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Clouds and Climate
So the planetary system of clouds interacts
strongly with the radiation passing both
upward and downward through the atmosphere. Though the atmosphere itself lets
sunlight pass through, clouds reflect significant
portions of that light back to space. This reflective cooling, operating by itself, lowers the
planet’s surface temperature by more than
20°C. A blanket of clouds can also introduce
warming at the surface by blocking the passage
of infrared radiation (heat) from the ground.
High, thin clouds are the most effective heaters.
Lack of reliable data on the heating and
cooling by clouds has long hindered the study
of climate and climate change. Recently,
however, the heating and cooling by clouds
was measured by V. Ramanathan (now at the
Scripps Institution of Oceanography) and
colleagues at the University of Chicago with
instruments aboard the Earth Radiation Budget
Satellite. They found that the reflective “white
clothes” cooling of clouds was about 50%
greater than their “blanket warming” effect.
It appears, then, that the net effect of Earth’s
clouds is to cool the planet.
Earth’s clouds are so deeply interwoven into
the climate system that they cannot be ignored
in even the most rudimentary climate models.
But where they fit in is the question. We know
they have both cooling and heating effects.
How do these effects balance?
A glance at the cloud-covered planet from
space illustrates how clouds brighten, and
therefore cool, the planetary environment by
reflecting sunlight. If we could somehow
cleanse the atmosphere of all cloud condensation nuclei and prevent the formation of clouds,
the reflectivity, or albedo, of the cloudless
planet would drop from its present value of
30% to around 10%. That would mean an
increase in the planet’s temperature. It is even
possible that the oceans might evaporate if
clouds went away.
But clouds also warm the Earth by holding
in outgoing long-wave (infrared) radiation. We
have already noted how, as farmers know, a
cloud layer is an excellent preventative for frost.
In deserts, though solar heating during the day
might be very extreme, loss of infrared radiation to the cloudless star-filled skies can lead to
surprisingly cool nights.
Now consider the opposite: a hypothetical
Earth cloaked everywhere with a uniform, thin,
high-altitude cloud deck. Let us assume that
these clouds have low reflectivity, so that the
planetary albedo is unchanged from its present
value of 30%. (The reflectivity of clouds is
frequently higher than 30%, but here we wish to
concentrate on their heating effect.) On this
cloud-covered planet the surface temperature
would be 27° C (49° F) higher than in the real
world. It would be a hot planet indeed!
Changes in Earth’s Climate from
Clouds and from CO2
Changes in climate from pollution or natural
causes are sometimes expressed in terms of the
heating of the Earth system that would attend a
doubling of carbon dioxide in the atmosphere.
Our burning of wood, coal, gas, and oil will
almost certainly cause a doubling of atmospheric CO2 over its 1950 levels sometime in
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CLOUDS AND CLIMATE CHANGE
the middle of the next century, and that will
cause a change in climate.
We know that, on the average, clouds cool
the Earth. Satellite measurements of cloud
radiation indicate that the cloud cooling effect is
about four times as large as the estimated
heating introduced by the doubling of atmospheric CO2. So small alterations in the way
clouds and radiation interact can play significant roles as feedback mechanisms.
a certain point, the thermostat senses it and
turns on the furnace, the temperature rises, and
when the thermostat again senses a temperature threshold, the furnace shuts off. It has even
been suggested by the British naturalist James
Lovelock that the planet has evolved feedback
loops to stabilize its climate system. This socalled Gaia Hypothesis is described in greater
detail later.
Figure 8 is a wiring diagram for a simple
feedback control system. Each block represents
a component or machine in the system. Part of
the output of the second block is fed back
through block B to provide a transformed
signal that is added to or subtracted from the
system’s input. The important point is that the
system’s output affects the input.
Feedback systems have some interesting and
frequently surprising characteristics. The way
the system behaves depends on whether the
output from Block B is added to (positive
feedback) or subtracted from (negative feedback) the input. Systems with negative feedback tend to be sluggish and respond more
slowly to input than do systems with positive
feedback. Positive feedback, on the other hand,
is associated with amplification, rapid response,
even overshooting and instability. Some
Cloud/Climate Feedbacks
Scientists believe that during past ice ages
cool oceanic currents flowing toward the
equator caused a migration toward the tropics
of cooling clouds like the oceanic clouds
mentioned above and that this might well have
amplified the general cooling trend. This is an
example of positive feedback. There are numerous positive and negative feedbacks in the
planetary climate system. This section discusses
some of those involving clouds.
Feedback Loops in General
A feedback is a process that responds to a
system change by enhancing or diminishing the
change. When a fraction of the output from a
system feeds back to the system, it either speeds
it up (positive feedback) or slows it down
(negative feedback). Feedback loops are ubiquitous in nature. The concept lies at the heart of
all biological systems. Any time a living organism causes a change in its environment and
then reacts to that change there is feedback. A
dog is following a trail. If he deviates from the
trail, the odor decreases (negative feedback),
causing him to adjust his direction until the
odor becomes stronger.
Feedback control systems also play important roles in such human enterprises as engineering, economics, sociology, and political
science. A household heating system is a
feedback system. The temperature drops to
ELEMENTARY FEEDBACK
CONTROL SYSTEM
B
±
Input
A
Output
Figure 8. General block diagram of a feedback loop. Box A
is the forward, or main, transfer function; block B is the
feedback transfer function. The feedback system’s input is
to the left, and its output is on the right.
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CLOUDS AND CLIMATE
Center for Atmospheric Research in Boulder,
Colorado, using global climate models called
general circulation models (GCMs). These
simulations have suggested that the central
regions of North America may experience
severe drying when atmospheric carbon
dioxide doubles. This would decrease cloudiness, which would lead to increased solar
heating on the soil of the Great Plains, amplifying the tendency toward drying.
systems exhibit both negative and positive
feedback and can be more responsive to some
kinds of inputs than others.
Feedback Loops in Climate
Some of the feedback loops in the control
system of Earth’s climate machine are:
1. Ice/albedo feedback. As planetary temperature falls, ice sheets and oceanic pack ice build.
The bright ice reflects more solar radiation back
into space, and the cooling is amplified.
5. Cloud height and amount/temperature
feedback. Computer simulations show that
increases in greenhouse gases cause clouds to
be higher and deeper, especially in the tropics.
Surprisingly, there tends also to be a reduction
in cloud cover, leading to greater heating by
sunshine and to higher clouds, which also heat
the Earth. Some scientists have estimated that
these two feedbacks together provide a slight
net positive feedback, making the Earth slightly
warmer than it would otherwise be.
2. Water vapor/temperature feedback. This is
another positive feedback loop. We learned
earlier in this module that the saturation vapor
pressure of water increases with temperature.
Water vapor, like carbon dioxide, is a greenhouse gas, absorbing infrared radiation. Increasing global temperatures cause additional
water to evaporate, and the water vapor
content of the atmosphere rises. The greenhouse effect of the increased water vapor “feeds
back,” and amplifies, the warming, by holding
heat.
6. Cloud wetness/temperature feedback. Global
warming will lead to increased atmospheric
water vapor, so one might expect the volume of
clouds to increase accordingly. Larger clouds
are brighter, so they would reflect more solar
radiation and counter the warming trend, a
negative feedback.
3. Ocean temperature/cloud feedback. During
the ice ages, low clouds and midlatitude
cyclonic clouds over oceans may have been
driven toward the equator by cool oceanic
currents. During the height of the last ice age,
the midlatitude Atlantic Ocean was 5° to 10°C
cooler than at present. The new cloud banks
over the cooler oceans would have amplified
the cooling tendency (positive feedback) in a
way similar to the ice/albedo feedback. If the
positive feedback is strong enough, there is
the possibility of runaway; fortunately, in this
case, there were apparently negative feedbacks
that prevented the ice age from amplifying
endlessly.
The cloud–climate control system for Earth,
with some of the feedback loops mentioned
above, is outlined in Figure 9. The strength of
the feedbacks is uncertain. In a recent review
paper on greenhouse warming, John Mitchell,
of the United Kingdom Meteorological Office in
Bracknell, England, says that the formation of
clouds and their radiative properties depend on
many small-scale processes that cannot be
determined in large-scale models. Current
models have only extremely crude representations of clouds, cloud water, and cloud radiative properties, and this is one of the largest
sources of uncertainty in scientists’ attempts
to simulate Earth’s climate. Resolving this
4. Drought/cloud feedback. Scientists carry out
climate simulations on large computers like the
CRAY Y-MP supercomputer at the National
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CLOUDS AND CLIMATE CHANGE
uncertainty has become the number one priority for research in the area of global change.
processes. The sulfur gases spread throughout
the lower part of the atmosphere in the form of
dimethylsulphide (DMS), which provides cloud
condensation nuclei. Charlson and colleagues
suggest that as the planetary temperature alters,
the production of DMS and, therefore, cloud
droplet concentration, would also change. A
rising temperature means more DMS, which
means more reflective clouds, which, according
to Ramanathan, tends to cool the planet. The
result would be to dampen out swings in
planetary temperature.
A Gaia Feedback Loop
Robert Charlson, a chemist and cloud
physicist at the University of Washington, and
colleagues have suggested a possible feedback
loop that brings Earth’s biosphere into the
climate picture. Tiny organisms in the sea,
phytoplankton, produce sulfur-containing
gases as a byproduct of their physiological
THE CLIMATE SYSTEM
+
+
B
Ice Albedo
0.4 W m-2/°C
C
H2O VaporTemperature
1.5 W m-2/°C
A
Forward Transfer
Fluctuation
in Solar
Radiation
+
–
D
Cloud Height-Temperature
Cloud Fraction-Temperature
E
Cloud MicrophysicsTemperature
0.27 °C/W m-2
Fluctuation in
Surface Temperature
0.9 W m-2/°C
0 to -1.9 W m-2/°C
Figure 9. A diagram showing four climate feedback loops: ice/albedo, water
vapor/temperature, cloud height/temperature, and cloud microphysics.
“A” in the center represents transfer of solar radiation out of the system.
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