Transcript Follow the Energy

PTYS 214 – Spring 2011
Announcements
 Homework #5 available for download at the class website
DUE Thursday, Feb. 24
Reminder: Extra Credit Presentations (up to 10pts)
Deadline: Thursday, Mar. 3 (must have selected a paper)
 Class website:
http://www.lpl.arizona.edu/undergrad/classes/spring2011/Pierazzo_214/
 Useful Reading: class website  “Reading Material”
http://en.wikipedia.org/wiki/Greenhouse_effect
http://www.lsbu.ac.uk/water/vibrat.html
http://www.atmosphere.mpg.de/enid/25s.html
Quiz #4
20
 Class Average: 3.2
15
 Low: 0
 High: 4
# Students
 Total Students: 24
10
5
0
0
1
2
3
Grade
Quizzes are worth 20% of the grade
4
5
Solar Spectrum at Earth’s Surface
Greenhouse gases absorb IR radiation at specific wavelengths
Greenhouse
gases and
radiation
 Solar radiation is
absorbed on its way
to the Earth’s surface
 Terrestrial IR
radiation is absorbed
on its way out
towards space
Effect of the Atmosphere
(Earth and Solar spectra are NOT to scale)
Red reaches Earth’s surface
Blue escapes to space
Atmospheric Greenhouse Effect
 The Greenhouse Effect increases the surface
temperature by returning part of the outgoing IR
radiation back to the surface
 The outgoing IR radiation includes Earth’s radiation
but also the IR part of the reflected solar spectrum
 The magnitude of the greenhouse effect depends
on the abundance of greenhouse gases (CO2, H2O,
O3, CH4, etc.)
Non-Greenhouse Gases
 The molecules/atoms that constitute the bulk of the
atmosphere: O2, N2 and Ar, do not interact with
infrared radiation significantly (scattering)
 While the oxygen and nitrogen molecules can vibrate,
because of their symmetry these vibrations do not
create any transient charge separation (dipole)
 Without such a transient dipole moment, they can
neither absorb nor emit infrared radiation
Water in the Earth’s Atmosphere
 The water content of the atmosphere varies about 100fold between the hot and humid tropics and the cold and
dry polar ice deserts
 Water vapor is the main absorber of radiation in the
atmosphere, accounting for about 70% of all
atmospheric absorption of radiation, mainly in the IR!
 Liquid water and ice droplets are also present in the
atmosphere as clouds
 Clouds both reflect sunlight, which cools the Earth, and
trap heat in the same way as greenhouse gases, and
thus warm the Earth
Cumulus cloud
Cirrus clouds
puffy, white clouds
Stratus clouds
grey, low-level clouds
high, wispy clouds
Clouds
and
Radiation
 Stratus Clouds
reflect sunlight  Cooling
 Low thick clouds have a high albedo, reflecting more sunlight
 Cirrus Clouds absorb and re-emit outgoing IR radiation
 Warming
 High, thin clouds have a low albedo, letting most solar
radiation through but absorbing and emitting IR
Atmospheric Greenhouse Effect
Vis
IR
UV
IR
IR, UV
all
IR
all
all
IR
IR
Activity:
The Greenhouse Effect
The Greenhouse Effect
2) Does the Sun give off more UV or IR photons?
IR photons – Why?
3) Does Earth’s surface emit radiation at night?
Of course!
5) Which has an easier time getting through the atmosphere,
Visible or IR?
Visible
6) What about radiation emitted by Earth?
It is IR, so it tends to be trapped by the atmosphere
8) What is the radiation heating Earth’s surface and
atmosphere?
Earth’s surface: mostly Visible and IR
Earth’s atmosphere: IR, UV
Energy Flow WITHOUT Greenhouse Effect
Earth without an atmosphere
What about other solar system objects
with an atmosphere?
Planet
Emission
Temperature
Surface
Temperature
Venus
282K
740K
Earth
255K
288K
Mars
210K
210K
Titan
82K
94K
Difference between Emission and Surface Temperatures
indicates the efficiency of the greenhouse effect
Back to the Habitable Zone
Consider a planet with:
– Earth’s atmospheric greenhouse warming
(33 K) and
– Earth’s planetary albedo (~ 0.3)
Where would the boundaries of
the Habitable Zone be for such planet?
Remember the Energy Balance Equation:
Eabs = Eout
(1 a)S  4σT
4
e
Eout
aEin
Ein
The solar constant, S, at any given distance
from the Sun, R, is determined by the Inverse
Square Law:
L(W)
S
2
2
4D (m )
D is the distance of a
planet from the star (the
Sun for our Solar System)
We can substitute the formula for the Solar flux to
the planetary energy balance equation and solve
for the distance:
(1 a)S  4σT
L
4
(1  a)
 4σTem
2
4D
4
em
L(1- a)
D
4
16Te
The temperature in this equation is the effective
emission temperature of the planet
The Habitable Zone
L(1- a)
D
4
16Tem
The distance at which liquid water can be found on a
planet’s surface varies with:
- the Star’s Luminosity, L
- the Planet’s Albedo, a
- the Planet’s Effective Emission Temperature, Tem
But liquid water depends on the temperature
on the surface…
The Habitable Zone
 We want to find the region around the Sun where water
could be in liquid form
 For that assume for the surface temperature that
273K < Ts < 373K
How does the surface temperature relate
to the emission temperature?
Ts = Tem + TGH
The Solar System Habitable Zone
L(1- a)
D
4
16Tem
where: Tem
= Ts - TGH
For an Earth-like planet: TGH = 33K
a = 0.3
The range of surface temperatures is limited by:
Min: Ts = 273K → Tem(min) → Dout
Max: Ts = 373K → Tem(max) → Din
Habitable Zone
Region around a star where a planetary body can
maintain liquid water on its surface
Dout
Din
Average surface temperature (Ts)
The average surface temperature (Ts) depends on three
main factors:
a) Solar luminosity (energy emission from star)
b) Planetary albedo (on Earth it is also affected by
clouds)
c) Greenhouse Effect (CO2, H2O , CH4, O3 etc.) – this
implies the presence of an atmosphere!
Complication:
The amount of atmospheric greenhouse warming (∆Tg)
and the planetary albedo (a) can change as a function
of surface temperature (Ts) through different feedbacks
in the climate system
Climate System
We can think about climate system as a number of
components (atmosphere, ocean, land, ice cover,
vegetation, etc.) which constantly interact with each other
Coupling of System Components
Positive Coupling
Car’s gas pedal
(+)
Car’s speed
A change in one component leads to a change of the
same direction in the linked component
Negative Coupling
Car’s break
pedal
(-)
Car’s speed
A change in one component leads to a change of the
opposite direction in the linked component
Negative Coupling in Climate
Earth’s albedo
(reflectivity)
(-)
Earth’s
surface
temperature
 An increase in Earth’s albedo causes a
corresponding decrease in the Earth’s surface
temperature by reflecting more sunlight back to
space
 Conversely, a decrease in albedo causes an
increase in surface temperature
Positive Coupling in Climate
Atmospheric
CO2
(+)
Greenhouse
effect
 An increase in atmospheric CO2 causes a
corresponding increase in the greenhouse effect,
and thus in Earth’s surface temperature
 Conversely, a decrease in atmospheric CO2
causes a decrease in the greenhouse effect
Feedbacks
In nature component A affects component B but
component B also affects component A
This “two-way” interaction is called a feedback loop
A
B
Loops can be stable or unstable
Unstable Loops
(+)
positive coupling
World
Population
Number
of Births
positive coupling
(+)
Positive feedback loop:
An unstable system which changes further
following a perturbation
Stable Loops
(-)
negative coupling
Number of
Preys
Number of
Predators
positive coupling
(+)
Negative feedback loop:
A stable system which resists change
following a perturbation
Multiple Feedback Systems
Odd numbers of negative couplings:
Overall negative (stable) loop
Even number of negative couplings:
Overall positive (unstable) loop