ATM S 111, Global Warming:
Understanding the Forecast
DARGAN M. W. FRIERSON
DEPARTMENT OF ATMOSPHERIC SCIENCES
DAY 1: MARCH 30, 2010
Reading Assignments
 From last time:
 Make sure you’ve read Rough Guide p. 3-19 “Climate Change:
A Primer”
The Big Picture
 The Outlook
 What Can We Do About It

 Next reading assignment:
 Rough Guide p. 20-31 “The Greenhouse Effect”
 If it’s about hotels in Melbourne you might have bought the
wrong Rough Guide
Outline of This Lecture
 How exactly the Sun heats the Earth
 How strong?
 Important concept of “albedo”: reflectivity
 How the greenhouse effect works
 How the Earth cools
 And how greenhouse gases lead to less cooling
 What are the main greenhouse gases?
 And which are changed by human activity?
From Before We Asked…
 What factors influence climate at a given place?
 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?

We’ll study this next
The Sun
 Driver of everything in the climate system!
We Are Small!
Fun facts (i.e., won’t
be on the HW/tests):
Sun has approximately:
• 100 times the radius
of the Earth
• 1 million times the
volume of Earth
• 300,000 times the
mass of Earth
The Sun is 10 light
minutes away from
Earth (100 Sun
diameters away)
Fun Facts: Other Heat Sources?
 The Sun is by far the main heat source for the
atmosphere/ocean system
 What are other ways the atmosphere and ocean can be
heated?


Heat from volcanoes or the solid Earth: 0.025% of the Sun’s
heating
Direct heating of the atmosphere/ocean from burning of fossil
fuels or nuclear power: 0.007% of the Sun


This is not the greenhouse effect
Tides: 0.002% of the Sun

Caused by the moon, lead to motions which eventually turn to heat
from friction
 So the Sun contributes over 99.97% of the energy input!
How Does Energy Arrive From the Sun?
 Energy from the Sun is “electromagnetic
radiation” or just “radiation” for short


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

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
 Types of electromagnetic radiation, from most
powerful to least powerful (or shortest wavelength to
longest wavelength)







Gamma rays
X-rays
Ultraviolet (UV) radiation
Visible light
Infrared radiation
Microwaves
Radio waves
Sun’s Radiation
 The Sun emits:
 Visible light (duh)
 Also “near infrared” radiation (infrared with very short
wavelength)
 A small (but dangerous) amount of ultraviolet radiation

This is what makes us sunburn!
 These three bands together we call “shortwave
radiation”
How Strong is the Sun?
 By the time it gets to the top of Earth’s atmosphere,
the Sun shines at a strength of 1366 Watts per
square meter
 Watt (abbreviated as W): unit of power or energy
per unit time
 1366 W/m2 is roughly what’s experienced in the
tropics when the sun is directly overhead
Average Solar Radiation
 The average incoming solar radiation is not 1366
W/m2 though

It’s only 342 W/m2 (exactly ¼ of this). Why?
Half the
planet is
dark at all
times…
High latitudes
get less direct
radiation, which
spreads out more
Here it’s
nighttime
Reason for seasons:
Winter is tilted
away from the Sun,
gets less direct
light, and thus is
colder
Next: When Solar Radiation Hits the Atmosphere
 So, the average incoming solar radiation is 342 W/m2
 What happens when this encounters the atmosphere?
 Only about 20% gets absorbed within the atmosphere
 This includes absorption of dangerous UV by the ozone layer
 50% is absorbed right at the surface
 Meaning much of the sunlight makes it directly through the
atmosphere!
 30% is reflected away, back into space
 What does the reflecting?
Key Concept for Climate: Albedo
 Albedo: fraction of incident light that’s
reflected away

Albedo ranges from 0 to 1:
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
 Clouds, ice, and snow have high albedo
 Cloud albedo varies
from 0.2 to 0.7
 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
 Remember we said 30% of incoming solar radiation is
reflected away?



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
 Total absorbed solar radiation is 70% of the
incoming solar radiation


Because 30% is reflected away
70% of 341 W/m2 = 240 W/m2
Summary So Far
 The Sun heats the Earth
 Some is reflected back, a bit is absorbed in the atmosphere
 But other than that, the atmosphere is pretty much
transparent when it comes to solar radiation (half is absorbed
right at the surface!)
 Clouds and snow/ice are primary contributors to the albedo of
Earth
 Next, how energy escapes from Earth and the
greenhouse effect
“Longwave Radiation”
 The Sun is the energy input to the climate system
 How does the Earth lose energy?
 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”
 Everything actually emits radiation
 Depends partly on the substance but mostly on temperature
Neck = hotter
Hair = colder
Infrared thermometer
Longwave Radiation
 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
Energy Into and Out of the Earth
 Heating/cooling of Earth
 The Earth is heated by the Sun (shortwave radiation)
 The Earth loses energy by longwave radiation (out to space)
“Energy Balance”
 If the energy into a system is greater than the
energy out, the temperature will increase

A temperature increase then results in an increase of energy out


Hotter things radiate more
This will happen until:
Energy in
Energy out
 When energy in equals energy out, we call this “energy
balance”
Energy Balance on Earth
 If the solar radiation into Earth is greater than the
outgoing longwave radiation, the temperature will
increase


A temperature increase then results in an increase of the
longwave radiation out (hotter things radiate more)
This will happen until:
Shortwave in
Longwave out
 Global warming upsets the energy balance of the planet
Energy Balance with No Atmosphere
 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)
 Missing piece: the greenhouse effect
 All longwave radiation doesn’t escape directly to space
The Greenhouse Effect
 Greenhouse gases block longwave radiation from
escaping directly to space


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
 Greenhouse gases cause the outgoing radiation to
happen at higher levels (no longer from the surface)


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
 Greenhouse effect is intuitive if you pay attention to
the weather!
Cloudy nights
cool less quickly


In the desert, temperatures plunge at night!

No clouds & little water vapor in the desert: little greenhouse effect
What are the Major Greenhouse Gases?
 Our atmosphere is mostly nitrogen (N2, 78%),
oxygen (O2, 21%), and argon (Ar, 0.9%)

But these are not greenhouse gases
 Molecules with 1 atom or 2 of the same atoms aren’t
greenhouse gases though

Just like the atmosphere is almost transparent to solar
radiation, the primary gases in our atmosphere are transparent
to longwave radiation
If our atmosphere was only nitrogen, oxygen,
and argon, this picture with no greenhouse
effect would be accurate!
Greenhouse Gases
 Polyatomic molecules are greenhouse gases
 Water vapor (H2O)
 Carbon dioxide (CO2)
 Methane (CH4)
 Nitrous oxide (N2O)
 Ozone (O3)
 Chlorofluorocarbons (the
ozone depleting chemicals which
have been banned)
The fact that they can rotate and vibrate means
they can absorb the right frequencies of longwave
Water Vapor
 Gas form of water
 AKA humidity
 The number one greenhouse gas!
 Powerful because there’s a lot of it
 Not controlled by humans!
 It’s a feedback not a forcing (topic of the next lecture)
 Observed to be increasing with global warming
Carbon Dioxide
 CO2
 It’s what we breathe out, what plants breathe in
 The primary contributor to the anthropogenic
(human-caused) greenhouse effect

63% of the anthropogenic greenhouse effect so far
 Increases primarily due to
fossil fuel burning (80%)
and biomass burning (e.g.,
forest fires; 20%)


Preindustrial value: 280 ppm
Current value: 386 ppm
Carbon Dioxide
 CO2 will also be the main problem in the future
 It’s extremely long-lived in the atmosphere
 50% of what we emit quickly gets taken up by the ocean or
land



We’ll discuss this more later
Most of the rest sticks around for over 100 years
Some of what we emit will still be in the atmosphere over
1000 years from now!
Methane
 CH4
 Natural gas like in stoves/heating systems
 Much more potent on a per molecule basis than CO2
 Only 1.7 ppm though – much smaller concentration than CO2
 Natural sources from marshes (swamp gas) and
other wetlands
 Increases anthropogenically due
to farm animals (cow burps),
landfills, natural gas leakage, rice
farming
Methane
 The lifetime of CH4 is significantly shorter than
carbon dioxide


Breaks down in the atmosphere in chemical reactions
Lifetime of methane is only 8 years
Methane concentrations have
been leveling off in recent years,
possibly due to drought in wetlands
at high latitudes
Global Warming Potential
 CO2 lifetime > 100 years
 Methane lifetime = 8 years
 But methane is a much stronger greenhouse gas
 How to put these on similar terms? Global
warming potential (or GWP)

Global warming potential is how much greenhouse effect
emissions of a given gas causes over a fixed amount of time
(usually 100 years)


Measured relative to CO2 (so CO2 = 1)
Methane’s global warming potential is 25

Much more potent than CO2 even though it doesn’t stay as long
Nitrous Oxide
 N2O
 Laughing gas
 Also more potent on a per molecule basis than CO2
 Global warming potential: 310
 Comes from agriculture, chemical industry,
deforestation
 Small concentrations of
only 0.3 ppm
Ozone
 Ozone or O3 occurs in two places in the atmosphere
 In the ozone layer very high up
This is “good ozone” which protects us from ultraviolet radiation &
skin cancer
 Remember ozone depletion is not global warming!


Near the surface where it is caused by air pollution: “bad ozone”
 Bad ozone is a greenhouse gas, and is more potent on a
per molecule basis than CO2

But very very short-lived

Fun fact: Global warming potential for ozone is not usually calculated
– rather it’s wrapped into the GWPs of the other gases that lead to its
chemical creation
CFCs
 CFCs or chlorofluorocarbons are the ozone
depleting chemicals

Have been almost entirely phased out
 CFCs are strong greenhouse gases
 Their reduction likely saved significant global warming in
addition to the ozone layer!
 Some replacements for CFCs (called HFCs) are
strong greenhouse gases though
 Global warming
potentials of up to
11,000!
The Natural Greenhouse Effect
 Contributions to the natural greenhouse effect:
 H2O (water vapor): 60%
 CO2 (carbon dioxide): 26%
 All others: 14%
 These numbers are computed with a very accurate
radiation model

First running with all substances, then removing each
individual gas
The Unnatural Greenhouse Effect
 Increasing levels of CO2 and other greenhouse gases
leads to a stronger greenhouse effect

With more greenhouse gases, it becomes harder for outgoing
radiation to escape to space
It’s like this picture from
before, but more.
More radiation trapped
before it gets out to space.
Longwave radiation is
emitted from a higher
(and colder) level on
average.
The Unnatural Greenhouse Effect
 Contributors to the “anthropogenic” greenhouse
effect

Numbers for the whole world up to this point:
Carbon dioxide: 63%
 Methane: 18%
 CFCs, HFCs: 12%
 Nitrous oxide: 6%

The Anthropogenic Greenhouse Effect
 Contributors to the “anthropogenic” greenhouse
effect
Numbers for the US
based on current (2008)
emissions

CO2 is the big problem in the US
currently.
Note how much lower the HFCs are
than on the previous slide. This is
b/c we basically don’t emit CFCs any
more.
From US EPA 2010 report (draft)
Summary
 The Earth is heated by the Sun
 This is shortwave radiation
 Albedo: key factor that determines how much radiation is
absorbed vs reflected
 Earth loses energy due to longwave radiation
 The greenhouse effect causes less heat loss due to longwave
radiation
 Greenhouse gases:
 Number one is water vapor
 Number two is CO2
 Global warming potential: key concept
Survey Question about Types of Radiation
 Which of the following are types of electromagnetic
radiation?







A. X-rays
B. Sonar
C. Radio waves
D. A + B
E. A + C
F. B + C
G. All