Climate feedbacksWe talked briefly about the positiv

Climate �feedbacks�
We talked briefly about the positive feedback processes of
climate
change in previous lectures. What is “feedback”?
Feedback is a concept that explains the interaction of the
climate
system that alters changes in climate. When the rate of climate
change
is amplified (either by warming or cooling), the process is
called
“positive feedback”. The upper figure demonstrates the basic
way that
these feedbacks operate.
On the other hand, when the rate of climate change is
suppressed, then
the process is called “negative feedback” (lower figure).
Primary Climate System Feedbacks
• Radiation feedback (hotter planet radiates
more energy out to space, E=sT4)
• Snow/ice-albedo feedback
• Water Vapor feedback

• Cloud feedback (high versus low clouds)
So, climate feedbacks are a loop of cause and effect; positive
(amplifier) and
negative feedbacks (stabilizer). Some feedback processes are
more
complicated than others. Here are a few important feedbacks
that affect our
climate system.
Temperatureà radiation feedback
Energy emitted = σT4
éTemperature
éradiation to
space
éCO2
êTemperature
The temperature of the Earth is increasing due to a rise in
greenhouse gases in
the atmosphere. Thus, how will the climate feedback system
change with this
temperature increase?
First, increases in temperature will alter radiation feedback

because the energy
emitted from a blackbody is proportionate to its temperature to
the fourth (σT4).
Feedback process: Increasing CO2 concentration in the
atmosphere – increasing
temperature – increasing associated energy radiation to space –
decreasing
temperature
Thus, increasing CO2 is a negative feedback process in the long
term. However,
this feedback process in the climate system is far more complex.
This is not the
only feedback loop that we know of.
Snow/sea ice albedo feedback
Melting of snow/sea ice directly affects the
albedo of the Earth (less ice = decrease in albedo)
Measuring Earth’s Albedo
https://earthobservatory.nasa.gov/IOTD/view.php
?id=84499
https://earthobservatory.nasa.gov/IOTD/view.php?id=84499
Also, we have seen how
recent warming has
been impacting the
arctic sea ice (see the
following two slides)

Polar amplification!
Global temperature departures from average
during January through May 2020, compared
with a 1951-1980 average. (Berkeley Earth).
Greater climate change observed near the pole responds to
changes in the
radiation balance (e.g. intensified greenhouse effect). This
phenomenon is
known as “polar amplification”.
Melting sea ice in the Arctic decreases the Earth’s albedo.
Changes in albedo are
likely contributing to significant temperature increases in the
northern
hemisphere. The increase in surface temperature is observed
mainly in the
higher latitude in the northern hemisphere, where most sea ice
is, and where
there is a greater continental distribution (more continent is
located in the
northern hemisphere than in the southern hemisphere. continent
heat capacity
is lower than the water body – ocean).
Polar Amplification
“Over the past 100 years, it is possible (33-66% confidence)
that there has been polar amplification, however, over the
past 50 years it is probable (66-90% confidence)”

[The Arctic Climate Impacts Assessment (ACIA), 2005, p22]
Although polar amplification has been a known phenomenon for
over 100 years,
such amplification has been more and more prominent in the
recent past.
Further reading:
Polar amplification effect
http://ossfoundation.us/projects/environment/global -
warming/arctic-polar-
amplification-effect
IPCC AR5 report about polar regions
(https://unfccc.int/files/science/workstreams/research/applicatio
n/pdf/5_wgiar
5_hezel_sbsta40_short.pdf)
Snow/sea ice albedo feedback
éTemperature
êsnow and ice
éCO2
Melting of snow/sea ice directly affects the albedo of the Earth
(less ice =
decrease in albedo).

Feedback process: Increasing CO2 concentration –> increasing
temperature –>
melting snow/sea ice –> decreasing albedo –> less energy
reflected to the
space –> further increasing temperature
Water vapor feedback
http://water.usgs.gov/edu/watercyclecondensation.html
Clausius-Clapeyron relationship
Warm air holds
more water vapor!
NASA: Sea Surface Temperature vs Water Vapor
https://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MY
D28M&d2=MYDAL2_M_SKY_WV
http://www.atmo.arizona.edu/students/co
urselinks/fall16/atmo336/lectures/sec1/ev
ap_cond.html
https://earthobservatory.nasa.gov/GlobalMaps/view.php?d1=MY
D28M&d2=MYDAL2_M_SKY_WV
Clausius-Clapayron relationship is a way of characterizing
discontinuous
phase transition between two phases of a matter of a single
constituent. This

concept explains the relationship between the temperature and
water
vapor, which is by far the most concerning greenhouse gas in
Earth’s
atmosphere. This figure shows how the water-holding capacity
of the
atmosphere (water vapor pressure) increases by 8% per
temperature
increase in Celsius. Importantly, this relationship is mainly a
function of
temperature, and not directly dependent on other parameters
like pressure
or density.
What does this figure tell us?
“warm air hold more water vapor!”
Measures of Humidity
(a)
(b)
The same concept can be explained by “relative humidity” and
“water vapor
capacity”.
• Vapor pressure – contribution of water vapor to total
atmospheric pressure
• Humidity – amount of water vapor in the air
Imagine you have a balloon that is perfectly sealed. No air or
water vapor goes in

or out of this balloon. In this figure, “water vapor capacity (red
solid line)”
indicates that your balloon is “saturated (= relatively humidity
100%)” at the
temperature and the amount of water vapor that exists in the
balloon. Now, your
balloon is first saturated at 16 degree Celsius or 60 degree
Fahrenheit with 10g of
water vapor per cubic meter. How can you change the saturation
status? It is easy
– you just need to change its temperature! If you heat up your
balloon, for
instance, to 100F, your balloon will no longer be saturated (a).
Instead, to
saturate this warm balloon (100F to be exact), you need 4 times
more water
vapor (b)!
• Relative humidity – how close the air is to saturation
– Saturation represents the maximum amount of water
vapor the air can hold
– Saturation depends on temperature
– Saturation vapor pressure
In this figure, water vapor capacity is depicted in the yellow
circle. The
amount of water vapor in the atmosphere does not change
regardless of
the temperature (blue circle). Instead, water vapor capacity
increases with

increasing temperature. Therefore, relative humidity decreases
when you
increase the temperature.
• Temperature and relative humidity are inversely related
This relationship is rather obvious if you plot the typical hourly
temperature
with the relative humidity for 24 hours. Temperature increases
in the morning
at around 8 am. You observe the highest temperature of the day
in the
afternoon. The temperature decreases when the sun sets. As you
can see,
relative humidity is almost a mirror image of temperature. The
relative
humidity is highest in the early morning when the temperature
is the lowest,
and is at its minimum when the temperature is at the highest of
the day.
• Temperature and relative
humidity are inversely
related
• Dew point temperature

You must have seen dew in the grass or a windshield in the
early morning, when
the temperature is lowest. This is because the air becomes
saturated and the
excess amount of water is condensed to form moisture! When
the temperature is
close to freezing level, the dew turns into frost. Both are
exactly the same
phenomenon.
Dew point temperature—the critical air temperature at which
saturation is
reached.
Also, when warm air rises, the temperature decreases
adiabatically. At some
point, the air becomes saturated, and the excess amount of water
is
condensed. This is called cloud!
Water vapor feedback
ñTemperature
ñH2O vapor
ñCO2

Feedback process: Increasing CO2 concentration – increasing
temperature – high
temperature can hold more H2O vapor (which is a greenhouse
gas!) – further
increasing temperature
Studies show that water vapor feedback roughly doubles the
amount of warming
caused by CO2!
Further reading:
https://www.skepticalscience.com/water-vapor-greenhouse-
gas.htm
Cloud Feedback
Cloud feedback is the coupling between cloudiness and surface
air
temperature in which a change in radiative forcing perturbs the
surface air
temperature, leading to a change in clouds, which could then
amplify or
diminish the initial temperature perturbation.
Cloud feedbacks are more complicated
éTemperature

éclouds
éCO2
or
êTemperature
Feedback process:
Increasing CO2 concentration –> increasing temperature –>
enhance cloud
formation (due to enhanced evaporation from the ocean) –>
clouds emit
infrared radiation back to the Earth’s surface (positive
feedback)
Or
- cloud reflects sunlight (negative feedback)
Condensation
• Conversion of vapor to
liquid water
• Surface tension makes it nearly
impossible to grow pure water
droplets
• Need supersaturated air
• Need particles to grow droplets
around, a cloud condensation

nuclei
• Liquid water can persist at
temperatures colder than 0�C
without a nuclei – supercooled
How big does a rain drop need to be to reach
Earth without evaporating?
The drop would have to be approximately .2 mm
or larger in diameter. Typical rain drops are 2
mm in diameter.
• Lifting condensation
level (LCL)
Adiabatic Processes
Large masses of air can be cooled to the dew point ONLY by
expanding as they
rise. Because of this limitation, adiabatic cooling is the only
prominent
mechanism for development of clouds and production of rain.
When warm air rises, it cools down. This is called adiabatic
cooling. When the
air cools, it holds less moisture (capacity decreases). As a
result, relative
humidity increases. The altitude at which air becomes saturated
(100%

relative humidity) is called lifting condensation level (LCL).
Perhaps you have seen clouds like those shown in the slide –
tall puffy clouds
with a flat bottom. This happens because rising warm air
continuously brings
moisture to higher altitudes and, at a given point, air becomes
saturated
(LCL). Clouds will form above the LCL.
Lenticular clouds
Examples of cloud formation
due to atmospheric lifting!
– Cirrus clouds
– Cumulus clouds
– Stratus clouds
Clouds
Not all clouds precipitate, but all precipitation comes from
clouds!
The Oxford English Dictionary:
(Cloud is) "a visible mass of condensed
watery vapor floating in the air at
some considerable height above the
general surface of the ground."
At any given time, about 50 percent of Earth is covered by

clouds. Clouds play an
important role in the global energy budget.
• Cloud types
– High clouds (over 6 km)
– Middle clouds (from 2 to
6 km)
– Low clouds (less than
2 km)
– Clouds of vertical
development
• Grow upward from
low bases to heights
of over 15 km
occasionally
Cloud Families
Cloud categories are largely based on altitude:
• High clouds—Altocumulus clouds—found above 6 kilometers
(i.e., cirrus
clouds)
• Middle clouds—between about 2 and 6 kilometers (i.e.,
altocumulus and alto
stratus).
• Low clouds—below 2 kilometers (i.e., stratocumulus and

nimbostratus).
• Clouds with vertical development (i.e., cumulus clouds).
Clouds
– Cirrus clouds (high clouds)
Feathery appearance.
Cirrus: Detached clouds in the form of white, delicate filaments,
mostly
white patches or narrow bands. These clouds may have a fibrous
(hair-
like) and/or silky sheen appearance. Although cirrus clouds may
look less
dense, considering that they form in the high altitude, they are
always
composed of ice crystals. Since ice crystals are a blackbody that
absorb
and re-radiate outgoing infrared radiation, having more cirrus
clouds
contribute to warming (positive feedback)!
– Cumulus clouds (middle to low
clouds)
Puffy white cloud that forms from
rising columns of air.
– Stratus clouds (low clouds)
Low clouds, usually below 6500

feet, that sometimes occur as
individual clouds but more often
appear as a general overcast.
Clouds
Cumulus: Detached, generally dense clouds and with sharp
outlines that
develop vertically in the form of rising mounds, domes or
towers with
bulging upper parts often resembling a cauliflower. The sunlit
parts of these
clouds are mostly brilliant white while their bases are relatively
dark and
horizontal. Precipitation of showers or snow may be associated
with cumulus
clouds.
Stratus: A generally gray cloud layer with a uniform base which
may, if thick
enough, produce drizzle, ice prisms, or snow grains. When the
sun is visible
through this cloud, its outline is clearly discernible. Often when
a layer of
stratus breaks up and dissipates blue sky is seen. We also call
stratus clouds
as overcast.
Both cumulus and stratus clouds are middle to low clouds and
can block sun
light from reaching the ground. (Imagine an overcast day. You
will feel cold
because there is less energy from the sun on ground.) With this,
having more

cumulus and stratus clouds contribute to a cooling effect
(negative
feedback)!
Contrails: Man-made clouds
Jet contrails = condensation trails caused by the exhaust from
airplanes that
contain water vapor, and are not much different from natural
clouds. If the air is
very cold (which it often is at high altitudes), then the water
vapor in the exhaust
will condense out into what is essentially a cirrus cloud.
Sailors have known for some time to look specifically at the
patterns and
persistence of jet contrails for weather forecasting. On days
where the contrails
disappear quickly or don't even form, they can expect
continuing good weather.
While on days where they persist, a change in the weather
pattern may be
expected.
Contrails: Man-made clouds
http://www.wrh.noaa.gov/fgz/science/contrail.php?wfo=fgz

If contrails persist for a long enough period of time, they can
spread out
across the sky due to the prevailing winds at the level at which
they
formed. The two figures show how contrails generated on this
particular
day spread out fairly quickly due to the stronger jet stream of
air aloft.
Persistence of contrails is neither an indication that they contain
some kind
of chemical, nor that it is some kind of spray. It is simply an
atmospheric
condition.
Contrails are a concern in climate studies as increased jet traffic
may result
in an increase in cloud cover (specifically, cirrus cloud
coverage). It has been
estimated that in certain heavy air-traffic corridors, cloud cover
has
increased by as much as 20 percent. However, the world’s goals
for
reducing aircraft emissions are still unclear as strategies vary
by nation.

Below, I am sharing an interesting and informative reading
about the
unknowns of contrails and the complicating relationship with jet
fuel
exhaust.
Greening the Friendly Skies
By Mark Betancourt, 4 November 2020
https://eos.org/features/greening-the-friendly-
skies?mkt_tok=eyJpIjoiTkRZeU1XWm1ObUZpWkdZMCIsInQi
OiIzeE9BNGxoaD
gzWFYrMVhiaGkrQ0s3YlVuMU9STHo2XC9GNlZwWlM2NWl
Mb1NZQmxYcmlw
TnFPV1lrbGxjOWx1Mnk5QlJQb0dJUzRBK1BLNVVkZW5Ndm
ZwMFk0UldBaTB
BM3lYTTJtc1lYSDJnWG5Tdjk3d3RscVRcL1wvTjU5dWt2dyJ9
The Coming Surge of Rocket Emissions
The launch plume from a test missile, photographed on 10
October 2013 by
astronaut Luca Parmitano, diffuses into the middle and upper
atmosphere
during the first several minutes after launch. As the number of
rocket
launches increases in the future, rocket engine emissions will

increase
proportionally. Credit: © European Space Agency/NASA
By Martin N. Ross and Darin W. Toohey 24 September 2019,
EOS
Due to the unique nature of the
combustion chemistry, it turns
out that rocket engines emit
even larger amounts of black
carbon than a modern jet
engine. This means that it is
more potent than contrails!
“With 114 launches in 2018, the
number of launches has been
growing at an average rate of
about 8% per year for the past
decade. Rocket emissions have
also been growing.” (EOS)
https://www.flickr.com/photos/volaremission/10202754596/in/p
hotolist-eHHBQP-gxzMu1-fdh3FL
mailto:[email protected]
Cloud feedbacks are more complicated
Because cloud feedbacks are more complicated than other
feedbacks, it is likely that it
causes uncertainty in climate predictions.
éTemperature

éclouds
éCO2
or
êTemperature

Climate feedbacksWe talked briefly about the positiv

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