in the polar regions during that time of the year is about 17 million
square kilometers. However much of this sea ice is redundant as far reflectance of the Sun's
radiation is concerned because there are clouds above it. The cloud coverage in the Arctic is
about 60 percent in the winter so only 40 percent of the 15 million square kilometers of sea ice
is reflecting the Sun's radiation. The cloud coverage in the Antarctic in December, January
and February is about 10 percent so 1.8 million square kilometers of the 2 million is effective.
Thus the sea ice area that is effective in reflecting the Sun's radiation is 7.8 million square
kilometers. This 7.8 million square kilometers of effective reflective sea ice is about 1.7 percent
of the cloud coverage. Thus a 1.8 percent increase in cloud coverage would more than replace
the total loss of all the polar sea ice. It would take only 0.66 of 1 percent increase in global
cloud coverage to replace a 3 million square kilometer loss of Arctic sea ice. Actually the polar
sea ice is even less important to Earth's energy budget than the above computation indicate.
The amount of energy reflected in the polar regions is much less because the angle of the Sun.
The figure below illustrates this point.
This gaph uses some rough approximations to obtain an order of magnitude figure for the
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http://www.applet-magic.com/cloudinesspod.htm
importance of the polar ice and snow field to Earth's energy budget. For this graph the
albedo of clouds is taken to be 0.8. This same value is used for ice and snow fields. The ice and
snow fields are presumed to cover the Earth from 67.5° of latitude to the poles. The albedo of
rest of the Earth is taken to be 0.2. In the graph the reflectance is shown in arbitrary units.
Using the above stated values for the albedos and snow and ice coverage, total disappearance
of the snow and ice fields would reduce the Earth's reflectance by only 5 percent. This is
without any change in clouds.
Seiji Kato and his associates at the Langley Center of NASA published in 2006 the results of
an investigation of the effect of decreasing sea ice in the Arctic on the amount of radiant
energy reflected from the Arctic. This study concluded that although there was a decrease in
sea ice in recent years there was an increase in cloudiness that more than made up for the loss
of albedo from the sea ice. Thus there was not only no ice-albedo positive feedback presumed
by climate modelers, there was in fact a negative feedback. Kato's result illustates the
admonition that in climatology every theory has to be checked empirically.
A small change in cloudiness over the rest of the Earth's surface can be far more important
than major changes in the area of the ice caps. It is important to keep such things in
perspective. Climate modelers have a distinct tendency to focus on a sensational minor topic
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Clouds, Cloudiness, Surface Temperature and Global Climate Change
http://www.applet-magic.com/cloudinesspod.htm
while neglecting the major topics of climate. Clouds and cloudiness are the major factors in
the Earth's climate. Clouds rule the Earth's climate. Everything else, including the
atmospheric greenhouse gases, is marginal.
A very interesting and important discovery along these lines was made by Richard Lindzen
and his group at M.I.T. They found that in the central Pacific region when the sea surface
temperature rises there is less cirrus cloud cover and thus more energy radiates out into
space. This thermal vent is a negative feedback in the Earth's climate system and one that is
not incorporated in the computer climate models used to project global warming. It is
estimated that the mid-Pacific thermal vent would reduce by two thirds the projected global
temperature increases. Once again it is a matter of the cloud system ruling the Earth's climate
system. For more on this see Mid-Pacific Thermal Vent.
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