ATM S 111, Global Warming:
Understanding the Forecast
DARGAN M. W. FRIERSON
DEPARTMENT OF ATMOSPHERIC SCIENCES
DAY 1: OCTOBER 1, 2015
Outline
 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
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!
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 is 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 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…
Here it’s
nighttime
High latitudes
get less direct
radiation, which
spreads out more
Reason for Seasons
 Directness of solar radiation is key for seasons as well
 Winter is tilted away from the Sun, gets less direct light, and
thus is colder
Winter solstice
December 21-22
Equinox
March 20-21
or Sept 22-23
Summer solstice
June 20-21
Current sunlight
South pole sky soon after equinox (October maybe? – Sun goes around low in the sky)
Winter in the Northern Hemisphere
 When the Earth is tilted away from the Sun…
 The Sun is lower in the sky
 Days are shorter
When Solar Radiation Hits the Atmosphere
 Average incoming solar radiation = 342 W/m2
 Only 20% gets absorbed in the atmosphere
 This includes absorption of dangerous
UV by the ozone layer
 50% is absorbed at the surface
 Meaning much of the sunlight makes it
directly through the atmosphere!
 30% is reflected back to 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?


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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
 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 rather transparent 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
Longwave radiation
 From my cat
Temperature & Radiation
 Higher temperature = more radiation and more
energetic radiation (shorter wavelengths)
 Explains the Sun’s radiation too

Sun is really hot 
It emits much more radiation
 It emits shortwave radiation instead of longwave radiation like
the Earth

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
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
 If the Earth is pushed out of energy balance, it
warms or cools in response

Warming can occur from increased solar radiation or
increased greenhouse effect