Chapter 5, What Holds the Atmosphere Up? Module Six
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- Chapter 5, What Holds the Atmosphere Up? Module Six
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- How the greenhouse effect works within the temperature
structure of Earths atmosphere The greenhouse effect is powered by
the lapse rate Atmospheric scientists call the change in
temperature of the air with altitude the lapse rate It is about 6C
colder per kilometer of altitude The lower part of the atmosphere
is called the troposphere
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- Atmosphere The troposphere is the lower part of the atmosphere
It contains about 90% of the air It contains all of the weather The
boundary of low temperature is about 17 km high on average The
boundary where the air temperature reaches its coldest point is the
tropopause Commercial airplanes fly in the tropopause
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- Atmosphere with altitude
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- Atmospheric layers Troposphere about 10 km high, contains 90%
of air and all of the weather Tropopause boundary where air is the
coldest, commercial aircraft area Stratosphere air begins to warm
up because of ozone content Mesosphere not much effect on the
weather Exosphere ditto
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- No temperature contrast, no greenhouse effect Remember the
layer model with a skin temperature Think of the skin altitude for
the air column as some kind of average altitude from which the IR
escapes to space The idea of a skin layer in the atmosphere is
fuzzier than using a glass pane in the layer model but it still a
useful concept
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- Lapse rate vs. strength of GH effect If we increase the GHG
concentration of the atmosphere, the IR radiation to space will
originate from a higher altitude (skin altitude). The increase in
skin altitude increases the ground temperature. If the temperature
of the atmosphere was the same at all altitudes, then raising the
skin temperature would have no impact on the ground temperature.
More CO2 higher skin altitude warmer ground
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- Pressure as a function of altitude The pressure in the
atmosphere depends primarily on the weight of the air over your
head The weight of the overhead air at sea level is more than The
weight of the overhead air at the top of a mountain The pressure of
the air is non-linear with altitude (unlike scuba diving, where the
pressure is linear with depth)
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- Pressure is a non-linear (exponential) function
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- What to remember When a gas is depressurized (less pressure)
the gas expands When a gas expands, it cools When you pressurize a
gas it heats up
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- Expansion, Compression and Heat If we had a gas inside a
container with a piston, and pressurized the gas, it would heat up,
even in an insulated container with no heat entering or leaving A
closed system with no heat coming in or out is called adiabatic If
gas is compressed adiabatically, it warms up. It takes work to
compress a gas, the work energy is transferred to heat When it
expands, it cools, reversing the process and the gas cools
down
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- Water vapor and latent heat Remember chemistry and the phase
change diagram, where energy is added, the substance stayed at the
same temperature until it completely changed phase, solid to
liquid, or liquid to gas. The energy that was added is called
latent heat Latent heat of fusion between solid and liquid Latent
heat of vaporization between liquid and gas In one direction the
heat is added, in the other direction the heat is released.
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- Phase changes Solid + heat liquid (latent heat of fusion)
melting Liquid + heat gas (latent heat of vaporization) boiling
When the phase change goes in the other direction, the same amount
of energy is released during condensation or freezing Vapor liquid
+ heat released Liquid solid + heat released
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- Latent heat You charge up an air parcel with latent heat when
you evaporate water into it (vapor contains the latent heat- not
sensible heat) You get the heat back when the water condenses and
the latent heat is released A thermometer does not measure latent
heat A thermometer measures sensible heat (what you can sense)
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- Equilibrium conditions When water is in equilibrium between
liquid and vapor, its called saturated, or 100% relative humidity,
and the equilibrium vapor pressure of water will be high.
Undersaturated occurs when it is cold, the amount of water vapor is
lower than the equilibrium value Supersaturated occurs when vapor
pressure is higher than equilibrium, and the vapor tends to
condense into precipitation
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- Convection Convection occurs when you heat a fluid from below
or cool it from above (either a liquid or a gas) Fluid expands as
temperature increases, density decreases Unstable condition causes
the fluid column to turn over Warm fluid rises to the top The
Atmosphere tends to mix when it convects
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- Air is compressible The air is not all the same temperature
Pressure is higher at the bottom because o f the weight of the air
column Compressed air at the bottom heats up Because the air is
well mixed, the moving air will always find itself at the same
temperature as the rest of the air in the column This is what
static stability looks like in a column of compressible air the
same temperature as the rest of the column
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- Convection in the atmosphere Driven by sunlight hitting the
ground Warms the air at the bottom of the column Warm air begins to
rise, as it rises, it expands, and cools While ascending, it
remains lighter and warmer than the air around it If it does not
mix on the way up, the air can get all the way to the top of the
column If it mixes on the way up, the whole column warms up
uniformly
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- Moist Convection The latent heat in water vapor drives most of
the drama in our weather
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- Moist convection Air at the surface of the Earth with a
relative humidity of 100% rises due to convection As the
temperature drops, the equilibrium amount of water vapor decreases
Supersaturation drives water to condense into droplets or ice The
story of cloud formation will continue in chapter 7
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- Water vapor It changes the temperature of the air It
systematically changes the lapse rate Dry convection has a lapse
rate of about 10C temperature change per km of altitude Add the
latent heat in moist convection, the lapse rate decreases to about
6C per km It is possible that the lapse rate of the atmosphere
could be different in a changing climate
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- Take home points, Chapter 5 Air in the upper troposphere is
colder than air at the ground because of the process of moist
convection. The process includes the following: Convection is
driven by sunlight heating the air near the ground The air rises
and cools because it expands Water vapor condenses, releasing heat
as the air rises
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- Continued The moist convecting air gets colder with altitude,
but not as much as if it were dry If the air did not get colder
with altitude at all, there would be no greenhouse effect
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- Revisit the layers of the atmosphere Troposphere Stratosphere
Mesosphere Entering outer space: Ionosphere Exosphere
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- Chapter 6, Weather and Climate How the Weather Affects the
Climate
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- Chaos 10 days is the limit for predicting weather because
weather is chaotic an extreme sensitivity to initial conditions, so
that small differences between two states tend to amplify, and the
states diverge from each other The butterfly effect, a puff of air
from a butterflys wing eventually resulting in a giant storm
somewhere that would not have happened if the butterfly had never
existed
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- Butterfly effect First observed in a weather simulation model
The model stopped running Edward Lorenz restarted it by typing in
the variables like temperature and wind speed He had small,
insignificant changes, such as rounding errors The model diverged
completely from the results of the initial simulation
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- Edward Norton Lorenz Mathematician Edward Norton Lorenz was an
American mathematician and meteorologist, and a pioneer of chaos
theory. He discovered the strange attractor notion and coined the
term butterfly effect. WikipediaWikipedia Born: May 23, 1917, West
Hartford, CT BornWest Hartford, CT Died: April 16, 2008, Cambridge,
MA DiedCambridge, MA Books: The essence of chaos BooksThe essence
of chaos Education: Massachusetts Institute of Technology,
Dartmouth College, Harvard University EducationMassachusetts
Institute of Technology Dartmouth CollegeHarvard University
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- Weather Forecasts rely on computer models Small imperfections
in the initial conditions and the model cause the model weather to
diverge from the real weather By about 10 days the prediction is
worthless To overcome the error, run the model may times with tiny
variation in initial conditions an ensemble of model runs
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- Climate Defined as some time average of the weather
Climatological January (or any other month) would be the average of
many Januaries The weather is chaotic, but the climate generally is
not The weather would predict rain on a particular day, whereas the
climatologist may predict a rainy season
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- Averaging Layer Model Real World Warm and cold Summers and
winters Day and night Completely balanced energy budget Averaging
is valid Some places much hotter Some place much colder Radiative
energy budget at some place could be wildly out of balance Will
averaging change the answer to something unreasonable?
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- Averaging a non-linear system Top panel averaging radiative
energy flux (S-B equation) over a large temperature range
introduces a large bias. Bottom panel over the temperature range of
normal Earth conditions, the blackbody radiation energy flux is
closer to linear, so averaging over a small range would be less of
a problem
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- The Fluctuating Heat Budget Not stable like a model, but
fluctuates widely Solar energy comes in only during the day
Sunshine varies seasonally and by location Infrared is radiated day
and night The energy budget is in balance over a 24 hour period But
at any time in any spot on the planet, the energy is usually out of
balance
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- Seasonal variations The seasons are caused by the tilt of the
Earth relative to its orbit around the sun, the obliquity
Wintertime, days are shorter and the sun is lower Added over a day
the winter hemisphere has less sunlight
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- Seasons are NOT caused by the Earths distance from the sun The
eccentricity cycle refers to the shape of the Earths orbit around
the sun It varies from elliptical, to circular Currently we are in
a near circular orbit The Earth is actually closer to the sun in
January than it is in July Seasons are not caused by proximity to
the sun
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- Earths seasons are caused by the tilt of the poles relative to
the orbit, and not by its distance to the Sun
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- Incoming flux depends on latitude and day of the year Northern
hemisphere summer is in the middle of the plot, which shows flux as
a function of latitude and time of the year.
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- Interesting to note from the plot Highest daily fluxes are at
the poles during the summer Poles get six months of sunlight Sun
whirls around in a circle above the horizon (not overhead) Why isnt
it a tropical garden in the summer?
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- Thermal Inertia Damps out the temperature swing between day and
night Damps out the temperature swing as the seasons change Even
damps out the temperature change of global warming
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- Oceans Has a tremendous capacity to absorb and release heat
from the atmosphere Land not so much diffusion through the soil is
slow and only affects the first meter or two Cool water surface
turns over and has convective mixing to about 100 meters Maritime
areas have milder seasons Middle of large continents have more
intense seasonal cycles
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- Averaging a seasonal cycle Out of balance because of the heat
distribution from the water and from the wind The outgoing heat in
the tropics cant keep up with the incoming solar radiation The heat
is carried to cooler, higher latitudes by water and winds The Earth
can vent the excess heat to space from the higher latitudes
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- Heat carried to higher latitudes for venting to space
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- The Coriolis Acceleration
http://www.youtube.com/watch?v=i2mec3vgeaI
http://www.youtube.com/watch?v=aeY9tY9vKgs
http://www.youtube.com/watch?v=iqpV1236_Q0 Two clips on the
Coriolis Effect and one shows a Foucault Pendulum, demonstrating
the rotation of the Earth.
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- Coriolis Effect The water and the air feel the most effect at
the poles (incredibly high tides in higher latitudes, nearly no
tide difference at the equator) At the equator there is no apparent
rotation The middle latitudes fall somewhere between these two
extremes
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- Modeling the Weather Fluids are governed by Newtons Laws of
Motion because fluid has mass and inertia Inertia is the
sluggishness of matter to resist changes in motion Tendency to keep
moving if its moving, or remain stationary if it is already
stationary To change speed or direction, motion requires a force
such as gravity or a change in pressure (weather)
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- Bathtub vs. Earth Bathtub flows more quickly than the Earth
rotates, so does not feel the Earths rotation Flows in the
atmosphere and ocean persist long enough to feel the effect of a
rotating Earth Ocean flows can be driven by friction with the wind
Coriolis acceleration tries to deflect the flow to the right in the
northern hemisphere After a few rotations, a steady state is
reached where the fluid flows 90 degrees to the wind
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- The eventual steady state Top the fluid initially flows in the
direction of the wind. Middle after a while the Coriolis force
swings the fluid to the right. Eventually, the fluid itself flows
90 degrees to the wind or pressure force, and the Coriolis force
just balances the wind or the pressure force. Bottom the steady
state where the flow stops changing and remains steady.
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- Geostrophic Flow In a rotating world the fluid will eventually
end up flowing completely crossways to the direction that Its
pushed. This condition is called geostrophic flow. A geostrophic
flow balances the forces on it against each other.
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- Geostrophic cells on weather maps Cells of high pressure and
low pressure with flow going around them Low pressure, pressure
force points inward, 90 to the right of that the winds flow
counterclockwise in the N. hemisphere cyclonic direction of flow
High pressure, pressure force points outward, and the flow is
clockwise around the high pressure anticyclonic direction
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- Surface wind field from a climate model (computer
generated)
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- Parameterization, assumptions in models Assume that cloud
formation is a function of humidity in the air, humidity is a
parameter that would control cloudiness Effects of turbulent mixing
Air-sea processes such as heat transfer Biology modeling
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- Take home points chapter 6 The energy budget to space of a
particular location on Earth is probably out of balance,
fluctuating through the daily and seasonal cycles and with the
weather, This is in contrast to the Layer Model. The annual average
energy budget for some location on Earth may not balance either,
because excess heat from the tropics is carried to high latitudes
by winds and ocean currents. The global warming forecast requires
simulating the effects of weather, which is a really difficult
computational challenge.
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- Chapter 7, Feedbacks Complexity in the Earth system arises form
the way pieces of it interact with each other
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- Positive and Negative Feedbacks A feedback is a loop of cause
and effect At the center of a feedback is a state variable (average
temperature of the Earth) A positive feedback makes the temperature
change larger than it would be without the feedback A negative
feedback counteracts some of the external forcing, and tends to
stabilize the state variable
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- Feedbacks: A positive feedback is an amplifier A negative
feedback is a stabilizer
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- Stefan-Boltzmann Feedback Negative feedback a stabilizer The
radiated infrared heat attempts to pull the temperature back
down
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- Ice Albedo Feedback Positive feedback an amplifier Ice albedo
feed works on the state variable of temperature. An input
perturbation, such as a rise GHG, drives temperature up. Ice melts,
reducing the albedo, and warming the ground up a bit. The direction
of the input and the feedback loop agree with each other. It can
also go in the other direction, perturbation cools things down and
feedback agrees.
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- Water Vapor Feedbacks Positive Negative Water is involved in a
positive feedback loop acting on global temperature Warming allows
more water to evaporate before it rains Water vapor is a GHG
Doubles the climate impact of rising CO2 concentrations Without the
water vapor feedback, climate would be less sensitive to CO2 There
is a negative feedback loop that controls the amount of water vapor
in the atmosphere at any given temperature, having to do with
rainfall and evaporation (the hydrological cycle)
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- At the center of the feedback loop is a state variable
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- Runaway Greenhouse Effect It is possible for the water-vapor
feedback to feed into itself Means the end of a planets water
Earths climate uses the high latitudes as cooling fins to avoid the
runaway greenhouse effect A runaway greenhouse effect stops if the
vapor concentration in the air reaches saturation with liquid water
or ice, so that any further evaporation would just lead to rainfall
or snow
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- Phase diagram shows that Venus had a runaway GH effect, but not
Earth and Mars Triple point of water Pressure: 0.006207 atm
Temperature: 0.01C (273.16 K)
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- Earth retained its water Earth has its water because of the
structure of the atmosphere The tropopause acts as a cold trap,
making sure that water vapor rains or snows out before getting too
close to space The oceans are protected by a thin layer of cold air
for billions of years now The Hadley circulation controls the
distribution of atmospheric water vapor warm air rises at the
equator, it cools and water condenses
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- Clouds Cirrus high altitude thin and wispy, barely noticeable,
and made of ice crystals Cumulus clouds storm clouds are towers,
the result of focused upward blasts of convection Stratus clouds
low clouds layered, formed by broad diffuse upward motion spread
out over large geographical areas
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- Clouds: Interfere with both incoming visible light, and
outgoing IR light In the IR, clouds act as blackbodies, warming the
planet Incoming visible light is reflected back to space, cooling
the planet The overall impact of a cloud depends on which of these
two effects is stronger, which in turn depends on what type of
cloud it is
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- Earths Energy Budget The difference between Earths energy
budget between absorbed and scattered sunlight is that when light
is scattered back to space, its energy is never converted to heat,
so it never enters into the planets heat budget
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- Clouds: Vary by meteorological conditions and human pollution
Cloud droplet size is an important factor The smaller the drop, the
better it scatters light Rain clouds look dark because they have
large droplets, and are optically thick Cloud droplets are affected
by cloud condensation nuclei (seeds) that help droplets form Sea
salt, pollen, dust, smoke, and sulfur compounds from
phytoplankton
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- Human footprints Sulfate aerosols from coal fired power plants
Internal combustion engines Forest fires, heating fires and cooking
fires Contrails (short for condensation trails) jet airplanes
passing through clean air containing water vapor Persistent
spreading contrails are thought to have a significant effect on
global climate
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- Generalities - You cant see through low clouds meaning they are
optically thick You can see through high clouds, optically thin
High clouds warm, low clouds cool Clouds that form in dirty air
tend to be better light scatterers with a higher albedo, cooling
the planet Clouds are the largest source of uncertainty in climate
models
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- Ocean Currents, el Nio climate oscillation Periodic flip flop
between two states of the ocean called el Nio and la Nia Ocean
interaction with the atmosphere, corresponding atmospheric cycle
called the Southern oscillation ENSO el Nio Southern Oscillation
The state of the ENSO affects climate patterns around the
world
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- El Nio climate oscillation La Nia El Nio Cool surface water
Productivity high Fisheries good Equatorial E W wind Tilted
thermocline Wetter weather Warm surface water Less fertile
Fisheries collapse Winds diminish Thermocline collapses Drier
weather
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- Meridional overturning circulation in the North Atlantic Gulf
stream carries warm water from tropics to the North Atlantic Water
cools and sinks, making more room for warm water Greenland ice
cores show instability in Meridional overturning synchronous with
large temperature swings (~ 10C) within a few years 8.2k event
(8200 years ago) catastrophic freshwater release to the North
Atlantic Circulation will slow down with melting ice
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- Terrestrial Biosphere Feedbacks Changes in vegetation could
alter the albedo of the land surface when ice melts Land surface
stores carbon Trees evaporate water through transpiration (a self-
replicating cycle) Droughts, vegetation dies, soil dries, and the
water shortage is a positive feedback
- Slide 73
- Carbon Cycle Feedbacks The subject of the next three chapters
(Module 7)
- Slide 74
- Feedbacks in the Paleoclimate Record Models tend to
under-predict the extremes of climate variation in the real world
climate The future may surprise us
- Slide 75
- Take home points chapter 7 Positive feedbacks act as amplifiers
of variability, whereas negative feedbacks act as stabilizers. The
water-vapor feedback doubles or triples the expected warming owing
to rising CO2 concentrations. The ice albedo feedback amplifies the
warming in high latitudes by a factor of three or four.
- Slide 76
- Continued Clouds have a potentially huge impact on climate.
Clouds are expected to exert an amplifying feedback to climate
warming, although the strength of this feedback is uncertain.
Clouds are the largest source of uncertainty in model estimates of
the climate sensitivity.