What is the Greenhouse Effect?
Review of last lecture
– The two basic motions of the Earth
– What causes the four seasons: the Earth’s tilt
–
and the 3 ways it affects the solar insolation
(change of length of the day, beam spreading,
beam depletion)
Change of the Earth’s orbit at longer time
scales (Milankovitch cycles): eccentricity, axial
tilt, and precession
Satellite Measurements of the Earth’s
Radiation Budget
NASA’s Earth Radiation
Budget Satellite (ERBS)
1985-1989
Earth’s energy budget (averaged over the
whole globe and over a long time)
Yellow:
shortwave
Red:
longwave
Sensible
heat 7%
Net Longwave 21%
Latent heat
23%
• At the top of the atmosphere:
•
Incoming shortwave = Reflected Shortwave + Emitted longwave
At the surface:
Incoming shortwave = Reflected shortwave + Net emitted longwave (emitted - incoming)
+ Latent heat flux + sensible heat flux
Atmospheric influences on radiation
Reflection
Scattering
Absorption
(absorber
warms)
Reflection
• reflection – redirection of energy w/o
•
•
absorption
all objects reflect visible light 
effectiveness varies
albedo – % of visible light reflected
• There are two types of reflection (solid
surface):
– Specular: light is reflected with
equal intensity (e.g. mirror)
– Diffuse reflection OR scattering:
light is reflected in multiple
directions, weakly (e.g. snow)
Atmospheric Scattering
3 Types of Scattering:
1.
2.
3.
Raleigh
Mie
Non-Selective
A discussion of each type follows…
Rayleigh Scattering
•
•
•
involves gases smaller than insolation wavelength
scatters light in all directions
most effective at short wavelengths (violet, blue)…
hence, blue sky
The Earth has an atmosphere. So it has
Rayleigh scattering and its sky appears
blue
The Moon has no atmosphere. So it
has no Rayleigh scattering and its sky
appears dark
•
Rayleigh scattering also explains reddish-orange sunsets
when light travels through thick slice of atmosphere
Monet: Impressions, Sunrise
Grainstacks at different
time of the day
2) Mie scattering
– involves aerosols (e.g. dust, smoke) larger
than gas molecules
– forward scatter
– equally effective across visible spectrum
– explains hazy, gray days
3) Non-selective scattering
– Happens when atmospheric particles are much larger
than the wavelength of incoming radiation (e.g. water
droplets in clouds)
– Act like lenses; scatter all wavelengths equally to
create a white appearance
– That’s why clouds appear white
Video: The greenhouse effect
• http://www.youtube.com/watch?v=ZzCA60Wno
Mk
Atmospheric Absorption - The Greenhouse Effect
Transparent
to solar
(shortwave)
radiation
Opaque to
earth’s
(longwave)
radiation
Major GH gases:
CO2, H20(v), CH4
The importance of methane (CH4)
• 23 times more powerful as a greenhouse gas than
•
CO2
The livestock sector is a major player, which
accounts for 35-40% global anthropogenic
emissions of methane (their burps!)
• The livestock sector is responsible for 18% of
•
total greenhouse gas emissions
Therefore, consuming less meat can help reducing
global warming than not driving cars.
Seasonal variation of surface radiation
Surface “Sensible” and
“Latent” heat transfers
First, recall 2 other methods of
energy transfer in addition to
radiation:
1. Conduction
– This is how excess heat in ground is
transferred to the atmosphere via an
extremely thin layer of air in contact
with the surface
2. Convection
– Once the heat is transferred from the
surface to the air via conduction,
convection takes over from here via
“sensible” and “latent” heat transfers
Sensible Heat
• Heat energy which is readily detected
• Magnitude is related to an object’s specific heat
– The amount of energy needed to change the temperature of
an object a particular amount in J/kg/K
• Related to mass
– Higher mass requires more energy for heating
• Sensible heat transfer occurs from warmer to cooler
areas (i.e., from ground upward)
Latent Heat
• Energy required to induce changes of state
in a substance
• In atmospheric processes, invariably
involves water
• When water is present, latent heat of
evaporation redirects some energy which
would be used for sensible heat
– Wet environments are cooler relative to
their insolation amounts
• Latent heat of evaporation is stored in water
vapor
– Released as latent heat of condensation
when that change of state is induced
• Latent heat transfer occurs from regions of
wetter-to-drier
Seasonal variation of surface energy budget
Storage change = net radiation - latent heat flux sensible heat flux
Summary
• Earth’s energy balance at the top of the atmosphere and at
the surface. What percentage of solar energy is absorbed by
the surface?
• Atmospheric influences on radiation (3 ways)
• The three types of atmospheric scattering. What causes the
blue sky? Why causes the reddish-orange sunsets?
• What cause the greenhouse effect? What are the major
greenhouse gases? Why is methane important?
• Sensible heat flux (dry flux from warm to cold regions) and
latent heat flux (wet flux from wet to dry regions)