Mike Warner
Department of Atmospheric Sciences
Day 1: June 25, 2013
ATM S 111, Global Warming:
Understanding the Forecast
Outline
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How exactly the Sun heats the Earth
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How strong?
Important concept of “albedo”: reflectivity
How the greenhouse effect works
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How the Earth cools
And how greenhouse gases lead to less cooling
From Before We Asked…
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What factors influence climate at a given place?
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Sunshine (and latitude)
Topography/mountains
Proximity to oceans and large lakes
Ocean currents
Presence of trees/vegetation
Etc.
But what are the main factors that control the global
climate?
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We’ll study this next
The Sun
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Driver of everything in the climate system!
How Does Energy Arrive From the Sun?
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Energy from the Sun is “electromagnetic radiation”
or just “radiation” for short
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Goes through space at the speed of light
Radiation is absorbed or reflected once it gets to Earth
Radiation with shorter wavelengths are more
energetic
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And radiation is classified in terms of its wavelength
 This has long wavelength and low energy
 This has short wavelength and high energy
Types of Radiation
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Types of electromagnetic radiation, from most to least
powerful (or shortest wavelength to longest wavelength)
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Gamma rays
X-rays
Ultraviolet (UV) radiation
Visible light
Infrared radiation
Microwaves
Radio waves
Sun’s Radiation
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The Sun emits:
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Visible light (duh)
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Also “near infrared” radiation (infrared with very short
wavelength)
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A small (but dangerous) amount of ultraviolet radiation
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This is what makes us sunburn!
These three bands together we call “shortwave
radiation”
How Strong is the Sun?
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By the time it gets to the top of Earth’s atmosphere, the
Sun shines at a strength of 1366 Watts per square
meter
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Watt (abbreviated as W): unit of power or energy per
unit time
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1366 W/m2 is roughly what’s experienced in the tropics
when the sun is directly overhead
Average Solar Radiation
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The average incoming solar radiation is not 1366 W/m2
though
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It’s only 342 W/m2 (exactly ¼ of this). Why?
Half the
planet is
dark at all
times…
Here it’s
nighttime
High latitudes
get less direct
radiation, which
spreads out more
Reason for Seasons
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Directness of solar radiation is key for seasons as well
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Winter is tilted away from the Sun, gets less direct light, and thus
is colder
Winter solstice
December 21-22
Current sunlight
Equinox
March 20-21
or Sept 22-23
Summer solstice
June 20-21
When Solar Radiation Hits the Atmosphere
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Average incoming solar radiation = 342 W/m2
Only 20% gets absorbed in the atmosphere
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50% is absorbed at the surface
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This includes absorption of dangerous
UV by the ozone layer
Meaning much of the sunlight makes it
directly through the atmosphere!
30% is reflected back to space
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What does the reflecting?
Key Concept for Climate: Albedo
Albedo: fraction of incident light that’s
reflected away
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Albedo ranges from 0 to 1:
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0 = no reflection
1 = all reflection
Things that are white tend to reflect more
(high albedo)
Darker things absorb more radiation (low
albedo)
Albedo Values for Earth
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Clouds, ice, and snow have high albedo
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Cloud albedo varies
from 0.2 to 0.7
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Thicker clouds have
higher albedo (reflect
more)
Snow has albedo ranging
from 0.4 to 0.9
(depending on how old the snow is) and ice is approximately 0.4
Ocean is very dark (< 0.1), as are forests (0.15)
Desert has albedo of 0.3
Relative Contributions to Earth Albedo
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Remember we said 30% of incoming solar radiation is reflected
away?
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20% is from clouds
5% is by the surface
5% is by the atmosphere (things like dust from deserts and air
pollution are key players here)
Total Solar Input
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Total absorbed solar radiation is 70% of the incoming
solar radiation
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Because 30% is reflected away
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70% of 341 W/m2 = 240 W/m2
Summary So Far
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The Sun heats the Earth
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Some is reflected back, a bit is absorbed in the atmosphere
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But other than that, the atmosphere is pretty much transparent
when it comes to solar radiation (half is absorbed right at the
surface!)
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Clouds and snow/ice are primary contributors to the albedo of
Earth
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Next, how energy escapes from Earth and the greenhouse
effect
“Longwave Radiation”
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The Sun is the energy input to the climate system
How does the Earth lose energy?
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Turns out it’s also by radiation!
But it’s not visible light like
from the Sun, it’s infrared
radiation AKA “longwave
radiation”
Infrared satellite image 
“Longwave Radiation”
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Everything actually emits radiation
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Depends partly on the substance but mostly on temperature
Neck = hotter
Hair = colder
Infrared thermometer
Longwave Radiation
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Higher temperature means more radiation
Eyes and inner ears are warmest: they
radiate the most
Nose is the coldest: it radiates less
Thermal night vision technology detects longwave radiation
Temperature & Radiation
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Higher temperature = more radiation and more energetic
radiation (shorter wavelengths)
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Explains the Sun’s radiation too
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Sun is really hot 
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It emits much more radiation
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It emits shortwave radiation instead of longwave radiation like the
Earth
Energy Into and Out of the Earth
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Heating/cooling of Earth
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The Earth is heated by the Sun (shortwave radiation)
The Earth loses energy by longwave radiation (out to space)
“Energy Balance”
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If the energy into a system is greater than the energy
out, the temperature will increase
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A temperature increase then results in an increase of energy out
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Hotter things radiate more
This will happen until:
Energy in
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Energy out
When energy in equals energy out, we call this “energy
balance”
Energy Balance on Earth
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If the solar radiation into Earth is greater than the
outgoing longwave radiation, the temperature will
increase
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A temperature increase then results in an increase of the
longwave radiation out (hotter things radiate more)
This will happen until:
Shortwave in
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Longwave out
Global warming upsets the energy balance of the planet
Energy Balance with No Atmosphere
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If there was no atmosphere, for energy balance to
occur, the mean temperature of Earth would be 0o F (-18o
C)
-18o C (0o F)
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Missing piece: the greenhouse effect
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All longwave radiation doesn’t escape directly to space
The Greenhouse Effect
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Greenhouse gases block longwave radiation from
escaping directly to space
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These gases re-radiate both upward and downward
The extra radiation causes additional warming of the surface
Extra downward
radiation due to
greenhouse gases
15o C (59o F)
The Greenhouse Effect
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Greenhouse gases cause the outgoing radiation to happen
at higher levels (no longer from the surface)
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Air gets much colder as you go upward
So the radiation to space is much less (colder  less emission)
15o C (59o F)
The Greenhouse Effect
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Greenhouse effect is intuitive if you pay attention to the
weather!
Cloudy nights
cool less quickly
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In the desert, temperatures plunge at night!
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No clouds & little water vapor in the desert: little greenhouse effect
Summary
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The Earth is heated by the Sun
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Earth loses energy due to longwave radiation
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This is shortwave radiation
Albedo: key factor that determines how much radiation is
absorbed vs reflected
The greenhouse effect causes less heat loss due to longwave
radiation
If the Earth is pushed out of energy balance, it warms or
cools in response
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Warming can occur from increased solar radiation or increased
greenhouse effect